Saturday, July 16, 2011

Front or back focus problems? You can't adjust that with your DSLR! Just get intimate with your lens.


Hi Pentaxian friends.

This is like the number one complaint, all across the Internet, for photographers. It's not all that bad, I assure you. You can still take excellent pictures with a lens that front or back focuses. Of course the ideal is to have all your lenses calibrated to your camera or your camera calibrated to your lenses. Most of the newer semi-pro to pro DSLRs have the capability to adjust the front and back focus problem.

What is front or back focusing?

Front focusing: Auto focus lock occurs at a distance closer than the point you are focusing on.

Back focusing: Auto focus lock occurs at a distance further away than the point you are focusing on.

Why does this happens?
With interchangeable lens cameras, you have two separate components that have to work together. You have a lens and you have a camera body. (They both  really are mechanical devices but about 60% of it is electronic). The factory has a tolerance when adjusting cameras and lenses. Let's say, for the purpose of this write up, that a tolerance of +/-2% is acceptable and  not perceivable. Each camera is tested at factory specifications and as long as the focus mechanism is within plus or minus 2%, everything is fine. The same is true for lenses. Let's assume that the tolerance is also +/-2% for lenses as well. If your camera is actually calibrated to a +2% and the lens you attach to it is calibrated to -2%, you wind up with a perfect match with +/- 0%. But what if your camera is factory calibrated to +2% and your lens just happens to also be factory calibrated to + you have a problem. The system (camera and attached lens) are now +4% and this is not acceptable and you will have front or back focusing problems.

You likely have more than one lens and some are focusing right on the money and some have front or back focusing problems. If your camera has the capability of adjusting this problem internally, the solution is fairly easy. Several charts are available from as much as $139.00 to free for the download.  Obviously, some of the charts may not give you the accuracy you want, but not necessarily. Some people judge equipment and results by the price they pay. If that makes them more comfortable with the results, so be it.  Many charts are available and I even have one I designed myself. It's all about contrast detection and set-up. If a chart is well designed, a $139.00 or $1.00 chart will give the same results. It's not rocket science.

Previous posts about Front and Back focusing: Here and  Here.

Just get intimate with your lens!

You can still get excellent results with a not-so-perfect lens.  Just give yourself some time to get fully acquainted with your lens. You know, you can override your camera automatic decisions. Set the camera to where the shutter can be released even if you are not it focus. (All cameras have that function embedded in menus). Then learn, by trial and error, how much your lens front or back focuses in certain situations. Once you know approximately, make it an habit to let the camera focus first, and manually move the focus to before or after the point you are trying to focus on. It's like learning on how to hold a camera for slow shutter speed. With practice, you can get there in less than a week.  If a shallow depth of field is required, you will shoot with a large aperture, which makes even more difficult, but if you can, use a smaller aperture...that way more of the space ahead and behind your focus point will be within  acceptable accuracy.

If you're made of money, send your entire arsenal, camera and lenses, to the manufacturer for calibration.  They'll be happy to oblige and take your money. Then, every time you buy a new lens, send it back with the camera and when you update your camera to a new one, send all your lenses and the cameras for calibration. While your equipment is out for calibration, use your smart phone!
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