Now what am I doing wrong?

Hi guys,

I recently acquired a Pentax-M 1:1.7 50mm whih has taken a while to clean and refurbish and is now in pristine condition (thanks to patient advise from Dangermouse).

It physically fits my K10D, but doesn't quite work properly. Let me explain further :-

I've set the use of aperture ring choice to "permitted".
I've set the camera to MF
I've tried all the settings on the Mode Dial.

Whatever I set up on the aperture ring the aperture remains at 1.7 (fully open) and the exposure is all to pot.
Off the camera the aperture is snappy and free moving and can be set anywhere twixt f1.7 and f28 in 1/2 stops.

I didn't expect full AF but I did assume that the aperture would flip to the one manually selected and adjust the exposure accordingly.

Am I missing something?

Bernard

Yep

If you look at the camera from the front during the exposure (you might need to set it to manual and pick a long exposure to see it), you'll notice that the aperture does indeed stop down to whatever you set on the aperture ring.

However, that is while the shutter is open.

The metering takes place before the exposure, and this normally takes place with the aperture wide open. So you can set it to whatever you like, and the exposure the camera picks will still be based on a wide open aperture.

What Doug says is exactly right - the green button tells the camera to stop the lens down to the set aperture and take a meter reading, and pick a shutter speed based on the amount of light it measures.

This is great in theory, but sadly it doesn't work right on the K10D, nor the K20D I believe.

You'll find that (IIRC) the more you stop the lens down, the more overexposed the result is, even using the technique Doug describes.

The only way of using a K or M lens is in manual - meter wide open, and then calculate the shutter speed yourself for your selected aperture.

It's a pain - I got rid of all my M lenses once I knew about this problem.

A-Series lenses are no problem at all, because (when set to A), the camera can set their aperture just before the exposure.

So the camera still meters wide open, but because the camera knows what the aperture is going to be (because it's setting it), it can calculate the correct exposure, rather than attempting to measure it by stopping the lens down.

Note that even A-series (or FA for that matter) lenses will behave like K or M lenses if you don't set them to A.

what he said
works on the k-r

Thanks Dougf8 I now have a lens I can use, albeit with the obvious limitations. No doubt in due course I might find the cash to buy a more up to date lens.

ChrisA, your comments noted and verified. I did note with curiosity that the exposure from f2.8 to f16 ran 1/125, 1/45,1/15, 1/6, 1/2, 1.5sec rather than 1/125, 1/60, 1/30, 1/15, 1/8, 1/4 that I expected (for the light at the time of day I did the test).

Geordie01 At least this suggests that the Pentax range is consistant


Query- Do I need to go through the proceedure before each shot or can I set the speed manually with the front wheel and expect it to stay on setting. I'm sure I can


For anyone being picky I did have a typo in my 1st post. This lens stops down to f22 not f28

regards
Bernard

Look on the bright side.

This is an excellent lens for training you to estimate correct exposure.

M lenses work happily in AE mode so long as they are wide open.

G

Bernard, once set, the settings will stick until you press the green button again or adjust shutter speed manually.

Yes, this is exactly what people find. From not too bad at large apertures, to horribly overexposed at small ones.

There's simply no way round it with the K10. Calculate your own exposures in manual, or get rid of the Ms and get As (or later).

I've yet to find out if it's fixed on the K-5, but I don't have many manual lenses left, so for me it's not much of an issue.

I think it's why it's so easy to pick up M series lenses for next to nothing - optically the Pentax SMC M lenses are excellent, they just don't have the convenience factor.

By the same token, the equivalent A-series lenses will cost a lot more, and retain their value, since they have the same great optics, and all the auto-exposure modes work just fine.

I'm enjoying the Vivitar 55mm macro on the front of a K-5 at the moment. All manual and iq is excellent. I'm getting a bit faster at the controls. Focus, expose, refocus and shoot and the stop down meeting seems accurate. It's less of a bother with an A lens, of course.

Thanks guys I think collectively you've answered all my current queries.
I spent many years with a manual/clockwork camera back in the 60's/70's and was perfectly happy to "guess" my exposure requirements. Pleased to say I got a fair number within a reasonable tolerance for printing (albeit 5x7 with the occasional 10x.

I'm still thrown by the features avalable on digital cameras (the K10D was a 60th birthday present from all my family).

regards
Bernard

I've now discovered a modification to the above instructions :-


Under Menu/ Custom Settings/Green button on page 4/6 set to 2

ieSets the shutter speed with fixed aperture to adjust exposure.


Then the curious exposure setting previously refered to become more in line with expectation


regards
Bernard

I find I do almost what I do with any manual camera, i.e., meter from something neutral such as grass or mid-gray concrete and then go from there. The inaccuracy of the metering with the K10/K20 is a right royal pain but once you have mastered the histogram it is possible to be quite accurate and consistent. I don't even bother using the green button any more.

K.

Interesting. According to the manual (p158 ), that setting is not relevant when using a pre-A lens, or an A lens that is not set to A.

Those three custom settings refer to Hyper-Manual mode, which is available only in A mode.

If you have results that suggest that this isn't true, I'd be interested to see them, with the conditions carefully ensured to be consistent.

The camera would need to be on a tripod, with carefully controlled lighting.

My K10D has been set to setting 2 ever since I got it, and it still gives the exposure errors in stop-down mode, so unless there's something different between the cameras or their respective firmware versions, I'm a bit sceptical.

Hi Guys,
I would appear I was mis-guided in my intial euphoria.

This weekend I set up a static test as ChrisA suggested, in my case - Camera on a tripod focussed on a green bush in the garden with dull overcast skies (which I have taken to be consistent lighting over the test period). ASA set to ISO200
The lens allows :- 1.7 - 2.8 - 4 - 5.6 - 8 - 11 - 16 - 22 and a few in between which I ignored.

With the Green Button bit set to 1 I got shutter speeds of :-

350 - 125 - 30 - 10 - 4 - 1 - 1.5 - 1.5 respectively
with it set to 2 the only change was 0.7 rather that 1 in the above sequence.

Notes 1. All these tests were run twice for each bit setting - rising f number - falling f number over a period of 10 minutes.
2. The eyepiece was blacked out and the speed read from the top display
3. Runing the tests again and reading the speeds via the viewfinder gave varying results (probably because I wear spectacles) and I think this was the reason for my original test results indiating a different slope.
4. In all tests the exposure for f16 and f22 were identical and on viewing the front of the lens whilst pressing the green button I noted that the aperture was the same (Taking the lens off the camera the aperture could be seen to change between f16-f22).

At the end of the day I do have a lens that I can use albeit with a bit of pre-checking and as Womble points out after while I should get consistent results (I generally did this with my earlier film cameras, the differnce being the several days delay in checking your estimates)

regards
Bernard

'M' lenses don't meter correctly with the green button
(Search the US forum for details).

You could just meter at say f/4 or f/5.6 and apply a constant each time to give the correct exposure.