Focus screen placement for autofocus through viewfinder

Spy_Black

Hi folks, I have a question about AF through the viewfinder. I notice my D600 back-focuses regardless of lens use (G, D, Ai, Third-Party), but looking through the viewfinder, I can see it focusing exactly where I've place the focus spot (I use single spot). Looking at it through live view or after taking a shot, the image is back-focused.

I deduct from this therefore that AF through the finder is dependent on how the focus screen is spaced. For those of you who are aware, you know that there are tiny spacer shims behind the focusing screen that position it relative to the pentaprism (or pentamirror, depending on your camera).

So my question is, which way would the screen have to go to correct for this? Although I don't mind taking the screen off and removing a shim if that will do it, I would do it only if I know the screen would need to be closer to the pantaprism in order to reach correct AF. I don't want to bother messing with it otherwise.

So does anyone know if I would have to remove or add a spacer shim to compensate for back-focus? Obviously if I have to add one I have to bring it to a shop, but if it's simple enough for me to remove one, I'll do that. I believe that there are different thickness spacers used, I would simply start with the thinnest one first.

Thanks for any info on this.

Spy_Black

Actually, looking closer at the mirror assembly, I've notice an arrangement that seems like it may place the AF system on the mirror, unless I'm looking at the metering system, so my AF assumption may be incorrect. However that wouldn't explain why it looks correctly focused in the viewfinder when the AF system focuses through the viewfinder, so it's probably metering associated on the mirror assembly. Any info on this is appreciated.

Pistnbroke

If you have focus adjust issues you dont want to move the screen you need to use the focus adjust proceedure. If you dont know how to do it its on the Nikon site ,,you only need a ruler or expanding tape.
PS make sure you send it to nikon to get the shutter replaced as if you dont it will be worthless when you sell it without paperwork from nikon

heartyfisher

What you have is the classic "backfocus issue" nothing to do with the focusing screen at all ! dont touch it !!
I am almost certain you will not need to adjust anything physically. Make your adjustments in the camera menu microadjustments for the lense in question. if and only if that is insufficient than send the camera and lense in for adjustment.

In a DSLR there are 2 focusing systems. one is contrast based and its on the sensor and thus most direct(thats what you see in live view) . the second is phase detection based and its behind the mirror and its much faster and more accurate but its indirect thus needs to be calibrated (thats what you see in the viewfinder). In both cases it has nothing to do with the focusing screen and pentaprism.

of course in the past when we had split screens for focusing then it was the split screen that was assisting with manual focus.. but thats another story...

Ironheart

@Spy_Black, love your posts on the main blog, BTW. Look at page 259 in the D600 manual, it even tells you which way you are moving the focal plane. If all of your lenses are "off" I would start with the "default" value, in small increments. You may then still have to go in and individually tweak (not twerk) your favorite lenses.

Spy_Black

Thanks for your replies. Unfortunately the focus adjust does jack in correcting the issue, it somewhat minimizes it but doesn't fix the problem. It would also need to be done to every lens I use, which are a lot. It appears to me, and I may be wrong, that the AF system works off the focusing screen. I say this because when I focus through the viewfinder, the image is in focus EXACTLY where the AF dot is. The recorded shot however is somewhere behind it. This is irregardless of lens, AF, MF, G, D or third-party. Below is an example. In this shot I focused on the number 6. In the viewfinder, it was focused EXACTLY on the number 6. But this is what I get. So it seems to me the screen needs to be positioned closer or further away from the pentaprism. The only reason I'm aware of the shims is because I had replaced the screen on my D5100 with a KatzEye screen. So the question is, do I pull a shim out or need to put one in? I suppose I can just go ahead and pull one out to see, as I still have the KatzEye tool to pop the screen retainer, but I figured before I go messing with that I would try and find out if pulling one out (my only immediate option) would do the trick. Otherwise I have to bring it to a shop and be without the camera for X amont of time. If all I need to do is pull out a shim, it's a 5 minute job.

However, if heartyfisher above is correct (and I saw that whole mechanism he's referring to), then the screen won't have any effect on the recorded image. I don't understand however why the image is in focus on the focusing screen exactly where I place the AF dot, and image winds up back-focused.

heartyfisher

LOL .. I know you dont believe me .. but I will say it again, the Focusing screen has nothing to do with AF in a DSLR. I would suggest you take it in to Nikon to be fixed.. tell them that the AF screen is out too so that will have to change the shims.. I would suggest you dont touch it bec that would void the warrantee. Take in your lenses as well..

Try it with your D5100, take out the focusing screen the camera will still Autofocus without it. Since you changed the AF screen the warrantee is void anyway.

Ironheart

.... I don't understand however why the image is in focus on the focusing screen exactly where I place the AF dot, and image winds up back-focused.

The reason for this is that once the mirror is flipped up, the lens and the sensor are the only things left in the path. In a SLR the distance between the eyepiece plus the mirror and pentaprism has to be precisely calibrated to the distance when the mirror is open.

http://en.m.wikipedia.org/wiki/Single-lens_reflex_camera#/image/File:SLR_cross_section.svg

heartyfisher

there is a submirror for the AF system ..this is a better diagram ..

Spy_Black

Oh it's not that I don't believe you heartyfisher, it's that it's rather confusing that the image is properly focused on the screen yet winds up back-focused. I'll have to deduct that the autofocus mechanism AND the screen are both out calibration. I guess I'll just have to deal without the body for a while.

heartyfisher

Yup looks like it .. thats my guess as well..

PB_PM

Oh it's not that I don't believe you heartyfisher, it's that it's rather confusing that the image is properly focused on the screen yet winds up back-focused. I'll have to deduct that the autofocus mechanism AND the screen are both out calibration. I guess I'll just have to deal without the body for a while.

That's very common. The matte viewfinder screens of auto focus SLR's and DSLR's are deceiving enough as it is, even when there are no misalignments. Rule of thumb is, never trust the viewfinder. If you want to make sure your shots are going to be absolutely tack sharp more often switch to liveview (non-action shooting).

paulr

Heartyfisher,

Try it with your D5100, take out the focusing screen the camera will still Autofocus without it. Since you changed the AF screen the warrantee is void anyway.

You raise an interesting point with regard to changing Focus screens and Nikon warranties and I wanted to change the focus screen on a D4 and contacted Nikon Uk through my NPS membership, I asked if changing the screen would invalidate the warranty and was told no, even though I would have to use a third party screen, Nikon do not do an alternative screen for the D4. Needless to say this would only apply in the Uk, Nikon may have different opinions in different countries.

heartyfisher

Thats good to know .. I am thinking of getting another screen :-) but it takes a long time for me to decide .. :-)