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08-26-2011, 11:17 PM   #1
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Should my Vivitar Series 1 underexpose, and by how much? (explained)

OK, I have a Vivitar Series 1 70-210, version 3 which is f2.8 to 4.
Now, it has auto aperture, but it does not communicate it's focal length to the camera, so the camera doesn't know when you zoom using the old push pull. So, the camera thinks I can still be at f2.8 at 210mm, but I know it can't be, it's actually f4.

A few questions:
In my experience, the camera does not massively underexpose, when in Av, even at 210mm. But it SHOULD right... because the aperture is a stop slower than the camera thinks? I mean, it does a LITTLE bit, but not heaps. Normally, if I just set the +/- to +0.5 it will be spot on.

The other question is, does anyone know how far my lens extends before dropping its aperture? As in, can it get to 100mm at f2.8? Can it get to 150 at f3.5 etc?

I know the camera recognises an f3.2 setting, which I've never seen on another lens.

08-26-2011, 11:58 PM   #2
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I have the same lens, and while I only use it in manual that will not alter the following: (1) the camera measures the light received and sets the shutter speed--it does not care whether the lens is set at f/2.8 while the effective f/stop is for example f/4, and (2) the lens effective f stop changes progressively with focal length.
08-27-2011, 01:35 AM   #3
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Perhaps you can verify this by spot metering, placing it on a tripod. If you move the zoom in/out in A mode, the meter should change to what the correct shutter speed as the variable aperture changes from f2.8 to f4. On a fixed aperture zoom, this should not change.

As dms said, it doesn't care what the aperture is set to. The meter is always correct wide open because it measures on the lens wide open. But, if you step down at 210mm, it might be incorrect because the aperture lever may be pushed short or longer than the correct measurement.
08-27-2011, 04:51 AM   #4
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Depending on the camera you have, you may see consistent mis-metering with some non-A-type lenses. The K10D and K20D are notorious for this. But that's an issue with the metering system, not the lens-camera communications (or lack thereof). When in M-mode and using the Green button, the camera stops-down the lens and takes a reading. When in any Auto mode, the camera defaults to Av mode, and takes its reading with the lens wide-open (K- or M-type lenses) or wherever you have the aperture set (screwmount and preset lenses).

Depending on whether the lens base or adapter is shiny metal or painted, i.e. on how conductive it is, the camera may REPORT an aperture of f/2.2 or f/3.8 or whatever. THAT results from the lack of lens-camera communication -- but the camera isn't using such mis-information to base any metering or exposure calculations, it's only a report. When you use a non-A-type lens, just ignore what the camera says about the aperture, it's bogus -- and as dms said, the camera doesn't care either.

QuoteOriginally posted by 0rangepeel Quote
But, if you step down at 210mm, it might be incorrect because the aperture lever may be pushed short or longer than the correct measurement.
Not quite. AFAIK the aperture link is either open or closed, with no proportional nor partial closure. If the camera is in M-mode and finds a lever on the lens, it just shoves that aperture lever all the way, and the lens closes down to wherever the aperture ring is set. If the camera is in any Auto mode, it doesn't shove the lever. That's all.

My recommendation: Shoot; chimp (review); delete; adjust exposure; shoot again. Soon you'll know what mis-metering to expect at certain apertures on certain lenses.

08-30-2011, 04:38 AM   #5
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QuoteOriginally posted by Pentaxman645 Quote
OK, I have a Vivitar Series 1 70-210, version 3 which is f2.8 to 4.
Now, it has auto aperture, but it does not communicate it's focal length to the camera, so the camera doesn't know when you zoom using the old push pull. So, the camera thinks I can still be at f2.8 at 210mm, but I know it can't be, it's actually f4.

A few questions:
In my experience, the camera does not massively underexpose, when in Av, even at 210mm. But it SHOULD right... because the aperture is a stop slower than the camera thinks? I mean, it does a LITTLE bit, but not heaps. Normally, if I just set the +/- to +0.5 it will be spot on.

The other question is, does anyone know how far my lens extends before dropping its aperture? As in, can it get to 100mm at f2.8? Can it get to 150 at f3.5 etc?

I know the camera recognises an f3.2 setting, which I've never seen on another lens.
a couple of points here, to clarify things

as others have mentioned, since the camera reads the actual light coming through the lens, it will compensate for the variable aperture.

The other thing to confirm, is whether the lens (some do) changes aperture fed to the camera as you zoom. this is easy to verify, set it to 70mm and F2.8 and see if it shows F2.8 all the way to 210mm. If it does, then it has sliding contacts on the zoom barrel that change the pinout for aperture coding, if not, then it always shows F2.8 wide open.

that is fine until.

1) as some mention, you use a K10 or K20 which need to know the actual aperture to meter correctly, and
2) you fo to use flash, because P-TTL will get royally screwed up with the wrong aperture fed into things
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