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Old 03-09-2013, 04:16 PM
Jorda75078 Jorda75078 is offline
Larry Jordan
 
Join Date: Apr 2011
Location: Inverness, FL
Posts: 17
Default Who would want to do autofocus fine tuning (micro adjustment)?

I have been shooting with professional photographers and photo trek leaders when the subject of autofocus came up. While all of them agreed that autofocus is a good thing, especially when shooting fast-moving subjects, they did not always feel that autofocus fine tuning of a camera with a lens is necessary to get the best autofocus results. Because I am an engineer by profession and a fanatic about precision, and I always like to get the best results I can get from a camera and lens, I do autofocus fine tuning with all of my camera bodies with each of my lens. However, I do understand that there is a contingent of folks who will never do that fine tuning.

By the way, just for clarity, the following names are also used for autofocus fine tuning:
- Adjusting your lens
- Micro adjustment
These titles all describe the same camera and lens calibration matching that is stored by the camera body for retrieval when you next mount that same lens. Most Nikon and Canon prosumer and professional cameras have this capability.

So why would you want to take the time and go to the effort to do autofocus calibration of your camera with each lens if the camera has this feature? The reason most people do this procedure is to assure themselves that the camera and each lens they use on that camera focuses on the subject they want the camera to focus on when they push the shutter button. Pros fine tune to make sure they are getting optimal results from their expensive lenses to make sharper images.

You would think that cameras and lens that cost thousands of dollars each would be a perfect match and would focus where they indicate they are focusing every time you shoot an image, but that is not always the case. Whether you use the optical viewfinder or live view when you do your focusing can make a big difference in the accuracy of the autofocus results you get. We will delve into the details of how DSLR cameras perform autofocus in these two modes in another article. For now we will stick with autofocus using the optical viewfinder because it is faster than the autofocus in live view, and it is the mode of choice for a lot of nature photography.

DSLR cameras use a design called phase detection for autofocus when you are using the optical viewfinder, and the design is not always perfect for a variety of reasons. The biggest reason for the imperfection is slight variations of alignment between a camera body and a lens. This can be caused by a difference in temperature where you are shooting versus the temperature in the facility where the equipment was originally manufactured and aligned. The ware that occurs with lens/camera age and the changing of lens can also change the autofocus perfection of the camera/lens combination. Using a third-party lens can result in imperfect autofocus. Finally, impact to a camera body or lens can change the operation of the autofocus, making the camera/lens combination focus at a different point than you think it is focusing.

The imperfections in autofocus usually become an issue when you are shooting in low light with a low aperture or with a telephoto lens. A combination of the two is the worst case.

If you only shoot landscapes at an aperture of f/8, then the depth of field of the lens can mask imperfections in autofocus. In that case, the depth of field for a subject at 50 feet could be from 12 feet to infinity so having the camera autofocus improperly by a few feet would probably not show up in the final image.

However, when shooting a subject at 50 feet with a 200mm telephoto at an aperture of f/4 on a full-frame camera, the depth of field is only 4.5 feet. Shooting that same subject with a 400mm lens and the same camera and lens settings gives you a depth of field of 1.1 feet. In both of these cases, having the camera/lens miss focus by a few feet or even a few inches can produce results that you would not want. For example, if you aim at a flying birdís head with that 400mm lens and the autofocus is off by a foot, then part of the birdís wings may not be in focus. You may tolerate that kind of miss focus, but many folks want better results than that.

(See the posted table to for more examples of depth of field with various lenses and settings.)

If you ask professional photographers such as Jim Stamates, Thom Hogan or Greg Downing if they do autofocus fine tuning of their cameras and lens to assure a match between the two, they will answer yes. Thom says he does autofocus fine tuning at the start of each major field trip and especially on trips to environments that are significantly different from the last trip he took. He does the fine tuning at his destination locations. Now that is perfection is action.


Larry Jordan

www.larryjordan.smugmug.com
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