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Camera AF Microadjustment - for free
Checking for front / back focus and fine-tuning camera autofocus.
Newer DSLRs have the facility to individually tune the AF settings for different lenses via a custom setting (the micro adjust function).
The AF fine-tune adjustment for auto focus functionality is available in many newer cameras that have a 'liveview' capability.
This article contains several different (free) methods for AF adjustment
Note... You can use the Moire fringe technique to check any (digital) camera AF system for back or front focus.
Cameras with micro-adjustment (or AF fine tuning) currently include:
Canon 1D X, 1DIII, 1DIV, 1DsMkIII, 5DII, 5D3, 50D, 7D, Nikon D3, D3x, D4, D300, D700, D800 Sony A900, Pentax K20 (note that the EOS 60D does not include this feature) The EOS 1D X / 5D3 now allows separate AF Microadjustment for both the wide angle and the telephoto settings of a zoom lens.
Sometimes, different lenses might consistently not focus perfectly (front or back focus). This can now be adjusted for individual lenses. Some cameras even allow for multiple copies of the same lens to be individually adjusted.
We've got a downloadable lens calibration chart to make this process easier. The lens calibration target is easy to set up and use with an LCD monitor.
If you're looking for specific hardware to help with the process, we have a review of the SpyderLensCal
It's important to realise that any system of parts with individual tolerances can exhibit significant variance if the assorted 'errors' all stack up in one direction (they can just cancel out too). This is a key element of engineering design for manufacturing.
Whilst expensive lenses and an expensive camera should 'just work' there may be room for improvement.
Previously you could get Canon to calibrate your lenses and bodies for you, but this entailed sending the camera off for the work.
The 1D Mark 3 and 1Ds Mark 3 both allow customisation of the AF settings, although you should note that if you try a whole lot of lenses and they all require a considerable adjustment, then it may be that your camera body needs fixing.
The adjustment is in the Custom functions menus (C.Fn III-7 AF Microadjustment).
A setting of 0 will clear all AF adjustment information, 1 will enable the global adjustment, while 2 will set individual lenses.
If all lenses front or back focus a little then you can apply a global adjustment.
How to check focus accuracy
Canon have a 1D /1Ds3 'Optimising Camera settings' document [PDF] (German translation [PDF]) available which has some useful background info on many of the adjustments and settings you can make to these two cameras.
I was sent details of an excellent post on OPF by Bart van der Wolf, covering his use of a fine graphics design on an LCD screen.
I've -part- of the autofocus test image I've been using at the right.
AF Microadjustment procedures
The principle is that you display the square GIF image (at 100% full size) and focus on the computer screen, using liveview (zoomed if need be) and maximise the appearance of Moire interference patterns. Do not make the image 'fit' your computer screen, it needs to be unscaled.
You will need to have the camera mounted on a tripod and directly facing the computer screen. Take some care to get the screen square on and lined up with the camera.
The interference patterns come about from the interaction between the image pixels on your screen and the pixels of your sensor. They may not look exactly the same as in the examples below, but you should notice a distinct peak in the amount of detail visible - that is the focus point.
You then switch off liveview and part press the shutter button to activate AF.
Look carefully at the lens distance indicator as you do this ... if the lens and camera combination is spot on, then there will will be no movement of the lens focusing ring and the image will not change.
I tried this firstly with my 24-70, set at 70mm (Canon suggest setting zooms at their longest setting)
... no movement of the lens ring at all. The lens is spot on. Reactivating liveview showed the patterns I'd seen after manually focusing. There could be a slight difference since the interference technique is very sensitive. If you are not sure, then try the test again with an adjustment of + or - 1. you should see a difference.
Next I moved the camera closer to the screen, making sure it was properly 'square on' to the centre of the pattern. I fitted my EF14mm 2.8L II lens
Note - Camera-to-subject distance should ideally be no less than 50 times the focal length of the lens. For a 50mm lens, that would be at least 2.5 meters (25m for a 500mm)
It's difficult to show graphics here, but the first image gives an idea of the rear display when manually focused with liveview, while the second shows the view after getting the camera to autofocus (where an adjustment is needed).
I've exaggerated the difference slightly for showing here.
It's actually only a few centimetres difference in focal distance, but the interference effect allows you to get critically sharp focus.
I noticed some patterns in a quick check with a CRT (if this works fine - please let me know?), but I'd prefer a LCD (a laptop is useful for testing longer lenses). My 23" Apple Cinema display shows patterns much better with my Canon 1Ds3 than my 15" MacBook Pro with its higher resolution (pixels per inch) screen.
The exact pattern you see when sharply focused, depends on your LCD screen and its pixels, since it's the interference between the screen version of the image (and its individual pixels) and the pixels of your sensor that result in the aliasing. It was different with each lens and at different distances.
The effect should be very obvious to see - you are looking for a peak in the pattern's visibility, not any particular amount of pattern.
After a quick test, the following settings were altered
It's worth testing your lenses in different conditions and trying a few 'real world' photos as well. I'd not even noticed the error on the 14mm and a few quick test shots at f/2.8 show a just perceptible increase in sharpness.
I repeated each measurement several times just to be sure it was real and not a 'glitch' in the AF.
If you find yourself wishing there was a finer gradation of adjustment than offered, I'd seriously suggest ending the process and going out to take some photos :-)
Alternative AF setting technique using the Moire technique
An alternative way of testing is to always start with your lens set at infinity.
I've seen it suggested that by starting at infinity and letting the AF work at the start, you are getting more consistent results.
AF check on cameras without liveview/AF adjustment
If you want to try this with a camera without liveview then just shoot a picture of the screen using AF and then two more with the focus ring manually moved +/- 5cm. Hopefully the AF version should show some fringing not visible in the other two shots.
If all your lenses show a slight shift then it -might- be worth getting your camera serviced?
Some Examples showing what you might see
Here are two examples taken with the 1Ds Mk3 and 16-35 2.8L (mk1) @f/2.8 and 35mm
The first picture shows correct AF
One additional feature of this method is that if you don't have the camera square to the screen (i.e. sensor parallel to the screen), you will get noticeable asymmetry in the pattern. The screen above shows that the bottom left corner of the screen is slightly closer (or further away) than the top right corner. This method is very sensitive in this respect (much more so than you could get by simply looking at a shot for focus errors)
The second picture below, was manually set to front focus by at least 10cm. This is quite a bit, but I've done it to show the effect you are looking for. I've also taken the photo from closer than you might use for a real test.
Both of these shots were shot tethered and the images you see above are screenshots of the 'Quick Preview' from EOS utility.
I did this so that I could use an un-resampled image, since any resampling of the image can introduce new moire (these effects are not easy to show here!).
Remember too, that both images show some asymmetry in the patterns which indicates that I've not got the camera fully square on to the screen.
It can help if you use a monitor that has been calibrated to some extent. (We have quite a few articles all about aspects of colour management on this site)
Uncalibrated monitors often have a colour temperature up towards 10000K, which is very blue
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The nominal 'standard' monitor setting of 6500K is much closer to daylight.
I did see it suggested that if you do a lot of work in tungsten lighting, then setting your monitor colour temperature to 5000K before doing the adjustments might produce better results, particularly if using a lens with noticeable chromatic aberration.
It just works!
Thanks to all the people who've written in with questions, and refinements to the technique. I'll be sure to keep the page updated if any new techniques or tips come in.
I've heard examples of people finding that their 'average' lens made into the 'sharp' category after trying this ;-)
This is Canon's suggested technique - I cover much more about using similar techniques in my review of the SpyderLensCal device.
Thorough, but slower... ;-)
This is from a suggestion on the Birdphotographers list by Arash Hazeghi
Make sure you have the latest version of the Canon EOS utility installed. You need to point the camera at a fine detail target, ideally at least 50 times the focal length away
Make sure that the camera is square on to the target, and that the target is sufficiently flat. Something like a bank note usually has a lot of fine detail, if you don't have a convenient ISO chart available.
Connect the camera to your computer via the USB port, cancel any image download pop-up/application
Some notes - I've not tried this particular method yet. If anyone finds it particularly useful, or has any suggestions to make it better, then please do let me know?
If you've come directly to this method, do read some of the caveats in the notes from Canon just above this section.
Some more experimental techniques I've come across:
So, how good is AF?
Possibly better than you'd thought, but for anyone really getting into checking the minutiae of AF performance, I'd suggest reading this article.
In particular bear it in mind when you look at people's sample images. I've never been a 'detail' person, but I know enough about measuring the performance of optical systems to always take peoples' reports of lens quality (without a detailed description of their methodologies) with a pinch of salt ;-)
Of course, if you had a camera that incorporates both Phase Detect and Contrast Detect AF, then you could just point it at a target and it would self calibrate the AF.
There are other approaches to the moire one above that I use. However I'd suggest you try a 'free' technique first...
Datacolor produce device to make the process easier - we have a full review of the SpyderLensCal.
The review also has a more general discussion about using targets for focus adjustment.
Despite my initial distrust of such devices I can see situations where the SpyderLensCal would be of use, particularly for longer lenses outside.
Much as I might like the Moire technique, I know some people prefer an actual target.
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Experiments with this device also suggested that there was no real justification for 'precison' versions such as the Lensalign Pro. I'm adjusting lens microfocus, not setting up a lab to test mirrors for space telescopes...
The views in this article represent those of Keith Cooper.
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