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05-01-2011, 05:40 PM   #1
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31mm FA limited f1.8 lens question

I'm using a K-x and my 31mm FA limited 1.8 lens always shows an F stop of f1.8 or F--- on the exif information when I take photos regardless if I'm at other F stops. In manual mode its F--- and in other modes (aperture, shutter, sensitivity, auto, program) its always f1.8. Does anyone know why it would do this? I have validated the shutter does stop up/down when changing the aperture ring. It happens both in RAW and JPEG. The only time it shows actual aperture is when the lens is set to "A" position.

thanks for your help

05-01-2011, 05:52 PM - 1 Like   #2
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well the bottom line is: leave your camera on the"A" setting and you won't have any problems. When you set the aperture ring to anything other than the "A" setting camera has no idea what aperture you are using which is why you are getting "f--" indicator.

Modern pentax DSLR cameras have a "crippled" lens mount which doesn't have the mechanical linkage to the aperture mechanism that tells the camera what f number the lens is set to. This linkage is similar to the AI metering coupling "claws" on older nikkor AI-S lenses although in a smart move pentax decided to make this linkage a part of the lens mount itself. Pentax abandoned the use of this linkage in order to simplify their cameras, hence the "crippled" KAF3 lens mount which is what we use today.

Last edited by Digitalis; 05-01-2011 at 06:04 PM.
05-01-2011, 07:22 PM - 1 Like   #3
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When you take the lens off of "A", the ONLY mode you can use unless you want all your photos shot wide open (in this case, f1.8), is M. None of the others will stop the lens down at shutter press time. It's just like using any other Manual lens (with the exception of the M42s set on M).

05-02-2011, 01:15 AM   #4
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QuoteOriginally posted by Digitalis Quote
<snip>
Modern pentax DSLR cameras have a "crippled" lens mount which doesn't have the mechanical linkage to the aperture mechanism that tells the camera what f number the lens is set to. This linkage is similar to the AI metering coupling "claws" on older nikkor AI-S lenses although in a smart move pentax decided to make this linkage a part of the lens mount itself. Pentax abandoned the use of this linkage in order to simplify their cameras, hence the "crippled" KAF3 lens mount which is what we use today.
Actually, the "claws" were pre AI. AI lenses had aperture coupling built into the mount that was designed to be used with AI bodies. The claws were just there to provide backwards compatibility with older bodies like the early F2. My 50mm 1.4 AI Nikkor lens doesn't use the prong with my Nikkormat FT3, but it used the prong with a friend's old F2 photomic.

I've always understood that the older Pentax lens mounts with the mechanical aperture coupling were equivalent to the Nikkor AI. But I could be wrong.

Regards,
Richard.

05-02-2011, 03:11 AM   #5
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QuoteOriginally posted by RichardS Quote
Actually, the "claws" were pre AI. AI lenses had aperture coupling built into the mount that was designed to be used with AI bodies. The claws were just there to provide backwards compatibility with older bodies like the early F2. My 50mm 1.4 AI Nikkor lens doesn't use the prong with my Nikkormat FT3, but it used the prong with a friend's old F2 photomic.

you are right however I was actually just using the nikkor AI-S series lenses as a example,it's true that nikon did use a mount integral register for communicating aperture values however with the photomic it requires the "claws" because of the way that camera's metering system coupling was designed.

The K series are basically the full equivalent of nikkor AI series lenses in terms of lens/body integration.

Last edited by Digitalis; 05-02-2011 at 11:49 PM.
05-02-2011, 05:33 PM   #6
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the bottom line is I need to use the "A" setting on the lens and then use aperture priority if I want to use a specific f stop - correct?
05-02-2011, 07:29 PM   #7
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QuoteOriginally posted by olivemike Quote
the bottom line is I need to use the "A" setting on the lens and then use aperture priority if I want to use a specific f stop - correct?
That's what I would do.
05-02-2011, 07:33 PM - 1 Like   #8
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QuoteOriginally posted by Digitalis Quote
you are right however I was actually just using the nikkor AI-S series lenses as a example,it's true that nikon did use a mount integral register for communicating aperture values however with the photomic it requires the "claws" because of the way that camera's metering system coupling was designed. The

The K series are basically the full equivalent of nikkor AI series lenses in terms of lens/body integration.
Understood. I was just letting my inner pedantic engineer come to the surface. I try to keep him hidden from polite society and succeed most of the time.

All the best,
Richard.

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