Depth of Field Is Oh-So-Shallow

2010-01-10 • SEND FEEDBACK •
Related: circle of confusion, depth of field, How-To, optics

See Making Sharp Images for everything you need to know on blur and sharpness.

See also Depth of Field.

A reader who just purchased a Zeiss ZF 35mm f/2 Distagon (see Guide to Zeiss ZF/ZE Lenses) is working hard to understand how to get maximum sharpness, using the depth of field scale and hyperfocal distance. It takes two stops to double depth of field.

Forget the hyperfocal distance, and consider instead what is most important in the image—skew focus towards that area and let the other areas fall where they may.

Stop down to f/8 for depth of field, more if necessary, keeping in mind diffraction and pixel size issues. For best results, f/11 is the limit for smaller sensor cameras with photosites in the 5-6 micron range, f/16 is OK for cameras with larger photosites. More than that is OK, too, but the entire image will look hazy as contrast really drops.

A point of much consternation is the depth of field scale marked on the lens, which allegedly shows what will be “sharp”. The marks are a useful reference point, and that’s it. Most such scales presume an acceptable “circle of confusion” (spot size) of 30 microns. A Nikon D300 has photosites that are 5.5 microns. That means that an “acceptably sharp” dot is about 30 times larger (in area) than a photosite. In other words, “acceptably sharp” of 30 microns for a 12 megapixel D300 includes detail down to a resolution of 0.4 megapixels!

Mouse over to see a depiction of a 30-micron circle of confusion on a grid representing 5.5-micron photosites.

Simulated circle of confusion

The circle of confusion ranges from the tightly focused dot at the plane of focus to the big blob depicted here; the size grows away from the plane of focus. Stopping down shrinks the dot, but diffraction enlarges it! That’s why stopping down too far is self-defeating.

To exploit the sensor resolution, you’ll need to be much more conservative. Mentally adjusting by two stops gives a more realistic gauge to what will be sharp in a digital image. For example, use the f/4 depth of field marks when shooting at f/8, f/5.6 marks for f/11, etc.

Ultimately it all depends on how large a print will be made. For small prints up to 12 X 8 inches, 30 microns may well be acceptable. For 30 X 20 inch prints, it’s unrealistic. It is easy to stare at actual pixels on a computer monitor and be disappointed—be sure to consider the print size and keep things in perspective.

Depth of field is razor thin at close range