Part 4: Understanding functions of your DSLR camera

This week, I am spending time talking about the different functions of your camera, including cell phones (See Monday’s blog). Tuesday, I covered the funner parts of your DSLR camera, talking about aperture, shutter speed, and the ISO settings.

Today we are continuing on this subject of camera functions by talking about: Focusing modes, and then the exposure compensation control. I just hope that many of you know that you have these functions on your DSLR. Let’s learn about these functions:

3. Focusing Modes (Single Point vs. Spectrum)

This relates to how the autofocus system works. You may have the experience of turning on a DSLR camera and, when you go to focus the camera, in order to take a test shot, a bunch of different indicators flash upon the LCD or Electronic Viewfinder (EVF). These indicators are the different points of the spectrum that have been activated and the camera calculates that certain areas are the ones that you may want in focus, and these are typically represented by red or green boxes over different parts of the image. What typically works better (and by that, I mean, is more reliable and less annoying), is to go into your camera’s menu system, turn off the spectrum focusing option, and switch your camera so that it focuses just on a single point (typically in the center of the frame, although you can adjust this, such as placing the single focusing point over the point where a key subject is or will be in your image so that you get that subject in focus).

4. Back Focus

It seems that a lot of DSLR cameras are set up by their manufacturers so that the shutter button handles both the focusing part AND the exposure part of taking a photo. This can be fine, for a while, and you can get pretty adept at subtly pressing the shutter button half way to focus on your target subject before applying a bit more pressure on the same button to take the photo. However, there may come a time when this system ends up costing you valuable photo opportunities. For instance, when doing light painting photography, you’ll be working in relative darkness, taking time to set up your camera and focusing on just the right point in the image where you want tack-sharp clarity. Then comes the moment when you’ll press the shutter button to begin the long exposure so that you can walk out in front of the camera to wave your torch around, to capture the spectacular movements of light. However, just as you go to press the shutter button, you fail to put the right amount of pressure through the button, and the camera treats it like you’ve requested a change of focus, and the autofocus system kicks in, taking the camera out of the perfectly adjusted focus point.

On the more sophisticated DSLRs, you can save yourself this sort of aggravation by decoupling the autofocus feature from the shutter button and assigning the autofocus to one of the other option buttons. The reason why this method is called “Back Focusing” is because the button that is usually selected for the job of focusing is typically on the back of the camera, but in close-enough proximity to the shutter button so that you can easily engage the newly assigned autofocus button with your thumb while your forefinger remains the trigger finger to engage the shutter button. It does take a little getting used to, but it does enhance your workflow and the way in which you operate your camera.

If you find that your focus is sharp when you are zoomed in but soft when zoomed out, your back focus needs adjusting. This normally only happens to cameras with detachable lenses — consumer-level camera users shouldn’t have to worry about it.

Technical Note: Back focus refers to the “focal flange length”. This is the distance between the rear lens element and the CCD.

You will need:

  • A camera with a back focus ring. It will be located toward the rear of the lens housing.
  • A back focus chart like the one pictured is helpful, but any object with sharp contrast will do.

How to Adjust the Camera Back Focus

  1. Set your camera on a tripod or stable mount, with your subject (back focus chart or other contrasting object) at least 20 metres/70 feet away (or as far as possible).
  2. Your iris should be wide open, so it’s better to perform this operation in low light. Alternatively, add some shutter speed or a ND filter.
  3. If your lens has a 2X extender, switch it to 1X.
  4. Zoom in on your subject.
  5. Adjust the focus normally until the picture is sharp. If you’re using a back focus chart, the centre of the chart will appear blurry – your focus is sharpest when the blurred circle is smallest. (You can simulate this effect by looking at the chart above and defocusing your eyes.)
  6. Zoom out.
  7. Loosen the back-focus ring’s locking screw, and adjust the ring until the picture is sharp.
  8. Repeat steps 3-6 until the focus is consistently sharp.
  9. Tighten the back-focus locking screw.

5. Exposure Compensation

A snowy scene frequently confuses your camera's meter. To the left is a shot taken at normal exposure. To the right is one taken after adding in a stop of exposure compensation (overexposure).

You may not use this feature all of the time, but there are certainly occasions when you’ll want to take advantage of the exposure compensation setting to help improve the overall quality of your image. The exposure compensation settings are measured in values, with zero in the middle, then you either go to the plus values, to brighten the image, or into the minus values, to darken the image. Why would you want to do this, when you’ve already adjusted the brightness with either the aperture, shutter speed, and/or ISO settings? The problem is, with modern DSLR cameras, the algorithms they use tend to result in overcompensation of light quality with the resulting image. If you’re photographing in dark conditions, such as at night or in the evening (when you get those darker blues, for instance), without using exposure compensation, the camera will calculate that any source of light, such as street lights, lanterns, etc., will be rendered extremely bright, as the DSLR overcompensates to make sure the light can be seen in the dark environment. Professional photographers will often deal with this by using the exposure compensation feature, and dialing down into the minus values, typically going to -1 of exposure compensation, in order to tone down those light sources in the resulting image. Conversely, when out in a really bright environment, such as in snow, an exposure compensation value of +1, or even +2, will help to combat the camera’s tendency to overcompensate in the other way. What you’ll typically find is that without adjusting the exposure compensation settings, anything that’s white in your scene will most likely be rendered a really ugly grey color. By adding a value of +1 or +2 of exposure compensation, you’re able to bring back that brilliant white.


Photos using these examples:

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