Today’s Question: Could you please discuss the use of the DoF button on the front of the camera? I see how the image in the viewfinder darkens as one stops down, but now what? How do you use that?
Tim’s Quick Answer: The depth of field (DoF) preview button available on many SLR cameras enables you to preview the effect on depth of field caused by stopping down the aperture. It can be difficult to see well because of the darkening of the view through the viewfinder, but the change is there.
More Detail: With a digital SLR, under normal circumstances the aperture remains in the wide-open position until you actually take a photo, at which point the aperture closes down to the setting you have established and the camera takes the picture (with a variety of other tasks also being performed by the camera, of couse).
Because of this behavior, when you are looking through the viewfinder you are viewing the scene at maximum brightness in terms of the available light and the maximum lens aperture opening. That also means you are viewing the scene with minimum depth of field, since you are looking at an image projected by the lens based on a wide-open aperture.
The DoF preview button causes the aperture to be closed down based on the current exposure settings on the camera. The result is a change in the visible depth of field, based on the overall conditions (such as distance to subject). In other words, you are seeing the scene the way the image sensor will see the scene. However, you don’t have the benefit of longer exposure times with your eyes, which can make it difficult to evaluate the darkened scene you are viewing through the viewfinder.
Because of this issue, my recommendation is to employ the Live View feature of your camera in conjunction with the DoF preview button, if your camera offers a Live View display. If you make sure the exposure preview option for your Live View display is enabled, you should be able to achieve a preview that represents a good exposure (except under very dark conditions) even when holding the DoF preview button.
I do find the DoF preview feature to be tremendously valuable when I want to confirm the range of depth of field I can actually achieve for a given scene. This is especially important for situations where depth of field is a particular challenge, such as when you are focusing very close to a subject. When you combine the Live View display with the DoF preview feature, you have an excellent way to evaluate the potential depth of field for a scene, or to fine-tune your exposure settings to achieve exactly the result you want for your photo.