cbtsam Loc: Monkton, MD

I have an AF Micro Nikkor 105mm 2.8D lens.

On the aperture ring, it shows stops from 2.8 to 32. When the lens is focused at infinity, my D810's aperture selector allows me to stop down to f/32. However, when cranked out to 1:1, I can turn the selector further, selecting up to f/57, according to the info in the viewfinder. If I set the aperture to f/57 at 1:1, and crank the focus to infinity, the camera shows that the aperture has changed to f/32. On the other hand, when I set the aperture to f/11, it does not change as the focus goes from 1:1 to infinity.

A few questions about this: 1) Do the aperture blades change position as I crank the focus out from 1:1 to infinity at f/57? 2) Why is there no change with f/11? 3) If I shoot at f/57, do I get greater depth of field than I would at f/32? 4) I've read that optimal sharpness is to be had around f/5.6 to f/11, but at 1:1 or thereabouts, this leaves me without much depth of field, and thus very little apparrent sharpness; how far can I stop down without totally undermining sharpness.

There are probably a few other questions to be raised here, but I'm too tired to try to think them out. As usual, I can use all the help I can get.

jim quist Loc: Missouri

okay, putting all of that aside. I shoot with a bellows for great macro, and I also use a 100 macro lens. stay away from say a 2.8. and move to a f22.

redhogbill Loc: antelope, calif

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Apaflo Loc: Anchorage, Alaska

The "focal length" of a lens is defined by the extension (from the sensor) to set the focus at infinity. To get 1:1 magnification requires twice that extension. And at the closer focus distance the "focal length" is different. Since f/stop is the ratio of the focal length to the aperture, the fstop necessarily changes too, and will be 2 f/stops larger at 1:1 magnification. The formula is

actual_fstop = marked_fstop * (1 + magnification)

So your 105mm f/2.8 lens is only that at low magnifications. At about 1:4 the difference starts to be significant, at 1:2 it clearly has effect, and that effect increases progressively as the magnification goes up!

You do get all the expected changes. Greater DOF being one, but greater diffraction being another. Hence at 1:1 the aperture wide open is going to be what you'd have expected at f/5.6 for DOF. And on a Full Frame sensor the diffraction you would expect to see at f/22 will occur at f/11. With careful sharpening you can usually reverse the effects of diffraction by about 2 fstops, so at 1:1 magnification if you might find a setting of f/22 workable even though it is actually f/45. Higher f/stops might be less sharp though. And higher magnifications too...

Today's technology means we can live with the thinner DOF if the subject is static, by using focus stacking.

cbtsam Loc: Monkton, MD

Thanks; that's helpful, although it raises another question in my little brain: does it mean that, when I focus as close as possible with, say, my 50mm lens, the maximum f/stop changes from what it was at infinity, albeit by just a tiny, even insignificant, bit?

Apaflo Loc: Anchorage, Alaska

If you've got an older 50mm that has an aperture ring and doesn't do macro there actually is a slight change at the closest focusing distances, but as you say "tiny, even insignificant". If the lens is not an Internal Focus design (Nikon labels those with "IF" ), you could literally measure the change in the extension from when it is focused at infinity and when it at minimum focus disance, then calculate the magnification from that, and from that figure calculate the change in f/stop too.

If this is a newer Nikon lens then the camea will show something close to what it actually is. You'll get a different indicated maximum f/stop when at minimum focus distance than when focused at infinity.

Another interesting, if a bit frustrating, point if you want to get down to that level of precision is that formula for deterining actual aperture from the magnification isn't quite correct! The value for magnification has to be divided by another parameter, known as "pupillary magnification". It will only be 1 for a symmetric lens and might be greater than 1, but will usually be less than 1. The frustrating part is that it changes with focus distance and about the only way we would ever figure out what it is would be to carefully measure the aperture and work backwards! Not much help...