Backfocus with split-screen

[ChrisLilley]

Looks like my recently-installed split-prism focusing screen needs adjustment. Intended focus was half-way along the stems of the letter"m" in Extreme, in line with the bottom of the bar of the letter "e". Actual sharpest focus is midway between "lock" and "SanDisk"

This is the full photo, which shows you nothing at this size::


(100% crop here, to save folks who don't like high resolution images)

Nikon D90, Cosina Voigtländer SL APO-Lanthar 90mm f/3.5 Close Focus plus 49-52mm kood step-up ring plus Nikon 4T close-up lens @ ISO 500, 1/200s f/8

Any suggestions? Thicker/thinner shims (there where two shims when I took out the original screen, which I put back). There are also supposed to be cams somewhere under the mirror which can be adjusted with an allen key. Or maybe just taking it out and putting it back in again?

[Orio]

I have the same problem. And I am having a very hard time finding the metal shims.

[koji]

I saw Canon has many different shims for finders of EOS camera,
but forgot their model names. Can you ask Canon for shims directory?

[Orio]

[koji]

Canon supplies over two dozen shims for each EOS camera, each has 0.01
mm difference. Customer centre mush have those shims. I do not think
they put them on their catalogue (they wont admit camera's true precision
particularly for finders ).

But you can ask customer support centre for shims, thou you may have to
bring your own camera to fix font/rear focus problem.

When I cleaned my 5D finder, I found one shim used inside between penta-
prism and finder frame, finder screen is sitting on this frame with spring loaded
another metal frame to hold the screen.

----

When I bought a BrightScreen, My 5D became back-focus. So I complained it,
then sent me a metal shim used for their screen. BUT 5D became front-focus.
So I sanded this shim to just focus correctly, but their screen was not my taste.
This shim is installed body's finder frame and screen. (I still have this screen!)

Basically (simple conceptual diagram)

.......Penta-prism........
........Canon shim....... This is the shim I am talking about, Canon has many kinds!!!!
........metal frame......
........(shim)................................ extra I installed (it is not there when you buy a camera)
........finder screen...................... ^^^^ When I installed a BrightScreen
........metal frame to hold finder....
....... mirror box.........
..........mirror.............
..........bottom............

Now I use Ee-S screen only, but I attached Nikon DK-17M magnifier to 5D finder eyepiece.

[poilu]

the out of focus is not so big, about 2mm
maybe you will mess everything by trying to shim
just focus on the nose to get the eyes in focus

[ChrisLilley]

[rawhead]

I had this same problem, although much bigger, with my K20D and KatzEye. After sending the cam to Pentax (stilll under warranty) for adjustment, and getting it back without anything being done on it (grrrr ), and lightly scratching my $160 replacement screen during the multiple sessions of taking it out and putting it back in, I broke down and sent the whole thing to KatzEye to get them professionally calibrate the MF and install the screen.


Needless to say, my K20D + KatzEye is now BANG on with the MF.


I don't know about Nikon, but with Pentax, I was told I should order the entire range of shims (very cheap) from Pentax service, if I wanted to do the adjustment on my own.

[sichko]

[LucisPictor]

I put two small stripes of tape at the edges of my screen. That has worked fine.

[horvlas]

Hello. I've just bought a split screen to my D80, and I have a 6mm backfocus. Should I install new shim under the new screen, or remove one? (There are two metal shims under the new screen, originally).

Luispictor: Did you use casette tape strip? If I have to install more, how much thickness dou you think?

[Screamin Scott]

I just ordered one of the Chinese screens yesterday although I have a replacement screen to a N90s that I'm about to cut down. The Chinese screen is a split microprism & the N90s screen is an "E" type Matt Fresnel/grid screen...I'll be using them on a Nikon D70s which I'm told has a stock brass shim. The camera however also has a set screw to adjust the focus (using an Allen wrench). From what I've read, the Chinese screen may need focus adjustments, but the Nikon screen I'm going to cut down should not...At least I hope so. I don't want to have to be adjusting the focus screw everytime I decide to swap out screens...

[ChrisLilley]

Having shot some test charts today, the AF (well, the green dot - I only have one AF lens and its a slow one) is fairly close, maybe a bit front focused, but its hard to tell because there is a range of focus where the dot is lit. So, lets say its ok.

The MF appears to be severely off, significant back focus.

To check I just did the same test with my D40 (which also has a split-prism screen); both the green dot and focusing by eye/prisms give similar results, with no significant back/front focus. In other words I can trust what my eyes tell me, to give a correct result.

My D40 had no shims under the the stock screen. My D90 had two different shims. I'm wondering about putting in the new screen with just one shim and seeing if that makes it better or worse. Or fiddling with the adjustment cams.

[rawhead]

[horvlas]

Just remember, AF BF/FF issue and MF BF/FF issue are compeltely separate; one won't affect the other. Sounds like you have a MF BF issue, so the only way to fix it is to get a shim with the proper thickness. It's not the fault of the screen, but rather the camera itself.[/quote]

OK, but how much thickness do you think to disappear the 6mm backfocus?

[rick_oleson]

I used Scotch tape to shim my screen - it's about 0.05mm thick. I managed to get it as close as my eyesight can discern in my particular camera.

[Orio]

[rawhead]

[ChrisLilley]

[rick_oleson]

[horvlas]

[rawhead]

[horvlas]

Recently I noticed that, when I use a manual focus lens, each lens has different result with the split screen. Volna-9 and Industar-61 at minimum distance(0.2m) f2.8 almost perfect, Helios-44-7 2/58, f2 0.5m: 2mm backfocus. Nikkor 1.4/50 0.6m distance: 4-6mm backfocus. Why??? I also noticed, when I stop down manual mode the blades, the screen darks and the picture in the spit circle moves a bit, when I open the picture in the split circle moves back and displays good. Maybe a split screen fault? (I also noticed this fact with my split screened ZENIT)

[rick_oleson]

That's pretty weird, focusing screens are pretty simple things. All a focusing screen is, is a piece of frosted material located the same distance (through the mirror) as the film plane or image sensor from the lens mount. It doesn't know what lens you have mounted, and it doesn't move up or down when you change lenses. It seems it would either be located correctly (exactly matching the position of the sensor) or not, regardless of what lens you are using. Maybe I'm missing something.

As for the prism darkening when you stop down, of course that's what prisms do. When the lens is open (usually f/5.6 or larger, the cutoff point depends on the design of the specific screen), both sides of the prism are viewing through the lens - one sees light coming through the right hand portion of the lens, the other sees light coming through the left. When the diaphragm closes off the light paths that the prism is using, you begin to view the back side of the aperture blades through the prism. The split image is still doing what it's made to do, but what you are now looking at is black. The rest of the screen gradually darkens too, just because there is less light coming through the lens as you stop it down.

[horvlas]

Yes I know it should darken, but the picture in the split circle is moving a bit to the left, so it displays splitted a bit, when I open to wide, the picture in the circle isn't splitted.