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Canon 1Ds Mark III AF Microadjustment
Checking for front / back focus and tuning the autofocus on a 1Ds3 (or 1D3)
The 1Ds3 (and 1D Mark 3) has the facility to individually tune the AF settings for different lenses via a custom setting (the micro adjust function).
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.
I'd toyed with the idea of looking at the micro-adjustment settings for some time, but just hadn't quite got round to setting things up (since it didn't seem a problem I'd noticed)...
Note... You can use this technique to check any (digital) camera AF system for back or front focus.
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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.
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 whole one 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.
Please don't link to this file - the filename may change ;-)
The principle is that you display this graphic (at full size) and focus on the screen, using liveview (zoomed if need be) and maximise the appearance of Moire interference patterns.
These patterns come about from the interaction between the image pixels on your screen and the pixels of your sensor.
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.
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 lens
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.
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.
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?
[update added 3/3/08 after several people asked for more examples]
Here are two examples taken with the 1Ds Mk3 and 16-35 2.8L (mk1) @f/2.8 and 35mm
First 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)
The second 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.
Buying the 1Ds Mk3
We make a specific point of not selling hardware, but if you found the info on our site of help, please consider buying a 1Ds3 via our link with Amazon.
It won't cost any more (nor less we're afraid) but will contribute towards the running costs of our site
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!).
Both images show some asymmetry in the patterns which indicates that I've not got the camera fully square on to the screen.
I've heard examples of people finding that their 'average' lens made into the 'sharp' category after trying this ;-)
Other 1Ds3 Info
- 1Ds3 News page - Any 1Ds3 info we find
- Using the EOS 1Ds3 - Keith's thoughts on moving from a 1Ds to a 1Ds3
- Main Canon rumours page - All kinds of stuff (take with big pinch of salt!)
- 1Ds3 first impressions - some very good points in a DPR post
- Diffraction and optimal aperture - by Bob Atkins
- Print size and diffraction
- 1Ds3 review - good article by Phil Holland
- Flickr 1Ds3 images
- 1Ds3 and D3 - thoughts and comparisons about high ISO at LL
- Canon PDF about getting the best out of the 1D3 and 1Ds3
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