Auto Focus Microadjustment

[WAFKT]

<p class="MsoNoteLevel1CxSpFirst"]Does anyone
take advantage of this feature on occasion?<span> Did you find after testing the AF that there were certain
lenses/body combinations that benefited from a little microadjustment applied?<span> What was your approach for performing this adjustment accurately?
<p class="MsoNoteLevel1CxSpLast"]I&rsquo;ve been looking at the [url="http://www.lensalign.com]Lensalign[/url]product.<span> The manufacturer suggest that most
every lens can benefit from AF Microadjustments, even if it appears that the
lens is focusing correctly to the eye. I've heard of people using other makeshift means to make a microadjustments, but possibly not with the same accurate results as with something like the Lensalign.
<p class="MsoNoteLevel1CxSpLast"]I realise that the AF calibration varies slightly from the wide to narrow end of a zoom lens, and so when calibrating or making micro adjustments to a zoom lens it's recommended to either calibrated in the middle of the focal length range or at the focal length most used. But what about working distance; should microadjustments be made at (or around) the working distance most used (e.g. if the lens is going to be used primarily for closer working distances - like macro or portrait photography; versus at greater working distances - like sports or wildlife), or if calibrated for one working distance is the calibration consistent for all working distances at the same focal length?

[Sean Setters]

Does anyone
take advantage of this feature on occasion?<span> Did you find after testing the AF that there were certain
lenses/body combinations that benefited from a little microadjustment applied?<span> What was your approach for performing this adjustment accurately?

After shooting with my 70-200 f/2.8 L IS for 2 years on 2 different bodies, I decided it was front focusing a bit. So I set up a tripod in one end of the largest room in my apartment and I set up a test subject in the other (my photo backpack). I set the lens to 200mm, and started testing. Sure enough the lens was front focusing. I dialed in the adjustment until it looked like it was just right. I've been ecstatic with the results. My keeper rate went up significantly after the adjustment (even though I didn't do the adjustment under optimal conditions).

...should microadjustments be made at (or around) the working distance most used (e.g. if the lens is going to be used primarily for closer working distances - like macro or portrait photography; versus at greater working distances - like sports or wildlife), or if calibrated for one working distance is the calibration consistent for all working distances at the same focal length?

From what I understand, microadjustments should be made from the focal length most used on a zoom lens. I think the microadjustment will help at all focal lengths, but you'll have the best results from the focal length you did the microadjustment at.

[WAFKT]

Thanks Sean, you bring up a good point (perhaps not directly or intentionally, but good non-the-less) - all my AF lenses are f/2.8 and f/1.2 (and I shoot wide open in most cases - indoor sports with $#!+&yen; lighting) so the very shallow depth of field would obviously make back or front focusing more of a pronounced issue. Recently I haven't been very impressed with the image quality I'm getting from my EF 70-200mm f/2.8L IS USM (surprising considering the great reputation this lens has) or my EF 50mm f/1.2L USM for that matter as well. I can see a noticeable difference at 70mm using my EF 24-70mm f/2.8L USM (very crisp and sharp images) vs. my 70-200 (edges of things are blurry). It also varies depending upon which body I'm using these on; images with the 50 looks awesome from my 5D Mark II, but not as great from my 50D (this goes beyond differences in resolution and control of noise) - additionally images with the 70-200 from my 50D were great, but now that I use that lens primarily on my 1D Mark III they don't seem as sharp anymore.I'm only assuming that the issue is slight back or front focusing - thought it wouldn't hurt to see if AF adjustments were necessary and might fix the issue.

From what I understand, microadjustments should be made from the focal length most used on a zoom lens. I think the microadjustment will help at all focal lengths, but you'll have the best results from the focal length you did the microadjustment at.

I'm aware of that, but what about working distance? If I make AF microadjustments to my 70-200 @ 200mm with a Lensalign or some other sort of target at 10-12 feet away will that ensure proper focusing when shooting something at a more typical distance of 60-100 feet away. Or should I be making microadjustments with the target close to say 70 ft away?

[Bob]

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WAFKT[/b]
<p class="MsoNormal" style="margin: 0in 0in 10pt;"]<span style="line-height: 115%; font-family: 'Arial','sans-serif'; font-size: 12pt;"]I have performed numerous micro adjustments on various lenses.<span style="mso-spacerun: yes;"] The most important part of the process allows you to see the front and back focus range from the selected focus point.<span style="mso-spacerun: yes;"] <o></o>
<p class="MsoNormal" style="margin: 0in 0in 10pt;"]<span style="line-height: 115%; font-family: 'Arial','sans-serif'; font-size: 12pt;"]I use measurement scales (rulers) at a 30 to 45 degree angle. Also the depth of field needs to be as narrow as possible to emphasize what is in and out of focus.<span style="mso-spacerun: yes;"] Example:<span style="mso-spacerun: yes;"] With a scale (yard stick) set to a 45 degree angle, focus at the 18 inch point, (you can put a larger object next to this point to help with focus).<span style="mso-spacerun: yes;"] Take several photos from a tripod, (mirror locked-up, timer release shutter at your normal working distance), and see where the sharpest focus is located.<span style="mso-spacerun: yes;"] This will allow you to check for front or rear focus and by how much.<span style="mso-spacerun: yes;"] Then follow the procedure in the camera&rsquo;s manual to set the micro adjustment to correct any out of calibration focus. Note: you may want the lens to be slightly rear focus.
<p class="MsoNormal" style="margin: 0in 0in 10pt;"]<span style="line-height: 115%; font-family: 'Arial','sans-serif'; font-size: 12pt;"]
<p class="MsoNormal" style="margin: 0in 0in 10pt;"]<span style="line-height: 115%; font-family: 'Arial','sans-serif'; font-size: 12pt;"]Bob<o></o>

[wickerprints]

I think it bears mentioning that certain lenses exhibit a pronounced degree of focus shift, and this must also be considered when performing microfocus adjustment.


Focus shift tends to be more apparent in very fast lenses, such as the EF 50/1.2L USM. For those of you who are not familiar with the term, basically what happens is that if you focus on a target with the diaphragm wide open (e.g., f/1.2), but take the image at a smaller aperture (e.g., f/1.4 - 2.8), then the focus will not match to what you chose. However, if you take the image wide open, there is no shift, and if you stop down far enough (e.g., f/5.6), then the DOF is large enough to mask the effect of the shift. Note that this is strictly a property of the lens, not the camera body.


The repercussions of this phenomenon (which is in fact present in many lenses but is not noticeable) is that certain lenses must be microfocus adjusted carefully with respect to the intended f-number, subject distance, and focal length (for zooms). For instance, if you perform your microfocus adjustment tests at f/1.2, you will have calibrated the AF system for precise focus at f/1.2, only to discover that AF is not precise at f/1.4 - 2.8, and you may wonder why this happened. If you calibrate the AF at, say, f/1.8, then you will discover it will be off at f/1.2. Presently it is not possible to specify multiple corrections based on f-number (or subject distance or focal length), so you are just going to have to pick a single correction value that best corresponds to your anticipated shooting conditions.

[Alan]

Presently it is not possible to specify multiple corrections based on f-number (or subject distance or focal length), so you are just going to have to pick a single correction value that best corresponds to your anticipated shooting conditions.
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wickerprints, that's an interesting point you've made.


I've used my 70-200 f/2.8 at 70 mm, 200 mm, everything in between, from f/2.8 up to f/11. These are my anticipated shooting conditions, and I would think anyone using this lens would never be limited by one f stop or distance, etc.


So, I'm just curious what is the big deal for anyone to find precise focus for such a lens.

[wickerprints]

wickerprints, that's an interesting point you've made.


I've used my 70-200 f/2.8 at 70 mm, 200 mm, everything in between, from f/2.8 up to f/11. These are my anticipated shooting conditions, and I would think anyone using this lens would never be limited by one f stop or distance, etc.


So, I'm just curious what is the big deal for anyone to find precise focus for such a lens.
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Of course, the versatility of zooms is one of the reasons why many photographers like to use them. Remember those multiple choice tests, where the instructions say "choose the BEST answer?" That's the situation we find ourselves in when using microadjustment. You just have to pick the one best solution, presently.


Also, bear in mind that some lens+body combinations are just off, period, and sometimes consistently so across all shooting parameters. Adjustment can rectify this situation. Sending the lens to Canon for recalibration may also help but it may not, because you don't know if it is your camera body that is contributing to the issue.


Why is it important? Arguably for wide-angle zooms, critical focus isn't necessary because you have enough DOF. But for something like an 85/1.2L II or a 300/2.8L IS, microfocus adjustment can make the difference between getting a high hit rate versus a bunch of out of focus shots wide open. Some people shoot narrow DOF in fast-paced conditions and precise AF is always appreciated.


Not all of my lenses have been adjusted. The only one I did adjust was my 70-200/2.8L IS (+10), because it was consistently off. My other ones were mostly fine, except the 24-105/4L IS which was backfocusing on one end and frontfocusing on the other (though I can't remember which was which). So I compromised and left it at 0. In most real-world shooting conditions I would be hard pressed to notice this behavior, but it does make a tiny difference in how sharp I can get a photo.