Camera AF Microadjustment - for freeChecking 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 adjustmentTopics (this page) Latest articles: Jane Fonda and Me... A print from previously unseen photographs taken on the set of the making of Barbarella in 1967 Using tilt lenses - What happens when you tilt a lens. Includes sets of tables with lens settings. Producing a black and white print - from camera to print. 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 imageSometimes, 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 focusIt'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 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 accuracyCanon 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.
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. AF Microadjustment proceduresThe 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.
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
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
Anything whatsoever you order from Amazon helps us run our site - thanks to everyone who has purchased items, however small.
Amazon UK link / Amazon USA link / Amazon Canada link 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.
AF check on cameras without liveview/AF adjustmentIf 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 seeHere 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.
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.
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. Check latest price/availability from Amazon.com RWCM Other Amazon sites See some other books Keith has on the shelf, on our Books Page 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.
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 AdjustmentThis is Canon's suggested technique
Thorough, but slower... ;-)
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 ;-)
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
Keith is always happy to discuss matters raised in his articles. You can use our Contact Form Site News: 'What's New' RSS Feed | Rumours RSS feed | Site updates: Twitter | Keith's Blog The views above represent those of Keith Cooper. Articles copyright ©2003-9 Keith Cooper. Northlight Images prides itself on its independence when giving advice. We do not sell hardware or software and have no direct commercial links with any of the software or hardware vendors that may be mentioned here. See our Review Policy for more information.We have a version of Google's Search engine, optimised for photography related items that may be of use in finding more information. Explore our site... Digital Black and White photography and printing. Some of Keiths thoughts, techniques and tips for those interested in a digital approach to black and white. We've lots of original Photography Articles and Photography Reviews on the site to help anyone interested in photography.
We've a gallery of landscape photography and fine art prints that are there to enjoy... Northlight Images is based in the UK and supplies Commercial Photography services, Photography training and Colour Management Consultancy. Tel +44 116 2919092 or use the Contact Form New site features are always covered on the What's New page. Visiting Leicester or wondering where it is? We have views from the Leicester traffic cameras. |
|||