Adjusting Mirror Stop Screw on a Nikon DSLR?

<p>Hello,<br>

I have a newish Nikon D610 that I suspect has a misadjusted mirror stop as I am getting consistent back focusing (actual focus point in photo is further away from lens than how it appeared in viewfinder).<br>

I had this problem with my D300 and I carefully adjusted the mirror stop screw with a hex wrench and eliminated this problem in a trial and error adjustment that took an hour once I figured out which direction to turn for what effect....<br>

Has anyone else tried Adjusting Mirror Stop Screw on a Nikon DSLR to fix consistent focus issues under conditions of using lenses wide open close subject distance?</p>

 

<p>I guess the "proper" way to adjust the focusing screen manual focus calibration is to put shims under the focusing screen (you could ask Nikon service to do it, but they might claim it's in specs). I have used the hex screw on the lower right side of the mirror chamber to make adjustments and it worked fine and didn't cause any unwanted side effects that I'm aware of, but the adjustments were very small. This method was recommended by Katz Eye optics. According to them the adjustment that is needed to correct the error that I had in manual focusing doesn't affect the AF. I did not notice any adverse effects in either camera that I adjusted in this way. However, it would be nice if Nikon service could do this or at least advise on the procedure.</p>

<p>If you have done it once already, with success, then why not. I have only adjusted my old F100. That was a bit more tricky, as the time for actual feecback from the adjustmentis, is related to the film development turnaround time. In the end, the film sample proved at least improvement in the focus accuracy.<br>

Note that there may be another stop post for the helper/af- mirror. I'd adjust the main mirror first.<br>

Another possibility: Wide open and close distance: Do note that adjusments based on these factors may lead to unwanted side effects at longer distances. Why not to adjust your camera focus for medium distances (if needed) and use af fine-tune value (decrease) for specific situations when shooting close and lens wide open. A software parameter change instead of a mechanical position change?</p>

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<p><em>"and use af fine-tune"</em></p>

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<p><em><br /></em>dont get that point,<br>

either you use AF or MF, only here correct mirroralignment is important</p>

<p>Mag, it should read as:</p>

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<p>...and use af fine-tune value (decrease) for specific situations when shooting close and lens wide open...</p>

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<p><br />To clarify my earlier post...Then adjust the af fine-tune value back to its normal state when not doing wide open and close shooting. This applies naturally if one is doing focus operation by autofocus system, or is relying on the on-focus indicator.<br>

Now OP should find out how his other lenses are focusing with that camera. If other lenses show similar back focus issues generally, then I'd do the mirror adjustment.</p>

<p>I have never done AF fine-tune, so maybe i dont understand it, I think it tunes the AF for some lenses, but the AF doesnt use the mirror, so I dont see the point, backfocus from missaligned mirror can only be an issue in manual focus mode</p>

YES, the issue is with all (3 nikons 1.8g's and a sigma 1.4) manual focus AND autofocus mode.

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<p>I've done this on earlier bodies like the F100, D700 and D800 with a simple hex wrench. Sadly, more recent bodies like the D750 don't have the same setup this but fortunately, it wasn't necessary with recent improvement in their tolerances. I can't speak to the D610 but if in doubt, send it out to Nikon USA or APS for adjustment to factory specs.</p>

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<p><em>"YES, the issue is with all (3 nikons 1.8g's and a sigma 1.4) manual focus AND autofocus mode."</em></p>

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<p><em><br /></em>Maybe you are a little unclear how the AF of your Nikon works, it uses an own sensor and has <strong>NOTHING</strong> to do with the mirror.</p>

<p>Autofocus does use <em>two</em> mirrors, the light goes through the primary mirror and the second mirror is under the main mirror, reflecting the light on the AF sensor. I can easily see how it would be possible to misalign the mirrors and get a camera that doesn't autofocus correctly (and might be difficult for service to check since they are not expecting users to turn the screws). Also it's possible that the adjustment of mirrors may lead to uneven focus across the viewfinder image. So I would be careful and if you get an unwanted side effect from the adjustment, step back. Shimming is probably the best thing to do to adjust manual focus screen calibration since it affects the screen position evenly.</p>

<p>I've only adjusted the mirror stop on an F2, where you can check for alignment between viewfinder screen and image-plane by use of a makeshift ground-glass screen in the back of the camera. Obviously this isn't possible with a DSLR. Unless you take Live View as your reference plane.</p>

<p>Ilkka is correct that shifting the mirror stop may lead to focus being different from top to bottom of the screen. This is because the mirror stop adjusts the angle of the mirror, and not its absolute position. However for visual focus almost nobody uses the periphery of the viewfinder, so in practise this isn't too much of an issue.</p>

<p>The AF system I'm not sure about. Nikon's AF sensors are down in a slit in the base of the mirror-box, and the secondary mirror and semi-transparent part of the main mirror are also placed centrally on the main mirror. Therefore I would <em>imagine</em> that altering the angle of the mirror wouldn't affect the top-to-bottom accuracy of AF all that much, but I would certainly check for AF accuracy right across the AF area after any adjustment. And be darned sure I could return the set-screw to its original position.</p>

<p>There ought to be a way to check for absolute 45 degree setting of the mirror using a laser, but finding a reference plane from the outside of the camera body is surely problematic.</p>

<p>P.S. I'm not convinced that Nikon's factory adjustment/alignment procedures are controlled tightly enough. After my experience with two different D7200 bodies, one of which had awful AF accuracy and the other near-perfect AF. So it wouldn't surprise me if the mirror setting from new was incorrect.</p>

<p>I believe on my D300, the main mirror and autofocus mirror adjusted in tandem and the autofocus mirror may have had an additional dedicated screw adjustment, which I did not need to use.<br>

I have not adjusted yet or even taken a look at the setup on my D610.</p>

<p>Factory AF adjustment is made with Nikon's own service software.</p>

<p>Viewfinder is roughly adjusted with shims and fine tuned with the mirror stops, always at infinity. Only reason to fine tune it if you have a split image focusing screen installed because that is the only way to see that it is ever so slightly off. With a standard focusing screen you can not see if the focusing screen needs to be fine tuned. Adjustments are done with a collimator and double checked with Nikon's reference 50mm lens. After the mirror stops have been adjusted the AF adjustment is always readjusted.</p>

<p>I've had several cameras adjusted by Nikon. For free when they were under warranty and for a minor fee when not. If you don't have Nikon's proprietary software to readjust the AF, don't touch the mirror stops.</p>

<p>If you are getting constant back focusing you have to do tripod tests. Many people move when focusing. And then we have focus shift and spherical aberration. Basically you focus at one point and when the lens is stopped down and the image taken, the point of focus has moved further away. This is NOT the camera backfocusing. It's a reality of large aperture lenses design and it's common. </p>

 

<p>Older DSLRs like the D700 have 2 separate mirror stops (one for the main mirror, the other for the AF mirror that piggybacks on the main mirror). I'll use this to illustrate how I use to do it when both stops were "user" adjustable with a hex wrench.<br>

Since the only thing that can't be easily adjusted is the lens to sensor distance, you must first determine with a lens set up as manual focus, the distance setting that gives you exactly "perfect" focus. Optimally, I'd use a prime like a 50 mm, set at f/4 using a resolution test chart at about 10 feet away. If your viewfinder's focusing screen doesn't match this focus, the main mirror stop must first be adjusted. Checking focus should be initially done on the focusing screen's ultrafine micro-prism with at least one (if not two "piggybacked" giving 4x) accessory 2x critical focus magnifiers (Nikon DG-2) easily purchased used on eBay or Amazon. Once this is correctly adjusted, adjust the secondary AF mirror so that the camera sets the focus to match the established "perfect" focus setting when AF is engaged. Modern Nikon DSLRs usually achieve with the menu's AF micro-focus adjustment rather than playing with the secondary AF mirror which is typically done by the factory or a service tech.</p>

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<p>"Since the only thing that can't be easily adjusted is the lens to sensor distance..."</p>

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<p>Surely this <em>can</em> be adjusted by shimming or shaving the bayonet mount? I'd have thought this was a fairly easy thing to do after removing the lens mount, but of course you'd have to know by exactly how much and in which direction the register needed to be changed. It couldn't be practically done on the fly or by trial and error as the mirror stop can. But it would be a better solution (theoretically) than simply altering the mirror angle away from 45 degrees or by some software fudge.</p>

<p>If one needs to alter the bayonet flange to sensor distance, there is something seriously wrong as I would expect this would be the last thing needing any modification.</p>

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<p>Only reason to fine tune it if you have a split image focusing screen installed because that is the only way to see that it is ever so slightly off. With a standard focusing screen you can not see if the focusing screen needs to be fine tuned.<br>

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<p>I disagre. The reason to fine tune is because at a subject distance of <strong>appx</strong> 2 to 3 feet, my actual "on film" focus point is a good 3 inches further away from camera than that indicated <strong>CLEARLY</strong> on my matte focus screen. <br>

A 3 inch focus differential is a big difference when taking portraits with lens wide open. You eyes are basically going to be out of focus consistently. <br>

<br>

Furthermore, anyone consistently moving their camera forward 3 inches from their brain's *push shutter* decision to their finger pushing shutter has neurological challenges to overcome, and yes THAT group would need a tripod to assess if there is a problem to begin with.</p>

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<p>than simply altering the mirror angle away from 45 degrees<br>

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<p><strong>Or</strong> can it be assumed that if there is a consistent back or front focus shift with precise manual focusing, the main mirror is <strong>not</strong> at 45 degrees?</p>

<p>The entire focusing screen of most DSLRs is typically composed of very fine microprisms that usually show exact focus with a 2x eyepiece magnifier but preferably two piggy-backed 2X magnifiers. These are much finer than the coarse microprism spots found in older film cameras like the F4 that can be detected without the use of extra magnification. Works great on my D750 if I want to confirm the focusing screen focus matches the AF focus. Of course, the center portion of the screen is what you'll be evaluating. </p>

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<p>I disagre. The reason to fine tune is because at a subject distance of <strong>appx</strong> 2 to 3 feet, my actual "on film" focus point is a good 3 inches further away from camera than that indicated <strong>CLEARLY</strong> on my matte focus screen. <br /> A 3 inch focus differential is a big difference when taking portraits with lens wide open. You eyes are basically going to be out of focus consistently.</p>

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<p>I'm talking about <strong>fine-tuning</strong>. If your focus screen shows focus 3" off at 3 ft then something is very wrong. That's not fine-tuning, thats coarse adjustment. If your camera is under warranty it needs to go to Nikon asap.</p>

<p>What I wrote in earlier posts is based on my experience talking to experienced Nikon techs, standing next to them while they are adjusting my cameras focusing screens, AF etc, having studied service manuals from Nikon which outlines recommended procedures and working on cameras myself.</p>

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