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03-18-2010, 04:59 PM - 2 Likes   #1
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The things you learn. Katz Eye screen

I did post this over at the "other forum" but thought it worth posting here as well:

I just purchased a Katz Eye focus screen for my K-7 and received it yesterday. The screen is a ground glass screen with 1/3rd grid lines for composition and Opti-Brite treatment. There are no other focus aids other than the ground glass screen, there is no split prism or microprism collar.

This screen is wonderful as it is simple and very effective for getting that "snap intop focus" look throught he VF.

I did some preliminary tests as far as exposure and manual focus accuracy. Exposure looks spot on but I noticed an issue with the manual focus accuracy. When the focus is correct in the VF, the resultant recorded image on the sensor was focused in front of the intended target. This was using the A50mm f1.2 lens which will obviously have very narrow DOF and allow such accurate focusing ability.

My first thought was that the focus screen is too thick and therefore the image was being focused in front of the correct focus plane, but on reflection and talking to Rachael Katz from Katz Eye, this is of course an incorrect assumption. Silly me!! The focus screen thickness is irrelevent as the focusing takes place on the ground glass surface which is on top of the screen next to the pentaprism, just the same as the original screen focus would be and therefore has no bearing on the issue.

My second thought was that the pentaprism is out of calibration, but Rachael Katz advises that there is a shim between the focus screen and the pentaprism bottom face and these shims come in various thicknesses so as there is adjustment available!

I removed my shim and measured it and found that it was .40mm. When removed, the resultant image target recorded on the sensor was now behind the focused target image in the VF by about the same margin as it was in front with the shim installed.

So, it would appear that the correct shim thickness should be about .20mm to .25mm and this is borne out by what Rachael Katz has found with her dealings with the K-7. Apparently, some K-7's are shipped with the .40mm shim and some with .20mm and .25mm. It looks as though mine should have been with the .20mm or .25mm shim and this is why it is out of calibration.

Rachael also states that using the Katz Eye screen due to it's excellent focusability, you can see where even as little as .05mm can make the difference whether you have the focus accuracy exactly spot on! I would suggest that you need a very fast lens to utilize this sort of focus accuracy with the Katz Eye screen, probably sub f2.

What I did notice is that the DA*16-50 f2.8 lens was less susceptible to slight focus inaccuracies due to the extra DOF, but it would still be advantageous to have the correct focus shim applied. I think my point here is that you are less likely to see focus inaccuracies with an f2.8 lens or slower than if you used a faster lens(obviously).

The Katz Eye screen seems to give better focus "snap" and therefore better focus accuracy, me thinks. I mean, I never really noticed that I had any focus inaccuracies previously, but using the Katz Eye certainly pointed it out quite easily. Maybe this is why a did get a few ever so slightly misfocused shots (but within realms of acceptibility) previously and put it down to user error. The "inaccuracies" may be, and probably are, masked by the extra DOF afforded by using smaller apertures like f4 etc.

If your shim is incorrect, this may also play a part in any focus anomolies experienced when trying to adjust for AF Adjustment and why some people find it difficult to accurately get correct AF using the AF adjustment feature. I am not saying that the shim issue is prevalent, but it may be worth checking if you are having any issues with focus and discount this part of the equation.

It is a reasonably simple exercise to swap shims and not much different to changing the focus screen, if you are reasonably competent. I would be very careful when doing it as you may damage the bottom of the pentaprism if you are not careful. Use a plastic set of tweezers rather than anything metal. Katz Eye supply an excellent set of plastic tweezers for swapping the focus screen, so you can utilize this for the job.

03-18-2010, 05:10 PM   #2
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Is there an "Old News" sub-forum somewhere? How about a "Things That Have Been Discussed To Death" one?

For many people even using no shim still doesn't fix the problem. Many have had to send back their focusing screens as a result of this. On my camera, I removed the factory shim altogether and focus is perfect.
03-18-2010, 05:18 PM   #3
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QuoteOriginally posted by GoremanX Quote
Is there an "Old News" sub-forum somewhere? How about a "Things That Have Been Discussed To Death" one?
Well, excuuuuuse me for breathing. I didn't know this had been discussed before. I had never heard of the shim issue anywhere other than that the DS(?) had a shim. I never noticed tham in any of my other cams, *ist D, K10D or K20D.

QuoteQuote:
For many people even using no shim still doesn't fix the problem.
I am sure that this can happen. Did I say it couldn't?

QuoteQuote:
Many have had to send back their focusing screens as a result of this. On my camera, I removed the factory shim altogether and focus is perfect.
Well, there you go.
03-18-2010, 05:21 PM   #4
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QuoteOriginally posted by Lance B Quote
Well, excuuuuuse me for breathing. I didn't know this had been discussed before. I had never heard of the shim issue anywhere other than that the DS(?) had a shim. I never noticed tham in any of my other cams, *ist D, K10D or K20D.
You're right. My apologies for my rudeness.

03-18-2010, 05:22 PM   #5
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QuoteOriginally posted by GoremanX Quote
You're right. My apologies for my rudeness.
No probs, mate.
03-18-2010, 05:37 PM   #6
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What's a millimeter? Can't you guys explain this in fractions of inches instead?

METRIC!? HELL NO! WE WON'T GO!

Too much work, and too hard.
03-18-2010, 05:43 PM   #7
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QuoteOriginally posted by Ira Quote
What's a millimeter? Can't you guys explain this in fractions of inches instead?

METRIC!? HELL NO! WE WON'T GO!

Too much work, and too hard.
It's easy:

1 liter (1000ml) of water at 10 degrees celcius weighs 1 kilogram (1000g) and fits perfectly in a cubic container measuring 10cm on every side (1000 cubic centimeters).

Make sense now?
03-18-2010, 05:50 PM   #8
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Only if you can say it again in 10 different languages....

03-18-2010, 05:52 PM   #9
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Don't bother. Dividing by 10 makes no sense in one country.
03-18-2010, 05:57 PM   #10
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QuoteOriginally posted by Lance B Quote
Apparently, some K-7's are shipped with the .40mm shim and some with .20mm and .25mm. It looks as though mine should have been with the .20mm or .25mm shim and this is why it is out of calibration.
I find this entire post very interesting, but I am intrigued as to why there would varying shim sizes. Any thoughts here? Seems that would make QC more difficult or at the very least allow for a larger variance.
03-18-2010, 06:00 PM   #11
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QuoteOriginally posted by joeyc Quote
I find this entire post very interesting, but I am intrigued as to why there would varying shim sizes. Any thoughts here? Seems that would make QC more difficult or at the very least allow for a larger variance.
Because it's nearly impossible to have the distance between the CMOS sensor, the autofocus sensor and the focusing screen be completely identical while keeping the product affordable. This is true of all SLR cameras, which by their very nature introduce some variance in their component location. Those are 3 components which are in 3 completely different plcaes within the camera. We're talking super-thin shims here and tons of precision.

edit: http://www.focusingscreen.com/work/test-fsen.htm
03-18-2010, 06:02 PM   #12
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The Katz Eye screen is made of glass not plastic?

I would imagine its not as prone to scratching duing handling or installation ?
03-18-2010, 06:05 PM   #13
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QuoteOriginally posted by Warren s Quote
The Katz Eye screen is made of glass not plastic?

I would imagine its not as prone to scratching duing handling or installation ?
I'm pretty sure it's actually plastic, just like almost all modern focusing screens. I could be wrong on this, but I doubt it. My K3 focusing screen from focusingscreen.com is plastic, I know this because I dropped it on the floor and my dog sniffed it with his wet nose and stepped on it and it got all scratched to hell.
03-18-2010, 06:45 PM   #14
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Hey Lance, nice info you posted here.

I noticed the same thing w/ my K7, and my friends' K7s have the same problem. We bought those split scressn from focusingscreen.com, but we all feel the focusing is just a tad bit off. It's more obvious when using a fast lens as you addressed above.

I thought about taking out the stock shim and putting in a thinner one, but haven't given it a try yet. I might do so over the weekend. Needs to find a clean place w/ nobody moving around. W/ two little kids around, I can't do this at home
03-18-2010, 06:54 PM   #15
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QuoteOriginally posted by GoremanX Quote
It's easy:

1 liter (1000ml) of water at 10 degrees celcius weighs 1 kilogram (1000g) and fits perfectly in a cubic container measuring 10cm on every side (1000 cubic centimeters).

Make sense now?

Dont you mean Litre???
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