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05-26-2010, 06:06 AM   #1
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manual lens behavior - can you explain?

Dear all,

Abstract: I've found a Cosina manual lens that behaves like some kind of semi-A lens. If I could understand why, I'd change all my other manual lenses to behave in the same way.

I have a K20D and a growing set of lenses. I have bought some manual lenses on K-mount (no M42): Pentax M135/3.5, K135/2.5, M50/1.7 and more recently Cosina Cosinon-S 50/1.8

I have used the manual lenses in the M mode of the camera, with the green button, but I've found that too inconvenient: at each shot you must manually focus (if needed), press the green button (can't easily check if needed) and only then take the picture.

With the Cosinon-S 50/1.8 I've found that I can put the camera on "A" mode, adjust the aperture ring e.g. to 2.8 or 4, where its optical characteristics are excellent and benefit from automatically adjusted exposure time. The exposures are off (too dark) by a consistent amount (but dependent on aperture, 1 EV at 2.8, 2 EV at 4) for which I set a compensation once for a whole bunch of photos done at the same aperture.

On short:
[Preparation]
1. Put the lens aperture ring to 4
2. Compensate with +2EV
[Shot]
3. Repeat: focus, shot, change ISO from time to time, etc.
No need for the annoying little (as in hard to find) green button.

From what I've read on the web and judging on the behavior of the other three manual lenses I have the camera was supposed to shot the lens wide open in A mode, fortunately it doesn't do that, it appropriately closes the aperture at every shot.
Also, I am pretty sure that this is not about the trick with incompletely screwing the lens into the mount, as the lens is obviously open for the focusing phase, not dark even at F16. Also, the mount side of the lens is indistinguishable from the M50/1.7 one, nothing seems to me to be misaligned..
Also, the lens is not an A-mount lens, no aperture is displayed by the camera, nor recorded in the photo's exif (on camera's screen I get that F-- as for any other manual lens).

Can someone please explain? I would love to make all my other manual lenses behave in this very convenient way.

05-26-2010, 07:00 AM   #2
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Indeed, from memory, the camera does not close a M lens in Av mode...

Just tried right now with my K7, and it's indeed always wide open, for whatever aperture I turn the ring to...

Interesting... What firmware do you have?
05-26-2010, 07:43 AM   #3
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I seem to recall on my vivitar 85F1.4 when taking it out of A mode, the exposures are also very dark on my K7, but somewhat consistent when metering in M mode. Not so sure about aperture control in Av mode though, I need to go back and check this.


On the K10D I know Av mode does not set aperture when not in A, it shoots wide open.

You should set the consina lens to A also and use the aperture dial on the camera to control it
05-26-2010, 11:18 AM   #4
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QuoteOriginally posted by chrisg Quote
I have used the manual lenses in the M mode of the camera, with the green button, but I've found that too inconvenient: at each shot you must manually focus (if needed), press the green button (can't easily check if needed) and only then take the picture.
That's not really true. You only need to hit the green button if you need to alter the exposure between shot - like when the light changes. Typically you might takes *dozens* of shots without needing to change exposure. In general, you'd need to re-focus between shots *far* more often than you'd need to re-meter.

05-26-2010, 02:58 PM   #5
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It seems to be related to the mechanical aperture control of the K mount

Hi,

Thank you for your replies.
I have made some more tests.
The most visible difference between the cosinon-s 50/1.8 and pentax M-50/1.7 in this respect is the behavior to the optical preview in Av mode when the aperture ring is set e.g. at 16:
the cosinon stops down, the blades close, so the optical preview works properly, while the pentax M-50/1.7 doesn't.
In M,B and X modes both lenses have this desirable behavior of properly doing optical preview.

I have looked with the camera with no lens attached to the mechanical actuator that releases the aperture lever on the lens mount. It seems to do the same full swing movement in both M and Av modes. With a normal M-lens attached I cannot see of course what does it do, but I hear almost the same noise (but for the noise of the blades) when doing the optical preview - why then it has no effect in Av mode?

I would be grateful for any hint.
05-26-2010, 03:09 PM   #6
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I suspect your Cosina lens has a non-conducting mount. I just tried putting a piece of paper between the lens and camera mounts to insulate the lowermost (only relevant) connector. I got your Cosina behavior on a normally Pentax-behaving lens.

So you just need some non-conducting paint to fix all your lenses like this.

Last edited by drougge; 05-26-2010 at 03:10 PM. Reason: I should proofread better
05-26-2010, 03:18 PM   #7
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QuoteOriginally posted by chrisg Quote
Hi,

Thank you for your replies.
I have made some more tests.
The most visible difference between the cosinon-s 50/1.8 and pentax M-50/1.7 in this respect is the behavior to the optical preview in Av mode when the aperture ring is set e.g. at 16:
the cosinon stops down, the blades close, so the optical preview works properly, while the pentax M-50/1.7 doesn't.
In M,B and X modes both lenses have this desirable behavior of properly doing optical preview.

I have looked with the camera with no lens attached to the mechanical actuator that releases the aperture lever on the lens mount. It seems to do the same full swing movement in both M and Av modes. With a normal M-lens attached I cannot see of course what does it do, but I hear almost the same noise (but for the noise of the blades) when doing the optical preview - why then it has no effect in Av mode?

I would be grateful for any hint.
thats very interesting. does metering work aok for both in Av ? say with ring set narrower to f8 or f11 ?
05-26-2010, 05:16 PM   #8
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So everyone is now making paper gaskets for their lenses and synchronizing The Wizard Of Oz to The Darkside Of The Moon.

No. That's cool. Really.

05-27-2010, 01:30 AM   #9
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My K20d has the latest firmware ( version 1.04).
The Cosinon-S 50/1.8 lens has no "A" setting, just normal aperture steps from 1.8 to 16 - this reply is for Lowell Goudge.

I wish covering the bottom-most contact on the mount would replicate this behavior in other lenses - it didn't do it for my M50/1.7.

Apart for trying to isolate contacts, I have also experimented with partially (un)mounting the lenses. On progressive mounting the lens with the camera turned on, one can see how the thing on the camera side starts to open the aperture. Doing optical preview for the Cosinon with the camera in "A" mode closes completely the aperture to the selected value on the aperture ring, while for the Pentax M lens it only make the blades close a little. The servos sound can be still heard, not so much the sound of the blades.

@tokyoso: the metering needs an aperture-dependent exposure compensation, that was +1EV for 2.8, +2EV for 4 and +3EV for 5.6 ; I can't go higher with the exposure compensation, so I get too dark pictures if I select a smaller aperture. My explanation for this is that the metering is done with the lens open, while the picture is taken with the camera closing down the lens to the aperture mechanically selected by the aperture ring.
I don't really see this as a limitation. I like the bokeh and the look on the camera of the old lenses, but shutting them wide-open is a source of too many problems. Shooting them at about 4 or 5.6 seems to bring my lenses in a region of much better characteristics (CA, resolution) and if one keeps the same aperture for longer - as I tend to do even with my FA and DA lenses - the cheap manual lens becomes a very useful lens, were the only thing that one must control is the focus.
05-27-2010, 02:22 AM   #10
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Are you sure you managed to cover the contact? Try this experiment:

Turn the camera on with no lens. Do DOF preview, and observe the lever moving. Take a screwdriver (or whatever metal you have handy) and short the lowermost contant to the mount, and do DOF preview again. Observe the lever only twitching slightly. This works on my K20D with 1.04, and I'm sure it'll work on yours as well.

Having done this, feel confident that the difference really is the conductivity of the mount, and try isolating the contact with your M50/1.7 again. Maybe the paper (or whatever you used) moved too much when mounting the lens?
05-27-2010, 01:42 PM   #11
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Success - after isolating all contacts

Thank you for your hints and explanations.

I've replicated your experiment, and went further: shorting any of the contacts to the mount switched the aperture control in optical preview in Av mode from a full swing to just a small tremble.
Therefore I've isolated all contacts, on the lens side.
Thus any of my manual K and M lenses behave like some kind of automatic M42 in Av:
- exposure time adjusts continuously without the need to press the green button.
- shooting other than wide open in Av mode becomes possible, while the focusing is done wide open - the lens is stopped down at shot-time by the camera to whatever is selected on the aperture ring; constant exposure compensation must be dialed in (the lower the aperture, the higher the compensation).

The modification can be done by gluing some transparent adhesive tape on the lens where it makes contact with the contacts on the camera mount and it is thus fully reversible as opposed to cutting/removing the aperture lever.
It doesn't bring any disadvantages that I know of; the behavior in M mode remains unchanged. What one gains is a workable Av for the first few F-stops
05-27-2010, 02:48 PM   #12
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QuoteOriginally posted by chrisg Quote
It doesn't bring any disadvantages that I know of; the behavior in M mode remains unchanged. What one gains is a workable Av for the first few F-stops
Have you checked catch in focus? That may not work with this mod.
05-27-2010, 03:31 PM   #13
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You are of course right, all contacts need to be isolated. I was sure my piece of paper wasn't doing that, but I didn't really care one way or the other, so I guess it must have.
05-28-2010, 02:15 PM   #14
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Indeed, the catch in focus works no more. Not a big problem for me, I adjust the focus better than the camera! - and not even slower, on other than perfect light.
Again and again I wish the software of this camera would be open.
Forcing catch-in-focus for the case of all contacts isolated would be then easy.
Also, if the only thing that prevents one to convert K and M lenses to A is the offset curve of the aperture lever, it would be so easy to fix this in software.
05-29-2010, 06:47 AM   #15
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The hardware way

Hi, Chrisg,
I have added your functionality to off brand manual K lenses by opening the lens mount, removing and saving the "hold open" lever. This does not need a change to the body contacts.
It gives Av mode,the catch focus indicator works fine up to about f11 and M mode works too.
The lens no longer has wide open manual focussing when stopped down, so no good for a telephoto lens.
I like the classical Aperture Priority feel of this mod (Av on the ist ds). You might not need Ev comp if you use spot metering and AE Lock.
It is a good mod for a general purpose old manual zoom lens if you want to allow others to take shots on your camera.
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