[Lance B]

Hi Lance,

Nice to see you here too. Haven't been to the other forum for a long time, but I'm sure you are still very active there

I am but not as much. Not as nice over there as it once was. Too many good people, like you, have left and there are too many horrid types joining in.

The kids are great, all grown up now. Thanks for asking btw.

Good to hear.

I didn't know you could order this thing. I got a couple of plastic shims along w/ the split screen from focusingscreen.com. Thought about giving them a try first and see how. Thanks for sharing this.

Glad to be of service.

[Lance B]

Lance, thanks for posting -- it inspired me to finally do something about the poorly-focusing cheapo eBay focus screen I had in my K20D.

I like old lenses. I like weird lenses. I like fast lenses. And most importantly, I need *cheap* lenses. So I have picked up a variety of old Takumars (including an SMC Tak 55/1.8 and an Auto Tak 85/1.8), and I also managed to finagle one of those Cosina 55/1.2 lenses. I love them all, but they are quite difficult to focus.

I was never able to get very good focus on the stock screen, so I ordered an eBay replacement screen. I think it was the jinfinance model. But when the two sides of the split prism were lined up, my fast lenses were NEVER in focus.

I read a lot on the subject, and figured out that because I had front-focus, the existing focus screen shim was TOO thick. Bah, I thought. I can't just modify the one that's there.

Eventually I got annoyed and put back the stock focus screen, relying on the focus-confirmation beep rather than my eyes.

So anyhow last night, finally, after reading this tread I decided to take action. I took out the stock screen and the shim, put back the split-prism screen without any shim, and took some test shots. Aha, now I was back-focusing a bit.

I put a few layers of masking tape onto wax paper, then cut out some VERY NARROW (less than 1mm) strips of tape with a razor blade, leaving a tail of wax paper off one side. Peeled the wax paper backing off, and mounted two narrow strips to the top side of my split prism screen, one along the top edge, and one along the bottom, right at the very edges.

It took some experimentation, but now I have PERFECT manual focus using the prism. It turned out to need three layers of blue 3M painter's tape.

I'm very pleased!

Hey, that is great news and a very ingenious way of fixing the problem!

[Lance B]

Lance -

Can you post from exactly where you purchased the shims and do you have an illustration or photo of where these shims are inside the camera? It seems my Katz Eye screen does not agree with my AF beep and I'd like to fix that, but I want to know exactly what I'm doing and have the right materials first.

Thanks.

I purchased them from Pentax in Australia. You will obviously need to go to Pentax USA.

Here is a good link for reference so you can hopefully see what I am talking about:

Focusing Screen--How to adjust focusing screen--

Please refer to the diagrams in the link above. L1 is the focus screen light path length to the top of the focus screen closest to the bottom of the pentaprsim as shown in the diagram and L2 is the light path to the sensor. For correct manual focus, L1 must equal L2. This has no bearing on auto focus other than to show whether the AF corresponds to manual focus by having the image sharp in the VF as the AF is done with a seperate module. The red dot auto focus confirmation is not 100% reliable, IMO and having the focus screen correctly calibrated is a must. This is where the shims come into play. They go between the focus screen and the bottom of the pentaprism. The shims are a qui

For auto focus (AF) to be correct, L3 must equal L2. AF can be adjusted in the K20D and the K7 via the AF Adjustment feature, but I think it best to get the focus screen calibrated correctly before doing any AF calibration.

Here is a photo of the shim:

[Lance B]

So where do you get these shims from. Does Katz supply them?

From Pentax.

[Lance B]

Lance - which Katz Eye screen is this?
I looked at the ones for Pentax and didn't see any without focus aids. Is this one custom?

Yes, it is a custom screen, but it is easily ordered from Katz Eye. Just contact Rachael via the link on the Katz Eye site and ask for what you require. Racheal will send you a link and a code for you to quote on your request through the site. Rachael is very helpful and knowledgeable.

[Wheatfield]

Please refer to the diagrams in the link above. L1 is the focus screen light path length to the top of the focus screen closest to the bottom of the pentaprsim as shown in the diagram

For clarity, I've linked to that diagram:



Note that L1 is pointing towards the bottom of the screen nearest the reflex mirror, not the top nearest the pentaprism.
Now, take a moment and actually look at a focusing screen.
Note which side the ground glass is on.
You will find that the groundglass side is, in fact, on the bottom of the screen nearest the mirror.
Consider that if the groundglss side was, in fact, nearest the prism, there would be no need at all for the shim.
I'm sorry to have to call you out on this, and it may seem a very minor thing, but you are passing off bad information, and you are just plain wrong.
Now, take some time to educate yourself and stop passing your nonsense off as fact.

[Lance B]

For clarity, I've linked to that diagram:



Note that L1 is pointing towards the bottom of the screen nearest the reflex mirror, not the top nearest the pentaprism.
Now, take a moment and actually look at a focusing screen.
Note which side the ground glass is on.
You will find that the groundglass side is, in fact, on the bottom of the screen nearest the mirror.
Consider that if the groundglss side was, in fact, nearest the prism, there would be no need at all for the shim.
I'm sorry to have to call you out on this, and it may seem a very minor thing, but you are passing off bad information, and you are just plain wrong.
Now, take some time to educate yourself and stop passing your nonsense off as fact.

Have you actually seen where and how the screen is placed in the VF? Did you actually look at a screen and the way it is inserted into the camera before making such a claim? The ground glass is nearest the pentaprism of the K-7, I know as I have just looked at it. So, seeing as the ground glass is nearest the pentaprism, then this does necessitate the shim for calibration purposes. If, as you incorrectly believe, that there is no need for a shim if the ground glass is on the Pentaprism side, then how come, when I remove the shim, that the focus changes???? Magic?? Unlucky??? No, the shim does make a difference and that is precisely why Pentax offer them in varying thicknesses. Do you actually think before you type?
The diagram is for representation purposes only and the arrows are not necessarily refering to the way Pentax does things.
Now, it is you and not I that is passing off bad information and before you make bold statements, maybe you should check your facts before passing off nonsense as fact.
I think you owe me an apology.

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Last edited by Lance B; 03-20-2010 at 06:03 AM.

[creampuff]

Note that L1 is pointing towards the bottom of the screen nearest the reflex mirror, not the top nearest the pentaprism.
Now, take a moment and actually look at a focusing screen.
Note which side the ground glass is on.
You will find that the groundglass side is, in fact, on the bottom of the screen nearest the mirror.
Consider that if the groundglss side was, in fact, nearest the prism, there would be no need at all for the shim.
I'm sorry to have to call you out on this, and it may seem a very minor thing, but you are passing off bad information, and you are just plain wrong.
Now, take some time to educate yourself and stop passing your nonsense off as fact.

I have to agree with LanceB that you're dead wrong on this one.
The orientation where the groundglass and other focusing aids like microprisms and split image rangefinders is positioned is always closest to the pentaprism and not facing down towards the mirror. Just remove any stock focusing screen from any Pentax camera and it is immediately evident that the groundglass side faces upwards, not down, with the side facing the mirror being flat. Having the groundglass side facing upwards does not negate the need to have a shim to correct any focus error. I had to use a shim between the focusing screen and the pentaprism to make sure that what I focused manually coincided with the camera's AF. Focusing manually with my K3 screen without a shim alone, focus was off whereas the camera's AF was spot on.

Considering the length of time that you've stated that you've been photographer and the fact that you've used many different cameras, I'm surprised you can get the issue of focusing screen orientation wrong.

[holance]

I have to agree with LanceB that you're dead wrong on this one.
The orientation where the groundglass and other focusing aids like microprisms and split image rangefinders is positioned is always closest to the pentaprism and not facing down towards the mirror. Just remove any stock focusing screen from any Pentax camera and it is immediately evident that the groundglass side faces upwards, not down, with the side facing the mirror being flat. Having the groundglass side facing upwards does not negate the need to have a shim to correct any focus error. I had to use a shim between the focusing screen and the pentaprism to make sure that what I focused manually coincided with the camera's AF. Focusing manually with my K3 screen without a shim alone, focus was off whereas the camera's AF was spot on.

Considering the length of time that you've stated that you've been photographer and the fact that you've used many different cameras, I'm surprised you can get the issue of focusing screen orientation wrong.

Agree. Because the L1 is the distance between the mirror and the flat side of the focusing screen. If the focusing screen is thinner than it should be, you need to add the shim on the top of the focusing screen(the ground/grind side) so the L1 can be decreased. That why we can only correct the back focus but not the front focus.

[GoremanX]

Agree. Because the L1 is the distance between the mirror and the flat side of the focusing screen. If the focusing screen is thinner than it should be, you need to add the shim on the top of the focusing screen(the ground/grind side) so the L1 can be decreased. That why we can only correct the back focus but not the front focus.

Your logic is flawed. Where we place the shim has no bearing on anything. When you place the shim above the focusing screen, you're moving both surfaces of the focusing screen (top and bottom) downwards by the thickness of the shim. When you place the shim below the focusing screen... you're not moving it at all. It still sits against the top edge of the mounting surface. All that would do is make the swivel bracket close at a different place.

I don't know where the ground glass is on a focusing screen, it's hard to tell on my original K-7 focusing screen. I'm literally holding it in my hands and checking, and both surfaces feel pretty smooth to me. I'm not going to try this with my K3 focusing screen. But I'm pretty sure the focusing happens on the bottom surface of the focusing screen. Otherwise, how would the microlenses have any effect on the resulting image?

[Wheatfield]

I have to agree with LanceB that you're dead wrong on this one.
The orientation where the groundglass and other focusing aids like microprisms and split image rangefinders is positioned is always closest to the pentaprism and not facing down towards the mirror. Just remove any stock focusing screen from any Pentax camera and it is immediately evident that the groundglass side faces upwards, not down, with the side facing the mirror being flat. Having the groundglass side facing upwards does not negate the need to have a shim to correct any focus error. I had to use a shim between the focusing screen and the pentaprism to make sure that what I focused manually coincided with the camera's AF. Focusing manually with my K3 screen without a shim alone, focus was off whereas the camera's AF was spot on.

Considering the length of time that you've stated that you've been photographer and the fact that you've used many different cameras, I'm surprised you can get the issue of focusing screen orientation wrong.

Well my friend, I have a K7 screen sitting on the desk in front of me. When it is oriented to go into the camera (tab on the left) the groundglass side is facing downwards. On my 4x5 view camera, the ground glass side of the screen is nearest the lens. On my Nikon F2s, the groundglass side of the screen is nearest the mirror, on my LX, the ground glass is nearest the mirror.
I don't really care if you agree with me or not. You can believe me, or you can believe something else and be wrong. It doesn't affect my life.
All you have to do is take a screen, orient it so that it will go into a camera and look at where the ground side is facing.
I mean really people, this isn't rocket science, just take the screen out of your camera and look at it.

[holance]

Your logic is flawed. Where we place the shim has no bearing on anything. When you place the shim above the focusing screen, you're moving both surfaces of the focusing screen (top and bottom) downwards by the thickness of the shim. When you place the shim below the focusing screen... you're not moving it at all. It still sits against the top edge of the mounting surface. All that would do is make the swivel bracket close at a different place.

I don't know where the ground glass is on a focusing screen, it's hard to tell on my original K-7 focusing screen. I'm literally holding it in my hands and checking, and both surfaces feel pretty smooth to me. I'm not going to try this with my K3 focusing screen. But I'm pretty sure the focusing happens on the bottom surface of the focusing screen. Otherwise, how would the microlenses have any effect on the resulting image?

Actually that's what I mean, you can only move the focusing screen downwards to decrease the L1 and solve the back focusing problem on the focusing screen.

[GoremanX]

Actually that's what I mean, you can only move the focusing screen downwards to decrease the L1 and solve the back focusing problem on the focusing screen.

Every Pentax DSLR I know of has a shim in place from the factory. On my K-7, this is a silver metal plate that's between the focusing screen and the mounting surface. This factory shim can be removed and replaced with a different one. So your options don't include just adding shims, you can remove it too, and replace it with a thinner one. Therefore you can in fact fix both front and back focusing problems. In my case, I had to remove this factory shim altogether for the focus to be right with the K3 screen from focusingscreen.com

[Lance B]

Well my friend, I have a K7 screen sitting on the desk in front of me. When it is oriented to go into the camera (tab on the left) the groundglass side is facing downwards. On my 4x5 view camera, the ground glass side of the screen is nearest the lens. On my Nikon F2s, the groundglass side of the screen is nearest the mirror, on my LX, the ground glass is nearest the mirror.
I don't really care if you agree with me or not. You can believe me, or you can believe something else and be wrong. It doesn't affect my life.
All you have to do is take a screen, orient it so that it will go into a camera and look at where the ground side is facing.
I mean really people, this isn't rocket science, just take the screen out of your camera and look at it.

Yours would have to be the only Pentax DSLR ever that has had the ground glass (rough surface used for focusing) facing down on the opposite side of the pentaprism. All my Pentax DSLR's have the ground glass (rough surface used for focusing) facing upwards towards the pentaprism. This is borne out by Rachael Katz of Katz Eye focus screens and exactly why the thickness of the glass has no bearing on focus. I quote from an email Rachael Katz; "On all Pentax DSLR cameras, the ground glass surface (aka as the diffusion surface and serving as the focal plane) is on the top, against the pentaprism (or pentamirror) and the bottom surface (the side visible through the lens mount) is the Fresnel lens surface. In fact, this is the arrangement in every DSLR that we have studied to date, from every manufacturer, with the notable exception of the Olympus cameras with side-swing mirrors."

I further add from Rachael; "ground glass surface is the rough surface, located at the focal plane, that is used for focusing. It is located on the top of the focusing screen in Pentax DSLRs. The Fresnel surface is on the other side, located downward in Pentax DSLRs, which serves to expand the image from the lens to fill the viewfinder. The grid lines, when supplied, are applied to the upper ground glass surface."

I think you are confusing the Fresnal surface - used for expanding the image to fill the VF - with the ground glass surface for focusing.

The adjustment shim is placed between the ground glass face of the focus screen and the pentaprism in order to calibrate the screen. The reason that the shim makes a difference is because it changes the distance from the subject to the ground glass of the focus screen. I repeat again, the shims are available in various thicknesses from .10mm to .50mm inclusive in .05mm increments in order to adjust distance from subject to Fresnal surface

Looking at the figure below, the ground glass surface is the surface on top of the yellow rectangle which represents the focus screen. The shim goes between the top surface of the focus screen - top of the yellow rectange - and the bottom of the pentaprism in order to adjust for back or front focus.



In this figure below, the shim goes where the "transparent pad put here" is indicating. This adjusts the distance from subject to focus surface of the screen. If the shim were put below the screen (nearest the mirror) this would have no effect whatsoever on the distance between the subject and the focus surface as it doesn't alter anything.

[Wheatfield]

After all this, I could have saved myself some embarrassment. This would be one of those rare times when I don't know what the hell I'm talking about.
Lance, I owe you an apology.
You are 100% correct on this one, at least as it applies to the Pentax DSLRs that I have floating around my house.
At some point, I'll pull out my old F2s and see if it is as I think it is or if I am just plain confused.