That really is great news.
And it's my birthday soon!!

Actually John, although it appears they do, I'd argue that they don't, at least not with stop-down metering with the K10D.

I expect the reference you're thinking of is p210 in the K10D manual, where it refers to exposure errors with wide open metering...

"The shutter speed changes in relation to the open aperture but an exposure error may occur"

However, the reference in the manual to stop-down metering is on p158, in reference to the green button:

"Shutter speed is adjusted to appropriate exposure according to lens aperture when lens aperture is not set to A position."

I think my F4, 1/60 exposure reading (wide open) above is probably an example of the former error, which they do mention explicitly.

The overexposure while stopping down isn't consistent with their use of the word "appropriate", in my view.

Indeed, although wouldn't it be nice with an old M lens set to manual, to poke the green button and have it get it right!!

Me too!!

You know, I always find great respect for someone that makes an honest mistake, and 'fesses up!!

No worries - and we all live in glass houses, so no stone throwing from me

As it happens, I have the SMC-A 100mm F4 macro, and as it happens, it's overcast today with very consistent, even lighting. So a direct comparison of A-mode, and manual with a lens similar to yours (is yours the F4 or the F2.8?) was possible.

I've just done a very quick check with my K10D... I picked a reasonably well lit, not particularly contrasty scene, lit naturally from the window, tripod, covered viewfinder...

... and wrote down the metered exposures at ISO 200, using the lens first set to A, varying the aperture from the camera in Av, then using the aperture ring with the camera in manual, using the green button for stop-down metering:

F-----A-------M
4-----1/80--1/60
5.6---1/40--1/30
8-----1/20--1/10
11----1/10--1/5
16----1/5----0.4
22----0.4----0.8
32----0.8----1.3

Note the exact reciprocity in 'A'*, and the overexposure using stop down metering. I repeated the 'A' metering to ensure that the light hadn't changed, and it hadn't.

This trend of progressive overexposure with decreasing aperture exactly corresponds with what I found with several lenses when I did all this with the K10 when I first got it.


I am intrigued that you find the stop-down metering accurate - was this with the K-7? Either way, I'd urge you to have a bit more of a careful look at this.

Maybe the accuracy of stop-down metering has been improved with the K-7 which would be good news indeed.

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* this reciprocity shouldn't be a surprise of course - since the camera meters wide open, it's calculating the exposure required for the smaller apertures, not measuring it.

Not only that, but it operates in Av (still wide open though), even if it's set to P, Sv, Tv, or TAv in these circumstances.

The K10D that is. Is the 20D or -7 any different?

The point is, this technique does not work with K10D or later (not sure about the earlier dSLRs).

It meters wide open, regardless of what you set the aperture ring to. It's because of the KAF2 mount - there is no aperture coupling between the lens and the camera that enables the camera to see (or react to) what aperture the lens is set to, until it takes the picture, at which point the stop down lever will simply move as far as it can.

You can turn the aperture ring all you like, but the aperture does not get smaller until you release the shutter. Then the camera stops the lens down to whatever the aperture is set to, and opens the shutter.

Try it if you don't believe me - look at the camera from the front while you turn the ring

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Edit: ISTR reading somewhere that M42 lenses are permanently stopped down to whatever aperture you set them to. I've never owned one, and I have no idea whether I'm either accurately remembering what I read, or if I did, if what I remember reading is true. However, if it is so, then they will behave as you say. Perhaps someone could confirm.

Not so - not with the K10D (and, I believe, the K20D) anyway.

All the "aperture ring allowed in Custom menu" option means is that the shutter will release - it won't, using manual lenses (pre-A) with that option not set.

The camera can't read the aperture from the lens on pre-A lenses, or on A series lenses not set to A. Indeed even with the lens set to A, the camera doesn't read the aperture, it sets it.

Using the aperture ring to set the aperture, all metering is done wide open, unless you use the stop-down metering feature, with which you get the now well-documented exposure issues with pre-A lenses. (Edit: and Stop-down metering is only available in Manual mode).

The restrictions are all listed on p210 of the K10D manual. Sadly the exposure issues with stop-down metering are not described in the manual. They're fairly well known, though - search this or PentaxForums (or DPR I think) for something like "K10D manual lens exposure problems" and you'll get dozens of hits.

That is impressive. I've been very lucky with my eyesight (still no need for glasses even at my advanced age of {redacted}, and I've always had very good low light vision too. It's not as good as it was, but although it's still better than people a lot younger than I, manual focusing fast enough in very low light isn't as easy as I'd like.

Sadly, I probably can't blame the (edit: camera) lenses for my failures...

I think the test is to switch to MF, focus manually, and then press the button. If the focus confirmation comes up straight away, you pass the test. If you have to turn it a little, you failed it.

I tried it a few times - passed a few times, failed a few times too.

I think I'm aware of the confirmation more than I actively look for it. The AF snaps in, occasionally does a fine adjustment if the light is bad, and I'm aware of it finishing. I think I use the aural as well as visual cues without really noticing them, unless it takes an inordinate length of times thinking about it, as of course it does sometimes in near darkness or with low-contrast targets.

I take pictures in semi-darkness quite a lot, at dance events. Manual focus without a split screen or focus confirmation would be hard.

Having now tried it several times, it appears that the behaviour in AF is different than in MF.

Set to MF, with half-press held, the hexagon appears and disappears as you turn the focus ring. As you'd expect, the shutter will release whether in focus or not.

Set to AF, it focuses, and then the green hexagon remains lit after quick-shifting. You can still release the shutter at any time after the initial focus confirmation.

This behaviour is the same whether you have the Custom Menu set to AF on half-press or not - if not, holding the AF button pressed has the same effect as keeping the shutter release half-pressed if it is.


Worth knowing about this difference, perhaps - it might be possible to disturb the focus unintentionally while half-pressed in AF, and think from the hexagon that it was still in focus when it isn't.

Is it the same with the K20D and K-7?

Agreed... however,
Not with my K10D.

With the half-press held, the green in-focus hexagon remains, even when I've quick-shifted way out of focus.

The behaviour is the same with both my 18-55 (Mk1) and 55-300.