[Lowell Goudge]

Years ago, I did a lot of work with the K10 and exposure, because it was all over the map, or so it seemed at the time,

What you need to do, is with both spot metering, and center weighted metering, meter and shoot at every aperture, using a uniformly lit block wall, or paved surface (I.e. a huge grey card). It is very important to characterize the K10 with your lenses. Exposure is highly dependent on the aperture which you meter, and viewfinder focusing screen.

With a variable aperture zoom lens, it is important to shoot and meter at each focal length, because the aperture changes, and exposure at 55 mm will be different from 300 mm as an example.

I will search for a post to reference

Edit note

Check here

https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/124627-k-5...ml#post1289087

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Last edited by Lowell Goudge; 03-23-2021 at 04:23 AM.

[Dartmoor Dave]

With a variable aperture zoom lens, it is important to shoot and meter at each focal length, because the aperture changes, and exposure at 55 mm will be different from 300 mm as an example.

That will only be the case with a legacy manual zoom lens with a variable maximum aperture, where the K10D body doesn't know either the lens's maximum aperture or the actual taking aperture and so can only use stop-down metering. In that situation, non-linearity caused by the focusing screen's optimisiation for a bright viewfinder image will cause progressive amounts of exposure error at smaller apertures.

In this case, the OP is having trouble with a modern autofocus lens that transmits the relevant data for open aperture metering to the camera, so stop-down metering isn't an issue. And with the stock screen at least, the K10D has got no issues at all metering a modern zoom with a slow maximum aperture -- it's designed to work with the 18-55mm f/3.5-5.6 after all. This forum's K10D Club is full of members happily using various modern zooms with slow apertures at the long end, with no exposure problems at all.

Assuming that the OP's lens is sending the aperture data to the camera correctly (which he hasn't confirmed yet), the underexposure can only be caused by non-linearity resulting from the optimisation of the Katzeye screen for a bright viewfinder image. Darkening of the centre split prism at f/5.8 couldn't possibly cause underexposure, but only overexposure. And the Katzeye's characteristics must be wildly non-linear indeed if it works fine with the OP's 20-40mm at f/4 but then causes two stops of error with the one stop (plus a tiny fraction) darker 50-300mm at f/5.8.

I'm afraid I'm going to have to keep stressing this distinction, otherwise in a few months time we'll have people in other threads declaring it to be true writ that split prisms cause underexposure with slow lenses -- when they don't and can't.

[Lowell Goudge]

That will only be the case with a legacy manual zoom lens with a variable maximum aperture, where the K10D body doesn't know either the lens's maximum aperture or the actual taking aperture and so can only use stop-down metering. In that situation, non-linearity caused by the focusing screen's optimisiation for a bright viewfinder image will cause progressive amounts of exposure error at smaller apertures.

In this case, the OP is having trouble with a modern autofocus lens that transmits the relevant data for open aperture metering to the camera, so stop-down metering isn't an issue. And with the stock screen at least, the K10D has got no issues at all metering a modern zoom with a slow maximum aperture -- it's designed to work with the 18-55mm f/3.5-5.6 after all. This forum's K10D Club is full of members happily using various modern zooms with slow apertures at the long end, with no exposure problems at all.

Assuming that the OP's lens is sending the aperture data to the camera correctly (which he hasn't confirmed yet), the underexposure can only be caused by non-linearity resulting from the optimisation of the Katzeye screen for a bright viewfinder image. Darkening of the centre split prism at f/5.8 couldn't possibly cause underexposure, but only overexposure. And the Katzeye's characteristics must be wildly non-linear indeed if it works fine with the OP's 20-40mm at f/4 but then causes two stops of error with the one stop (plus a tiny fraction) darker 50-300mm at f/5.8.

I'm afraid I'm going to have to keep stressing this distinction, otherwise in a few months time we'll have people in other threads declaring it to be true writ that split prisms cause underexposure with slow lenses -- when they don't and can't.

dave, the point i am making is that especially with the K10 is to test the lens, and characterize it. there are a lot of non linearities with the K10 exposure and havng a miodified focusing screen does not help because the exposure meter does get the light from the focusing screen.

I agree on the whole with your comment about the split image, if it goes dark yes it would tend to over exposure, but the OP should test to evaluate the functionality of the camera plus lens. that is the only way to discuss the point properly, with data

[Dartmoor Dave]

dave, the point i am making is that especially with the K10 is to test the lens, and characterize it. there are a lot of non linearities with the K10 exposure and havng a miodified focusing screen does not help because the exposure meter does get the light from the focusing screen.

I agree on the whole with your comment about the split image, if it goes dark yes it would tend to over exposure, but the OP should test to evaluate the functionality of the camera plus lens. that is the only way to discuss the point properly, with data

All very fair points Lowell, and I get the feeling that I've started sounding a bit shouty in this thread, for which I apologise. When I rejoined the forum I promised myself that I'd stay out of threads like this where I tend to get unnecessarily worked up, but oh look -- here I go again! Totally obsessing about the whole thing!

I think I'll bow out now, because I don't want to be that guy on the forum who gets argumentative for no reason when everyone else is making completely fair and valid points. Your advice to the OP to do some more rigorous testing is an excellent suggestion, and they should certainly do that.

[DonV]

D.Dave,
thanks for your posts, I think it is important point out uncertain, inaccurate, or suspect information; or as you say- it will become "common knowledge" and be around and relied on-wrongly- for years. JMHO.

All good points in the above posts by all, as is often the case, there is likely no "single" reason for the inaccuracies encountered.

[stevebrot]

I suppose it's possible that the main ground glass area of the Katzeye screen is so optimised for a bright viewfinder image with fast lenses that it causes two or three stops of underexposure when used with an f/5.8 widest aperture.

I shot with the KatzEye (w/Optibrite) on the K10D for about five years with no metering issues except for spot metering and stop-down metering in general. The DA 18-55/3.5-5.6 was definitely in the mix using matrix metering. FWIW, I did linearity studies with both the stock and Katzeye screens as part of evaluation of the latter. Both were quite linear when used for other than stop-down metering.

That said...I will also bow out, given that the issue is somewhat foggy, coupled with the age of the kit.


Steve

[photogem]

fwiw its a plain old HD, not PLM HD

have tried to replicate behaviour with other lenses, with a uniform scene (sky, wall etc) there is no exposure difference calculated by the camera when swapping between spot, c/w and matrix - only with the 55-300

also, in case folks think it might be important, the K10d has a Katzeye split imagescreen (which hasnt been an issue before)

I shot with the Katzeye split-image-screen in my K10D as well as in my K5.

It is as Steve said: Spotmetering does not work with any split-image-screen. It is common knowledge.
So something is complety odd here, it cannot be that the DA 55-300 (which is not the KAF4-PLM Version) will bring correct results with spot metering and wrong results with matrix. Impossible!

Maybe you mix up: "spot in the AF Active Area" and spot-light-metering?

Spot in both cases but for two completly different functions.

Spot AF is not going through the focusing screen because the AF-sensors sit below the mirror.
Spot Metering is in trouble with a split-image-focusing-screen because light-metering is done through the center of the screen: The split image is located there and in the way.

[LittleSkink]

feel bad that I left this thread to run without updating, sorry about that life kind of got in the way

In no particular order:

I think the spot metering is a red herring /aberation - it clearly works differently compared to the c/w and matrix, which is expected behaviour in a slow lens / katzeye combination

The camera itself meters fine with a variety of other AF / MF lenses, though 20-40 f2.8-4 is my slowest lens. The camera does seem to behave to provent highlights blowing out, which I believe is expected behviour (for a Pentax)

So I have tried to "characterise" the lens with some real field use rather than blank walls and grey cards - apologies I havent been able to upload some example images, they are on my PC just need to get them online somewhere/somehow to link to

What I know: C/W and Matrix appear to align / agree in most situations. Adjusting aperture right down to f32 maintains EV, so aperture and metering adjustment appears to be working fine. Reported Exif data appears to be correct, so lens is talking to camera. The lens always underexposes in all situations. 200-300mm is worse than 55mm by half a stop or more. Underexposure varies, between >1ish to over 2 stops. At its worse underexposure will cause significant black blinkies, these have happened wide open at 300mm. I need to evaluate if stopping down maintains that behaviour, but mostly I shoot wide open or there abouts anyhow.

The lens appears crisp and handles decently (oh, except the tiresome AF) so I am minded to keep it and dial in +1EV, I shoot raw so its not hard to fix in PP if the blinkies stay away. The shop is willing to let me return it for refund - if nothing changes I am minded to keep it, if some sort of failure is imminent then maybe not

One thing that may or may not be true (I lack convincing evidence) was I cleaned all the contacts of DSLR and lens really carefully and "think" things might have got a little better