Out of curiosity, I calculated the hyperfocal distances for my lenses, and then compared those calculated values with the depth-of-field indicators on the lenses. (That is, I focused the lens at infinity, and then estimated the distance that the f/16 mark was pointing at.) I tried this with four lenses, and on three of them, my calculated number seemed to agree with what I read off the lens; but on the other lens, the value was way off. Why might this be?

Here's the details. I used this formula to calculate the hyperfocal distance:

hyperfocal-distance = (focal length squared) divided by (f-number times circle-of-confusion-diameter)

I used f/16 as the f-number, and 0.03mm as the circle of confusion diameter, in each case. So my formula simplifies to (focal length squared) divided by 0.48 mm.

My lenses are

Nikkor 20mm f/2.8 AF Nikkor 50mm f/1.8 AF Nikkor 85mm f/1.8 AF-D Nikkor 105mm f/2.8 Micro

the hyperfocal numbers I got are (rounded a little)

800mm 5200mm 15000mm 23000mm

It's the third one (the 85mm) that doesn't fit; looking at the lens, it appears that the hyperfocal distance is closer to 8000mm.

So there you have it. Might there be something about the design of the 85mm lens that renders the formula invalid, or am I missing something?

-- Eric Hanchrow (offby1@blarg.net), February 03, 2000

Answers

Focal length aside, and with the exception of special, deliberately soft-focus lenses, there is no way that the design of a lens can alter Hyperfocal distance or Depth-of-field.

It's quite possible that a different circle-of-confusion diameter was chosen for one lens against the others. 30 microns seems a bit generous, 20 would be a more realistic figure, but having said that, choice of circle-of-confusion is fairly arbitrary anyway. It really should take into account the final enlargement ratio of the print or projected size of a slide, and the viewing distance. For instance, what appears sharp in a 6"x4" matt finish proof at arms length will not be so sharp in a 20"x16" glossy a foot away. Subject and lens contrast will affect apparent sharpness as well.

-- Pete Andrews (p.l.andrews@bham.ac.uk), February 04, 2000.

Eric, how did you determine that the hyperfocal distance is something like 8000mm for the 85mm lens? The formula you used is based on general optical principles and some usually reasonable approximations. Do you have any evidence that is not correct for your 85?

-- Peter Wagemans (p.wagemans@kpn.com), February 09, 2000.

(That is, I focused the lens at infinity, and then estimated the distance that the f/16 mark was pointing at.)

To check the hyperfocal distance at f/16 on your lens, wouldn't you line up infinity with the f/16 Depth of Field mark on the lens and read the hyperfocal distance off the focus mark?

-- Walter Newton (waltern@spectralogic.com), February 09, 2000.