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03-14-2015, 03:41 PM   #1
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Odd experience with older 3rd party lens on K5

The camera is, as in the subject line, a K5. The lens is a Sigma 70-210, manual focus, which advertises itself as f4 to f5.6 with a 52mm thread, made in Japan.

I dug it out of mothballs because I wanted to play around with manual focus and see how sharp I could get it without the camera's help, and something odd happened - when I zoom it all the way to 210mm, the f stop continues to report to the camera as f4.0. One would expect it would report as f5.6. The metering works fine; the pics are equally well exposed at both f stops. I do notice a change of shutter speed when I do the zoom, as if the camera knows there's less light coming in and is adjusting accordingly, but the f stop is rock solid on 4.0.

Either this lens is lying its pretty little head off (not talking to the camera properly) or the APS-C sensor is simply not detecting the closed-down aperture somehow and still thinks it has an unrestricted light path. The latter makes no sense. It hardly matters to me, as the thing is exposing properly, but I do wonder what's actually going on and whether an exorcist is required.

03-14-2015, 03:48 PM   #2
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It's most likely the case that the lens isn't properly communicating the change in aperture to the camera. It's all in the electronic contacts on the lens, not in the meter/sensor. If you stop-down meter with the aperture ring away from A, however, you should still be able to get an accurate exposure reading.

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03-14-2015, 04:24 PM   #3
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Certainly this old lens doesn't have a data contact. The lens does not have the means to communicate the real change in aperture when zooming. In Av, the camera meters at the shutter press and will give the exposure it measures. The fact that it thinks it's f4 and records f4 is besides the point. If you look at the shutter speeds however you should see the difference in aperture between 70mm and 210mm reflected. The other thing is if you use manual mode, set the exposure at 70mm with the green button and then zoom in, then you will see underexposure due to the variable aperture.
03-14-2015, 04:44 PM   #4
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If it thinks it is anything, i.e. if it reports an aperture at all rather than "F--", then it must at least be a 'A' lens. Don't at least some of these old manual focus zooms with a variable aperture report the change as they zoom? (Not with a data pin, but by a mechanical slider that changes which contacts are conductive.) At least the Pentax-branded ones maybe?

03-14-2015, 05:49 PM   #5
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QuoteOriginally posted by vonBaloney Quote
If it thinks it is anything, i.e. if it reports an aperture at all rather than "F--", then it must at least be a 'A' lens. Don't at least some of these old manual focus zooms with a variable aperture report the change as they zoom? (Not with a data pin, but by a mechanical slider that changes which contacts are conductive.) At least the Pentax-branded ones maybe?
Not that I'm aware of. There are no pins other than the "A" pin on the "A" series lenses. It's either bare metal (flange) for conducting or a pit drilled out and filled with an insulator for non-conducting.

If I put a DA lens on a six pin Tele-converter it behaves as if it were an "A" series lens. I have to set a focal length for shake reduction, use manual focus and the f-stop set in the viewfinder remains at the widest for the lens regardless of the zoom setting e.g. f/4 instead of f/5.8.
03-14-2015, 07:02 PM   #6
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QuoteOriginally posted by Not a Number Quote
Not that I'm aware of. There are no pins other than the "A" pin on the "A" series lenses. It's either bare metal (flange) for conducting or a pit drilled out and filled with an insulator for non-conducting.

If I put a DA lens on a six pin Tele-converter it behaves as if it were an "A" series lens. I have to set a focal length for shake reduction, use manual focus and the f-stop set in the viewfinder remains at the widest for the lens regardless of the zoom setting e.g. f/4 instead of f/5.8.
I thought there were some that actually changed (essentially) the pin configuration. Seems like I've seen that before, but I could be wrong. Like the PKA adaptall mounts can read anywhere from f/1.5 to f/4 depending on the lens they are on even though the visible contacts are always the same (the position of some conducting slider underneath changes depending of the max aperture of the lens). Of course they still remain fixed for a particular even on a zoom with variable aperture.
03-14-2015, 07:57 PM   #7
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My older (MF) A lenses [Vivitar and Adaptall-2) work the same way. They don't know about the effective f stop--but of course they do get the correct exposure--unless you use M mode and then change the FL and take the shot.
03-15-2015, 07:43 AM   #8
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I don't know about Pentax but Minolta (at least from the X-700 on) in auto modes would do a final stop-down exposure check just before releasing the shutter so the exposure would be "right" no matter what the aperture setting control was telling the camera.

03-16-2015, 05:24 AM   #9
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As others have pointed out, it's the "seventh" pin on the mount, i.e., the encoding/programming of it, which some earlier lenses didn't handle quite correctly. Mostly a problem for older zooms. Doesn't matter much, the camera should still meter and expose correctly in the auto modes, it will just think it got a little darker (like a cloud moved in overhead, or you put an ND filter on the front) whenever you zoom to a longer focal length.

The only downside is that the aperture will be reported incorrectly to you in the display and EXIF data, not a big deal IMO.

Also as mentioned by others, if you are in M mode and meter using stop down metering, but then change the focal length at the last second before firing the shutter, that will throw off the exposure as well.
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