AP's resident tech guru, Prashphutita Greco, answers readers' questions.
Q: Having recently bought a new Canon EOS 6D, I noticed I get a lot of out-of-focus shots using this camera compared to my previous DSLR (a Canon 600D). The lenses I use include a 24-105mm f/4 L and 50mm f/1.8. I’ve tried Autofocus Micro-adjustment (AFMA) on the body and lens combination using the DotTune method and I’ve found the required adjustments for the body and lens are – 24-105mm L lens: Wide angle: -4 & Telephoto: -14; 50mm lens: -10. After my adjustments the pictures have improved, but not 100%. Both these lens were extremely sharp on the 600D. I’ve read elsewhere that Canon lenses rarely need AFMA (less than one out of 20). Given that both lenses have been adjusted so severely on the 6D, does that point to a camera body which needs adjustment? Can a camera body be adjusted, and if so, how?
- Lam Van, Adelaide, SA.
A: This is a very complicated subject and in part it relies on having an in-depth understanding of the various types of AF systems, and their specific technologies in each camera model. Again, all of the “behaviour” can be changed according to the firmware which is controlling the hardware.
What settings do you have on the camera – AF mode (One-Shot, AI Servo, AI Focus) – and which AF sensors are active? Are you selecting an aperture where the lens is likely to be at its best? Typically that’s between two and three stops down from maximum or wide-open aperture for the zoom lens – between f/8 and f/11. Are you using the camera in “all-sectors” mode – a combination of all AF sensors active, and AI Servo or AI Focus? Making its own decisions, the camera can choose an AF point other than what you would select.
Canon EOS 6D with 24-105mm f4L lens.
Have you selected only the central AF (cross-type) sensor, which is the most accurate and fastest, to obtain more precise focus? Are you running the latest version of firmware in your camera? It’s possible that the manufacturer (Canon) has incorporated a solution to an identified problem with the AF, even if it isn’t listed as one of the areas addressed by the firmware.
You may have an issue with the usual subjects you shoot, and under certain lighting conditions – for example, under very low light or when there’s little contrast between the subject and the background, any camera’s auto-focus system will struggle.
For a zoom lens the AFMA required will vary from one end of the zoom range to the other. And for any lens the AFMA will vary according to the distance at which the lens is focused. Have you also attempted the adjustment procedures with the well-established, conventional approach, such as focusing on a calibrated ruler placed on an incline?
As an experiment, try using manual focus with all the best techniques (like the camera mounted on a sturdy tripod, with mirror lock-up and 10-second delay, using a remote cable release) to establish that critical focus – and sharp-looking images – are still possible with those lenses, on that camera, under appropriate conditions. While the very earliest implementations of autofocus on various manufacturers’ cameras suffered from major limitations and problems of different kinds, continued R&D has resulted in the vastly improved systems that are in use today, especially with contrast detection (as used in LiveView mode), phase detection, and Dual Pixel CMOS AF. However, you must avoid the trap of relying on AF as being infallible in all situations.
Your current camera is a 20-megapixel full-frame, whereas your previous camera had an 18-megapixel APS-C sized sensor. By cropping only the central portion of the entire image circle produced by the lens, many of the weaknesses, distortions, and aberrations inherent in any lens (which can be worst away from the centre) will not be revealed. This could be another reason why all your lenses now suddenly seem to perform more poorly.
There are weaknesses and limitations with the DotTune approach. It might be better to try something like Michael Tapes’ LensAlign system. Best overall currently in my view, including for ease and speed, is Reikan FoCal. Mechanical tolerances for high-quality, high-performance cameras and lenses both need to be maintained to hundredths of a millimetre. Impact damage, or even just vibration (perhaps caused by transporting an unshielded lens long distances by car on a bumpy road) can shake it out of its factory alignment.
To maintain acceptable costs for mass-produced items a range of allowable variation from the nominal design value exists. It could be that the variation in your old camera was nearly equal, and opposite, to that in your lenses, so they cancelled each other out, giving you “sharp” results. Conversely, even if your new camera is within specifications, it could be towards the maximum end of its range, and adding to (making worse) the variation in your lenses.
Canon’s recommended AFMA procedure (in its online “QuickGuide to Accurate EOS Autofocus”), similar to the DotTune method, relies on manually focusing via LiveView, then switching to One-Shot AF using only the central AF point. Next, observation is required as to whether the AF moves the lens’ focusing ring or scale. If so, in which direction? Multiple images need to be taken at various micro-adjustment settings, then examined at full-size on a monitor. This process needs to be repeated until you obtain satisfactory results. It can become tedious! For this reason automated (software) solutions have been devised.
You could also send your camera and lenses to Canon for AFMA treatment on its optical test bench. However, you’d expect this still won’t be ideal over all focal lengths, distances, or wavelengths of light. While slower, the Contrast Detection system typically used in LiveView is usually more accurate than Phase Detection AF. See this excellent blog from lensrentals.com for more info.
A lens calibration target can help you test the accuracy of your camera's autofocus.
When performing AFMA, generally it’s advisable to have the target at a distance from the lens of 50 times the focal length. You might achieve a better compromise by choosing a distance you’d typically use to take pictures with that lens. Remember, too, that AF will always have some errors and inaccuracies.
It isn’t necessarily perfect and repeatable from one picture to the next, even if absolutely nothing has changed. As always, the best strategy, if that image is important, is to take a series of pictures in burst mode, then later choose the one which has the best focus by viewing on a large monitor at 100% (“Actual Pixels”). In a controlled situation, manual focusing (assisted by using magnified LiveView mode) still remains the most accurate, consistent and repeatable type of focus.
The real problem could still be in the camera body with misalignment of the sensor plane, again due to the extremely fine tolerances required and the realities of mass-production. There are no user adjustments for that (you need specialised experience and expensive equipment or you could turn your camera into a brick). If you purchased your camera from a local store, they might be agreeable to you testing your lenses on different bodies until you find one that is a good match for those lenses.
Got a question?
Does photography have you stumped? If you have a query about cameras, lenses, imaging software, digital imaging, or anything else, ask Prashphutita Greco at qanda@australianphotography.com. Please include your name, suburb, state and phone number.