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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.
Article contains several different (free) methods for AF adjustment
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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 1DIII, 1DsMkIII, 5DII, 50D, Nikon D3, D3x, D300, D700, Sony A900, Pentax K20
Procedure and Downloadable AF Test image
Sometimes, different lenses might consistently not focus perfectly (front or back focus).
This can now be adjusted for individual lenses. We've got a downloadable lens calibration chart to make this easier. The lens calibration target is easy to set up and use with an LCD monitor.
AF Microadjustment - curing back / front focus
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.
Note - I'm using the 1Ds3 as an example here - this procedure works with any camera that has some way of fine tuning the AF performance, and a 'live view' mode
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.
The large image plays tricks with your visual system, so be wary of this if you have any difficulties caused by repetitive patterns.
Download (zip file) a 1000x1000 pixels square version.
It will expand into a 1000x1000 GIF file
Please don't link to this file - the filename will change ;-)
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.
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.
- Note - this won't work with a print of the image! You need a screen view
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).
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.
After a quick test, the following settings were altered
- EF14mm 2.8L II - a setting of +8 (backward)
- EF15mm 2.8 (fisheye) - no correction required
- EF16-35 2.8L @35mm - a setting of +5 (backward)
- EF24-70 2.8L @70mm - no correction required
- EF70-200 2.8L @200mm - no correction required
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.
Do remember that all AF systems have a degree of tolerance in them, so don't get carried away spending hours chasing 'perfect' focusing.
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
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An alternative way of testing is to always start with your lens set at infinity.
You line up the target and activate AF.
Then turn off AF and activate liveview.
You should see some form of Moire pattern.
Turn the focus ring to a slightly closer distance - does this initially make the pattern more obvious?
If so then you have an element of backfocusing.
If it makes the pattern less obvious then you have front focusing (if any movement of the focus ring makes things worse, then smile to yourself, since the AF for that lens is already spot on :-)
Adjust the AF setting to compensate (positive numbers on my 1Ds3 for back focus). Repeat the process until any movement of the focus ring lessens the visibility of the Moire.
The amount of movement required may be very small with some lenses
I've seen it suggested that by starting at infinity and letting the AF work at the start, you are getting more consistent results.
- My own feeling is that the first way of doing it worked just fine... YMMV ;-)
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
This is using the camera after the micro-adjustment I set the other day.
Note that these two images are intended to give an idea of the effect you are looking for, not the testing methodology (you can use the LCD screen and Liveview)
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.
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My usual suggestion is Bruce Fraser's Real World Colour Management. My own copy is well thumbed. It's my first port of call if I'm asked a question and I feel I don't quite understand an issue well enough to be absolutely sure of an answer.
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It's actually quite important that 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
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.
- I've not tested this, so if anyone has the patience to do so, then please do let me know what you find out?
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 ;-)
Canon Suggested Micro Focus Adjustment
This is Canon's suggested technique
- Mount the camera on a good tripod.
- Set up a target for the camera to focus on. The reference target should have sufficient contrast for the AF system to detect. It should be flat and parallel to the camera's focal plane, and centred.
- Lighting should be bright / even.
- Camera-to-subject distance should be no less than 50 times the focal length of the lens. For a 50mm lens, that would be at least 2.5 meters.
- Set the lens for AF and the camera for One-Shot AF, and manually select the centre focusing point.
- Shoot at the maximum aperture of the lens via manual mode or aperture-priority. Adjust exposure level to get an accurate exposure. Use low ISO setting.
- If the lens has an image stabilizer, turn it off.
- Use a remote switch or the camera's self-timer to fire the shutter. Use mirror lock up as well.
- Take three sets of images at microadjustment settings of -5, 0 and +5, i.e, three consecutive images at -5, three consecutive images at 0, and three consecutive images at +5.
- Look at the images on your screen at 100% magnification.
- Take additional sets of test images at different microadjustment settings if necessary until the sharpest image is achieved.
- Register the corresponding microadjustment settings in the camera.
Thorough, but slower... ;-)
| AF Adjustment notes from Canon
For best results, manually set the focus on the lens to infinity for every exposure before allowing the camera to autofocus the reference target. Expect some minor variations in focusing accuracy within each set of three test images, even though they were all taken at the same microadjustment setting. This is completely normal, and is due to the tolerances of the camera's AF system. Expect smaller microadjustment settings to have a greater effect with telephoto lenses, and vice versa for wide-angle lenses. If you are attempting to set microadjustments for a zoom lens, it is important to realize that the camera's setting may only be accurate for the focal length setting you test. The instruction book suggests testing at the longest focal length of the lens, but you may find it more efficient to choose the focal length you use most often. |
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 ;-)
Other ways ;-)
You could always try a more expensive approach to the moire one above that I use. However I'd seriously suggest you try a 'free' technique first...
If you are feeling relatively clumsy or inept, there is (expensive) help at hand in the form of the Lensalign Pro ;-)
This device has been designed to appeal to the techy side of photography, where having a new gadget hopefully trumps mere photographic ability.
From a marketing point of view I take my hat off to them for coming up with a gizmo that will appeal to a particular market demographic ;-)
Other information
- Our Camera news and rumours index page
- All our equipment and software reviews
- Photography articles on this site
- 1Ds3 News page - Any 1Ds3 info we find
- Using the EOS 1Ds3 - Keith's thoughts on moving from a 1Ds to a 1Ds3
- Camera profiling for ACR
- Diffraction and optimal aperture - by Bob Atkins
- Print size and diffraction
- Canon PDF about getting the best out of the 1D3 and 1Ds3
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