Front-focusing with Katz Eye screen -- curable?

[troyz]

[EDIT: All fixed! (see below)]

Hello all.

I just installed a Katz Eye screen in my K20D. While it's fun to have optical focusing aids again, I have a front-focus problem when using the screen.

With an FA77 lens, it seems to be about 1cm off at about 80cm:



(cropped image; setup is w/ tripod, inverted central column, camera aimed at floor, see third post for details)

with similar (but harder to quantify ) problems at infinity.

I suspect the source of the problem is a misaligned focusing screen frame in the body; probably always there but undetectable with the stock screen.

(1) Anyone else experience this?

(2) Is there any way to fix the problem (presumably moving the focusing screen away from the mirror)?

Would be nice if there was a magic cam adjustment somewhere, but I doubt that's the case.

[Marc Sabatella]

I don't totally understand what your test pictures are showing, and would *highly* recommend using a good focus test chart like the one at Home when doing this sort of thing. It's designed to work well for AF, but also works fine for MF.

EDIT: I finally figured out your picture - we're looking straight down, and one label is closer to us than the other (raised off the floor, presumably). Clever. But you're still much better off with a good focus test chart.

Anyhow, doesn't the Katz Eye come with shims to use to adjust the position of the screen? Many third-party screens do. I think you need to get it further from the mirror. If there is a shim between the screen and prism, remove it.

Also, I've heard it claimed the Katz Eye screen don't suffer the following problem, but it is worth checking: most stock focusing screens show entirely too much DOF at large apertures. With my A50/1.7 wide open at f/1.7, the screen might show 20 lines of text in focus when looking at a newspaper at an angle, but the picture itself will show only 0 lines in focus. Not until I stop down to about f/3.5 does the focus screen show what the picture will show. Until that point, the DOF preview is useless - it doesn't show any aperture changes at all. So you have to accept that something might be in focus in the viewfinder but not in the picture, and learn how to anticipate this effect and not just make sure your subejct is in focus, but in the right position (toward the front, or toward the back, depending on how your screen is adjusted) of the focus zone.

Again, supposedly the Katz Eye doens't suffer that, but it's certainly worth a test.

[troyz]

Originally Posted by Marc Sabatella

EDIT: I finally figured out your picture - we're looking straight down, and one label is closer to us than the other (raised off the floor, presumably). Clever. But you're still much better off with a good focus test chart.

Anyhow, doesn't the Katz Eye come with shims to use to adjust the position of the screen? Many third-party screens do. I think you need to get it further from the mirror. If there is a shim between the screen and prism, remove it.

I guess I could have explained the test setup better; I focused on the soap box on the right, and added shim material under the soap box on the left until the sensor image became sharp. This setup doesn't locate the plane of focus exactly but it provides a big easy focus target parallel to the sensor plane and verifies what I noticed immediately in real-world usage.

No shims came with Katz Eye, and I didn't notice any shims between the original screen and the viewfinder assembly; they could be present but adhered to the viewfinder assembly or just didn't become dislodged during installation.

[Wheatfield]

Try removing the screen and then replacing it, on the off chance it comes to rest in a better place.
If that doesn't fix it, you will need to have your viewfinder aligned. I've seen this a couple of times now, where the finder doean't allow accurate manual fousing.

[morfic]

Front ofucs is bad. if reseating does not cure it it's too thick (the screen that is):

Haoda sent me this in an email when i had similar issues with his screen on a 30D:

"There are some backup shims in the package.
By adding or removing, can you see a lot difference?

The correct case should like

if front focus, remove the shim
if back focus, add the shim

Best,
Haoda"

Too bad the single shim he included was of the same thickness as Canon's brass shim. That bad experience is why i chose to only spend a few bucks on a chinese focus screen from jinfinance off ebay, it's still in transit, so i don't know yet how i'll fair this time, since the 30D was not even deisgned for focus screen replacements, it made testing even more difficult, since each time i had a chance to seat it majorly wrong w/o knowing it.

Good luck, hope reseating does it for you.

[ftpaddict]

My K100D has a metal spacer between the latch and the focusing screen itself. Maybe your camera has one too, and you've placed it the wrong way 'round. Try swapping positions, i.e put the spacer under the focusing screen, or above it. I forget what the proper order is.

If it doesn't, then dayum. Send it back and have it replaced.

[lbam]

Hmm, I seem to be having the same problem with my screen, front focusing. I did notice a shim in the housing so I'll try putting it under the screen instead of on top... excellent advice!

[lbam]

Ok, with the original (brass coloured) shim in place, shot's seem to focus on a point in front of my desired target...
With the shim removed, the camera seems to capture an image slightly behind the desired target...

I guess I need a slimmer shim

I'll try cleaning the shim, and screen again to reduce their offset distance from the prism...

[troyz]

Originally Posted by lbam

Ok, with the original (brass coloured) shim in place, shot's seem to focus on a point in front of my desired target...
With the shim removed, the camera seems to capture an image slightly behind the desired target...

I guess I need a slimmer shim

I'll try cleaning the shim, and screen again to reduce their offset distance from the prism...

Thanks for the info! Please let me know if you find a solution.

I'll keep an eye out for the shim when I finally get around to working on the focus screen again. (I didn't notice it the first time, but I wasn't looking for it).

[Miserere]

This might be a dumb question but...did you place the screen the right way 'round?

Also, is your diopter adjustment set correctly? One last question: Could you manual focus OK with the standard screen before?

OK, I lied, I have one extra question: Your test image is focussed manually, right?

[Tom S.]

The screen has a tab on it - it will only go in one way.

Others have mentioned focusing problems that Katzeye customer service was able to resolve. It may involve a shim, or a replacement screen. Have you contacted them?

[lbam]

Ok, it's correct the shim will only go one way round. I tried the diopter adjustment, makes no difference apart from stopping you from seeing the focus screen at all (lol I did hope at one point though).

Today, at work I decided to take matters into my own hands. I took the shim out and reduced it's thickness by rubbing it on a super fine grade, flat, knife sharpening stone.
I re-fitted the shim only to find I needed less thickness, so I did it again, and again, and again.. 8-(
In the end it's still just focusing in front of my desired subject, but seems to be as good as I want to take it... the shim is getting quite thin now, and will become vulnerable to bending if I go much further...
This has been a learning experience, my Katz eye isn't as clean as it was (not from shim dust as I washed it thoroughly after each sanding), but hairs from somewhere... sigh
I'm actually quite happy now, the Nokton F1.4 lens has been handy for checking focus as it's depth of field is so narrow..
I now (testing on work colleges much to their dismay), am able to get pretty sharp portraits wide open, and am loving the manual focus action.
I'd do it all again, and am pretty good inside the camera now, but it was certainly scary at some points..

[patrickhh]

Originally Posted by troyz

(1) Anyone else experience this?

I have the exact same problem and a friend who has a K20D and a Katzeye screen also has the same problem. Seems to be a fairly common issue with K20D.

(2) Is there any way to fix the problem (presumably moving the focusing screen away from the mirror)?

You can send the camera to Katzeye and have them adjust the viewfinder. I'm thinking about that because Pentax support doesn't want to do it. (they claim it's the third party's focusing screen's fault, of course)

[troyz]

lbam, patrickhh:

Thank you for this useful information! I'll probably send the camera into Katz Eye for calibration (unless I can somehow obtain a shim kit).

[troyz]

All fixed!

Katz eye confirmed the front-focus problem, swapped out the shim (was 0.35mm, now 0.25mm), and reinstalled the screen. Focus is now quite accurate -- if there's any error, it's too small for me to measure.

I'm very pleased with the service. E-mail from Katz was very friendly and clear, and the service was fast -- my camera arrived at Katz Eye Wednesday evening, was serviced on Thursday, and arrived at my desk on Friday.