K10D Stop down metering exposure problem

I get the same metering when stopping down my SMC Takumar 50/1.4 as stopping down my SMC-A 50/1.4 (green button metering or stop down check with the on/off switch) at different apertures (ok at f4). This in M-mode of course.

// Mats

Hello from "DownUnder"
I have recently acquired a K10D with the 18-55 Pentax kit lens and a superb old SMC 70-210 F4.0 lens. In addition I have in my box of old lenses a SMC F1.4 50mm and an SMC F3.5 35mm (v. sharp)
After reading the posts in this forum I fitted the 1.4 50mm and made a few trial exposures at various apertures. All were correctly exposed with loads of headroom. Am I missing something?
BTW I have applied the recent 1.11 firnware update.

Ron McDermott
Qld, Australia

Hi I have been following this stop down metering discussion for a while because I am very tempted to buy a pentax k10d.

I wonder if the inconsistencies concerning stop down measurement and actual exposure for non-A pentax k mounts could be due to the function of the aperture lever.

According to the k-mount website http://www.bdimitrov.de/kmp/technology/K-mount/Ka.htm
the old pre-A k mounts had a linear response of the aperture actuator with respect to the diameter of the aperture, while the kA-mount lenses have a linear response with respect to the area of the aperture.

Since the digital cameras use the "crippled" KA mount (no metering index lever)- and therefore don't receive any infomation as to what the actua chosenl aperture is, AND stop down using the apereture actuator AS IF it is an A type mount this must lead to, in some instances, quite substantial differences in exposure.

My guess is that the actuator of the camera only moves the way it should if an A-type lens was attatched whereas the actual pre-A mount will work if the actuator moves to or beyond the required aperture, which won' be at the same position in those two instances.

I also guess that the inconsistencies experienced with different k-mount lenses (pentax or others) are due to the fact that prior to the A-mount the only factors required for correct operation were:

- A correct mechanical coupling between lens and camera body (the index lever at the right side of the camera body when you look at the mount) telling the camera body the chosen aperture.

- A coupling between aperture actuator and body which insured that the lens was wide open when mounted, and that it could close all down, when the picture was taken.

This would explain the differences between correct metering and progressive overexposure some have reported.

It would also, assuming that Pentax (or others) produced non-A lenses with apertures having a linear response concerning aperture area, explain why some don't have these troubles at all, but it wold be due to the lenses and not to the camera house.

Well it does seem that some are getting the problems and some are not.

Do you have any A- series lenses? If so, I'd be interested to see a comparison between the following situations:

1. Camera in Manual, aperture set using aperture ring, exposure chosen by camera using green button stop-down metering.

2. Camera in Manual, lens in A-mode, apertures set from camera, exposures set for correct exposure using green button.

Obviously, camera on tripod, viewfinder covered up for the metering, lighting conditions consistent.

My experience with several different A-series lenses is that the results in each case are not the same; the A-mode exposures come out right, and follow the law of reciprocity as the aperture is gradually reduced. With stop down metering, reciprocity does not apply, and the exposures are often wrong.

I haven't yet posted the results of the tests I've done since applying the 1.11 firmware update, but it's still not right on my camera.

Hi ChrisA
My lens is a KA - originally fron a K2 circa 1976 or 7.
My tests - v. quick and dirty at apertures from F22 to 1.4 were all consistent and accurate although at 1.4 the shot was a little over exposed as I ran out of shutter range!
Lens delivers beautiful skin sharp to the edges and lovely bokeh. I am very happy with this portrait lens.
I used Av and simply set the lens aperture ring to the aperture and the camera did the rest - a NoBrainer.
Ron McDermott

Did you actually note any change of shutter speeds at the different apertures?

According to the manual (p210), in Aperture Priority mode (Av), with A and M lenses, using the aperture ring to select the aperture, "the aperture remains open regardless of the aperture ring position".

On mine, the shutter speed indicated does not change when stopping the lens down with the aperture ring, with the camera in Av.

So if you're saying that you get correct variation of shutter speed with manually set aperture (ie using aperture ring on the lens, not setting it from the camera with the lens in A mode), then I am now officially confused.

Very interesting. I wasn't aware of the difference between A and pre-A lenses you describe.

However, if it accounted for the problems, would you not expect that using the same A-series lens, first in A mode, and then in non-A mode, the results would be consistent?

In my experience, they aren't. I'll post the results of my tests post-firmware 1.11 when I get a chance.

Whatever auto setting you use, the camera will be set to Av if using an M or K series lens. The shutter speed will vary according to the light level, but the aperture will always be wide open.

In manual, you can check the exposure by using the stop down lever (why is everyone intent on using the green button?) and whilst holding that down you release the shutter.

Of course you have to set aperture ring permitted in the custom finction menu.

That should work, but it's ages since I messed with it as it's much more efficient to use an A lens on the A setting!

johnriley
You are absolutely correct as to the camera defaulting to Av for ANY KA lens in any Auto mode.
My observation was that the shutter speeds trackes accurately with changes in aperture.
BTW My previous camera was an Olympus 8080. I always shot RAW in manusl mode and measured exposure using the Histogram in EVF preview. NB this is estimated histogram and the final histogram could sometimes differ markedly. However I have found the matrix autoexposure so accurate that it is only in v. rare cases that I reshoot with a corrected exposure.
My confidence level has reached a point where I will be content to shoot - P mode MTF setting, subject to the demands of the subject.
Ron

buris,

good thought. However if my understanding of the K mount is correct it shouldn't matter.

The original K mount works thusly

As you look from the back of the lens, there is a small actuator on the right hand side which activates stop down. This is spring loaded so that it defaults to the "stopped down" position.

When this actuator is in the up position (as it is when composing) the lens is wide open.

When the shutter fires, the camera moves a cam out of the way and the lens stops down to shooting aperture. The aperture it closes down to is determined by the position of the aperture ring.

The cam always moves all the way and the final size of the aperture is determined solely by the position of the aperture ring. How it gets there doesn't matter since the shutter is closed during this process.

In later models with the KA mount this was modified as you say, but in true Pentax style the modifications are unimportant to cameras which can't make use of them.


As it happens I have two examples of the SMC-M 50mm F2. One is the early version with classic M series diameter based stop down, one is a later one which was obviously built with some A series components since it stops down like an A series lens (see Bojidar's KMP for a description of these lenses). When I get home tomorrow I'll check and see if these two behave the same.


John, I am very grateful to you for pointing out that you can use DOF preview to get a real meter readout, but it gives the same result either way (though the DOF preview makes compensating for it less painful!)

There were no KA lenses at the time of the K2 (I bought one in 1975).

KA lenses appeared with the Super A (the clue is in the name ) in 1984. (I bought one in 1985).

The SMCP-A 1:1.4 50mm came out in 1984.

Incidentally, Boz lists the Super A as coming out in 1983, but since there were no KA lenses then, and since cameras almost always came with a normal lens in those days, I suspect he is wrong. But perhaps it was announced in 1983, and appeared on the shelves in 1984. This sort of thing is not unusual with Pentax, as we know!

G

George Lazarette
Many thanks for your clarification. I was confused having come back to Pentax after spendins 2 decades using professionsl medium format cameras, Gasselblad, Mamiya RB/Z67, even Kowa 6.
My lens is certainly a K lens.
Ron McDermott

Thanks for the replies to my posting, there are however a few points Iwould like to point out.
As I understand the point of having an A setting on the A-type lens is that it is possible, provided there is some kind of predictability in the relationship between the aperture actuator and the actual aperture, to let the processor in the camera decide the aperture when shooting in Auto or Shutter priority.
The way it usually is realised is by having the aperture set at the smallest setting of the lens.
The "A" setting on pentax A lenses, is (as far as I know) an example of this, therefore the actuator from the body has to stop down differently, given different chosen apertures by the camera program, if it didn't, the aperture would have to be controlled by some other means (electric contacts?) in order to change the aperture.

That the function is as described is how I understand the explanation on the k-mount homepage (see link in my previous posting).

Another aspect is that the lack of the coding contacts in the mount also gives problems for the camera body - in order to function corectly the camera needs 3 informations from the lens: the minimum and maximum apertures and whether or not the aperture ring is set to the minimum
For non A type lenses none of these informations are given, or rather the information given are:
minimum aperture: 22 maximum
aperture: 1.2
since all of the lens contacts are "on".

If my guess is correct, then it should be possible to see apertures suggested by the camera meter, which are larger than the max aperture of the lens in some instances.
(Slow shutter time, abundant light)

Another aspect is the question of how an(old) A type lens will work in Non A mode I assume You mean if the aperture control is set to a value other than A - in that case I would not expect similar results, because even if the aperture lever works the new cameras do not have the aperture indexing lever so what the actual chosen aperture is will not be known of the camera.
I must say that I do not(yet) own a pentax k10d, so I am not able to test the hypotheses myself.

I cannot quite see how you manage to do this on the K10d - you would have to be very dextrous. Can you please explain more precisely. ? I think I understand using the preview lever but it does not store the captured image, it only allows one to check settings such as white balance etc. - or is there more to it than that ?

I had a go at this - it's pretty easy to do if you're hand-holding the camera. Index finger on the shutter release, middle finger to move the lever. Easy enough to hold the lever against its spring while you push the button.

I wouldn't be surprised, though, if the tendency for camera shake was greater, owing to the increased muscle tension while holding the lever in its preview position.

More serious, though, I'd have thought, is use on a tripod, where holding the lever in the stop-down position is more likely to shake the whole assembly. Obviously the tripod will work best if you can let go while you take the shot.