OM10 Lens markings for focus are all wrong

Hey folks. I decided to clean the focusing screen of my OM10 camera. I removed the small plastic circuit cover and unscrewed the screen. (In passing the mirror cushion foam doesn't need to be removed, but I digress). I removed and cleaned the fresnel lens gently with a suitable clean brush and blower and all is well. However then I researched which way up the fresnel lens focusing screen should be replaced, and it's tough to find. First I reinstalled it with the shiny/fresnel side down to mirror and the matte side up to pentaprism.

I went to do a focus test and discovered the anomaly described below, so I reinverted it so that the shiny/fresnel lens is up to pentaprism and the matte is down to mirror.

However in either case the behaviour is unchanged. Now, inspecting the focusing screen it does appear that the central focusing spot is in the centre of the screen so I am guessing it's not very relevant which way up the screen in placed anyway, though I would love to know. However this problem exists in both cases, and certainly predates my cleaning as (Remarkably for a change) I did absolutely no damage

The problem is that the numbers on the focusing ring of the lens don't match with actual real world distance. So, I'm using an Olympus OM System Zukio Auto-S 1:1.8 50 mm lens. However I tried another lens and it appears to be the same (An older Olympus OM System G-Zukio Auto-W 1:3.5 28mm).

So the lenses works with normal readings on my OM1, but on the OM10 body concrete bricks 5 metres away focus at 2 metres, and a chair edge 1 metre away exactly focuses at 0.7.

But here's the thing, I have used this camera to photograph events and have always had good results, it's sortof my less fun more reliable body. I am not greatly concerned about the lens markings when using this body but I would like to avoid focusing errors and understand what's happening. Also any advice on the sanction orientation of the focusing screen would be good.

I don't have an OM-10, but I can tell you that on the three cameras where I have changed focusing screens (OM-2n, Rolleiflex, Hasselblad) the matte side goes down. Very few lenses in my experience are accurate at showing exact distances when focused, and the only thing that matters is whether the photo turns out sharp. Might be time for a test film in your OM-10!

https://learncamerarepair.com/downloads/pdf/olympus-OM-10-service.pdf pdf page 66,67,68. matt side faces the pentaprism.

Until this morning I had it matt side up to pentaprism. But this resource says matt to mirror https://www.photo.net/discuss/threads/om-10-focussing-problem.85142/

However I accept the matt to prism fresnel/shiny down to mirror is in the service manual. Perhaps there is confusion between what up and down mean. Thanks shutterfinger. I'm doing a test roll as we speak but with matt to mirror. I will switch it half way through. I suspect the focus spot is central so no difference will result but I'll let you know after development. Thanks all.

spl said:

But this resource says matt to mirror

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It says dull side up. Dull side is the matt surface. Is the mirror above the pentaprism?
Factory service manuals are correct 99.99% of the time.

shutterfinger said:

It says dull side up. Dull side is the matt surface. Is the mirror above the pentaprism?
Factory service manuals are correct 99.99% of the time.

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It is if the camera is inverted to safely insert the focus screen. I didn't mean to be obtuse this was just my experience. Thanks for your advice.

My Olympus OM2 has the screen with the Fresnel side to the lens/mirror. As does every other 35mm & MF SLR that has passed through my hands.

I can't think the OM-10 is the sole exception to the rule.

Maybe it was just "Lost in Translation," or, as mentioned, confusion on which side is "up" if the camera is upside-down?

I own a number of Nikon lenses. I noticed that the marked distances are not always precise in terms of best actual focus. Nonetheless, they work perfectly to make well-focused images when the image seen on the focusing screen is well focused.

Note: The “focusing distance” marked on the lenses and cameras are usually referenced to the distance from the subject plane to the film plane. That is why many SLRs have a film-plane-indicator mark, such as are found on the top of Nikon SLR bodies.

The optical-system subject distances used in common formulas are measured from the subject plane to the first nodal point of the lens.

A few older cameras with non-interchangeable lenses give distance scales measured from the subject plane to some other reference point on the camera, often, but not always, to the front of the lens barrel.

In my experience, focusing screens require the textured surface (upon which the image forms) to be placed on the side closest to the lens. In the case of an SLR camera, the bottom surface is the image plane. The principle is: The length of the light path from the subject to the film is identical to that from the subject to the image-forming surface of the focusing screen.

Echoing post #7, I have never encountered an SLR with the textured image-forming surface on the top. But I’ve never owned an Olympus SLR. I recommend checking with a camera repair service. For example:

Zacks Camera Repair – Zacks Camera Repair Website

Have you checked with film to see whether the camera is focusing to the same distance as is indicated in the viewing system, or whether it is focusing to the distance marked on the lens?
I'm guessing that the replaced screen isn't sitting at the right plane.

Getting the focusing screen upside down may have different effects on the split-image at the center of the screen, and the matte area outside the central region. You should also check best focus with the matte area. It should agree with the split image.

Although lens focusing scales are not precise, an error such as object at 5 meters focuses at 2 meters on the focusing scale is very large. No camera should be that far off. The errors (5m object at 2m on scale, 1m object at 0.7m on scale) both suggest that the lens is moved to about 1mm more extension than it should; the focusing screen is 1mm too close to the lens. If the matte side is supposed to be up and it's been placed down, that could be the problem. Otherwise, it could be a spacer, if there is one. You can check the actual focus by placing a small groundglass or spare focusing screen on the inner set of film rails and viewing through the lens directly.

Fixed!!

TL;DR: Matt side up to pentaprism, shiny fresnel down to mirror, and even more importantly the rounded corners must be on the lens side and the angular corners on the film side. Because of the second problem (rotation) the lens focus scale was all wrong (2m for a 5m subject), and correcting the up/down makes all parts of the focusing screen be in agreement.

First off, it's impossible to put the focus screen in the wrong *location* but it may be possible to insert it upside-down. You might think (As many here said) that the matt side goes down to mirror (Possibly exposing the side that is easier to clean?) however when I had the lens inserted that way the focusing was unsatisfactory. Specifically, when the split focusing screen was aligned the image in the metering spot and outside looked not quite focused, so do you focus on the split line, or the metering spot, or the textured area, or the rest of the frame in the matt area?? Well, obviously they should all agree so I decided that matt down to mirror is wrong for this camera.

So I decided to go with the service manual, which is not surprisingly correct if badly worded ("Position the screen with its matted face coming on pentaprism side"). However I also noticed that there is a reference to "R Corner" and "B side mount", presumably meaning that the rounded corners go toward the bayonet mount.

So I put the rounded corners to the bayonet mount by rotating 180 degrees relative to where I was before, and put matt up to pentaprism. Now some exciting things happen:

1) When I focus on a subject 5 metres away the lens focus scale reads exactly 5m quite accurately.

2) The lens goes very slightly beyond infinity, which is normal, but when I focus on a lamppost at infinity the focus point is on the left edge of the infinity symbol, which is satisfactory.

3) At infinity focus, everything at infinity looks in focus a) in the split focus spot b) in the metering spot c) in the textured area around the metering spot d) in the matt area of the rest of the viewfinder.

I am so confident of the positioning that I have chosen not to waste the remainder of the test roll, I will use it tomorrow on a trip to the country.

So the correct positioning for the OM10 focusing screen is: Matt side to pentaprism, Shiny fresnel side to mirror, rounded corners to lens, angular corners to film.

The rotation of the focusing screen 180 degrees is what caused the lens to read 2 metres instead of 5 and 10 metres instead of infinity etc, not its inversion mirror to pentaprism. Clearly the fresnel lens is not symmetrical, I should not have assumed it to be so.

Thanks all for your help. It's astonishing how much disagreement exists on this, it may be different for different cameras, but I am 100% confident that it is now correct for the OM10 due in no small part to interpreting the slightly cryptic service manual, and will hopefully be 110% certain when I develop the test roll which will be all out of focus to date.

reddesert said:

The focusing screen is 1mm too close to the lens. If the matte side is supposed to be up and it's been placed down, that could be the problem.

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This makes perfect sense logically, however the lens focus scale markings were incorrect to the same extent in both ways up, matt up and matt down, no difference.

reddesert said:

Getting the focusing screen upside down may have different effects on the split-image at the center of the screen, and the matte area outside the central region. You should also check best focus with the matte area. It should agree with the split image.

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This is insightful and may well have been due to the up/down/mirror/prism inversion.

reddesert said:

You can check the actual focus by placing a small groundglass or spare focusing screen on the inner set of film rails and viewing through the lens directly.

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This is actually a great idea in preference to a test roll!

Yes different manufacturers have their own jargon then there's the translation from one language to another when there is no word in one language for one in the other.
Happy shooting!