Tuesday, December 1, 2009

Digital Camera AF systems

First of all, these are my thoughts, based on my experiences, and are not to be taken as fact.

On third-party camera lens inconsistency: Are ya'll sure it's the lens? Here's the reason I ask. I own a Nikon D300. It takes a pretty nice photo. Sometimes. In examining its inconsistency I found that at f8 and above, the depth of field is masking pretty consistent back-focus errors. The sensors in the body are focusing, but the image generated at the imager focal plane is "pushed back".

Now the D300 is a wonderful body, and it has this feature called AF Fine Tune, where you can walk the focus point back and forth by up to 20 "points". I'll call them points because they've got no real equivalent in actuality. The distance of focal inaccuracy varies based on the distance away from the camera body. So... the body can be adjusted. This is a great idea BUT it doesn't solve the fact that it's the BODY which has a problem. It demonstrates this problem with my Nikon 18-55mm kit lens, and with my Sigma 70-200mm f2.8 EX HSM Macro lens. One lens was $129, the other $899. You tell me which lens has the better engineering inside! Same issue with both lenses.

A Nikon body has a hex-head screw inside the mirror box which serves as a "stop". Actually, it has two of them, one for the viewfinder mirror, and another one, farther back, which sets the position of the AF system mirror. Ah HA! Now, as my camera has aged over the course of a year, and I've shot with it professionally, that little mirror has slapped countless times against its stop screw. I think it has moved the screw slightly.

Here's why this is important: even if you adjust the system using AF Fine Tune, you cannot manually focus properly. The eyeball result at the imager (from the big mirror) does not match the result from the AF sensor's mirror, and the AF sensor mirror controls what the lens thinks it should be doing. Nevermind that the stock D300 focusing screen is woefully inept for manual focus precision, it's simply impossible to come away with a manual focus setting which is "correct" at the imager plane when the system is out of whack.

With the AF Fine Tune you can set an overall camera "default", and you can set a plus/minus position for each lens. you can also set it to "On" or "Off". If it's "On" then it uses the stored lens setting. If it's "Off", it uses the system default. Since this setting behaves like a percentage, and not a physical distance, not only does the focus error change for every distance range, it changes for each lens' focal length too! 18mm behaves differently than 55mm which behaves differently than 200mm. General rule of thumb is this; the greater the distance, the larger the focus error. At least this is true with my D300 camera body.

So why is this important? Third party lenses are retro-engineered, based on the examination of OEM MFG's camera bodies and lenses. Physically, they need to be spot-on, or as close as possible to accurate. Now, given that the AF system's mirror can be misaligned quite easily, where the elements in a camera lens cannot (if they're properly installed and within their limits inside the lens' internal mechanism), does it make sense to blame the third party lens manufacturers, or the mass-produced camera bodies?

To sum it up, I feel if there's a sharp area somewhere in your photo, blame the camera, not the lens.

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