Understanding micro focus adjustment and Lens Align

- Modern autofocusing systems are pretty amazing. If you ever want to really appreciate autofocus, just switch your lens back to manual and try shooting that way for a day. You'll realize, oh wow, I'm really, really dependent on my camera being able to focus for me. But autofocus means I can work quickly, it means I can think about composition and tone and exposure without having to worry so much about focus. However, modern cameras are made on assembly lines in huge quantities. And so there are manufacturing tolerances that don't always get met.

That means that the autofocus system on your camera, when combined with a particular lens, might be a little bit off. This week on The Practicing Photographer, we're going to look at a certain type of camera that lets you adjust the autofocus system inside the camera itself to make up for any manufacturing tolerances that may have gone askew. And what I mean by tolerances, is at the factory, the camera maker allows a certain amount of autofocus drift. If the autofocus is inaccurate to a tiny little degree, they're OK with that, and for the most part that's never a problem because for the most part, the amount of autofocus drift that we're talking about is made up for with depth of field.

If you are shooting with a very fast lens, wide open, and your depth of field is measured in millimeters or centimeters, then that drift may become more apparent to you, and it might be time for you to turn to something called micro-focus adjustment. Now, different vendors call this different things, and not all cameras have this. Not all cameras from a vendor that does support this have this. I've got a Canon 7D here, this is one of the cameras that has it. Some Nikon cameras, some Pentax cameras, I believe some Olympus cameras, have an ability to do this. To get it to work, though, you need a focusing aid.

What micro-focus adjustment does, is lets you dial in a certain amount of correction for the autofocus system. But you've got to figure out how much correction to dial in. And so for that, we're going to use something called a LensAlign. This is a really clever little system that is made out of laminated cardboard. You have to assemble it, assembly is very, very easy, there are no tools required, there are no screws or anything, it's all just slots and notches that fit together to create this little contraption. What this does, is provide me kind of two different tools here.

This area of the LensAlign is used to ensure that the plane of the LensAlign is perfectly parallel to my focal plane on my camera. I need that to be sure that my focus system is pointed precisely perpendicularly to my target. And the way that works is, I've got a focusing target here that has a hole in the middle, and back here I have these red targets. And so what I do is I line up my camera so that my center focus spot is looking through this hole to reveal the center of this circle back here.

And as I move the camera around, the circle moves around, so I center up this circle inside there, and now I know that these are perfectly parallel. As I said, I'm shooting with a Canon 7D, I have a 1.2 millimeter, I'm sorry, f/1.2 50 millimeter lens on my camera, And I have used a special tool on the LensAlign website to determine the distance that the camera should be from the LensAlign itself. There's just a little form you fill out and it says, oh, the camera needs to be 4.1 feet from the LensAlign.

So I measured all that out, I set it all up, these are both just sitting on two tripods. I have looked through, I've got everything aligned. So what that means is that if I take a picture, I now know that this is perfectly parallel to my focal plane, and what I'm really doing is taking a picture of this thing here. This is the part that matters for doing my micro-focus adjustment. So let's go take a picture and see what we come up with. I've got my camera set in aperture priority because I need control of aperture. I'm going to open the lens up all the way to f/1.2.

I set my self-timer to a two-second timer to eliminate camera shake, because I'm trying to measure focus here, I don't want any vibration skewing my image in that regard. I'm also shooting tethered, because what needs to happen here is I need to shoot the image and then take it into the computer to evaluate the results. So I'm shooting tethered into Lightroom, so if the tethering works, I should get an image right away. And I'm in autofocus, I'm going to rack the focus a little bit, meaning I'm going to just turn it all one direction or another to be sure that the autofocus system completely engages and does its whole thing.

Half-press my shutter button, it just focused, I heard the beep, and now (beeping) I'm going to set it off on taking the picture. (shutter clicks) There it is, it should be tunneling its way through this cable and over here into Lightroom, and there it is. So, here you can see, I've just got this wide-open shot with the LensAlign in it, my focusing target here is in the dead center of the frame, and as I said, this is the part that I'm interested in. So I'm going to zoom in here to one-to-one. Now, the way that this is all set up, the zero should be in focus, and it is.

But look, this four back here is more in focus than this four here. Now, at this distance, my depth of field really should be divided more 50/50. Because actually if I go in closer I would probably find that the bottom of the zero is maybe a little sharper than the top. So I do have a little bit of a focusing problem here, my focus is a little more distant than I think I would like it to be, particularly if I'm doing macro or stuff where this shallow depth of field is going to matter. Some people will refine this process a little bit further by taking this image into Photoshop and doing the Emboss filter or Find Edges filter to exaggerate sharpness in the image.

I'm trusting this pretty well for this demonstration. These numbers on here, these zeroes and fours and eights and things over here, do not correspond in any way to any setting on my camera anywhere. They're just visual aids, I could be looking at the fours, I could be looking at these circles. If you're wondering about all this red stuff, and all this green stuff, that's just chromatic aberration. Wide open at this distance, focused this way, the lens is not able to focus all wavelengths of light to precisely the same point, so I'm getting this red and green stuff. That's not something I'm going to correct, but it's an interesting thing to find out about this lens.

So what I need to do now is come in here and dial in some correction. I'm going to go to the menu, and this is going to be different depending on what kind of camera you use, it may have a different name, the LensAlign website gives you a pretty comprehensive list of cameras that have a microfocus adjustment and it also tells you the name of the command which is nice. On this particular Canon, it's in my Custom Functions menu in the autofocus drive section, and it is somewhere in here, I had it selected and now I'm off.

AF Microadjustment. I have three options: I can disable it, I can adjust all by the same amount, or I can adjust by lens. Adjusting all by the same amount means I'm putting in a global focus adjustment that will apply to any lens that I put on the camera. I don't want to do that, because I've already tested some of my other lenses and I know they're OK. So I want to adjust by lens. So what that's saying, is that it's identified this as an EF50 millimeter 1.2L USM lens.

So anytime I put this lens on this camera, it's going to go, oh, it's that 50 millimeter that's got the screwed-up focus, and it's going to automatically remember this particular setting. I have a scale here from minus 20 to plus 20, I want to pull the focus back in this direction, so I'm going to dial in a minus adjustment. I don't know how much to do, I'm just going to start with minus five, and hit OK. And that's it, the setting is made. Let's take another image here. (beeping) (shutter clicks) Let that download, and now we'll see if there is any kind of difference.

So here's my new image, it still looks like I'm shifted more that direction than that direction, let's go back to the previous one. I'll move that hand out of the way. OK, I don't know if you can see it on your screen. I can tell a tiny, tiny little difference. Going from here to here, there is a tiny bit of sharpening increase there. But this is very, very subtle. And that's the first thing to know about micro-focus adjustment, it's micro, I mean really micro. You're not going to see suddenly, oh, my images are 1000 times sharper than they were before.

This really is for very precise focusing concerns. I'm going to go all the way to minus 15 and see what happens. (beeping) (shutter clicks) And as you can see, notice I was not moving or talking there because I really don't want to vibrate the floor. It's worth being that careful when you're doing this kind of stuff. Ah, interesting. At minus 15, this one is now noticeably sharper than this one, and look, my eight is even coming into focus. So that's too far, let's back off to minus 10 and see what that does.

And this is just how this works, you've just got to experiment. Again, these numbers don't correspond to anything, there is no standard for micro-focus adjustment. (beeping) (shutter clicks) So every vendor has their own little scale. I really recommend doing this tethered, this makes it much easier than carrying cards around. OK, this is looking pretty good. I feel like the amount of defocusing here is about the same as the amount of defocusing here, so I really have my focus centered. Again, what this is going to be great for is working with this lens with really really shallow depth of field.

I keep saying macro, but that's not the only use. At 1.2, focus is so critical on this lens because depth of field is so shallow. It can be the difference between an eye being in focus or out of focus in a portrait. It can be the difference between, uh-oh, I focused on the nose and the eye is out of focus. And here, we're talking about, I've pulled that back maybe a centimeter, which, at this shallow depth of field, can really be significant. So this lens has been adjusted, now I can go on to my next lens and try again. The camera will remain aligned, and that's the trickiest part of this whole process. So once that's set up, you're good to test a bunch of lenses.

However, if you're working with a zoom lens, things get more complicated, because the micro-focus adjustment may not be consistent throughout the focal length range of the zoom lens. So you're going to want to test zoomed all the way out, you're going to want to test zoomed all the way in, you're going to want test in the middle, and try and find some kind of average. Further complicating things is that you're going to need a different camera distance for each one of those focal lengths. So it's going to take you a while to do it. You need to be pretty precise if you're going to do this, and then you're still ultimately kind of just averaging and hazarding a guess that you're going to hope will apply to every focal length on that lens.

If the lens is not particularly fast, if it's a f/4 lens, well, your depth of field is going to be so deep at that point that it may not be worth bothering because you're looking at the kinds of distances we're adjusting here are measured in millimeters and centimeters. And again, at f/4, your depth of field is probably going to pave over that area anyway, and you're going to be OK. So, if you work with very fast lenses and you're really critical about focus, micro-focus adjustment can mean the difference between getting the shot and not getting the shot.

But to work with it, you are going to need a specialized tool like the LensAlign.