Revisiting the Nikon aperture matter.....

Started Mar 7, 2012 | Discussions thread
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Revisiting the Nikon aperture matter.....
Mar 7, 2012

From Ken Rockwell’s website:

AF [Nikon] cameras control the aperture by varying the precise position of the spring-loaded automatic diaphragm pin that pops out of the back of the lens. The correlation between this pin's position and the opening of the diaphragm also needs to be adjusted correctly. This is a separate adjustment from the the correlation between the aperture ring and diaphragm noted above. Any slight variation in this internal adjustment will vary the exposures you get with that lens. Therefore, some lenses may give slightly different exposures than others. This is a limitation of the Nikon AF system with which Nikon is stuck because they base their AF system on compatibility with manual lenses. Canon controls all this electronically.

I say "stuck" because the mechanical tolerances are quite tight and can lead to a third stop variation here and there.

You can try looking at this yourself. Look through the lens on an AF camera. There ought not be any aperture blades visible. Now go to M or A modes and select different apertures. Press the DOF button on a modern AF camera and see what happens. When you set the maximum aperture of the lens you ought not see any blade motion. At the first 1/3 stop down setting you ought to see a little bit of blade motion. At the next 2/3 stop down you ought to see twice as much.

For instance, with an f/2.8 lens set at f/2.8 you should see no motion of the diaphragm when set to f/2.8 on the camera. Set the camera to f/3.2 and you ought to see them move a little bit when pressing the DOF button. Now set f/3.5 on the camera and you should see twice as much motion.

If you see a lot of motion, even at the lens' maximum aperture, you may tend towards underexposure.

If you see no motion unless the camera is set to a couple of third stops down from maximum you may tend towards overexposure.

Unless you are a camera designer don't fret over this. A far better way to test this is to go photograph on slide film with two or more lenses you wish to compare.

The diaphragm ought to be in the same place when set to the same aperture on the lens, or on the camera.

Many zooms move the diaphragm while you zoom. It is often normal to see a bit of diaphragm even when set to maximum aperture at some ends of the zoom range.

What he’s saying not only makes sense, it sounds like something extremely plausible. This system introduces some imprecision that affects the exposure by a small amount.

It doesn’t stop there. Do a Google search, and you’ll find plenty of stories about problems with this mechanism: broken actuator mechanism, bent lever, stuck lever - all of these things requiring the camera to go in for repairs.

Some of you say that Nikon keeps this mechanical coupling for backward lens compatibility. I can understand that. But then why not do with the aperture control what they did with the autofocus? The D800 and D4 still keep the built-in autofocus motor and drive shaft that sit there doing nothing while we use our AF-I/AF-S lenses. Why not upgrade the aperture control to a purely electronic one, and keep the lever mechanism for backwards compatibility? Hell, it is entirely possible that Ken Rockwell is wrong, and that Nikon isn’t “stuck” with this aperture mechanism. It’s entirely possible that all they need to do is make lenses from here on out with their own diaphragm actuators.

Nikon D4 Nikon D800
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