BetterPhoto Q&A
Category: Problems with Photo Equipment - Tips & Tricks

Depth of Field Preview Button

To love this question, log in above
June 21, 2004

When you look through the viewfinder on an SLR, the lens is held wide open at its maximum aperture (smallest f-number). A number of things occur when you press the shutter release, before the shutter curtains travel (opening the shutter), including stopping the lens down to either the aperture you've selected (or the one the "program mode" has selected if you have it in a program mode).

What you see through the viewfinder with the lens wide open is the narrowest depth of field. If the Nikon N80 QD has a DOF preview button (I'm not familiar with that specific Nikon), then that will stop the lens down to the f-stop that will be used to make the photograph. You can then get a feel for what the depth of field will be at that aperture.

When you use a DOF preview the image in the viewfinder can become quite dim if the camera is set (or the program mode will use) a much narrower aperture than wide open. This is one of the reasons the camera body holds the lens wide open until you press the shutter release.

The DOF preview only gives you a "feel" for the depth of field. It can still look different in the photograph, especially if it's enlarged. Basic rule of thumb: if it appears out of focus in the viewfinder it will definitely be out of focus in the photograph. Near the front and rear edges of the field depth, things can appear to be in focus in the viewfinder but look decidedly "soft" in the photograph. This is because the apparent DOF you see through the viewfinder will be greater than what shows up in the photograph.

The reason for this has to do with DOF being a human perception. It varies with enlargement (one of several factors affecting it). DOF shrinks as the photograph is made larger. From 35mm film, photographs of even normal size are significantly larger than the viewfinder image.

As you shoot more photographs, you'll get a "feel" for certain common situations and what various lens apertures will do to the DOF. For example, I know that I need to stop down no less than f/5.6 to get reasonable DOF in most wedding photographs, both in the church and at the reception. Customer expectation is normally that DOF won't be very shallow. [There are some exceptions in which it's usually desirable to have a shallow DOF, but they're for a few special photos.] For landscapes I like to stop down to f/11 but can usually live with f/8 if necessary.

Additional Information:
There is another effect that kicks in at very small apertures called diffraction limiting which reduces resolving power of the optics . . . which means you start to lose detail optically before anything ever reaches the film. It has nothing to do with lens design and everything to do with the physics of light . . . its behavior when it passes through very small openings. Reason I mention this is so you don't walk away from this posting believing that stopping down to f/64 will give you a massively deep DOF and make everything razor sharp. At f/64 you would a deep DOF, but at common focal lengths it will also make the entire photograph appear slighly soft. That's why "pinhole" cameras produce soft photographs . . . the exceptremely small aperture of the pinhole.

For 35mm cameras, the focal length range most commonly used for general photography is 35mm to about 80mm. In that focal length range, f/16 is the point at which diffraction limiting of optical resolution starts to impinge on film resolving power, print resolution, and human acuity (eye resolution) with common print sizes . . . and it's why lenses in that range can usually stop down no farther than f/16.

-- John Lind

To love this comment, log in above
June 21, 2004

To love this comment, log in above
June 21, 2004