This section deals with the nuts and bolts of actually taking pictures. This is the part where most of us like to spend time on. :)
絞り - defines the size of the opening in the lens. Aperture is metered in f-numbers. f-number is a ratio between the diameter of aperture and the focal length of lens (e.g. for 50 mm lens f/2 means that diameter of aperture is 25 mm). Traditionally f-numbers are set in a sequence so that each higher number (called f-stops) reduces amount of light on the image sensor two times (i.e 1 ev). This sequence is - f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32 etc. Modern cameras also allow you to change aperture in 1/2 or 1/3 ev steps. 絞りはレンズの開口の大きさを決める。絞りはf値で表される。f値は絞りの直径とレンズの焦点距離の比である（すなわち焦点距離50mmのレンズでf値が2ということは、絞りの直径が25mmということである）。伝統的に、f値は一連の数として設定され、(stub) 最近のカメラは絞りを1/2evまたは1/3ev刻みで変更できる。
' - defines the exposure time i.e. the time for which the shutter is held open during the taking of a photograph to allow light to reach the sensor. Shutter speed is metered in seconds or fractions of a second (e.g. 1s, 1/30s, 1/60s, etc.). Many cameras also have bulb shuter speed, which means that shutter is opened as long as you keep shutter release button pressed. Traditionally shutter speed is set in a sequence so that each number (called f-stops) changes amount of light approximately two times (i.e 1 ev) from the previous number. This sequence is - 1s, 1/2s, 1/4s, 1/8s, 1/15s, 1/30s, 1/60s, 1/125s, 1/500s, 1/1000s, 1/2000s, 1/4000s etc. Modern cameras also allow to change shutter speed in 1/2 or 1/3 ev steps.
For a given set of illumination conditions and sensor ISO speed, there usually are several combinations of aperture and shutter speed which will make a correctly exposed image (e.g f/4 - 1/500s, f/5.6 - 1/125s, f/8 - 1/60s). Choices are more limited in dark or extremely bright conditions. For example in a night scene you may already have your ISO speed set to your cameras maximum and the lens aperture wide open, so your only choice to allow more light in is to increase the shutter speed.
For artistic/photographic reasons, you may want to use specific settings for some of these options. To ensure the same brightness is maintained, you will then need to inversely change one of the other options. For example, if you are shooting a flower and want the background to blur out, you will want a short depth of field... hence a more open aperture, say F4 or F8. This also lets more light in than the automatic settings would have chosen, so you will need to speed up the exposure time to compensate. Or, if you are doing a sports shot and want to show action blur, you will want a slower exposure time. Since this also lets in more light, you will also have to close down the aperture a bit to compensate and still get the correct exposure. This will be covered in more detail as we continue.
Sensitivity of the sensor - setting ISO speed[編集]
In analogue photography, ISO measures the sensitivity of the film used. What you do need to remember is that a higher ISO will give you a brighter image but that the image will have more noise.
Generally it's best to use the lowest ISO (normally 100) available unless you have a good reason to use a different one. There is no universal standard so ISO 100 on one camera may be good as ISO 50 on another. The only way you can find out what ISO level is acceptable to you is by experimenting.
If your camera has an automatic ISO selection function don't use it. Digital cameras are notoriously bad at automatically choosing ISO. If you let them do it you'll end up regretting it when the perfect shot ends up noisy.
ISO refers to film speed and a low ISO film is regarded as slow film and a high ISO film is fast film. Slow film offer better picture quality and fast film has a lot of grain referred to as noise in digital photography. Fast film does have its place when shooting in low light environments.
ISO is also known as ASA. ISO stands for International Standards Organisation and ASA stands for American Standards Association. In terms of photography they are both identical in that 100 ISO film and 100 ASA film is one and the same.
ISO settings in consumer-level/ entry-level digital cameras are significantly different from pro-level digital cameras. This is primarily due to the different manufacturing techniques used in high production, middle quality sensors and low product, high quality sensors. When you compare photos across cameras, an entry-level camera will typically have very high noise at around ISO400, whereas a digital SLR will likely not have a speck at ISO400. As such, ISO values can only be compared within camera, not across camera.
Prosumer SLR cameras tends to perform very well up to ISO800, and even at ISO1600, the noise is noticable but tolerable. As can be expected, ISO1600 offers a trememdous advantage to photographers in low light/ night environment.
There are normally three types of light-metering: evaluative, center weighted average and spot. Your camera uses these to decide on what exposure it should aim for.
Evaluative means your camera will try and choose an exposure which will work well for the entire image, however how this is implemented can vary vastly from camera to camera. Evaluative metering usually is preferable when you use automatic or semiautomatic (Av, Tv) exposure setting.
Spot is the exact opposite of the Evaluative setting, rather than taking into account the whole of the image it tries to choose an exposure that is appropriate for whatever appears in the center of the image ignoring the rest of the image. This metering method is useful if you set the camera to manual exposure mode. A typical camera has dynamic range of about 6 ev steps. Areas for which camera show +3 ev in picture will be absolutely white, and areas with -3 ev will be absolutely black. The easest way to set correct exposure will be to meter highlights (brightest white spots) in your composition and set exposure so that highlights read at +2.7 or +3 ev.
Center weighted falls between these two settings, the camera chooses an exposure which will work well for the entire image, but places more importance on getting the exposure right for the center section.
- オートマチックモード (A, Autopicture, different picture modes (portrait, landscape etc.), P (Program)), in which the camera sets your aperture and shutter speed.
- セミオート or priority modes:
- Shutter priority (Tv) - you set the shutter speed and the camera sets the correct aperture.
- Aperture priority (Av) - you set the aperture and the camera sets the shutter speed.
- マニュアルモード(M and B) - you set the aperture and shutter speed.
- Using a "soft" camera setting.
Camera movement (or camera shake) usually comes from hand-holding your camera while using a slow shutter speed. To minimize camera movement while using slow shutter speeds of 1/25th of a second or lower you should use a tripod. If you don't have a tripod with you, try placing the camera on a flat surface and using the camera's self timer (obviously, this works best when there is not much movement in the scene). Sometimes, if the lighting conditions are marginal where your shutter speed is right at 1/25th of a second, if you have a very steady hand, you can try holding your breath briefly as you snap the shutter. You can also try manually increasing the shutter speed at the expense of using a wider aperture (lower f-stop). Generally, a higher shutter speed decreases the effect of camera movement, but the lower f-stop decreases the depth of field and requires more careful focusing.
手ぶれは主に、シャッタースピードが遅い写真を手持ちで撮ったときに起こることが多い。1/25th of a second？よりも遅いシャッターを切るときには、手ぶれを防ぐために三脚を使うべきです。三脚を持っていないなら、カメラを平らなところに置き、セルフタイマーを使ってみてください。移動しないで撮るときには有効な方法です。（途中まで）
Lack of proper focus is another cause for lack of sharp photographs. Fortunately, most digital cameras today comes with autofocus capabilities. In a few fixed lens cameras and in almost any digital SLR, the autofocus can be disabled allowing you to focus the lens manually. Greater care must be exercised to make sure the object that you wish to be the center of attention in the photo is sharply focused.
In more inexpensive cameras, the only focus adjustment may be a normal and macro mode. The normal mode is usually represented by a mountain icon and the macro mode is usually represented by a flower icon. If your camera only supports these two modes, make sure it is in the proper mode before you snap the shutter to achieve the sharpest photos.
Camera with normal and macro focusing modes
Poor lighting conditions may cause your photos to lack definition and contrast. You may notice that even when you use a tripod and with your camera handling the autofocus, your photographs may not appear as sharp as you might wish. This is especially true for available light photographs where you do not wish to use a flash. The fault for this lies not with your camera but with the scene itself. In cases like this, try increasing the available light shining directly on the subject. This can improve the photograph by allowing the edges to be more sharply defined against the background.
In camera sharpening can also affect the sharpness of an image. Some higher end cameras offer a sharpness adjustment that usually ranges from +2 or +3 to -2 or -3 with zero in the center. Increasing the sharpness can give you crisper looking images, and decreasing it will give a more "soft focus" effect. Some photographers like to use a soft-focus effect when taking potraits, especially of women, because it tends to smooth out the appearance of the skin. Increasing the sharpness will not compensate for camera shake or lack of proper focus. Many photographers choose to leave the sharpness setting at zero and perform any sharpening in the post-processing stage.
What's the slowest exposure time you can get away with ?[編集]
"Camera shake" is what happens when you don't hold the camera sufficiently still. The longer the exposure time, the longer you have to hold the camera perfectly still, and the more chance there is for shake.
To see the effect, look at the following shots:
How long an exposure time you can use before being affected by camera shake depends very much on the individual. With a lot of practice it's possible to hold the camera still for as much as half a second, but most people should be able to hold a camera still for 1/20 of a second with a little practice. Beginners should be able to hold for 1/25 of a seconds without problem.
As a general rule of thumb, it is recommended to not use a shutter speed lower than the focal length of the lens on the camera. For example, if you use a 50mm lens, try not to use a shutter speed slower than 1/50. If you are using a zoom lens, use the lowest focal length number. For example, for a 12-24mm lens, don't use a shutter speed slower than 1/12 of a second
Another way to find out what shutter speed to use is by testing.
- Pick something which will stay motionless and has straight lines which can easily be used to detect blurring. A good option would be a piece of paper which has large lettering printed on it, or use a suitable book cover.
- Be somewhere that has a reasonable amount of light (not necessary - but then all of your photos won't have the same brightness). If light is sufficient, feel free to turn off the flash to allow you to take photos faster.
- Switch your camera to exposure-time/shutterspeed priority mode.
- Set the camera to a reasonably long exposure time (say 1 second).
- Take a photo of your target in the way you normally take photos. Don't go out of your way to try to be more stable unless you are willing to do that every time you take a photo.
- Reduce the exposure time by one or two steps (how long this is in seconds will depend on your camera).
- Repeat the last two steps until you get down to about 1/30th of a second.
Now load the photos onto your computer, the one taken at 1 second will be blurred unless you are some sort of statue, but as the exposure time decreases the amount of blurring decreases. Go through the photos in order until you find where the blurring is no longer noticeable. That photo will represent the maximum exposure time you can take photos without "camera shake". Don't worry if you didn't note down the exposure times while taking the photos because most modern digital camera will store the information alongside the photo using what is known as EXIF. Most photo-manipulation packages can read this EXIF information as can many stand-alone EXIF-readers.
(If you use Windows XP you can get the EXIF information by right clicking on the photo and choosing properties, then the summary tab and clicking on advanced.)
The EXIF-data will include a lot of information, but the field you'll want to find is "exposure time" which, logically, will tell you the exposure time used when that photograph was taken.
Now you know what exposure-time you can get away with. Many photographers have their own techniques for stabilizing the camera so that they can use longer exposure times. These normally involve levering body parts against each other for greater stability. One common technique is to hold the camera directly against the brow or nose. Another is to try to lean against the nearest stable object like a tree, wall, etc., and then hugging your elbows into your chest. How well these work for you will very much depend on you, and of course, practicing.
Now that you know how long an exposure time you can get away with using your normal technique, you can start experimenting with other ways to hold your camera that will allow for longer exposure times without causing camera shake.
When taking shots with exposure times longer than you normally use, it's a good idea to take several shots if you have the spare memory. The more shots you take the better the chances that you manage to hold sufficiently still for one of them to come out well.
As a general rule of thumb, the longest exposure time that tends not to be affected by camera shake is about 1/60th of a second or 1/125th of a second. At those values, the camera shutter is operating fast enough that the image captured is typically not affected by camera shake.
However, such values are typically achieved only at locations with good lighting, say, a typical afternoon outside. It is tricky to achieve this lighting indoors and photography at those shutter speeds without compromising on ISO (see below) or aperture is difficult at times
In addition to this, high end digital cameras are starting to appear with image stabilization or vibration reduction technologies. These can help to increase the longest exposure time to about 1/30th of a second or even 1/15th of a second when used right, and it is a huge advantage in indoors/ low light environment. Unfortunately, the technology is expensive at the moment, but it would eventually trickle into the more consumer-friendly models.
Let's say you want to shoot a little leaguer running to first base, and not have it blurred. If your camera is in a semi-automatic mode (for example shutter-speed priority), you can adjust the shutter-speed to be faster (i.e. shorter exposure time). The camera then tries to modify the other settings (flash and aperture size) to make the image have an appropriate brightness.
But there are limits to how far the camera can go. The amount the aperture can be opened is physically limited. Once the aperture has been opened to its widest and the camera is out of other options, the brightness can't be increased further. This means that when you decrease the exposure time beyond that point the image will get darker, or "underexposed".
Luckily your camera will be able to tell you when it reaches this point. How it does this varies from camera to camera but the most common way is to make the aperture size change to red on the display (you may have to have the LCD display turned on to see this). On many cameras this information is only updated when the shutter button is pressed half way. Further details on this should be available from your camera manual.
Just as in the case above, where short exposure time images can become darker, or underexposed, when you exceed the camera's ability to "open up" the aperture, the camera is limited by how much the aperture can be closed down. So, in very bright conditions, if the camera cannot close down further, images will be brighter, or "overexposed". Again, most cameras have a way of warning you when this point is reached, and unless in manual mode, will not allow you to shoot.
Hopefully by now you understand the basics of exposure time and how it affects your shots. If you have shots that you've taken before that have turned out blurred now is a good time to go and have a look at their EXIF information and pay attention to the enviroment the photos were taken in. Almost certainly, your blurred photos will have been taken in low light situations and the EXIF information will reveal that the exposure time was fairly long (longer than your "stability limit").
You should now be able to make educated guesses as to what your camera's automatic system was doing that resulted in blurred images. The camera detected low light so it probably tried to compensate by turning the flash on and opening the aperture as far as it could. However, even after doing this, the camera thought the photo would be too dark, so it increased the exposure time. Here it made the fatal mistake! Since the camera doesn't know how stable you can hold the camera or if it is on a tripod, it assumes that you can hold perfectly still for as long as you want.
Since you know that's not true, this is a perfect example of a situation where you should step in and take control. In most situations you don't want a blurred photo, (though you do have to consider what you want). If, for example, you would prefer a darker photo but without blur or camera shake, then all you have to do is switch to shutter-speed priority mode, set the exposure time to a setting which you are confortable with, and let the camera handle the rest of the settings.
What if you want the image brighter? Well, the simple answer is to increase the lighting. If you're indoors, then turn the lights on if they aren't on already or set up what you want to shoot near window light. If that's not an option, you can start playing with other settings such as the ISO. Increasing the ISO can increase the brightness of the shot by increasing the camera's sensitivity to light, but at the expense of causing more noise to appear in the image (more on ISO later).
All of these choices involve compromise, and your camera can't choose which compromise to make for you as it doesn't know why you want the photo and what's important about it. Only you can make that decision, and that's why you have to take control.
Why change the aperture ?[編集]
In general, cameras are fairly good at automatically changing the aperture to deal with the light situation.
However, as mentioned earlier, if you want to increase Depth of Field (the range around your subject that remains in focus) or improve the focus on distance shots, you might want to step in and make the aperture smaller.
Without getting into a lot of theory and mathematics, aperture widths are measured in what are called "F-Stops" written like this: f/1.0, f/2.8, f/4, f/5.6 f/8.0, f/11, f/16 etc. As these numbers get larger, the aperture gets smaller. So from these numbers f/1.0 means the aperture is the widest (wide open in fact), while f/16.0 is the smallest. When you increase the size of the aperture by setting it to a lower numbered f-stop, it is called opening up. When you decrease the size of the aperture by setting it to a higher numbered f-stop, it is called stopping down.
Put your camera into manual aperture priority and move the dial back and forth to adjust it. You will see the numbers in the view finder change as you open and close, and can see the range of options your specific camera has. Each of these numbers represents one "stop" more or less of light reaching the camera. Another, more indirect way of changing the aperture, is to switch to time mode and do the same drill, observing that the time notation changes for the amount of the light reaching the camera. The numbers show what part of a second the aperture is open. For example, t/90 means 1/90th of a second. You will see the numbers change something like this: t/90, t/125, t/180, t/250, t/350, t/500. Again, as the numbers change, so does the amount of light-- there's one "stop" less light with each shorter exposure.
If you use aperture-priority mode on your camera, beware of the same problems that you can get in fully automatic mode. Your camera may increase the exposure time significantly to compensate for less light through the aperature, so make sure the exposure time isn't too long when taking the shot.
If it is too long, then you have to use a tripod or other method to stablize your camera, and switch to manual mode. This will mean that you have to set both the aperture and exposure time by hand. But that's not too hard, now that you see that the numbers simply represent step changes in the amount of light reaching the camera.
Remember, as shown above, each step increment change of either the f/stop or time exposure represents one "stop" of light. As long as the change is proportional between these measurements, the amount of light is the same. I.e. If you open the lens up more, shorten the time the same number of steps to still get the same amount of light... if you close the lens more, increase the time the same number of steps to still get the same amount of light.
So, here's how you can set manually without fancy light meters. Simply find out what the camera's light meter is recommending in the automatic mode. For example, it may think you need t/250 with f/8. But you want to get a greater depth of field, say f/16. That is two steps smaller aperature than f/8, hence, two stops less light. So in manual mode, set f/16 (which is two clicks less light than recommended to get the right amount of light) and then INCREASE the amount of light two stops above what the automatic setting recommended, in this case, moving two clicks from t/250 to t/125. Actually, you can simply think of it as turning two "clicks" up or down for each setting and don't worry about the numbers.
Again, a simple solution for manual mode is to take the automatic mode setting, then switch to manual and "click" the same number of steps up or down to get your new aperature and speed settings.
Most camera lenses do not produce images of the best resolution with the aperture wide-open (e.g. f/1.4 on a 50 f/1.4 lens). Stopping down (increasing the f-stop setting) can improve their resolution, often drastically. This can be referred to MTF charts. Using f/8 or f/11 on most lenses produces an image with fine detail. Beyond this would actually degrade images due to diffraction, so don't overly stop-down unless you need greater depth of field.
The major tradeoff a photographer will encounter when fiddling with the aperture is this: low aperture allows more light in, but at the cost of Depth of Field; high aperture allows less light in, but have more Depth of Field.
Normally, you would want to shoot with aperture as low as possible, as not having enough light is typically more common than having too much light. Under some circumstances though, a low aperture gives more headaches than the increased light. One very obvious case is when you are taking photographs in macro mode.
In macrophotography, you are pushing the camera to capture details on a small object, typically close to the lens. In these cases, when the object is close to the lens, Depth of Field is critical. Having insufficient Depth of Field means only part of the object is in focussed. This is good if it's your artistic intention, but that's rarely the intention.
In these cases, you have to stop down to higher aperture numbers, and balance the tradeoffs elsewhere. Normally these come out from shutter speed (longer shutter speed) or ISO (higher ISO settings), which is why macrophotography tends to occur in controlled environment.
Thus, as a general rule of thumb, a aperture of around f/4 is probably adequate for normal photography; anything lower and you begin to get noticable loss of Depth-of-Field. At f/2.8, the Depth-of-Field is visably constrainted to fairly narrow bands.
For macrophotography, apertures around f/8 or f/10 or even higher are common. such high aperture values would mean a corresponding increase in shutter speeds.
There is another aspect of apertures that may not be obvious to casual users. Higher apertures gives you more Depth-of-Field, which means more things are in focus -- which means you have more room for errors in terms of focussing.
This is most obvious when you have a low-aperture lens, shooting a distance (really distant! Say, 150~200+ metres away) object, in poor light and/ or on a moving platform.
(Aside: One has to wonder why a photographer is in these conditions, but we digress... )
Even with autofocussing technologies in today's camera, getting a sharp picture in those circumstances is difficult. Autofocussing relies largely on contrast and not so much on image context, so it is easy for the autofocus to focus on the wrong portion of the subject.
If your aperture is set high enough, this isn't an issue; depth-of-field will ensure your subject is in focus. However, if you are shooting at low apertures -- say, f/2.8 -- the low depth-of-field will mean you need to be very precise in your focussing to ensure the subject is in focus. Make a mistake (easy enough when your lens is hand-held and shooting at 150+ metres), and your subject slips out of the depth-of-field and you end up with a blurred shot.
As such, using a high aperture gives you more margin of error when it comes to focussing on a subject. As usual, you have to adjust for the (generally) lower amounts of light a higher aperture implies, and it becomes a balancing act.
Changing what automatic/semi-automatic mode aims for[編集]
When you use automatic, shutter-priority, or aperture-priority modes the camera has a fixed idea about what exposure (brightness) it should aim for. What if you want to change this without having to use manual mode ?
Luckily most cameras come with a feature known as "exposure compensation" which allows you to underexpose or overexpose shots easily. However how much you should change the exposure depends on the situation and on your camera, so the best way you can learn is by experimentation.
Many digital cameras tend to overexpose and it's common for photographers to use an exposure compensation of -1/3 to -2/3 as default.
Many photographers use "bracketing", this is the name given to when you take several photos with a range of exposures to ensure one photo comes out with the correct exposure. Automatic mode normally gets fairly close to a good exposure so taking photos at one level above and below its defaults will probably get you a good shot.
Speeding up recovery time ?[編集]
recovery time (also called latency) is the time taken between shots (ie. after you take a photo how long do you have to wait before you take another)
You can speed up the recovery by turning some or all of the following things off,
- image sharpening
- getting a better memory card
Latency time is one of the downfalls of point and shoot type digital cameas, but as time goes by and technology improves the latency time has been reduced dramatically. More advanced digital SLR cameras are as fast as their film counterparts ever were with some professional SLR's offering up to eight frames per second with a buffer of forty frames.
The general opinion is that image sharpening should be left to the post-processing stage. Some cameras offer in-built sharpening, however these are normally less sophisticated than those available on most photo-manipulation packages. There is no technical advantage in using in-built sharpening over using a computer based sharpening tool. Actually image sharpening must be the last step in image manipulation, so if you will need some postprocessing the in camera sharpening will only decrease the quality of the final image.
Photo effects/Color modes[編集]
These let you take shots in Black and White, Sepia, vivid mode, etc. The basic rule is don't use them, shoot in colour and apply these effects in post-processing if you want them. Like with sharpening, your computer is able to take advantage of more sophisticated algorithms resulting in a better end-result.
White balance often confuses beginners; after all, white is white, isn't it?
Actually, it isn't. Your camera sees what colour an object is; unfortunately this won't be the same colour as humans see. This is because the human brain makes people see non-white colours as white.
The colour of an object is dependant on the light source, however humans automatically adjust so that anything which roughly combines the primary light colours together appears white.
"White-balance" is the name given to the technique cameras use to try and depict the image as humans see it. Most cameras will have at least three options (daylight, flouresecent, and tungsten) and may have many more which are designed to adjust to the light source.
Some cameras will also have a custom option, the custom option will allows you to setup a ""White-balance" for a specific lighting situation. The way it works is that you have to show it something which is white (or more often a grey card) and using that it makes corrections to the photos taken.
Luckily automatic white-balance detection is fairly sophisticated so most of the time leaving white-balance on auto mode will get you the result you want.
Depth of Field and achieving blurring effects[編集]
The depth of field (DOF)is the distance in front of and behind the subject which appears to be in focus. Areas which are out of DOF looks blurred. Sometimes it's very useful to have shallow DOF to separate the subject from the background (and/or foreground). Photographers try to get the subject in focus and the background slightly blurred.
DOF for given camera depends on three factors:
- Distance to subject. Getting closer to subject will make DOF shallower;
- Lens focal length. The longer focal length will make DOF shallower;
- Aperture. The larger aperture will make DOF shallower;
Despite there are three factors wich affect DOF, only aperture can help you control it. If you choose longer lens to get all subject in frame you will need increase distance to your subject and these two factors will compensate effects of each other.
So to have shallow DOF you need:
- fill as much of the frame as possible with your subject (it is usually easer with long lens)
- set big aperture (f4.0, f2.8 or even f1.4 if you have such lens)
Worth noting that cameras with bigger sensors will have shallower DOF for the same f-number, because for the same focal length of the lens you will need to go nearer to your subject to fill frame. So with digital SLR getting shallow DOF is much easer than with compact Point & shoot camera.
It is also possible, relatively easily, to introduce blurring at the post-processing stage although the effect may not be as good compared to that done in-camera.
Taking infra-red photos[編集]
Some digital cameras have sensors which are capable of detecting infrared, if your camera is capable of doing so then you should be able to obtain a infrared filter relatively cheaply (about £10 GBP/ $15 USD) which allows you to take infrared photos.
The simplest way you can tell if your camera is infrared-sensitive is to point an infrared source into your lens and see if it is visible on your LCD screen. Almost any infrared source can be used including remote controls (point the emitter at your lens and hold down a button on your remote).
A more technicial explanation follows:
All digital camera sensors are capable of detecting infrared; this is actually a problem with early digital cameras, because they pick up infrared and as such alters the captured image.
To deal with this issue, camera manufacturers build something called a hot mirror into digital cameras. Effectively this is an IR filter, designed to filter out IR so that the final images are not affected by the extra 'light' coming in.
Hot mirrors aren't 100% effective, thus some cameras are more sensitive to IR then others. Of course, if you are daring enough, you can open up your digital camera and replace the hot mirror with a glass of matching optical index, but this is not recommended and not advised. Doing that will almost certainly void your warranty, if not your entire camera.
IR filters are filters which filter out non-IR light; the filter itself appears black or reddish-black. When you hold it up to a light source, you may see a dim image of the light source showing through, but pretty much nothing else will show up.
But attach the filter to a IR-sensitive camera, and when you take photographs, you will see some images captured. How does this work?
Essentially, it's the same idea how photographs are taken. IR light are reflected off objects the same way 'normal' light is, and the IR filter will remove the 'normal' light, leaving the camera to intepret only the IR light. This results in an image that can be eerie in the extreme: Trees are white (IR reflects off leaves strongly), dark clothing becomes white and vice versa, things simply aren't what you expect.
IR photography does have some issues to take note of. First, IR light tends to be lower in intensity, so you need to compensate with lower aperture, longer shutter speed, and/or higher ISO. Also, IR focuses slightly differently from normal, which is why some SLR lens have IR markings for them. So you may end up with slightly out-of-focused images.
The last issue is not technical, but moral. IR can and does reflect straight through clothing, especially thin clothing. This can lead to some morally questionable photography, although it's really really much harder to achieve than people are led to believe. The ability does exist though and it has to be left to the individual photographer's morals to determine what to take in IR.
- An action shot
- Obviously we want a short exposure time here, to compensate we would open up the apeture. We might also use the flash, but remember using the flash will mean you have to wait for the flash to warm up/fire.
- A landscape shot
- This time we'd want a large aperture as we want to photograph a large area. If we want to increase the brightness of our shot then we'll need to increase the exposure time (as the flash probably won't be powerful enough to light up our scene). In a landscape shot the most likely source of motion is the camera person, so it's best to take these shots using a tripod to avoid motion.
- A framed painting
- If there is glass in front of the painting you will almost certainly want to turn the flash off otherwise you'll get effects like this:
- Which obviously you want to avoid. Using a wide-aperture is an obvious move, if you have a tripod or otherwise stable place where you can place the camera a long-exposure time will also help. If you still don't have enough light then you might consider increasing the ISO but going over ISO 400 is likely to cause a noticable noise effect on the final image.
- Another solution would be to take the photo at an angle to reduce the reflection of the flash, however "straightening" the photo in post-processing may take more effort than it is worth.
- Glass enclosed exhibits
- Most of what we've said about framed paintings also applies here, however you're more likely to be able to get away with taking photos at angles. Consider the following two images which are both taken with flash,
- In the second the reflection from the glass is almost unnoticable, while in the first it destroys the balance of the image. However all is not lost in the first image either as the majority of the flash reflection happens away from our main target we should be able to remove most of the flash reflections in post-processing:
- Another problem with this sort of image is that the glass can often reflect other items in the room including the photographer. One way to avoid this is to take the photo while holding the camera very close to the glass. A secondary advantage of this approach is that you can make it appear that there is no glass between yourself and the exhibit for example in the following photo,
Another way to dramatically reduce reflections is with the use of a polarizer.
- Use a low depth of field and try and avoid using an on camera flash. If you can't increase the surrounding lighting use an external (preferaby bounce flash) flash instead. The reason for this is that using the on-camera flash will create harsh shadowing and is responsible for red-eye.