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02-27-2013, 12:23 AM   #1
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Metering with old A zoom having variable aperture

I have a Vivitar series 1 70-210 f2.8-4 lens and was wondering how to properly use it in Av mode. If I use Av mode, it shows f2.8 as maximum aperture throught the range which is wrong and hence it underexposes by 1 stop at longer end. I understand that there is no electronic in that lens so camera wouldn't know exact maximum aperture at a particular focal length.

Just wondering, is there a way I can simplify shooting without worry about focal length in Av mode. I know I can use M mode with stop down metering but then I lose advantage of A mode and also auto ISO capability.

Thanks
Yusuf

02-27-2013, 12:37 AM   #2
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QuoteOriginally posted by yusuf Quote
I have a Vivitar series 1 70-210 f2.8-4 lens and was wondering how to properly use it in Av mode. If I use Av mode, it shows f2.8 as maximum aperture throught the range which is wrong and hence it underexposes by 1 stop at longer end. I understand that there is no electronic in that lens so camera wouldn't know exact maximum aperture at a particular focal length.

Just wondering, is there a way I can simplify shooting without worry about focal length in Av mode. I know I can use M mode with stop down metering but then I lose advantage of A mode and also auto ISO capability.
Well, can't help with that, but I find it curious. I guess I've never had a manual-focus variable-aperture zoom with 'A' contacts -- this would have been a problem even back when the lens was new with film bodies. Seems like it should auto-adjust the shorting of the contacts as it zooms to give the correct max aperture. Do all old MF variable-aperture zooms with 'A' contacts have this problem? (Even the Pentax ones?)
02-27-2013, 12:58 AM   #3
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QuoteOriginally posted by yusuf Quote
Just wondering, is there a way I can simplify shooting without worry about focal length in Av mode.
I would suggest adjusting the EV compensation
so that the exposure is exact when the lens is in the middle of its range.

That way, you'll just overexpose by an easily corrected half-stop on the wide end,
and underexpose by a similar amount at the long end.

It helps to shoot RAW, but even JPEGs should be correctable.
02-27-2013, 01:19 AM   #4
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QuoteOriginally posted by vonBaloney Quote
Well, can't help with that, but I find it curious. I guess I've never had a manual-focus variable-aperture zoom with 'A' contacts -- this would have been a problem even back when the lens was new with film bodies. Seems like it should auto-adjust the shorting of the contacts as it zooms to give the correct max aperture. Do all old MF variable-aperture zooms with 'A' contacts have this problem? (Even the Pentax ones?)
Older film bodies have full k mount to sense aperture bar position so I doubt if they had problem. New DSLRs mount is crippled and hence the problem.

I think all the old MF 'A' lenses should be having this problem (correct me). They do not short contacts like modern zooms does.

02-27-2013, 04:11 AM   #5
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I would not worry about "proper" use in Av mode. Let the camera think it is always F2.8 and get on with more important things. The only exposure error you will see will be a function of each cameras internal compensation for metering non linearity

The K10 and K20 were really bad, exhibiting about a .7 EV shift between a lens at F2.8 and the same lens with a 1.4x TC which made the lens an F4 but the pass through contacts of the TC made the camera think it was still F2.8.

My other bodies , *istD, K7 and K5 do not exhibit that problem.

If you are really worried, take a series of test shots, using a uniformly lit surface (I like a block wall or paved roadway) and test at max, min and middle focal length using Av mode at all apertures and see using the measurement of grey scale, the shift in exposure if any. My sigma 70-200/2.8 was perfectly flat for all apertures from F2.8-32 but with no TC greyscale w 120(perfect) and shifted to 150 using the 1.4x and to 180 with the 2x TC. This represented a 0.7 amid 1.5 EV shift respectively,

If you are really concerned test the lens NAND set EV comp between the two errors you get (max and min)

Your biggest issue is P-TTL flash, which may require compensation as it really does need to know true aperture, there you will need to adjust flash and exposure. Compensation carefully to get it right at the long end
02-27-2013, 05:47 AM   #6
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I've done this and it will work fine. The lens will become a little darker at longer focal lengths, so the camera meter will just give you a longer exposure.

It's just like shooting with a filter or teleconverter, you don't need to tell the camera about it.

If you were using fully manual exposure with a light meter, then you would need to compensate. You would also need to compensate if you use a fully manual flash and set exposure via an exposure table.

I should think the lens should work fine in P-TTL because the exposure is calculated relative to the "darkness", and all A-lenses stop down in a proportional fashion. OTOH, I am not a P-TTL expert, so I'll defer to Lowell's comment above.

Only downside is your EXIF data will be off a little for the aperture value, if that matters to you.

Last edited by Tanzer; 02-27-2013 at 06:28 AM.
02-27-2013, 05:58 AM   #7
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I currently have two copies of this lens, one with a Pentax "P/K-A" mount and the other is a Pentax/Ricoh mount labeled "P/K-A R/K-A". I use them on both a K10 and K20 and while the exposure varies as I zoom the lens, usually dialing in a little EV adjustment will do the trick. The P/K-A mount usually needs a +.3EV when wide open when you zoom and the P/K-A R/K-A one needs about +.7EV. The curious thing is as you begin to stop down the lens the the need for any EV correction goes away on the P/K-A mount one, while the P/K-A R/K-A one increases to +1EV or even more. If you are shooting at approximately the same focal length once you get the EV dialed in your good to go.
02-27-2013, 07:10 AM   #8
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QuoteOriginally posted by Tanzer Quote
I've done this and it will work fine. The lens will become a little darker at longer focal lengths, so the camera meter will just give you a longer exposure.

It's just like shooting with a filter or teleconverter, you don't need to tell the camera about it.
Tanzer has a point here. When you tell the camera to stop down, it doesn't actually stop down until it takes the picture. Hence the need for the camera to know how much it's going to stop down, and compensate, since the shooting conditions don't match the metering conditions.

When you zoom in on a vari-aperture, the effective aperture really does change, and the metering condition will still match the shooting condition. Yusuf, have you actually witnessed this problem happening, or are you assuming it will happen? If it's really happening, I'm not sure this is the reason.

Lowell might have explained the reason, but I'm not sure. You kinda lost me there...

I've got several old MF zooms with an A setting. Maybe one of them is vari-aperture. I'll have to try this out when I get a chance.

02-27-2013, 11:10 AM   #9
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QuoteOriginally posted by Odinz Quote
I currently have two copies of this lens, one with a Pentax "P/K-A" mount and the other is a Pentax/Ricoh mount labeled "P/K-A R/K-A". I use them on both a K10 and K20 and while the exposure varies as I zoom the lens, usually dialing in a little EV adjustment will do the trick. The P/K-A mount usually needs a +.3EV when wide open when you zoom and the P/K-A R/K-A one needs about +.7EV. The curious thing is as you begin to stop down the lens the the need for any EV correction goes away on the P/K-A mount one, while the P/K-A R/K-A one increases to +1EV or even more. If you are shooting at approximately the same focal length once you get the EV dialed in your good to go.
The variation of exposure as you stop down, if this is in Av mode, it implies the aperture action is not quite linear with setting. My tamron 28-75/2.8 is spot on wide open but increases linearly with aperture to 1 stop over exposed at F32
02-27-2013, 11:13 AM   #10
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QuoteOriginally posted by Tanzer Quote

I should think the lens should work fine in P-TTL because the exposure is calculated relative to the "darkness", and all A-lenses stop down in a proportional fashion. OTOH, I am not a P-TTL expert, so I'll defer to Lowell's comment above.

Only downside is your EXIF data will be off a little for the aperture value, if that matters to you.
My experience with P-TTL and a 2x teleconverters shows the problem quite well. My lens on its own is spot on with flash exposure but with the 2x TC even though the camera sees the light etc, the flash power is calculated based upon the reported aperture of the lens, and not the true aperture so it under exposes. T get correct flash metering I have to set both the flash and camera EV comp to +1.
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