K-7 shims

Apparently, this has been discussed here before, but I thought I would post my experiences with my K-7 with regards the focus shims and Katz Eye screen. I never knew there were focus shims used for calibrating the focus screens of the Pentax DSLR's. I did know there was a shim for the DS(?), but I thought that was just a locating shim or some such thing. Anyway, I posted this over at the "other forum"

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.

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Cameras:K-7/K20D/*ist D/K10D
Lenses:DA14 f2.8, A16 f2.8, FA20 f2.8, FA31 f1.8, DA35 f2.8 macro, FA43 f1.9, FA50 f1.4, A50 f1.4, A50 f1.2, FA50 f2.8 macro, DA*55 f1.4, FA77 f1.8, DFA100 f2.8 macro, A100 f2.8, DA*300 f4, FA*300 f4.5, DA10-17 FE, DA12-24 f4, DA*16-50 f2.8, DA17-70 f4, FA*28-70 f2.8, DA*50-135 f2.8, DA*60-250 f4, FA*80-200 f2.8.
Flashes:AF540FGZ, AF360FGZ

G'Day Lance!

That's quite a saga. It appears from your description that it is AF that is in issue here, not focus per se were one to do that manually?

Also (knowing you had a K10 and K20), did you discover shims in those bodies as well?

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http://www.pbase.com/iberg

It's certainly interesting (and I'll be checking mine later) but if I understand correctly, it is about manual focus. There are three 'light paths' through an AF SLR: to the sensor/film; to the focusing screen via the mirror; and to the AF sensors (vi semi-silvered beam splitters in the mirror). They all need to be the same length (+/- not very much at all it seems) so if the bottom face of the focusing screen isn't in the right position, MF will be out. I can imagine though that it wouldn't help AF adjustment - your eye would be telling you its OOF and the AF thinks it is (or vice versa); and both might actually be wrong (in different directions) in the worst case.
Regards
Neal

It has nothing to do with AF. The A50mm f1.2 is a manual focus lens.

The discrepancy is due to the focussing screen not precisely correlating to the sensor plane (which is not adjustable via the AF micro-adjustment).

It's actually quite common, but most users don't notice as the standard screen doesn't have sufficient "snap" and also most lenses are quite slow in comparison to an f1.2! You do need a very shallow DOF setup to notice it, a long fast telephoto is a good lens to use.

Most SLR's have some form of focusing screen positional adjustment, usually via shims. My K10D was slightly out on manual focus and needed adjustment which was done by Pentax service under warranty.

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Best regards
Richard Day

Profile - link - (click on About for equipment profile) - My Flickr site - link

I've been through similar travails with my K20D. i spent a while looknig for genuine shims, but couldn't find any. SRS referred me to Johnsons who completely ignored my two emails.

I ended up using some strips of post-it note, which seem fairly close to perfect but not quite there.

Would love to find a supplier of K20D shims if anyone knows of one

FWIW My K20D had a (metal) shim in it already, despite reading several reports that it should have been shim-less.

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Tim
AF - Pentax K5, Sigma 10-20/4-5.6, Tamron 17-50/2.8, Sigma 30/1.4, Sigma 70-200/2.8, Tamron 70-300/4-5.6
MF - Vivitar CF 28/2.8, Tamron AD2 90/2.5, MTO 1000/11
Stuff - Metz 58 AF1, Cactus v4, Nikon SB24, Raynox 150, Sigma 1.4x TC, Sigma 2x TC, Kenko 2x macro TC, Redsnapper 283 tripod, iMac 27”, Macbook Pro 17”, iPad, iPhone 3G
Flickr • Fluidr • PPG • Street • Portfolio site
Feel free to edit any of my posted photos! If I post a photo for critique, I want brutal honesty. If you don't like it, please say so and tell me why!

I was thinking precisely the same thing, here is an image.



Basicly the Shims have only effect if you're using a focusing aid screen like a splitscreen, they need to be placed precisely at the right distance.
The standard screens don't have a focus aid so it won't do you much good besides that the focus bracket might look a bit sharper

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Stefan


K10D, K5
DA* 16-50, DA* 50-135, D-FA 100 Macro, DA 40 Ltd, DA 18-55
AF-540FGZ

Perfectly explained, Richard!

Here is a diagram of the various focus paths. To get correct sharpness in the VF that corresponds to the sensor image, L1 and L2 need to be exactly the same length. To achieve correct sharpness from auto focus, L3 needs to equal L2. For all to work together correctly, L1, L2 and L3 all need to be identical.

Pentax provide the AF Adjustment feature for calibrating L3 to correspond to L2. We now see that they also provide a way to calibrate L1 via shims.

Shims are inserted between the focus screen and the bottom of the pentaprism. A .05mm difference of shim thickness can be detected by a suitably fast lens, so I am told.

Here is a good link for explanation:

http://www.focusingscreen.com/work/test-fsen.htm




Here is the shim from my K-7:

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Cameras:K-7/K20D/*ist D/K10D
Lenses:DA14 f2.8, A16 f2.8, FA20 f2.8, FA31 f1.8, DA35 f2.8 macro, FA43 f1.9, FA50 f1.4, A50 f1.4, A50 f1.2, FA50 f2.8 macro, DA*55 f1.4, FA77 f1.8, DFA100 f2.8 macro, A100 f2.8, DA*300 f4, FA*300 f4.5, DA10-17 FE, DA12-24 f4, DA*16-50 f2.8, DA17-70 f4, FA*28-70 f2.8, DA*50-135 f2.8, DA*60-250 f4, FA*80-200 f2.8.
Flashes:AF540FGZ, AF360FGZ

The shims should be available from Pentax UK? I have purchased them from Pentax Australia and should receive them this week. I have purchased a set of 7 from .10mm to .40mm inclusive in .05mm steps, there are also .45mm and .50mm available.

I never knew that they had the shims until Rachael pointed it out to me and it is fantastic that they do, so now we can calibrate the whole focus system to correlate with each other. We can adjust AF via the AF Adjustment feature, and now the focus screen via shims.

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Cameras:K-7/K20D/*ist D/K10D
Lenses:DA14 f2.8, A16 f2.8, FA20 f2.8, FA31 f1.8, DA35 f2.8 macro, FA43 f1.9, FA50 f1.4, A50 f1.4, A50 f1.2, FA50 f2.8 macro, DA*55 f1.4, FA77 f1.8, DFA100 f2.8 macro, A100 f2.8, DA*300 f4, FA*300 f4.5, DA10-17 FE, DA12-24 f4, DA*16-50 f2.8, DA17-70 f4, FA*28-70 f2.8, DA*50-135 f2.8, DA*60-250 f4, FA*80-200 f2.8.
Flashes:AF540FGZ, AF360FGZ

Thanks all for the enlightenment - glad I asked!

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http://www.pbase.com/iberg

Thanks for confirmation Lance, I will send a few more emails to Johnsons or whoever is the UK Pentax rep now.

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Tim
AF - Pentax K5, Sigma 10-20/4-5.6, Tamron 17-50/2.8, Sigma 30/1.4, Sigma 70-200/2.8, Tamron 70-300/4-5.6
MF - Vivitar CF 28/2.8, Tamron AD2 90/2.5, MTO 1000/11
Stuff - Metz 58 AF1, Cactus v4, Nikon SB24, Raynox 150, Sigma 1.4x TC, Sigma 2x TC, Kenko 2x macro TC, Redsnapper 283 tripod, iMac 27”, Macbook Pro 17”, iPad, iPhone 3G
Flickr • Fluidr • PPG • Street • Portfolio site
Feel free to edit any of my posted photos! If I post a photo for critique, I want brutal honesty. If you don't like it, please say so and tell me why!

I found this topic fascinating, Ive had the focus screens out on three/four? Pentax bodies, MZ5n MZ7 istD and I think my K10D, and I dont recal finding a shim in any of them.

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My Names Alan, and I'm a lensaholic.
My PPG link
My Flckr link

How much did this set cost?

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Camera:K20D|Ist*DS|Spotmatic II|MZ-10
Pentax Lenses: DA16-45|DA50-200|50A 1.7
Tamron Lenses: 28-200
Takumar Lenses: SMC 55 1.8
Sigma Lenses: EX DG 50-500 'Bigma'|EX 50mm Macro
Flashes: Metz 58 AF-1|Samsung SEF-36PZF|Pentax AF-220T

I'd never noticed a shim in my K10D previously either, but I just checked it and sure enough there is one there!

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Cameras:K-7/K20D/*ist D/K10D
Lenses:DA14 f2.8, A16 f2.8, FA20 f2.8, FA31 f1.8, DA35 f2.8 macro, FA43 f1.9, FA50 f1.4, A50 f1.4, A50 f1.2, FA50 f2.8 macro, DA*55 f1.4, FA77 f1.8, DFA100 f2.8 macro, A100 f2.8, DA*300 f4, FA*300 f4.5, DA10-17 FE, DA12-24 f4, DA*16-50 f2.8, DA17-70 f4, FA*28-70 f2.8, DA*50-135 f2.8, DA*60-250 f4, FA*80-200 f2.8.
Flashes:AF540FGZ, AF360FGZ

Each one was AU$2.20 each, so a full set of 9 would be about AU$20.00 or about Euro13.20.

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Cameras:K-7/K20D/*ist D/K10D
Lenses:DA14 f2.8, A16 f2.8, FA20 f2.8, FA31 f1.8, DA35 f2.8 macro, FA43 f1.9, FA50 f1.4, A50 f1.4, A50 f1.2, FA50 f2.8 macro, DA*55 f1.4, FA77 f1.8, DFA100 f2.8 macro, A100 f2.8, DA*300 f4, FA*300 f4.5, DA10-17 FE, DA12-24 f4, DA*16-50 f2.8, DA17-70 f4, FA*28-70 f2.8, DA*50-135 f2.8, DA*60-250 f4, FA*80-200 f2.8.
Flashes:AF540FGZ, AF360FGZ

Good to see that Pentax charges a sensible price for the shims... I was scared that they would overprice them just like simple plastic things like lens caps, hotshoe covers and such.

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Camera:K20D|Ist*DS|Spotmatic II|MZ-10
Pentax Lenses: DA16-45|DA50-200|50A 1.7
Tamron Lenses: 28-200
Takumar Lenses: SMC 55 1.8
Sigma Lenses: EX DG 50-500 'Bigma'|EX 50mm Macro
Flashes: Metz 58 AF-1|Samsung SEF-36PZF|Pentax AF-220T