Washer (shim) for focusing screen - how to select?

Howdy · 07-07-2010, 12:08 PM

Greetings, fellow Pentaxians!

I have a cheap Chinese focusing screen for my Pentax K7 and I must say that this + O-ME53 magnifying eyecup has a great potential. There is one problem though - I am sure that I will need a proper washer underneath the focusing screen. To solve the problem, I have ordered a full set of shims from Pentax (I have all the widths available for the K7).

Now the problem: how to choose a proper shim?

The most obvious technique would be by trial and error but it is too inconsistent - the focus difference using SMC Takumar 50mm f/1.4 at f/1.4 (M42 mount) is just too small to see on the viewfinder but it is very easily visible using LiveView. The only option would be to shoot a set of pictures for each candidate shim and average the results. It is a lot of work and I am not sure about the results.

I have another idea - would it work if I used the bellows with the same 50mm lens? The great advantage is that I would have a razor depth of field and it would be very easy to find a proper shim. The big question to you is - does it make sense? Can this method be used to find a proper shim that would work for all ranges, not only macro shots?

Maybe you have other ideas?

tentacles · 07-07-2010, 12:37 PM

I am having a similar problem, I just put in a focusing screen and the prism is indicating focus in front of the subject ie you have to focus farther than where it lines up. I'm not sure if you are supposed to put a shim in front of or behind the screen in this case - I don't have a set of shims but I do have shim stock I could cut into shims.

Howdy · 07-07-2010, 12:57 PM

Originally posted by tentacles

I'm not sure if you are supposed to put a shim in front of or behind the screen in this case

This one is simple. The shim must go between a prism and a screen. A proper order is:

PRISM-SHIM-FOCUSING SCREEN-METAL CLIP TO HOLD ALL OF THIS TOGETHER-EYEPIECE-YOUR EYE

Originally posted by tentacles

I do have shim stock I could cut into shims

Are you sure we are talking about the same thing? A shim is a small metal part (at the very most 1 or 2mm thick; typically 0.5mm). Take a look at https://www.pentaxforums.com/forums/pentax-dslr-discussion/94154-things-you-l...tml#post961681 The shim is this small metal thingy shown on a picture by Lance B

The installation manual on the Focusingscreen.com shows the installation details --Pentax K7/K10D/K20D/ISTD Focusing Screen Installation Instruction--

Wheatfield · 07-07-2010, 02:02 PM

I chose the new shim for my Katz-Eye by trial and error. It isn't hard, and the whole process took about an hour.
I used a 50/1.2 lens at a shooting distance of 16 or so feet at a wall clock to dial in the focus.

Howdy · 07-07-2010, 04:15 PM

Originally posted by Wheatfield

I used a 50/1.2 lens at a shooting distance of 16 or so feet at a wall clock to dial in the focus.

Ehhhh.... 50 f/1.2..... (Must.... cure..... LBA.....)

Anyway, what do you think about the bellows method? If I am right, this could be by far the easiest and most accurate.

Wheatfield · 07-07-2010, 05:39 PM

I think it's overkill, and the very limited DOF might actually be counter productive.
I've always found that the best way to "test" equipment is to try to get as close to real world conditions as is possible.

troyz · 07-07-2010, 07:16 PM

Originally posted by Howdy

Ehhhh.... 50 f/1.2..... (Must.... cure..... LBA.....)

Anyway, what do you think about the bellows method? If I am right, this could be by far the easiest and most accurate.

The bellows method would decrease depth of field, but would not decrease depth of focus (see wikipedia for the distinction) so it won't necessarily lead to more accurate calibration.

As Wheatfield suggests, a 50/1.2 or 50/1.4 at normal shooting distances would be ideal.

BTW, in case it's not already clear, your camera probably already has a shim installed, in which case would need to replace it with one of different thickness.

Eruditass · 07-07-2010, 07:34 PM

you could use in-camera review. I'd just use a tripod and a bf/ff focus chart. portrait mode if necessary.

you could even try contrast detect, although I had problems at f2.8 200mm.. https://www.pentaxforums.com/forums/pentax-dslr-discussion/106817-contrast-de...king-down.html

Howdy · 07-08-2010, 01:35 AM

Originally posted by troyz

The bellows method would decrease depth of field, but would not decrease depth of focus (see wikipedia for the distinction) so it won't

Could you be more specific please? I have found the depth of field (DoFld) and depth of focus (DoFoc) articles on Wikipedia but I am not sure if I understand you correctly. My knowledge about optics is (at best) very limited.

First, according to Wikipedia: The image distances are measured from the camera's image plane to the lens's image nodal plane, which is not always easy to locate. (Depth of field - Wikipedia, the free encyclopedia). If I understand correctly, the nodal plane (whatever it is

) is a constant. When using bellows, the distance ("v") between this constant and an image plane is increased greatly. So,
DoFld = sf^2/(f^2+-X), where
X = Nc(s-f)

1/DoFld = (f^2+-Nc(s-f))/(sf^2) = 1/s +-Nc/(f^2)-+Nc/(sf) = +-Nc/(f^2)+(1-+Nc/f)*(1/s) = a+b/s
hence
DoFld = s/(as+b)
(a=+-Nc/(f^2), b=(1-+Nc/f))

s - distance of "ideal" focus
f - focal length
N - "f-number"
c - circle of confusion (c=0.02 for Pentax K7)

DoFoc = 2Ncv/f

From all of this, both DoFld and DoFoc do depend linearly on the use of bellows (in DoFld "a" is small, "b" is almost 1 and, as "s" is also small, DoFld is approximately DoFld=s/b). Obviously, the question is "how does "s" relate to "v"...

So, to sum it all up - The bellows method [...] would not decrease depth of focus is not true (or I am missing something).

rhodopsin · 07-08-2010, 03:02 AM

Somebody over on FM55 did work with Canon 5DII focus screen shims (washers), made measurements and produced a chart with graphs showing effects of shims on calibration using various focal length lenses. Focus was found to be affected most using wide angle lens. I think you are correct; the effect is even greater with macro.

MF Screen Calibration - FM Forums

troyz · 07-08-2010, 09:02 AM

Originally posted by Howdy

. . . Obviously, the question is "how does "s" relate to "v"...

So, to sum it all up - The bellows method [...] would not decrease depth of focus is not true (or I am missing something).

from Depth of focus - Wikipedia, the free encyclopedia :

"Depth of focus, however, is a measurement of how much the film or sensor plane can be displaced while an object remains in acceptably sharp focus."

Denoting depth of focus by t:

t = 2 Nc v/f
t = 2 Nc (1+m)

where
N = f-number
c = circle of confusion
v = image depth
f = focal length
m = subject magnification

---

m increases when you use the bellows, so t (depth of focus) increases with the bellows, which is undesirable because you won't be able to detect a difference between flange-to-sensor distance and flange-to-focus screen less than t.

Howdy · 07-08-2010, 02:06 PM

Thank you, troyz!

Now I think I get it!

Do you know any good method for selecting a proper shim thickness?

JohnBee · 07-08-2010, 05:53 PM

Originally posted by Howdy

Now the problem: how to choose a proper shim?

Whenever I get a new camera, I simply order the entire spread. Shims don't cost enough to nit-pick over getting the perfect shim and the added advantage of combining shims for micro-adjustments is a huge plus.

Aprox. price of total shim collection: 20USD.

troyz · 07-08-2010, 10:42 PM

In principle you could try to measure the focus error, determine the thickness of the shim you have now (with a micrometer?), and calculate the thickness of the replacement shim. . .

but in practice I think the trial-and-error method will be faster.

A link to Wheatfield's thread with shim part numbers and thicknesses in case you don't already have it:

https://www.pentaxforums.com/forums/pentax-dslr-discussion/88289-k7-screen-shims.html

(FWIW, I had Katz re-shim my K20D in their shop)

Wheatfield · 07-09-2010, 06:47 AM

Originally posted by troyz

In principle you could try to measure the focus error, determine the thickness of the shim you have now (with a micrometer?), and calculate the thickness of the replacement shim. . .

but in practice I think the trial-and-error method will be faster.

A link to Wheatfield's thread with shim part numbers and thicknesses in case you don't already have it:

https://www.pentaxforums.com/forums/pentax-dslr-discussion/88289-k7-screen-shims.html

(FWIW, I had Katz re-shim my K20D in their shop)

Apparently I used the 55/1.4 not the 50/1.2 for calibration.....

07-07-2010, 12:08 PM	#1
Howdy Forum Member Join Date: Feb 2010 Location: Poland Posts: 86	Washer (shim) for focusing screen - how to select? Greetings, fellow Pentaxians! I have a cheap Chinese focusing screen for my Pentax K7 and I must say that this + O-ME53 magnifying eyecup has a great potential. There is one problem though - I am sure that I will need a proper washer underneath the focusing screen. To solve the problem, I have ordered a full set of shims from Pentax (I have all the widths available for the K7). Now the problem: how to choose a proper shim? The most obvious technique would be by trial and error but it is too inconsistent - the focus difference using SMC Takumar 50mm f/1.4 at f/1.4 (M42 mount) is just too small to see on the viewfinder but it is very easily visible using LiveView. The only option would be to shoot a set of pictures for each candidate shim and average the results. It is a lot of work and I am not sure about the results. I have another idea - would it work if I used the bellows with the same 50mm lens? The great advantage is that I would have a razor depth of field and it would be very easy to find a proper shim. The big question to you is - does it make sense? Can this method be used to find a proper shim that would work for all ranges, not only macro shots? Maybe you have other ideas?

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Shim replacement works - K7 focus screen	Lance B	Pentax DSLR Discussion	28	12-29-2010 04:28 PM
Focusing manual lens with OEM focusing screen, is a split screen really needed?	skid2964	Pentax Camera and Field Accessories	16	09-17-2010 02:54 PM
For Sale - Sold: Minolta X-700 Focusing Screen (for you to cut your own split screen)	frank	Sold Items	3	09-24-2009 10:08 PM
K2000 focus screen copper shim	Eruditass	Pentax DSLR Discussion	5	09-15-2009 08:13 AM
K200 Focusing screen available shim thickness's	gregmoll	Pentax DSLR Discussion	0	07-03-2009 07:12 AM

07-07-2010, 12:37 PM	#2
tentacles Junior Member Join Date: Jun 2010 Location: Winnipeg, MB Photos: Albums Posts: 48	I am having a similar problem, I just put in a focusing screen and the prism is indicating focus in front of the subject ie you have to focus farther than where it lines up. I'm not sure if you are supposed to put a shim in front of or behind the screen in this case - I don't have a set of shims but I do have shim stock I could cut into shims.

07-07-2010, 12:57 PM	#3
Howdy Forum Member Join Date: Feb 2010 Location: Poland Posts: 86 Original Poster	Originally posted by tentacles I'm not sure if you are supposed to put a shim in front of or behind the screen in this case This one is simple. The shim must go between a prism and a screen. A proper order is: PRISM-SHIM-FOCUSING SCREEN-METAL CLIP TO HOLD ALL OF THIS TOGETHER-EYEPIECE-YOUR EYE Originally posted by tentacles I do have shim stock I could cut into shims Are you sure we are talking about the same thing? A shim is a small metal part (at the very most 1 or 2mm thick; typically 0.5mm). Take a look at https://www.pentaxforums.com/forums/pentax-dslr-discussion/94154-things-you-l...tml#post961681 The shim is this small metal thingy shown on a picture by Lance B The installation manual on the Focusingscreen.com shows the installation details --Pentax K7/K10D/K20D/ISTD Focusing Screen Installation Instruction--

07-07-2010, 02:02 PM	#4
Wheatfield Moderator Join Date: Apr 2008 Location: The wheatfields of Canada Posts: 15,987	I chose the new shim for my Katz-Eye by trial and error. It isn't hard, and the whole process took about an hour. I used a 50/1.2 lens at a shooting distance of 16 or so feet at a wall clock to dial in the focus.

07-07-2010, 04:15 PM	#5
Howdy Forum Member Join Date: Feb 2010 Location: Poland Posts: 86 Original Poster	Originally posted by Wheatfield I used a 50/1.2 lens at a shooting distance of 16 or so feet at a wall clock to dial in the focus. Ehhhh.... 50 f/1.2..... (Must.... cure..... LBA.....) Anyway, what do you think about the bellows method? If I am right, this could be by far the easiest and most accurate.

07-07-2010, 05:39 PM	#6
Wheatfield Moderator Join Date: Apr 2008 Location: The wheatfields of Canada Posts: 15,987	I think it's overkill, and the very limited DOF might actually be counter productive. I've always found that the best way to "test" equipment is to try to get as close to real world conditions as is possible.

07-07-2010, 07:16 PM	#7
troyz Veteran Member Join Date: May 2008 Posts: 389	Originally posted by Howdy Ehhhh.... 50 f/1.2..... (Must.... cure..... LBA.....) Anyway, what do you think about the bellows method? If I am right, this could be by far the easiest and most accurate. The bellows method would decrease depth of field, but would not decrease depth of focus (see wikipedia for the distinction) so it won't necessarily lead to more accurate calibration. As Wheatfield suggests, a 50/1.2 or 50/1.4 at normal shooting distances would be ideal. BTW, in case it's not already clear, your camera probably already has a shim installed, in which case would need to replace it with one of different thickness.

07-07-2010, 07:34 PM	#8
Eruditass Veteran Member Join Date: Jul 2009 Photos: Gallery Posts: 2,207	you could use in-camera review. I'd just use a tripod and a bf/ff focus chart. portrait mode if necessary. you could even try contrast detect, although I had problems at f2.8 200mm.. https://www.pentaxforums.com/forums/pentax-dslr-discussion/106817-contrast-de...king-down.html

07-08-2010, 01:35 AM	#9
Howdy Forum Member Join Date: Feb 2010 Location: Poland Posts: 86 Original Poster	Originally posted by troyz The bellows method would decrease depth of field, but would not decrease depth of focus (see wikipedia for the distinction) so it won't Could you be more specific please? I have found the depth of field (DoFld) and depth of focus (DoFoc) articles on Wikipedia but I am not sure if I understand you correctly. My knowledge about optics is (at best) very limited. First, according to Wikipedia: The image distances are measured from the camera's image plane to the lens's image nodal plane, which is not always easy to locate. (Depth of field - Wikipedia, the free encyclopedia). If I understand correctly, the nodal plane (whatever it is ) is a constant. When using bellows, the distance ("v") between this constant and an image plane is increased greatly. So, DoFld = sf^2/(f^2+-X), where X = Nc(s-f) 1/DoFld = (f^2+-Nc(s-f))/(sf^2) = 1/s +-Nc/(f^2)-+Nc/(sf) = +-Nc/(f^2)+(1-+Nc/f)(1/s) = a+b/s hence DoFld = s/(as+b) (a=+-Nc/(f^2), b=(1-+Nc/f)) s - distance of "ideal" focus f - focal length N - "f-number" c - circle of confusion (c=0.02 for Pentax K7) DoFoc = 2Ncv/f From all of this, both DoFld and DoFoc do depend linearly on the use of bellows (in DoFld "a" is small, "b" is almost 1 and, as "s" is also small, DoFld is approximately DoFld=s/b). Obviously, the question is "how does "s" relate to "v"... So, to sum it all up - The bellows method [...] would not decrease depth of focus* is not true (or I am missing something).

07-08-2010, 03:02 AM	#10
rhodopsin Veteran Member Join Date: Dec 2009 Location: Deep Forest Posts: 643	Somebody over on FM55 did work with Canon 5DII focus screen shims (washers), made measurements and produced a chart with graphs showing effects of shims on calibration using various focal length lenses. Focus was found to be affected most using wide angle lens. I think you are correct; the effect is even greater with macro. MF Screen Calibration - FM Forums

07-08-2010, 09:02 AM	#11
troyz Veteran Member Join Date: May 2008 Posts: 389	Originally posted by Howdy . . . Obviously, the question is "how does "s" relate to "v"... So, to sum it all up - The bellows method [...] would not decrease depth of focus is not true (or I am missing something). from Depth of focus - Wikipedia, the free encyclopedia : "Depth of focus, however, is a measurement of how much the film or sensor plane can be displaced while an object remains in acceptably sharp focus." Denoting depth of focus by t: t = 2 Nc v/f t = 2 Nc (1+m) where N = f-number c = circle of confusion v = image depth f = focal length m = subject magnification --- m increases when you use the bellows, so t (depth of focus) increases with the bellows, which is undesirable because you won't be able to detect a difference between flange-to-sensor distance and flange-to-focus screen less than t.

07-08-2010, 02:06 PM	#12
Howdy Forum Member Join Date: Feb 2010 Location: Poland Posts: 86 Original Poster	Thank you, troyz! Now I think I get it! Do you know any good method for selecting a proper shim thickness?

07-08-2010, 05:53 PM	#13
JohnBee Veteran Member Join Date: Sep 2008 Location: Newrfoundland Photos: Albums Posts: 4,667	Originally posted by Howdy Now the problem: how to choose a proper shim? Whenever I get a new camera, I simply order the entire spread. Shims don't cost enough to nit-pick over getting the perfect shim and the added advantage of combining shims for micro-adjustments is a huge plus. Aprox. price of total shim collection: 20USD.

07-08-2010, 10:42 PM	#14
troyz Veteran Member Join Date: May 2008 Posts: 389	In principle you could try to measure the focus error, determine the thickness of the shim you have now (with a micrometer?), and calculate the thickness of the replacement shim. . . but in practice I think the trial-and-error method will be faster. A link to Wheatfield's thread with shim part numbers and thicknesses in case you don't already have it: https://www.pentaxforums.com/forums/pentax-dslr-discussion/88289-k7-screen-shims.html (FWIW, I had Katz re-shim my K20D in their shop)

07-09-2010, 06:47 AM	#15
Wheatfield Moderator Join Date: Apr 2008 Location: The wheatfields of Canada Posts: 15,987	Originally posted by troyz In principle you could try to measure the focus error, determine the thickness of the shim you have now (with a micrometer?), and calculate the thickness of the replacement shim. . . but in practice I think the trial-and-error method will be faster. A link to Wheatfield's thread with shim part numbers and thicknesses in case you don't already have it: https://www.pentaxforums.com/forums/pentax-dslr-discussion/88289-k7-screen-shims.html (FWIW, I had Katz re-shim my K20D in their shop) Apparently I used the 55/1.4 not the 50/1.2 for calibration.....