Question for Matt on M/K lens metering

Having been playing around with a number of my older lenses recently, prior to selling some of them, I have to agree with Chris A and all the others - something isn't working with the metering when the lens is not set to A on the aperture ring.

Apologies to Chris, incidentally, for suggesting he was doing something wrong.

My question is, is this something that is likely to be controlled by firmware, and therefore fixable, or is it controlled by a lower level of software which can't be updated?

I don't imagine it is a mechanical problem.

G

No problem, George. It was a while before I was convinced that it wasn't finger trouble myself.

Unlike the software industry, where it's to be expected that new releases will be buggy, I think we kind of expect cameras to work. So I'm not surprised that there was a certain amount of scepticism.

Very much the question in my mind too.

There's a tantalising hint in the post from Chris@SRS after he met up with a rep...

https://www.pentaxuser.com/forum/viewtopic.php?t=4185

It would make the K10D even closer to perfect if this could be fixed.

It feels like there is something quite tense in this matter over at Pentax, as this issue is not mentioned anywhere (more than what the users are discussing on related forums) and when I´m trying to get some info from Pentax (at least here in Scandinavia), It´s difficult to get a reply to my emails.

I see that Pentax Sweden had added some info regarding the use of K and M lenses on digital bodies with the text; "metering errors may occour", which would be misleading if the earlier DSLR´s and the K100D/110D does not suffer from this metering error.
Also, it´s not specifically a K and M lens matter either as all lenses with an aperture ring gets this metering error when using the aperture ring. May it be FA, F, A, K, M, or M42 lenses.

// Mats

it's interesting to note that the error seems to get worse in lower light and/or at tighter F stops.

Note that as far as the camera is concerned in this mode there is no difference between F1.4 in the dark and F16 in bright sun, as it is metering purely on the amount of light passing through the lens at shooting aperture.

If you take a *ist series DSLR and use an M series lens in really low light, you'll notice (or at least I do on both mine and my Dads DL2) that it starts to underexpose as the light gets really bad.

To make a *ist do this is quite difficult and requires pretty poor lighting conditions, like say extreme macro under tungsten room lights at F16. I do wonder though if the K10D is just exhibiting the same problem more severely such that it becomes apparent under more normal shooting conditions.

sorry that this doesn't really answer any of the questions, just an observation I found interesting and thought I'd throw into the melting pot.

1. The error has to do with the screen.
It has been repeatedly shown that the error does not occur anymore when using an istDs screen.

2. Pentax does not seem to be much interested in the problem. I mentioned it to Pentax Germany and they said "well this may occur, these are sooo old lenses etc. and we have more important things to do, stop lamenting ...".

It is sad, but I don't have much hope that Pentax will fix this error in the K10.
Perhaps, hopefully, in a future body! Better still a stop-down coupler in the body!

To answer George's question...

It depends on exactly where the problem is.

If it really is a screen issue, then go and buy an *ist-D/Ds screen!! They're not expensive

Alas, I haven't had the time to sit down and quantify this. It's particularly difficult for me as I don't have any M or even A lenses (have F and FA though). If I can recreate the issue with F and FA, then if I can find a couple of hours I'll do some testing.

I would start by setting up some controlled lighting, let's say at three or four different Ev's. I'd do it with the "A" setting, and with three manual apertures. I'd probably also use the Expodisc to ascertain whether it's down to localised lighting, or light levels in general (the Expodisc creates a very uniform light level).

If it's down to a non-linearity in the light level measurement - this can be fixed in firmware. If the screen actually has anything to do with it, then fixing in firmware could be tricky (as you'd have two scenarios - one with the shipped K10D screen and one with an *ist-D screen).

If it really is down to the screen, then I suppose it's unlikely you'll ever hear about it. But, as I said, replacement screens for the older *ist-D and *ist-Ds are easily obtained and don't cost the earth.


In the past, firmware upgrades have often improved exposure issues even though such has not been mentioned on the firmware release. I have already found some situations where the exposure wasn't as good as I'd hoped on the K10D - and resorted to the Expodisc as an incident meter which gave spot-on readings.

I suppose we often underestimate the difficulties in getting a "right" exposure automatically and have largely lost the art of guaging light levels and using our head (e.g. I used to be able to reckon the light levels to within a stop when I used slide film... I've largely lost that knack :roll

Matt

Thanks, Matt,

If it's down to the screen, surely that would affect all lenses, not just pre-A lenses.

My guess is that this is an algorithm issue, and if the algorithm processing is done in firmware, then it should be fixable.

Come on, Pentax.

G

George,
Maybe it's down to stop-down-metering (whether that's on FA, F, A, or M). I think I did a quick check when this first came up and found that even the FA 24-90 exposed differently when doing stop down as opposed to wide-open metering.

As I still have my *ist-D, and an *ist-DS I can also try the screens... but it all comes down to time
Matt

Yes, given that it works OK with all lenses when set to A, that suggests it is not a focussing screen problem but a software problem.

But it's very strange, because the light that the camera is measuring is the same light that will be used to take the picture. There is no need to adjust for the working aperture, as there is when metering wide-open.

It's as though the camera is making an adjustment when none is needed.

G

Absolutely.

Not only does it work with A lenses set to A, it does not work with the same A lenses when not set to A, in other words, when using stop down metering to set the shutter speed.

Thanks for confirming that Chris. At least when I find the time it won't be wasted with my F and FA lenses

I'll try and set this up at work and have a go one lunchtime... will keep you posted...

Matt

That does not logically follow.
The metering works ok basically up to f4.
And basically all lenses are in this range in A mode (f1.4 - f4).

Only above f4, expec. above f5.6 the metering performs wrongly.

It is possible that they used the program code from the istDs for the metering, but the K10 has a different screen and needs a different assignment table or so. Then it could be fixed with the firmware.

But then what about those who have replaced the screen???? Any firmware update would break their K10D metering...
Matt

Sorry to break in here with what must be a really dumb question...

Why does the screen have anything at all to do with the metering?? :

because the meter cells are in the pentaprism housing, so the light they look at goes through the screen.