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01-24-2016, 11:37 PM   #1
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Stock un-shimmed focusing screen, a problem?

I've been reading about people having to shim their focusing screens.

If I understand the problem, an improperly shimmed focusing screen makes the viewfinderís focus slightly different than the actual focus of the lens, which can lead to out of focus photos when shooting lenses set to manual focus.

Is this something most people would notice or something that most people canít even perceive?
I know some people split hairs and worry about things that are nearly imperceptible.


Are there any example photos of it?


-------------

Also would this be a work around for it?

Since the viewfinder has a diopter knob for setting the viewfinderís focus for your eyesight.

Couldnít you use live view, manually focus, switch to the viewfinder and adjust the diopter until itís in focus?
Would that compensate for an out of alignment focusing screen?

Thanks.

01-25-2016, 12:17 AM   #2
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The diopter only makes it less or more in focus, it does not change the apparent distance. No different than looking w/ prescription glasses vs w/o the glasses. And the effect of shimming or not, or by an incorrect amount, is not always trivial.
01-25-2016, 12:23 AM - 2 Likes   #3
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QuoteOriginally posted by pentax360 Quote
I've been reading about people having to shim their focusing screens.

If I understand the problem, an improperly shimmed focusing screen makes the viewfinderís focus slightly different than the actual focus of the lens, which can lead to out of focus photos when shooting lenses set to manual focus.
Your understanding is correct.

QuoteOriginally posted by pentax360 Quote
Is this something most people would notice or something that most people canít even perceive?
The answer is complicated, but the short answer is yes, an improperly calibrated focus screen will usually result in poor manual focus performance that is easily detectable if fine focus is the goal.

QuoteOriginally posted by pentax360 Quote
Also would this be a work around for it?

Since the viewfinder has a diopter knob for setting the viewfinderís focus for your eyesight.

Couldnít you use live view, manually focus, switch to the viewfinder and adjust the diopter until itís in focus?
Would that compensate for an out of alignment focusing screen?
No...

Your work-around would work if the viewfinder used an aerial image, but it doesn't. The image is projected on to the bottom surface of the focus screen and the diopter works by adjusting the ocular focus to the surface of the focus screen.

Now that those points are out of the way, here is a quick primer on manual focus and shims.
  • Focus screen calibration is supposed to have been done at the factory
  • Most (all?) current model Pentax dSLRs will have a shim between the focus screen and the bottom of the pentaprism
  • The thickness of the shim allows adjustment of the reflected distance from lens to focus screen lower surface. The distance should be the same as that to the focal plane (the sensor surface)
  • Inaccurate calibration may be determined by comparing best effort manual focus with magnified live view
The above is pretty straightforward and I could end here except that accuracy is only part of attaining consistent manual focus results. The other component is precision. This is most easily understood as the probability of getting the same focus plane twice in a row. Precision is independent of accuracy in that the system calibration may be be spot on, but with a certain percentage of out-of-focus results clustered around the true focus plain. The worse the precision, the harder it is to get sharp results.
Poor calibration results in inaccurate focus. Poor precision results in few in-focus results regardless of calibration.
Back to the title of this thread:
"Stock un-shimmed focusing screen, a problem?"
Again, there is a short answer and that is no. I can't remember seeing a user report of a poorly calibrated stock focus screen.

Does that mean everything is fine? Unfortunately no. Both the stock focus screen and manual focus confirmation using the AF system suffer from relatively poor precision with the stock focus screen being the worst of the two when used with faster lenses (wider than f/4 maximum aperture).

Therein lies the context for most of the shimming discussions. In order to get reasonable fine focus results (better precision) from faster and/or longer lenses some users have replaced their stock focus screen with third party screens from various vendors. These screens usually have focus aides and/or are optimized for high precision with fast lenses. The thickness of these screens is seldom the same as the stock screen meaning they often require a calibration adjustment using some sort of replacement shim.

Make sense?


Steve
01-25-2016, 03:49 AM   #4
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QuoteOriginally posted by stevebrot Quote
Unfortunately no. Both the stock focus screen and manual focus confirmation using the AF system suffer from relatively poor precision with the stock focus screen being the worst of the two when used with faster lenses (wider than f/4 maximum aperture).

Therein lies the context for most of the shimming discussions. In order to get reasonable fine focus results (better precision) from faster and/or longer lenses some users have replaced their stock focus screen with third party screens from various vendors. These screens usually have focus aides and/or are optimized for high precision with fast lenses. The thickness of these screens is seldom the same as the stock screen meaning they often require a calibration adjustment using some sort of replacement shim.

Make sense?


Steve
Boom! This! Stock screen isn't designed for manual focussing (it's aimed more towards providing a bright view). The focus screen can swapped for one more optimised for manual focussing (there's no free lunch; these have their own disadvantages), and it's generally these that people are talking about calibrating by shimming.

01-25-2016, 12:30 PM   #5
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With Katz-Eye out of business, is there another source for having a new focusing screen shimmed and calibrated for you? One of the services they offered was to receive your camera, insert the newly purchased focusing screen and calibrate it with whatever shims were necessary. Thanks in advance for anyone who knows.
01-25-2016, 05:39 PM   #6
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Thanks for the excellent explanation, I think I understand all of it.
01-25-2016, 08:28 PM   #7
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QuoteOriginally posted by pentax360 Quote
IRT: stevebrot
Thanks for the excellent explanation, I think I understand all of it.
My pleasure! BTW, welcome to the Pentax Forums!


Steve
01-25-2016, 10:18 PM   #8
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thanks.

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