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06-07-2007, 12:52 PM   #1
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Interesting Lens Quirk

My friend has some pentax lenses from way back in the day that he wanted to try out on my k10d.

One of them was a 50mm f/1.7 (with no autofocus and manual aperture selection), which I had a fun time scampering around the house and shooting with. I noticed that even at smaller apertures, the camera seemed to be using a shorter exposure time than with my other lenses. So I set the 50mm lens to f/5.6 and took a picture. The k10d exposed for 1/125 of a second.

Then I pulled out my 18-55mm kit lens. I set it to 50mm, and set the aperture to f/5.6. I ensured that the ISO was the same (ISO 400). When I went to take the picture, the k10d wanted to expose for 1/20 of a second, and it was begging for the flash.

Can anybody explain the difference? Same focal length, same ISO, same aperture, but the camera was choosing a far different exposure length. The only thing I can think is that the software had already decided to process the image differently after the exposure, based on the fact that aperture was being selected manually. It's not a problem at all, I'm just new to the world of dslrs and I'm interested to know what's going on. Ideas? Thanks!

06-07-2007, 12:55 PM   #2
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Were you in manual mode or automatic?

In any automatic mode the pentax camerasl will all use full apature regardless of what you set the F stop to.
06-07-2007, 01:34 PM   #3
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I was in automatic mode both times.
06-07-2007, 02:01 PM   #4
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Of course it is a problem.
In theory the same light, ISO, and aperture can only result in the same shutter time.
There is a sort of bug with the K10D making stopped down metering with old lenses an adventure and in effect making a trial and error situation out of it.
You can search the forum and find several threads about this.

06-07-2007, 02:11 PM   #5
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Alright. I was thinking there might have been something I just didn't know about.

It was automatically choosing the exposure time though, and the pictures were coming out just fine, which is why I didn't think it was that much of an issue.
06-07-2007, 02:27 PM   #6
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Must use manual exposure

The problem will be that unless you use manual exposure mode, all your pictures will be exposed at the widest aperture, irrespective of what you have set the lens to.

Do a search , there are plenty of threads on this subject
06-07-2007, 02:48 PM   #7
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Funny names you guys use, daynebatten and pschlute...

As pschlute, and first, Lowell, mentioned the camera uses the largest aperture when set to automatic. My comment was not relevant to your experience, I'm sorry for that. Your experience has nothing to do with the problem I mentioned. But when you start to meter the light as it is supposed to be done with the M lens you'll run into it. Wide open everything is fine.

regards,
06-07-2007, 07:07 PM   #8
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Thanks guys. So I guess it's either shoot at F/1.7 with no depth of field control, or go to manual and do trial and error?

That's lame, but I've still got to salute Pentax for at least making the lens somewhat useable. That's more than can be said for other DSLRs.

06-07-2007, 07:20 PM   #9
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QuoteOriginally posted by daynebatten Quote
Thanks guys. So I guess it's either shoot at F/1.7 with no depth of field control, or go to manual and do trial and error?

That's lame, but I've still got to salute Pentax for at least making the lens somewhat useable. That's more than can be said for other DSLRs.
No need for trial and error, just set the function to use non automatic lenses, and when you press the green button in manual it will set the shutter for correct exposure
06-08-2007, 04:30 AM   #10
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QuoteOriginally posted by Lowell Goudge Quote
No need for trial and error, just set the function to use non automatic lenses, and when you press the green button in manual it will set the shutter for correct exposure
At some apertures it will work, at some it will not, hence my comment about "guesswork". Now it doesn't need to be that bad really but yu'll have to learn the deviances for each lens and aperture opening. Some long zooms work fine, most lenses does not.

To illustrate the problem (from one of the many earlier threads on this topic):


regards,
06-08-2007, 05:07 AM   #11
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QuoteOriginally posted by daynebatten Quote
So I guess it's either shoot at F/1.7 with no depth of field control
You can have DOF control, just set the DOF preview to "optical" (instead of digital) in the setup menu.
06-08-2007, 05:22 AM   #12
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most of my lenses are manual. i'm shooting a DS and i find the cam over exposes by 1-2 stops consistently. it's easy to fix by just adjusting the Ev comp.
pentax has done a fine job with backward compatability. you just need to learn to use it.
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