Nikon 60mm f/2.8D AF Micro-Nikkor

Helen B · Jan 16, 2008

jstuedle said:
... at the same aperture, the 105 has about half the DOF (depth of field) the 60 does. When DOF in of major importance, when I can I use the 60mm.

Are you comparing the two lenses at the same magnification and the same effective aperture? Can you give us an example?

Thanks,
Helen

jstuedle · Jan 16, 2008

No, the 105 is approximately a 2X lens, the 60 about a 1 X. Is my thinking askew here?

Helen B · Jan 16, 2008

What I meant was 'When you do your DoF comparisons are you doing them at the same magnification?' Ie is the size of the focused object exactly the same in each image, before any resizing during post processing. The other part of the comparison is the effective aperture - the comparison could be done at the same effective aperture rather than the same marked aperture.

Best,
Helen

jstuedle · Jan 16, 2008

What I meant was 'When you do your DoF comparisons are you doing them at the same magnification?' Ie is the size of the focused object exactly the same in each image, before any resizing during post processing. The other part of the comparison is the effective aperture - the comparison could be done at the same effective aperture rather than the same marked aperture.

Best,
Helen

To be honest, for the past several decades I choose my lens simply on the magnification based on focal length. DOF as a factor of focal length. If I was shooting at f/8 and 1:1 I based my selection on the desired DOF / required working distance. Did I make side by sides? No, this was based on what I was taught in the late 1960's. Could I be wrong? Yes, it's been know to happen before. But, I based my selection on magnification related to focal length, and not magnification relative to image/size on sensor. Please set me straight!

Helen B · Jan 16, 2008

Depth of field comparisons are rather slippery things unless the criteria used for the comparison are clear. In general, and at close distances (ie well away from the hyperfocal distance of the shorter lens), lens focal length has no effect on depth of field if the magnification, effective aperture and format are kept the same. As you approach the hyperfocal distance of the shorter lens, the shorter lens will give greater depth of field.

If you suppose that the 60 mm and the 105 mm AF Micro-Nikkors maintained their focal length, the depth of field at an effective aperture of f/11 and a magnification of 1x (ie a reproduction ratio of 1:1) is 1.3 mm for both lenses. At a magnification of 0.5x, the DoF is 4.0 mm for both lenses.* You can use one of the online calculators to find these results. As the DoF is not dependent on the focal length, it does not matter that the two lenses under consideration shorten their focal length as they focus more closely.

Best,
Helen

*Oops, I forgot to state the diameter of the circle of confusion that I used. It was 30 microns, which is typical for 35 mm.

jstuedle · Jan 16, 2008

If this is so, why is the working distance of the 105 more than 1 1/2 times the 60 at 1:1? Hence my assumption that the DOF would be greater at the lesser assumed magnification of the 60? I am now truly confused. In photography, we don't get something for nothing. There are trade-offs. Don't we trade less DOF for a greater working distance? This would be true everywhere else but macro.

M1M · Jan 16, 2008

You guys are confusing me :lol:

jstuedle · Jan 16, 2008

M1M said:
You guys are confusing me

Sorry, taking what I thought I knew for 40 years and challenging it, is confusing me. Sit back, we'll work it out.

Helen B · Jan 16, 2008

M1M said:
You guys are confusing me

Sorry about that. It's fairly simple. For close-up work the depth of field varies only with aperture and magnification for the same sensor or film size. If you know what magnification you want, only the aperture affects depth of field. The choice of lens focal length is purely a matter of convenience.

Double the F-number and you double the depth of field. Increase the magnification, and you decrease the depth of field. In slightly simplified terms:

DoF is proportional to N (1+1/m)

where N is the F-number and m is the magnification, for any given format. Comparisons between formats are slightly more difficult.

jstuedle said:
If this is so, why is the working distance of the 105 more than 1 1/2 times the 60 at 1:1? Hence my assumption that the DOF would be greater at the lesser assumed magnification of the 60? I am now truly confused. In photography, we don't get something for nothing. There are trade-offs. Don't we trade less DOF for a greater working distance? This would be true everywhere else but macro.

The working distance of the 105 mm is indeed greater than that of the 60 mm. DoF increases with working distance - in fact it varies with the square of the lens to object distance. Double the distance and you quadruple the DoF at the same aperture. DoF decreases with focal length, however. In fact DoF varies with the inverse of the square of the focal length. Double the focal length and you quarter the DoF at the same aperture. These two factors cancel each other out.

All the above applies to working distances that are not close to the hyperfocal distance for the lens under consideration.

Best,
Helen

jstuedle · Jan 16, 2008

OK Helen. I obviously have been wrong headed about this concept for about 40 years. The thought of the increased working distance countering the lens magnification makes total sense. Funny how you have a notion stuck in your head and we remain blind to the obvious. Thanks for spending the time to straighten out an old hard head.