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03-04-2015, 09:47 AM   #1
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Pentax-A SMC question

I recently bought a Pentax-A SMC 50mm f1.7. In initial testing, it seems to do very well. One thing confuses me: My K-50 wants me to input the focal length. Should I input 50mm, or should I input 35mm as this is an APS-C camera? I cannot see any real difference from test shots. Both seem underexposed a bit, but this is easily taken care of by kicking up the EV by 1.5-2.0.

Especially for the price, this seems so far like an exceptionally clear lens once stopped down a bit.

Any help would be appreciated. My other A lens seems to communicate its focal length directly to the camera computer.


Last edited by jgnfld; 03-04-2015 at 09:59 AM.
03-04-2015, 09:47 AM   #2
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50mm...
03-04-2015, 09:51 AM   #3
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50mm, it is just for the SR system.
03-04-2015, 09:52 AM   #4
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focal length does not change regardless of sensor size... just put 50mm for the camera to recognize the lens for SR purpose; and it does not affect much even if you put something close enough... like 55.

03-04-2015, 09:59 AM   #5
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QuoteOriginally posted by Macario Quote
50mm, it is just for the SR system.

"SR" ???
03-04-2015, 10:06 AM   #6
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Shake Reduction. The A50/1.7 is a nice lens, btw - my copy is sharp even wide-open.
03-04-2015, 10:07 AM   #7
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QuoteOriginally posted by jgnfld Quote
"SR" ???
The initials SR on the front of the K-50's body between the grip and the lens mount. It stands for "Shake Reduction".
03-04-2015, 10:36 AM   #8
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QuoteOriginally posted by pete-tarmigan Quote
The initials SR on the front of the K-50's body between the grip and the lens mount. It stands for "Shake Reduction".
Doh.

I was fixating on 'single reflex' or some such and that made no sense. For good reason it now appears!

03-04-2015, 10:51 AM   #9
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QuoteOriginally posted by jgnfld Quote
Doh.

I was fixating on 'single reflex' or some such and that made no sense. For good reason it now appears!
I, too, sometimes puzzle over the meaning of acronyms used on this website. For that reason I avoid using them. However, I took a typing class in high school, so a few extra keystrokes don't bother me. I guess typing is more laborious for those people using the typing method attributed to Messrs. "Hunt and Peck", so they resort to abbreviations.
03-04-2015, 11:01 AM   #10
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You would use 50 both for the SR system and because that is the number that goes into the EXIF for each image.
03-04-2015, 11:16 AM   #11
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As already mentioned you need to input 50mm. As a side note: the effective focal length on APS-C is actually 75mm, not 35mm. This is as the sensor size is smaller, it's only using a limited area of the sensor and the effective Field of View is reduced. So if you do want a standard lens a 28mm is a pretty close match (28 x 1.5 = 42).
03-04-2015, 11:28 AM   #12
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The other question here is why the lens isn't communicating the focal length to the camera. I thought A lenses did (pretty sure my A85 does, but I'll check when I get home). Try cleaning the contacts and see if that fixes it. If not, and the lens SHOULD communicate (someone who's sure please chime in), you may be looking at a repair.
03-04-2015, 11:55 AM - 1 Like   #13
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Pentax-A lenses don't have any facility for sending focal length data to the camera. The contacts on the KA mount exist solely to transmit minimum and maximum aperture. Only the autofocus lenses transmit their focal length.
03-04-2015, 12:51 PM   #14
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QuoteOriginally posted by jrpower10 Quote
The other question here is why the lens isn't communicating the focal length to the camera.
QuoteOriginally posted by aremmes Quote
The contacts on the KA mount exist solely to transmit minimum and maximum aperture. Only the autofocus lenses transmit their focal length.
Yes, the manual focus lenses support the 6 contacts that provide a passive map to minimum and maximum aperture. AF lenses support an additional "data" pin that allows communication between body and lens. Lens model, focal length, and focus distance are among the information that may be passed through the data pin.


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03-04-2015, 01:20 PM   #15
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QuoteOriginally posted by jrpower10 Quote
The other question here is why the lens isn't communicating the focal length to the camera. I thought A lenses did (pretty sure my A85 does, but I'll check when I get home). Try cleaning the contacts and see if that fixes it. If not, and the lens SHOULD communicate (someone who's sure please chime in), you may be looking at a repair.
Once I input the correct value, each time I mount the lens now, it recognizes it just fine w/o requiring further input. Whatever that means re contacts, etc.
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