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03-12-2007, 06:18 AM   #1
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Monday Morning Quiz

In your hands rests a sophisticated piece of equipment called the Single Lens Reflex camera (could be film or digital).

When you look through the viewfinder at some subject, the image of that subject comes in through the lens gets reflected by the mirror and passes through the focus screen, the penta-prism (or similar), the eyepiece and then it's processed by your eye.

When you press the shutter release the mirror swings up, the leading curtain of the shutter opens, the focused image strikes the film or sensor plane and the magic begins. When the exposure is finished, the rear shutter curtain closes, the mirror drops back down and once again the image is visible in the viewfinder.

During the exposure stage the image is (hopefully) properly focused on the film or sensor surface.

Where is the image properly focused during the view finder stage, i.e during focusing and composition operations? (It's first focus plane.)

It's not a trick question; if you are not sure or don't know, guess; but, please(!) participate. Feel free to include your 'logic'.

Have a great Monday!

03-12-2007, 06:24 AM   #2
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It's my understanding that the focusing screen is calibrated so that if the reflected image is in focus on the ground glass, the straight image is in focus on the film/sensor plane.
03-12-2007, 07:03 AM   #3
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My guess, is the image is focused on the focusing screen. Hopefully the screen is aligned so that it is the same distance from the rear element of the lens as is the film/sensor plane.
03-12-2007, 08:12 AM   #4
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Yup, what they said.

03-12-2007, 10:45 AM   #5
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I think it must be the focussing screen... the only other reasonable answer I could even think about would be your retina... but since your eye and the diopter adjustemnt, your contacts/glasses etc... do this bit of magic, I am sticking with your eye focusses on the image visible on the screen.
03-12-2007, 11:35 AM   #6
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Thanks John for making me think

Well, I wasn't sure but I thought it must be the focusing screen. But I wasn't sure and I did a web search and came across this informative article:
http://doug.kerr.home.att.net/pumpkin/Split_Prism.pdf
Which sort of confirmed my guess but reading further I found there is a fresnel lens behind the focusing screen that gathers light and sends it onto your eye. But actually from all I can find, the first place the image is focused is on the "ground glass" of the focus screen.

NaCl(that's my story and I'm sticking to it...unless of course it's wrong, then I eat crow)H2O
03-12-2007, 04:02 PM   #7
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OK, we agree; next '?'.

OK; I think we are all on the same page. Thank you for your responses and the Kerr reference-interesting reading one and all!

The image is on the ground glass side of the focusing screen when the mirror is down/shutter closed. And this image should be physically the same distance from the lens objective as the captured image on the film/sensor when the mirror rises and the shutter opens.

And further from Mr Kerr's paper we can see that the Fresnel 'lens' and 'micro prism' elements of an older split-ring focusing screen are also part of the focusing screen. The center and ring elements specifically.

I've replaced the focusing screens on my PZ-1p's. They are very thin glass and they have at least one alignment tab to prevent accidental improper installation. I have NOT changed the screens in my K10s, but I've read in several places that these screens are thin, soft plastic.

I'm a tenacious problem solver; I cannot let go sometimes. We all know of this past weekends 'focus-issue' thread pair (here and at DP Review). It started with several users making suggestions about macro focus techniques. I've now re-read the entire thread and some interesting (perhaps useful) information popped-up at the end of the thread.

The camera's origin is questionable. The OP repeats several times that the readers (specifically me) "don't know the circumstances of the camera acquisition". In his final post (as of this AM) he states the problem to be "with manual focus" and seems to say 'a correction in the same direction each time fixes the focus'.

If you followed all that, speculate on what might cause such a repeatable 'error condition'.

These are my speculations:

The focus screen is damaged and/or improperly installed: specifically upside-down or perhaps not securely mounted--i.e the holder not shut properly. If the screen was modified and mis-installed then the in-focus image WOULD be slightly off from the capture plane and the correction would always be in the same direction wrt turning the focus ring; this would also be a very small adjustment.

The camera is damaged in some other fashion which caused a loss of registration between the focus screen alignment and the film/sensor plane alignment: this might be a damaged mirror mount or something more substantial like a damaged frame.

Can anyone think of another cause; and which causes would be warrantied?
03-12-2007, 07:49 PM   #8
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QuoteOriginally posted by jfdavis58 Quote
*snip*

If you followed all that, speculate on what might cause such a repeatable 'error condition'.

These are my speculations:

The focus screen is damaged and/or improperly installed: specifically upside-down or perhaps not securely mounted--i.e the holder not shut properly. If the screen was modified and mis-installed then the in-focus image WOULD be slightly off from the capture plane and the correction would always be in the same direction wrt turning the focus ring; this would also be a very small adjustment.

The camera is damaged in some other fashion which caused a loss of registration between the focus screen alignment and the film/sensor plane alignment: this might be a damaged mirror mount or something more substantial like a damaged frame.

Can anyone think of another cause; and which causes would be warrantied?
Hi John I'm going to do an Occam's razor here. Assuming that the OP is correct, that with manual focus the exact same amount of correction has to be applied in every case, than obviously one light path is slightly longer than the other. IIRC the focusing screen is shimmed, when I was thinking of getting a Katzeye I remember reading that, and thinking at the time "what happens if you lose the shims?" I'm betting the OP (or someone else, maybe an original owner) replaced the original focusing screen, either with the Katzeye or the Pentax one (M-80?) but didn't replace the shims. Since the screen is tabbed to keep it's orientation correct, it seems unlikely that a new screen would be installed upside down. I don't know enough of the mechanics of the screen but I would imagine that if the spring clip was incorrectly fastened it would create distortions in the screen, not a simple focusing error. Same with a bent frame etc. However if the shims were missing, this one possibility would fit all of the known facts. This of course would be a non-warranty issue, since it was owner initiated. But if I'm right its ridiculously easy to diagnose and fix.

NaCl(a nice simple answer, my favorite kind)H2O

03-12-2007, 08:08 PM   #9
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Hadn't heard about the shims.

Guess I've got something of a suspicious mind about such things. I was thinking the alignment tabs might be snipped off--you know, they're just a pesky little thing. Or that the original owner might have sheared it off when he shut the frame--or bent the frame.....

But I like your simple solution answers and yes, I think the shims missing at re-assembly might also be the problem. I'm thinking about one of the Pentax replacement screens-I still see pretty well. I'll remember to post back if I decide to do the switch--especially about the shims.
03-13-2007, 08:17 AM   #10
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QuoteOriginally posted by jfdavis58 Quote
Hadn't heard about the shims.

Guess I've got something of a suspicious mind about such things. I was thinking the alignment tabs might be snipped off--you know, they're just a pesky little thing. Or that the original owner might have sheared it off when he shut the frame--or bent the frame.....

But I like your simple solution answers and yes, I think the shims missing at re-assembly might also be the problem. I'm thinking about one of the Pentax replacement screens-I still see pretty well. I'll remember to post back if I decide to do the switch--especially about the shims.
Actually I'm the suspicious sort too. I do warranty analysis between the company I work for and the NYC transit system. You wouldn't believe the things I've seen that NYCT tries to fob off on us as "warranty repair". On the other hand I've been in enough mechanical diagnosis situations to come to the conclusion that usually a simple solution will be more likely than a complex one.
I really can't think of anything else that doesn't involve catastrophic force. The other possibility is that something went wrong with the Shake Reduction system so that when it's turned off, it is slightly misaligned. Maybe some sort of very hard knock could misalign the locking magnets? The problem with this is that unless the image is so subtly distorted as to be undetectable with the unaided eye, all of the magnets (4 ?) would all have to be displaced the same amount so that the image wouldn't be distorted along one axis or another. That kind of coincidence boggles my mind.

NaCl(my thoughts on it anyway w/o at least a cursory examination of the camera)H2O
03-13-2007, 08:41 AM   #11
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Having replaced my screen with the Katz Eye I can confirm that there's a shim that keeps the screen in the right position. I don't know if it's adjustable or not, but if there is no shim in there you're missing a part.
03-13-2007, 09:52 AM   #12
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Well, that raises another question: where is the shim, above the focus screen on the penta-prism side or below the screen on the mirror side.

And a second question is either side of the frame equiped with a spring(s) or sufficient tension to hold a particular placement of the focusing screen?

An original screen, removed and then replaced would need to shift 'forward' or 'backward' in the light path to create a focus problem.
03-13-2007, 10:09 AM   #13
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Hi John,
I grabbed this link from another thread. Take a look at the pics in this file from Katz Eye. They mat be helpful.
http://www.katzeyeoptics.com/files/PENTAX.PDF
03-13-2007, 10:53 AM   #14
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Great, thanks! The shim stays in and above the focusing screen on the penta-prism side! It's not shown in the photos of the article/instructions (camera model differences), but it is noted on page 3 as a horseshoe shaped shim.

There is also some notation that the KatzEye screen might be thicker than the original--more force needed to lock the retaining frame into place.

I believe that the ground-glass side of the screen faces the mirror/lens/subject; this would imply that the KatzEye is etched more deeply than the original. Perhaps enough to be noticeable?

Clearly the KatzEye people don't feel the extra thickness is a problem, but could a poor installation damage the frame or frame hinge/latch enough so that a return to the original screen would cause focus problems?

Lot's to think about and watch for in the forums!!!
03-13-2007, 11:06 AM   #15
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I haven't removed the screen in my K10, but when I replaced the screens in both my DS's, I noticed how delicate and fragile both the frame, and screen are. I would assume the K100 and K10 are the same. I think that it would be very easy to damage the frame without even knowing it, if you are not careful with your installation.
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