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Welcome to this month’s edition of Tech Tips. This column is intended to be an ongoing exchange of helpful ideas and useful tips on various imaging technology and photo equipment issues.
Before we move on, let me remind you that you are invited to submit questions about photo equipment, imaging technology, or photo industry trends that may have a bearing on your work or interests. I cannot promise to answer everything, but I pledge to do my best to address the issues that concern you. (You can contact me through the e-mail link provided at the end of this article.)
Here are this month’s questions:
Q: Last month, you mentioned that autofocus SLRs are calibrated with a ‘tool lens.’ Can you provide more information on the calibration procedure?
A: AF system calibration involves a series of tests to determine the positioning accuracy of mechanical components such as the image sensor and reflex mirror assemblies. Once these potential problem issues have been eliminated, test images are taken using a “tool lens” with known performance characteristics. (The tool lens is typically a standard 50mm lens that has been verified to be performing as closely as possible to its design specifications in terms of AF accuracy.) When discrepancies are detected in the test images and mechanical issues have been eliminated, adjustment software is sometimes used to calibrate the camera’s AF system. This method enables technicians to ensure that the camera is performing well within tolerances for AF accuracy. Once a camera has been calibrated with the tool lens, it becomes possible to check the performance of other lenses the photographer may have.
Here are some more details on the positioning accuracy of mechanical components like your camera’s reflex mirror: Although this component and its related parts, like the sub-mirror that directs light to the AF sensor mechanism in the bottom of the camera, are checked during assembly to ensure that they are performing within stringent design tolerances, they are moving parts and therefore subject to normal wear and tear. Just as race car drivers need to maintain their cars if they want to keep them in top-notch condition, photojournalists should have their cameras checked regularly to make sure that everything is working according to spec. Be sure to check with your camera manufacturer to learn the range of after-sales service they provide. Several top manufacturers provide special services for full-time professional photographers.
Q: A customer of ours has bought an EOS-1D Mark II for sports photography. He is trying to set the camera so it will only fire in AI Servo mode when the image is in focus. He has set the custom function, which enables this (drive priority I think?) However, it still seems to be firing when the camera is not in focus. We tried another in store and it behaves in exactly the same way.
What seems to be happening is that when the shutter is pressed fully and the camera is refocusing between two definite AF locks (high-contrast subjects within range), it will not fire. But when AF is impossible (no contrast, too close, etc.) it will start firing again. Selecting all or different AF points does not help.
This wouldn't normally be a problem but he uses the camera for motocross photography, when the subject is only in view for a few seconds. He is finding that the one shot where the subject is perfectly framed is out of focus.
Is this normal, and is there any workaround he can use? I've told him it probably is quite normal and useful for pros, where for that split second any picture is better than no picture, but he's not having any of it!
A: None of the EOS cameras are designed to allow focus priority shutter release for single shots in AI Servo AF. This also applies to the first shot of a continuous burst in AI Servo. Canon's philosophy on this is that the camera should never be allowed to prevent the photographer from controlling the instant of shutter release in AI Servo, even if it means that single shots or the first shot in a burst may be slightly out of focus. Frankly, I don't expect this stance to be changed any time soon, if ever. I wrote an article describing AI Servo AF as it related to the original EOS-1, and that information has been preserved on the Web here:
AI Servo performance has greatly improved since the original EOS-1 was introduced in 1989, and the EOS-1D Mark II is currently Canon’s best performer in this area. Compared to the original EOS-1 camera, there are now 45 focusing points (including 7 cross-types) compared to only one, and the speed of the camera's circuitry is substantially faster. The 1D Mark II even incorporates two dedicated CPUs for AF, one for calculations and another to control lens drive. But it is still up to the photographer to control shutter release timing.
For best results, we strongly recommend that photographers press the shutter button halfway at least a second or two prior to shutter release, and to keep the button pressed halfway until pressing fully to take the picture. This technique allows the camera's AF system to track the subject more effectively, reduces the time lag before exposure begins, and increases the odds of getting sharp photos as consistently as possible in AI Servo mode. Another article covering this topic in depth is available here:
Hope this helps! That’s it for now. See you in July!
© Chuck Westfall
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