Calculating DOF for intermediate distance setting

[gerard_taillefer]

Looking at the DOF table for the EBC Fujinon 90 mm f3.5 lens (FUJI GW

690 III and GW 670 III) I can see that when the lens i focused at 5

meters and the aperture is 32, the DOF will be from 2.68 meters to

infinity. Great.

 

But sometimes you want to come even closer to your subject. The next

step on my lens and in the DOF table is 3 meters. This will give me a

closest distance of 2.01 but the DOF will stop at� 6.37 meters.

 

Neither the lens mount nor the DOF table in the manual has any

markings between 3 and 5 meters but I guess there might be a focusing

distance between those two �official� markings � perhaps 4 meters -

that could produce an even greater DOF than 2.68 to infinity, perhaps

something like 2.4 m to infinity?

 

Can anybody calculate the focusing distance needed and the resulting

DOF? Note:Jim Chow explained how to do this in his answer to a

similar question (August 25, 1999) but unfortunately I am not that

good at mathematics.

[janm]

Try f/Calc: http://tangentsoft.net/fcalc/

It's a nice little calculator for this sort of thing.

[john_lawson]

I use the focus+ program for my PalmPilot to calculate depth of field.

Using it for your example says that Fuji is using a circle of confusion of 50 microns. The program says that 2.6 meters is the close focus point at f/32. I've found that it is best to do your own test. I just tested my Pentax 67 105mm lens to find the best aperture and focus distance for objects at 15 feet, 30 feet and 2 miles (in the same picture). The result was to focus at 55 feet at f/22. This says that my personal criterion for circle of confusion is 30 microns even for 67 format(as well as for 35mm). Focus+ says that the depth of field should be 26 feet to infinity, but I found that 15 feet to infinity was OK when focussed at 55 feet.

[pete_andrews]

Try this <a href="http://www.avdesign.nl/DOF.html">online DOF calculator.</a><br>There are loads more of them out there, but that's one of the neatest I've come across.

[david b]

This doesn't directly answer your question, but I have a Fuji GW690III with the 90mm lens. The official DOF scale is wildly optimistic by my standards of acceptable sharpness. I'd forget it if I were you, and focus precisely on the one main subject part of the scene, etc.

Hyperfocal focussing especially give poor results to my eyes, and I know there are a lot of others who agree with me, not only the Fuji people but also Mamiya 7 users etc.

Just a cautionary note...

[ken_burns1]

Gerard:

 

The focussing distance scales on lenses are notoriously inaccurate, and DOF calculations (though theoretically accurate) are not always workable in practical applications. You might be better off to conduct a few test shots using focussing targets placed at the specific distances you want to calibrate. You might have to use a background building as your infinity target, but near targets can be easilly printed on a laser or inkjet printer and placed at the appropriate distances from the camera. I've made these types of tests to verify the accuracy (or more correctly, the inaccuracy) of the depth of field scales on all my lenses.