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01-31-2008, 06:33 PM   #1
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Two New Lenses and A Question

Well today I picked up my first primes (woot), Pentax SMC-M 50mm F1.7 and Vivitar 135mm F2.8 (also virtually all metal with lovely long focus throw), both in amazing condition for an even better price! I know understand what the big deal with the manual primes is. They're an absolute joy to use! With the 50mm my viewfinder becomes comfortable to use, and I doubt the DA 18-55mm will be used much anymore...
Some pics of the old lens, and of the two new babies...(click individual pics to see larger versions)













All pictures taken in my home-made lightbox thingymajig and taken with the 50mm F1.7 except the pictures of it which were taken with the DA 18-55mm.

I just have a small question, the 135mm seems to underexpose by about .7 to 1 stop, I was wandering if theres anyway I can correct it? Besides overexposing in manual mode? Is it just something I have to live with? Its the only negative thing about it and at the price I paid Im not complaining!

Thanks

01-31-2008, 07:23 PM   #2
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QuoteOriginally posted by Mikhail Quote
I just have a small question, the 135mm seems to underexpose by about .7 to 1 stop, I was wandering if theres anyway I can correct it? Besides overexposing in manual mode? Is it just something I have to live with? Its the only negative thing about it and at the price I paid Im not complaining!

Thanks
Is the under exposure constant over all apatures or does it change with apature? Is the vivitar automatic apature or manual.

You should do some tests, shooting against a uniformly lit surface (i use a block wall in sunlight) and map exposure at all f stops with metering, you can do this two ways, meter and record shutter, or take pictures and record grey scale reading.

Also set exposure steps for 1/2 stop if these are not automatic apature lenses, (i.e. no A setting) because this is the increments the apature clicks are in. sometimes iof the camera is set to 1/2 stops the camera ma take a sudden jump due to slight changes in lighting or non linearity in the apature
02-01-2008, 08:16 AM   #3
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QuoteOriginally posted by Lowell Goudge Quote
Is the under exposure constant over all apatures or does it change with apature? Is the vivitar automatic apature or manual.

You should do some tests, shooting against a uniformly lit surface (i use a block wall in sunlight) and map exposure at all f stops with metering, you can do this two ways, meter and record shutter, or take pictures and record grey scale reading.

Also set exposure steps for 1/2 stop if these are not automatic apature lenses, (i.e. no A setting) because this is the increments the apature clicks are in. sometimes iof the camera is set to 1/2 stops the camera ma take a sudden jump due to slight changes in lighting or non linearity in the apature
Okay thanks for that info. Im not totally sure if its across all f-stops. Just noticed it when shooting wide open (2.8) but I shall go run some test now.
And it doesnt have automatic aperture, manual like the SMC-M. But Ive always had my stop increasements set to 1/2 esp when in manual mode (often) as I find its quicker, and the 1/3 stop I may be off now and again can easily be adjusted in LR if needed, but its very rare.

Thanks
02-01-2008, 08:20 AM   #4
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I've experienced this under or over exposure issue with M series lenses as well. If you test as Lowell suggests and get used to how the lens reacts, it's not an issue. I believe it's because the K series cameras use a 'crippled' mount and just don't meter these lenses as accurately as an A series with the automatic meter coupling.

02-01-2008, 09:08 AM   #5
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QuoteOriginally posted by Peter Zack Quote
I've experienced this under or over exposure issue with M series lenses as well. If you test as Lowell suggests and get used to how the lens reacts, it's not an issue. I believe it's because the K series cameras use a 'crippled' mount and just don't meter these lenses as accurately as an A series with the automatic meter coupling.
Peter, what I have found, and I can only comment on *istD and K10D (as I own them) is that there is a definite issue on the K10D with respect to metering when ANY lens is not in the A setting.

the problem I have seen, and I have described many times, is as follows>

at very large apatures, F1.4 for example, the K10D has a tendancy to under expose by 1 stop when metering using the green button, (or DPOF to activate stopped down metering).

As you stop the lens down, by about F5.6 this can move from under exposure to over exxposure of 1-2 stops, with worst over exposure at about f8-11 and then it begins to correct a little, as you get to F22 or F32.

My *istD does not show this, and I have even seen it with the K10D on my brand new tamron 28-75 F2.8, when I take the apature ring out of A.

What I suspect (obviously can't prove and pentax won't say) is that they have some sort of problem with the meter, the geometry, light reflection etc... that causes a definite error as a function of F stop largely independant of focal length. They have put a correction factor in for this,. and can compensate for AE lenses. BUT since on a K mount they don't know the maximum apature, they do not know how to correct for the error.

I don't know if it exists as a problem on any other body (as I said, my *istD is very good at metering with K mount lenses) and I would really love for people to do testing with each body and lens.

As you indicate, I have "mapped" my lenses, and know what they do, so I am a little prepared for this, and until pentax addresses the issue, that will have to do.
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