Today’s Question: I understand that stopping down the lens aperture all the way will maximize depth of field, and opening up the aperture all the way will minimize depth of field. But how do I figure out what aperture to use for depth of field somewhere in between the extremes?
Tim’s Quick Answer: In theory you could calculate the depth of field for a given situation based on the specific conditions. In general, however, I find the depth of field preview feature to be a much better approach, especially in conjunction with the Live View display available on many cameras.
More Detail: The depth of field achieved for a scene depends on a variety of factors, including the lens aperture and the distance to the subject. Once you have configured the overall shot, the lens aperture is often your “final” way of controlling the depth of field.
There are a variety of tools available for calculating depth of field, including apps for smartphones that can be very helpful. However, if your camera includes a depth of field preview feature, that feature can provide a fast and easy way to evaluate the effect of different lens aperture settings.
By enabling the depth of field preview (often with a button near the lens mount) the lens aperture will stop down so you can see the impact on depth of field through the viewfinder.
Of course, because stopping down the lens will cause the scene through the viewfinder to be darkened, it can be a little challenging to actually evaluate the result. However, if you have a Live View feature with exposure simulation on your camera, you can easily evaluate the result using the LCD display on the camera. Simply enable the Live View display, ensure the exposure simulation feature is also enabled, and activate the depth of field preview.
With this approach you can simply adjust the lens aperture while in Aperture Priority mode to see the impact on depth of field. Based on this evaluation, you can then determine the specific aperture setting that will provide the look you prefer for the scene.