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Pictorial: AFMA (Auto Focus Micro Adjust) or AF Fine Tuning

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  • Pictorial: AFMA (Auto Focus Micro Adjust) or AF Fine Tuning

    Recently Ken posted a thread of close up shots of his boas after performing AF Fine Tuning on his Nikon DSLR (the same procedure is called Auto Focus Micro Adjust by other manufacturers) and it revealed astounding levels of detail! On a photography forum I visit, I posted a quick explanation of how I AFMA (I use a Canon camera and that's how it's referred to - but the process is the same for Nikon) my lenses to my DSLR and I thought I'd copy it here in case anyone was interested. I'm literally just copying/pasting so ignore things that reference past posts, etc... Also of note, this process is one you can follow AFTER becoming familiar with AFMA/AFFT. If you've never done it, I'd recommend learning about it first and probably doing it via the DotTune method a few times until you're familiar enough with it that you can do it anywhere/everywhere. Also, not all cameras have this feature (it's simply software). It's generally the mid and high level cameras that do. However, you can still perform DotTune once and see if the no-adjustment results might indicate there's a problem. Another option is to focus on something through the viewfinder and take a picture. Then use live view (look at the image live on the back screen) and magnify it 10x and let the camera focus zoomed in, and take a picture, and then compare those two pictures. The one through the viewfinder should have the same point of focus and be equally in-focus/sharp.

    Of note: Mirrorless cameras usually do NOT need this sort of adjustment. That is one of the advantages to mirrorless cameras - more accurate auto focus. The vast majority don't even have the option to perform this and I'm honestly not sure why those that do, do.

    So, here we go... AFMA "in the field"...

    Literally... in a field... behind my house.

    So I've posted a few times lately about performing AFMA in the field and how, once you've done AFMA a couple of times the traditional ways, doing it in the field really isn't hard. You just need to be familiar with the concept and when to dial in + or - compensation.

    I recently picked up the 135L and have yet to really use it on my 70D as it's been basically glued to my 6D. By the way, the 135/6D combo requires ZERO AFMA adjustment and it is DEAD ON. As in, couldn't possibly be MORE perfect. Anyway...

    I mounted my 135mm f/2 L to my 70D and woke up my dog, Maya, and headed outside. She ran around for a few minutes, sniffing everything... Like our backyard was some foreign place she's never been before. She was peeing on stuff. She pooped. And then she just started sniffing the air... like she always does. She's an air sniffer. I don't get it. Then again, I'm not a dog. So, I ask her to lay down and like the wonderful, obedient dog she is, she complies. And begins sniffing the air again.

    So, I turn on my 70D, position myself about 22 feet away (Canon recommends being 50x the focal length from your AFMA target), changed the aperture to f/2 (Canon recommends shooting wide open), put the center AF point over her eye (great target with a lot of contrast as she has white fur but her eyelids are a DARK black), and fired off a burst of shots. I picked the middle shot of the burst to inspect and this is what I saw...

    You can see how obedient she is. She's looking right at me, but her nose is off to the side because she can't stop herself from smelling the air! lol

    Anyway, downsized, it appears the focus is fine. But upon closer inspection...

    You can see that her nose is more in focus than her eyes. This means the camera/lens combination is "front focusing". It's also fairly extreme. So, rather than dial in +1 and repeating the process, I decided to dial in +4 and skip a few steps and save myself some time.

    Here's that shot...

    Even downsized, her eyes look a little sharper to me (I focused on her left eye). Upon closer inspection...

    Her nose is STILL sharper than her eyes. That means that the original shots plane of sharpest focus was actually in front of her face (which, when I looked at the original image on my pc at full magnification, I saw the foliage in front of her was indeed the sharpest part of the frame). So, I dialed in +8 and repeated.

    Those results were better. But still front focusing. I tried +9. Same thing. Then I jumped to +13 and it looked like it was now a little too far in the wrong direction (I was now back focusing). So I went in between the two prior values and tried +11. That was good! Just to be sure, I tried +10 and +12 as well. Then I tried +11 again. Here's +11 at the end of AFMA adjustments...

    And 100%

    *NOTE: These images were taken from a fairly far distance. That, plus using a camera that crops the image from the lens PLUS cropping the image further degrades image quality. So, the final picture isn't samurai sword sharp like it would be from my 6D (which doesn't crop the image as it's a Full Frame camera). Additionally, these were shot wide open at f/2. Had I stopped the lens down to f/8 or f/11, they would have been sharper due to having a deeper depth of field. I assume, based on the images Ken shared, that his lens was stopped down quite a bit.*

    All of the same processing has been applied to these images.

    As you can see, there's a DRAMATIC difference when viewed at 100% between 0 AFMA and +11 AFMA with this particular lens and this particular camera (remember, on my 6D, the lens is perfect without AFMA).

    What's great about using my dog is that if focus is slightly off, it shows up easily in her fur. And when focus is dead on, it shows in the fur around her eyes and her eyelashes. Also, I never insist that she's perfectly square to the camera. In fact, I like when she's facing slightly off camera. It makes front/back focusing easier to gauge. If her rear eye is sharper than her front eye, the camera is back focusing and I need - adjustment. If her nose or the fur along her mouth is sharper than the fur by her eye, the camera is front focusing and I need + adjustment.

    As an FYI... I do commit an AFMA faux pas. I do this handheld. I don't use a tripod because I never use a tripod when I shoot. I shoot just like I shoot my regular images to try and simulate real world use. I use back button focusing, AI Servo, and high speed continuous bursts for 3-6 images. For AMFA purposes, I generally pick the middle shot. If something doesn't seem right, I'll check the other images in the series or I'll reshoot. This has worked perfectly for me.

    So, whenever I pull my camera out, the prior AFMA value is still stored in there (I believe each camera can store up to 20 lenses). I'll take a couple of test shots of something with high contrast and check critical focus (tree bark, grout on the floor - I use the intersection of 4 floor tiles for this, or even my wife's eyes if she's willing to humor for 60 seconds or so). Generally little or no adjustment is needed from the prior setting. This situation took longer because I had never AFMA'd this lens before and it was such a LARGE adjustment. Also, Maya was air sniffing and I had to repeatedly get her attention and get her to look my way.

    I hope this is helpful for those who may be reluctant to use AFMA, or use it often, because you're not really sure what to do or what you're looking for. I hope it's also helpful to see the difference between "I haven't noticed any problems" (ie, +4 AFMA actually looks ok and I think most people would think the lens didn't need AFMA if that was the only picture I shared) and "that's pretty sharp!" I know I was a little scared before I tried it out. Just know that any changes you make can be immediately undone by setting the AFMA back to 0.

    By the way, I haven't noticed much difference in up to 2 AFMA points in real world usage (ie, NOT looking at an image at 100%). So I actually settled on +9 as my final AFMA value because there seems to be a bit of focus shift with this lens when getting closer to my subject. +7 was perfect close up. So +9 is my compromise.

    Hope you find this useful, or at least interesting :-)

  • #2
    Re: Pictorial: AFMA (Auto Focus Micro Adjust) or AF Fine Tuning

    Great write up jb and thanks for posting it here. While my antiquated camera doesn't have the fancy settings that yours (and most people's) have, I enjoy learning about photography and what it takes to make beautiful compositions. One of these days, I promise to devote some time into learning to take better photos, I know my business depends on it.

    Photo credit:Eddie Ard .....Banner Credit:Big PaPa Ernest