How to Understand Your Digital SLR

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When they were first introduced, digital SLR cameras were enormously expensive and a tool for professionals only. Since then, they have come down in price into the consumer price range. Because of this, many people buy digital SLRs without understanding how they work -- and, consequently, not making the most of them. This article will guide you through the most common functions they have, and show you how to learn to use one by experiment. The principles herein are the same for any camera, but you will probably not be able to set your shutter and aperture manually on most non-SLR cameras. Read on nonetheless.


  1. Look for a subject. We are going to be taking photographs at a wide range of settings, so it is important that you do not take photographs of anything too dark. Remember that human sight can perceive a much wider range of light intensities than a digital camera sensor (which is referred to as the cameras dynamic range). We can look through a window and still see what is inside at the same time, even though the light conditions inside are totally different from these outdoors. Cameras, and especially digital cameras, cannot. So on camera, you will only see what is outside through the window or what is inside. Consider this when picking your subject.

  1. Put your camera onto something. If you have a tripod, use one; if you have a solid surface to rest on, then do so. Not that a tripod is as necessary as many people say it is; but to observe the effects of various camera settings, it is best if you get several shots of exactly the same thing.

  1. Set your camera to Program (P) mode.

  1. Play with your cameras ISO speed. This will be the first camera setting that you will experiment with. You can set this in the cameras menus; many cameras will allow you to change this with a couple of button presses, too. The ISO speed is a measure of your cameras sensors sensitivity to light; a lower number is less sensitive, and a higher number is more sensitive. Take a photo of your subject at its lowest ISO speed (or "slowest", typically 50, 100 or 200), and then take one at its highest (800, 1600 or more). Observe the following:

    • The photo taken with the slower ISO speed will have forced the camera to use a slower shutter speed (which we will get to later on), while the photo taken with the faster ISO speed will have used a faster shutter speed. The difference between the two may have been significant enough to be audible. Being able to use a faster shutter speed means that you can, for example, freeze motion (and also avoid camera shake) in poorer light than you could with a slower one.

    • The photo taken with the slower ISO speed will have less noise (random discoloured pixels) than the one taken with a faster ISO speed (although digital SLRs, owing to their larger sensors, have much better high-ISO performance than small point-and-shoot digital cameras do). Hence, you are left with a trade-off between image quality and usability in low-light conditions. At a concert, for example, a higher ISO speed may well be more appropriate; in bright daylight, or when you are using a tripod and remote release, lower ISO speeds may be more appropriate.

  1. Set your camera to aperture-priority mode for a moment. We will get around to exactly what this means in a second. For Canon, it is called "Av" (for Aperture value) on your mode dial. For Nikon, it is called "A".

  1. Set your lens aperture (also called the diaphragm). This might be a dial on your lens with a series of numbers on it (which will typically fall anywhere between around 1.4 and 22 on most lenses); otherwise, refer to your cameras instructions. The diaphragm is just that: a opening towards the front of your lens that lets more or less light onto the sensor. The size of the diaphragm is expressed as a ratio of focal length to aperture size (hence, they are referred to as, for example, f/5.6); consequently, a smaller aperture (less light onto your sensor) is expressed by a larger number. So, take two photographs, one with a larger aperture, and then stop down and take one with a smaller aperture. Observe: 

    • The background of your subject is less sharp with a larger aperture than it is with the smaller one. This is called the depth of field. So, if you want to make a subject stand out from the background, use a large aperture to blur the background; if you need to get more of your scene in focus, use a smaller aperture.

    • The smaller aperture lets less light onto the sensor than the larger one would have, forcing the camera to compensate for this by using a slower shutter speed. This is what "aperture priority" exposure control is about. In automatic mode, the camera will adjust either the aperture or the shutter speed to get the right amount of light onto the sensor; Av mode forces the aperture to take priority and the camera will only adjust the shutter speed. However, this means you will not see the effect of the changed aperture in Av mode on overall exposure, because the camera would have automatically compensated for it. So try setting your camera into fully manual mode (M) to see the effect of the aperture on light.

    • Hence, there is a trade-off between depth of field and low-light performance. You can either have a wide open aperture, which will give you little depth of field but plenty of light onto the sensor, or a smaller one, which will do the opposite. There are also problems with diffraction effects stealing sharpness at very small apertures; as a general rule, do not use one smaller (remember: larger number!) than f/8.[1]

  1. Set your camera to fully-manual (M) mode. This will tell the camera to give up all control of trying to expose your picture properly. Most of the time, you will not need to use this (and should not; exposure control exists for a reason). But we will need to do this if we are going to show the effects of shutter speed.

  2. Play with your shutter speed. See your manual for the exact details of how to do this. Shutter speeds are numbers which go up in a sequence that roughly doubles each time, and normally expressed as a fraction of a second; i.e. 1 second, 1/2, 1/4, 1/16, 1/25, and so on (each one usually being called a "stop"). Take two pictures at shutter speeds a couple of stops apart. Observe:

    • The photo with the fast shutter speed will be darker. This can either be a good or bad thing, depending on lighting conditions.
    • The photo at the slower shutter speed might show some motion blur if you were holding it by hand. Even if you are setting your camera on a tripod, at very slow shutter speeds (of half a second or more, such as one would use at night), you might see some blur because of camera shake.
    • Hence, in very dark conditions, you will need to use a slower shutter speed; but such slow shutter speeds can cause motion blur. In brighter conditions, you will need to use a faster shutter speed, which will have the effect of freezing motion. This can be a good thing or a bad thing. If you need to get a longer exposure during the daytime, use an ND filter.

  1. Memorise these things. Think about it in terms of light; you can adjust either your aperture, ISO speed, or shutter speed to compensate for various strengths of light. Adjusting either of them will have effects on your image, for better or for worse. Memorise these effects, and for a time, think about these things while you are taking photographs, until they become second nature.
  2. Put your camera back into Program (P) mode for now. It is nice to know the above things, and in time you will learn when to use each of time. Most of the time, Program mode will be sufficient.
  3. Play with different lenses, if you have them. If you do not, the chances are good that you have a zoom. Either is fine. Fixed lenses of different sizes have different focal lengths; zoom lenses have a variable focal length. The focal length is the distance in mm between your lens elements (the glass inside) and your film/sensor. The perspective you get is completely different for each focal length.

    • The standard 50mm lens is more or less equal to the field of view of human sight, if they are paired with 35mm film or a full-frame sensor. However, be aware that most digital SLR sensors are smaller than regular 35mm film. Therefore, your effective focal lengths are multiplied by about 1.5 on most digital SLRs (this is called "FOV cropping"). Most of Canons sensors have a 1.6 crop factor. Most Nikon DSLRs have a 1.5 crop factor.
    • Wide-angle lenses, like a 28mm, lets you fit a lot of the scene onto your sensor. It has a wide view. It also creates the impression that you are looking at your object from a distance. It therefore is good for taking pictures of small rooms (makes these look bigger), landscapes, etc.
    • The telephoto lens, like 80mm or longer, will bring things closer to you. Therefore, this is used for portraits (because it forces you to be further away from the subject; the perspective at longer distances makes noses appear smaller), and wildlife photography. However, bear in mind what was said about apertures earlier; for a long telephoto lens to let in the same amount of light as a smaller lens, it has to be a lot larger. A 200-500mm f/2.8 zoom lens, for example, weighs nearly 35 pounds, and is still nearly twice as slow as the 50mm lenses of 30 years ago.[2] Short of these kinds of extremely expensive lenses, most consumer grade telephoto lenses tend to be slow (i.e. its aperture in relation to the focal length will be relatively small compared to that of a smaller lens), forcing you to use longer shutter speeds if it is in a low-light situation. This can be compensated for by using faster ISO speeds, as described above; this is a trade-off you have to make.

  1. Get out and take pictures. Now that you have a better understanding of how your camera works, and how to use it in situations that your camera cannot do automatically, you need to get outside and start using it.

  • Keep your camera in Program mode until you are proficient in other modes. This mode allows the most flexible option for you to favor shutter or aperture based on the creative results determined by shutter or aperture choice. Depth of field or action? Think motion or sharp detail. Shifting the P exposure is easy and fluid. Use semi-manual mode Tv (shutter priority for Canon DSLRs) or S (Nikon and other DSLRs) if the light is changing and you need say, 1/1000th of a second for sports action; use Av (aperture priority) when you want to pull a scene into focus with f/22 and the sun is playing hide and seek behind a cloud or if you want an out of focus background. Manual mode gives you the most control. Keep in mind a digital camera has 30 plus aperture and shutter settings, so be patient and persistent in Manual mode. Learn how to use your light meter in manual mode with the help of your cameras manual. Learn how to use a histogram to determine if you blew the highlights out or completely lost the details in shadows.
  • If you find you need to take photographs of unmoving objects (such as a city skyline) at night, using a tripod will allow you to use a lower ISO setting by keeping the camera still during longer shutter speed exposures. This can allow you to take beautiful nighttime shots without a less noticeable amount of noise.

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