Autofocus Microadjustment Techniques
This post is about all of the Autofocus Microadjustment Techniques that I am aware of at the time of this post.
Background – Because I do a lot of microstock focus is VERY important and one of the big reasons fro rejection. Compounding this is the fact that better selling images usually have a narrow DOF (dept of field) or focus area. You do this to make your subject “pop” in the image, provide a nice soft background, or sometimes to hide an ugly background.
When I upgraded to a Canon 7D I had all sorts of auto focus (AF) issues. It turn out to be the camera. But before I discovered that I thought I might fix the AF issue by using the microstock photography feature (MA) of the camera.
There are lots of good articles on the web about this. I will refer and comment to them. And there are at least a couple of products. I am familiar with one of these (actually two by the same guys. I will go into some detail on that.
The bottom line is that not all all lens/ camera combination will yield that perfect focus you want out of the box. And for Zoom lenses I have found that AF DOF usually shifts a bit, forward or backwards, as you zoom. They say to do your autofocus microadjustment adjustments at the smallest focal length. I believe that you should do it at the point where you do most of your shooting. For instance, on my 17-55mm f2.8 IS, I do most of my shots around 55mm.
Autofocus Microadjustment Methods –
There are about 5 that I know of. I will go into detail on the one I like the best. Note that I use a Canon DSLR (7D & 5DMkII) but the approach should be about the same.
- 1. Mount the camera on a good tripod.
2. 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.
3. Lighting should be bright / even.
4. 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.
5. Set the lens for AF and the camera for One-Shot AF, and manually select the centre focusing point.
6. 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.
7. If the lens has an image stabilizer, turn it off.
8. Use a remote switch or the camera’s self-timer to fire the shutter. Use mirror lock up as well.
9. Take three sets of images at autofocus 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.
10. Look at the images on your screen at 100% magnification.
11. Take additional sets of test images at different autofocus microadjustment settings if necessary until the sharpest image is achieved.
12. Register the corresponding autofocus microadjustment settings in the camera.
I found this method very tedious and time consume. And it did not give me a quantitative understanding of the relationship between each autofocus microadjustment adjustment and its effect on DOF.
The Moire Effect Technique – Again it can be found in Coopers’s Posting. This technique is just too cool to not use at least once. Cooper has a file that you can download and then put up on your computer screen. It looks something like this.
I guess it’s part of technique developed by Bart van der Wolf. You set up the camera per his instructions and use your camera AF. Then use your autofocus microadjustment settings to see if you can improve the adjustment. This is definitely fun to do (although my wife would not think so). But same problem as the previous method. I can’t get a good feel for the autofocus microadjustment versus what its doing to my DOF. The other problem is that with either of these methods is that there is no way to tell if your sensor is parallel to the focus target. It it is not then you are building in some inaccuracies.
The Printed target – There are several charts on the internet using printed targets. One really good one is Tim Jacksons D70 Focus Chart located here. He has a very thorough detailed explanation and logic on how and why this approach is a good one. Basically you print out a graduated or rulered chart and then set up your camera at a 45 degree angle and the shoot and adjust.
There is a similar method shown on Jeffery Friedl’s Page shown here.
These methods come close to what I was looking for in lens adjustment. It will give you that nice quantified correlation between autofocus microadjustment and DOF. I happily used this method for several months. It did a decent job of lens calibration. But….. I was always a little concerned with the 45 degree angle. Was I getting exactly 45 degrees. Is the relationship between the the MA adjustments really going to be linear with this set-up.
Professional Lens Focus Equipment – I think because I am an engineer and because focus is so important, especially in microstock, I decided to take the plunge and I bought the LensAlign Lite.
I will not go into a lot of detail about this because this product has been discontinued. Its too bad because it was their really affordable model and it did a good, although, not great job. The new feature offered over the previous techniques was the ability to do a basic alignment of the the focus target and the cameras sensor plane. The way this worked is both the lensalign target and the camera are on two separate tripods or stands. The target was also set up to sit on a table. Next you put a magnetically attached mirror on the target and while using liveview you move the camera around until you see the center of your lens in the center of the mirror reflection. The camera sensor plane is now roughly parallel to the target. Remove the mirror and use your camera AF on the parallel target. Take an image. Zoom in on the ruler to the right of target. This rule has its zero axis at the plane of the target. Look at the ruler to see where you DOF is. If you like it…you are done. If not. Adjust it. Do another AF, review, and so on. What I would do is take 5 images with the autofocus microadjustment (on the canon MA scale) at +10, +5, 0, -5, -10. Its fun because you can see the DOF very clearly moving up and down the ruler. Note that I have an easier way to do this now.
This is the current system that I have. And I like it. This system can be purchased here. I noticed that they just released a new model, the LensAnlign MkII, with a new sighting system. But I think I will stay with the Pro for a while. Its the same sort of system as the light but its MUCH easier to align. I use two tripods, one for the camera and one for the target. This system is a little sturdier and heavier than the lite. You start by looking through the back of the target through some small holes and you align it to point in the center of your camera lens. Next you go to your camera. I put the center focus point on the center of the target. I then go to live view and magnify. You can then finish the alignment by lining up the red dots in the red holes. With that aligned I am pretty confident that your sensor IS aligned with the target. And now you are ready to calibrate. And here is what I do…. and it goes pretty fast.
1. I cable the camera to my laptop and use Canon’s EOS utility to bring up the liveview on my laptop. I actually do this during the alignment step.
2. After it is aligned, I go to an appropriate magnifcation level. I move the liveview to the ruler (the focus spot will stay on the target.
3. Using the laptop EOS utility controls I move the lens focus to infinity. Then, again, using the laptop, I do an AF. and I check my live view. It should look something like this.
Actually my view has a little more to it than this and I can use smaller lettering or numbers on the ruler. I then can decide if I like the DOF or not. If not, I use the EOS utility to move the focus either in or out one click at at time. It turns out that each push of the focus button moves it the equivalent of one autofocus microadjustment adjustment.
4. Remember the number of movements you made. Close out of the EOS utility so you have control over the camera again. Go to the camera special functions and adjust the camera by that amount.
5. Open the Utility again, zoom to infinity, Try AF again. You should be right where you want. If not do it again but it will probably be a very small change to your first adjustment.
Note that I have also seen this product by Spyder.
It looks similar to the Lensalign system, But it looks like it relies on being level. Then I suppose you camera must be level as well, and the same height and ……. it sounds to me like there are too many details to this where the camera and target may not be aligned…..but I have not used it so take those comments for what they are….conjecture.