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01-25-2012, 05:13 AM   #1
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Simple focusing screen shim question

Does installing a shim between the focusing screen and the camera reduce back focus or front focus? If you replace a thin shim with a thick one, will this bring the point of accurate focus closer to you or further away? (For what it's worth I have installed at least three screens on different cameras - K10D, K7 and KX - and none of them have given precisely accurate results. The Katzeye was the least worst).

Thanks to anyone with any experience of this adjustment.

Tim

01-25-2012, 06:00 AM   #2
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I'm waiting for a selection of shims as my Katzeye is slightly off. I understand that using a thinner shim will move the focus toward the camera (ie reduce back-focus), which is what I need to do.

But I could be wrong...
01-25-2012, 06:14 AM   #3
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My camera was front-focusing with the factory installed shim. I then removed the shim and re-tested.
At that point I determined that it was back-focusing by about the same margin. I didn't measure the thickness of the original shim,
but from reading around, I assumed a default thickness of about .4 units. I had ordered 0.1, 0.15, 0.2, .25 and .3 washers from Pentax,
and decided to try the .2
The result was more less correct for my SMC-M 50mm/F1.7 @ F/1.7 and my SMC-M100mm/F2.8 @ F2.8.

So basically, you'll want to insert a thinner washer to correct for front-focusing (moving the focusing point further back), and a thicker washer
to correct for back-focusing.
Make some initial sample shots on a scene with your lenses using the distance scale on the lens, then by manually focusing. Compare your closest and infinity shots. Keep the files for later.
Next, do some serious test using some charts (or some setup that works for you), to determine which washer works best. Go out and re-take shots of the same scene you used for your initial samples.
You should be able to determine fairly easily if you have been successful. Make sure you test at close range and at infinity, and see how your manually focused shots
compare to shots using the distance scale on your lens. The distance scale on the lens can be off, but then you'd have an idea by how much (approximately).

It is important to try and keep all parameters the same, i.e., use a tripod, the same scene, time and lighting conditions. Also consider your physical condition.
Eyes can get tired and affect how you focus.
01-25-2012, 06:22 AM   #4
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Heres an explanation with diagrams from Focussing

The text is a little "Chinglish" but understandable together with the diagrams.

Focusing Screen--How to adjust focusing screen--

The shims are placed in between the underside of the prism and the focussing screen.
AF function is not affected by the focus screen or shims.

If you focus the subject in the viewfinder to be sharp, but the image is focussed closer to the foreground this is front focus. To correct this you need to fit a THINNER shim in place in between the prism and the focus screen.

If you focus the subject in the viewfinder to be sharp, but the image is focussed further to the backround this is back focus. To correct this you need to fit a THICKER shim in place in between the prism and the focus screen.


so to answer your question " If you replace a thin shim with a thick one, will this bring the point of accurate focus"... closer to you (ie you will be correcting a back focus problem)

01-25-2012, 07:06 AM   #5
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QuoteOriginally posted by Jonathan Mac Quote
I'm waiting for a selection of shims as my Katzeye is slightly off. I understand that using a thinner shim will move the focus toward the camera (ie reduce back-focus), which is what I need to do.

But I could be wrong...
Yes, I think you have it backwards. Think of the mirror being the center point of the path between the focusing screen and the sensor.
With a thicker washer, the path of the section between focusing screen and mirror would be shorter than the path between mirror and sensor.
Using that shorter distance between mirror and sensor would move the focal point in front of the sensor plane, ie. front focus.
A rather simplistic explanation, sorry.

Here are some more links on the issue:

https://www.pentaxforums.com/forums/pentax-k-5-forum/142225-focus-adjustment-...ml#post1485440
https://www.pentaxforums.com/forums/pentax-k-5-forum/131473-k-5-manual-focus.html
Focus Screen Shims/Washers for Canon 5D - FM Forums
01-25-2012, 08:37 AM   #6
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you need to be careful here, I assume you are discussing focusing error with MF lenses and not AF

the shim has no impact on AF accuracy, as AF takes light off the mirror not the focusing screen
01-25-2012, 09:04 AM   #7
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QuoteOriginally posted by timo Quote
Does installing a shim between the focusing screen and the camera reduce back focus or front focus? If you replace a thin shim with a thick one, will this bring the point of accurate focus closer to you or further away? (For what it's worth I have installed at least three screens on different cameras - K10D, K7 and KX - and none of them have given precisely accurate results. The Katzeye was the least worst).

Thanks to anyone with any experience of this adjustment.

Tim
The focusing screens are responsible for visual focus.
That is, how things appear in focus through the viewfinder. Which would include the accuracy of a split prism as well.

Having said that, the focusing screen does not affect the camera's auto focus, which is a separate system altogether.

As for the screen differences, I can testify that out of all the screens I've purchased and installed, none of them were accurate by default(Though a few came quite close). And so shimming and calibrating was always necessary to get things working properly.

PS. I purchase my shims through Pentax Canada. They're quite cheap and I usually buy a few copies of the most common sizes and multiples of the thinnest shims(the three thinnest) since its easier to combine a major shim with a very thin one to get the job done.

Hope this helps.

Last edited by JohnBee; 01-26-2012 at 09:14 AM.
01-26-2012, 08:26 AM   #8
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Where can I get a set of shims (K-x) in the USA? Thx.

01-26-2012, 09:15 AM   #9
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QuoteOriginally posted by hman Quote
Where can I get a set of shims (K-x) in the USA? Thx.
From Pentax.
Send them an email and tell them you're looking for focusing screen shims for your camera.
01-26-2012, 11:50 PM   #10
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QuoteOriginally posted by Lowell Goudge Quote
you need to be careful here, I assume you are discussing focusing error with MF lenses and not AF

the shim has no impact on AF accuracy, as AF takes light off the mirror not the focusing screen

Yes I totally understand that. My original question related entirely to manual focusing only, the point being that focus that appears accurate via the replacement MF-friendly focusing screens I have tried has never proven to be precisely accurate on closer pixel-peeping. It's not a problem with a wide-angle lens, or with any lens stopped down, but if you are using a fast 50 anywhere near wide-open, i.e. with very little DOF to play with, it is a real issue. In these circumstances using electronic focus confirmation or even catch-in-focus is not really accurate enough, and while LV is certainly a solution, it's a pretty laborious one in a non-studio situation.

So I was just trying to find out if thinner shim = more front focus and thicker shim = more back focus, or vice versa. Thanks to those who have replied.
01-27-2012, 02:22 AM   #11
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QuoteOriginally posted by timo Quote
So I was just trying to find out if thinner shim = more front focus and thicker shim = more back focus, or vice versa. Thanks to those who have replied.
Just in case the correct option hasn't got clear yet, I think the clearest explication in this thread has been posted by Steve1307:
If you focus the subject in the viewfinder to be sharp, but the image is focussed closer to the foreground this is front focus. To correct this you need to fit a THINNER shim in place in between the prism and the focus screen.

If you focus the subject in the viewfinder to be sharp, but the image is focussed further to the backround this is back focus. To correct this you need to fit a THICKER shim in place in between the prism and the focus screen.
Cheers - Klaus
05-05-2012, 05:13 AM   #12
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Hi everyone. I think I understand this procedure, but haven't read anyone mention the adjustment at the eye piece. My right and left eyes differ in focus and unfortunately I use the eye which needs maximum adjustment to the right to get close on focus through the eye piece. So would I need a thin or thick shim.
Thanks for the information, as this has never come up on any photo readings for me in the past.....and has hindered my manual focusing.
05-05-2012, 05:58 AM   #13
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QuoteOriginally posted by stefanostech Quote
Hi everyone. I think I understand this procedure, but haven't read anyone mention the adjustment at the eye piece. My right and left eyes differ in focus and unfortunately I use the eye which needs maximum adjustment to the right to get close on focus through the eye piece. So would I need a thin or thick shim.
Thanks for the information, as this has never come up on any photo readings for me in the past.....and has hindered my manual focusing.
I believe you are somewhat confused. Adding/subtracting shims between the camera and the focusing screen corrects the focus at the eye piece. In other words, the picture presented in the eyepiece will be correctly focused for someone with normal vision. Shims should NOT be used to correct for a person's vision problems because the results would be out of focus pictures. Vision problems should be corrected either with the sliding diopter adjuster located above the eyepiece or in extreme cases, with pieces that add onto the eye piece.
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