Photons Missing In Action

Started Nov 24, 2009 | Discussions thread
Marianne Oelund
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Part 2: Aperture Control Accuracy
In reply to Marianne Oelund, Dec 4, 2009

The efficiency issue discussed in Part 1 gives us an idea of how our lenses perform relative to their nominal apertures. This factor, however, is completely compensated for by the camera's exposure metering system, so will not affect the exposures we obtain for our images.

Over time, I've noticed some surprising differences in exposure between certain lenses, when they were used stopped down slightly. The reason is inaccuracies in the aperture control linkage between the camera body and the lens diaphragm.

It's important to understand that the camera's meter calculates exposure from the light level it sees when the lens is wide open. The camera then calculates the expected light level when the lens stops down for the exposure - it does not actually measure it. Thus, if the lens doesn't stop down precisely by the expected amount, the exposure will be incorrect.

The data that I'm discussing here can only be considered accurate for my particular copies of the lenses. Yours may differ, and results can vary simply by using a lens on different camera bodies. The point of this post, is only to demonstrate how much of an effect the aperture inaccuracies can have. Often, when reading about others' difficulties with exposure, I wonder if their lens aperture control is the culprit.

It is easy to check your own lenses. Make sure you are working under a steady light source (for example, typical household fluorescent lights must not be used). Using manual exposure, take a shot with the lens wide open, and again with the lens stopped down one stop, at half shutter speed. Continue to narrower apertures in this manner, if desired. Then check the image histograms; if they are all essentially identical, your lens is accurate. If you notice significant left-right shifts, then there is an accuracy control problem, which you may want to use exposure compensation to correct.

Here are some examples of aperture errors among my own lenses:

AIS 28mm f/2: Very accurate; slight underexposure up to 0.1 stop at f/22.
Zeiss ZF 35mm f/2: Slight overexposure, about 1/6 stop across all apertures.
AF 35mm f/2D: 0.4 stops overexposure to f/8, tapering to 0.3 stops at f/22.

45mm PC-E: This one is interesting since it's the only electronically-controlled aperture Nikkor that I own. Shows 1/6 stop overexposure up to f/5.6, then gradually dropping to 1/3 stop underexposure at f/32.

AF-S 50mm f/1.4G: Very accurate at f/5.6 and f/8, but otherwise overexposes 1/4 stop to 0.4 stop.

AF Micro 60mm f/2.8D: Very accurate through f/8, then gradually builds up to 1/2 stop overexposure at f/32.

AF 85mm f/1.8D: Overexposes 1/6 stop at f/2, then 0.4 to 0.5 stop through the rest of the aperture range.

AF 135mm f/2D: Overexposes 1/2 stop through f/5.6, tapering off to 1/3 stop at f/16.

AF-S 200 f/2G: Very accurate through f/5.6, then underexposes 1/6 to 0.3 stops through remainder of range.

14-24mm f/2.8G: Very slight underexposure up to 1/8 stop at f/22, except overexposes 1/6 stop at f/8.

24-70mm f/2.8G: Underexposes 0.3 stops through entire range, except 1/8 stop at f/8.

70-200mm f/2.8 VR I: Accurate through f/8, then gradually increasing underexposure to 0.4 stops at f/22.
AF-S 80-200mm f/2.8D: Overexposes about 1/2 stop through entire range.

As you can see, the most common errors are overexposure, and can definitely be enough to affect your images. The moral: Check your lenses .

For those few lenses which exhibit a constant error across the aperture range, I'm considering experimenting with an adjustment - but I'll need to check that their behavior is consistent between camera bodies first!

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