Pentax-A SMC question

jgnfld · 03-04-2015, 09:47 AM

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.

soycory · 03-04-2015, 09:47 AM

50mm...

Macario · 03-04-2015, 09:51 AM

50mm, it is just for the SR system.

aleonx3 · 03-04-2015, 09:52 AM

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.

jgnfld · 03-04-2015, 09:59 AM

Originally posted by Macario

50mm, it is just for the SR system.

"SR" ???

**paulh** · 03-04-2015, 10:06 AM

Shake Reduction. The A50/1.7 is a nice lens, btw - my copy is sharp even wide-open.

pete-tarmigan · 03-04-2015, 10:07 AM

Originally posted by jgnfld

"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".

jgnfld · 03-04-2015, 10:36 AM

Originally posted by pete-tarmigan

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!

pete-tarmigan · 03-04-2015, 10:51 AM

Originally posted by jgnfld

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.

**jatrax** · 03-04-2015, 11:01 AM

You would use 50 both for the SR system and because that is the number that goes into the EXIF for each image.

Unsane · 03-04-2015, 11:16 AM

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).

**jrpower10** · 03-04-2015, 11:28 AM

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.

aremmes · 03-04-2015, 11:55 AM - **1 Like**

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.

stevebrot · 03-04-2015, 12:51 PM

Originally posted by jrpower10

The other question here is why the lens isn't communicating the focal length to the camera.

Originally posted by aremmes

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.

Steve

jgnfld · 03-04-2015, 01:20 PM

Originally posted by jrpower10

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.

03-04-2015, 09:47 AM	#1
jgnfld Moderator Loyal Site Supporter Join Date: Feb 2015 Location: Avalon Peninsula, Newfoundland Photos: Gallery \| Albums Posts: 1,796	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.

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03-04-2015, 09:47 AM	#2
soycory Forum Member Join Date: Feb 2009 Location: Kentucky Posts: 93	50mm...

03-04-2015, 09:51 AM	#3
Macario Pentaxian Join Date: Feb 2011 Location: Hoek van Holland Photos: Gallery \| Albums Posts: 1,393	50mm, it is just for the SR system.

03-04-2015, 09:52 AM	#4
aleonx3 Veteran Member Join Date: Jul 2007 Location: Brampton, Ontario Photos: Gallery \| Albums Posts: 3,996	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
jgnfld Moderator Loyal Site Supporter Join Date: Feb 2015 Location: Avalon Peninsula, Newfoundland Photos: Gallery \| Albums Posts: 1,796 Original Poster	Originally posted by Macario 50mm, it is just for the SR system. "SR" ???

03-04-2015, 10:06 AM	#6
paulh Site Supporter Join Date: Sep 2010 Location: DFW Texas/Ventura County, CA Photos: Gallery \| Albums Posts: 33,301	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
pete-tarmigan Veteran Member Join Date: Apr 2009 Location: Conception Bay South, New-fun-land Photos: Gallery Posts: 1,272	Originally posted by jgnfld "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
jgnfld Moderator Loyal Site Supporter Join Date: Feb 2015 Location: Avalon Peninsula, Newfoundland Photos: Gallery \| Albums Posts: 1,796 Original Poster	Originally posted by pete-tarmigan 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
pete-tarmigan Veteran Member Join Date: Apr 2009 Location: Conception Bay South, New-fun-land Photos: Gallery Posts: 1,272	Originally posted by jgnfld 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
jatrax Site Supporter Join Date: May 2010 Location: Washington Cascades Photos: Gallery \| Albums Posts: 12,991	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
Unsane Junior Member Join Date: Sep 2014 Location: Somewhere in the Welsh Marches Photos: Albums Posts: 48	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
jrpower10 Site Supporter Join Date: Sep 2011 Location: Richland, Washington, USA Posts: 935	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
aremmes Senior Member Join Date: Oct 2013 Location: Philadelphia Photos: Albums Posts: 217	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
stevebrot Otis Memorial Pentaxian Join Date: Mar 2007 Location: Vancouver (USA) Photos: Gallery \| Albums Posts: 42,007	Originally posted by jrpower10 The other question here is why the lens isn't communicating the focal length to the camera. Originally posted by aremmes 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. Steve

03-04-2015, 01:20 PM	#15
jgnfld Moderator Loyal Site Supporter Join Date: Feb 2015 Location: Avalon Peninsula, Newfoundland Photos: Gallery \| Albums Posts: 1,796 Original Poster	Originally posted by jrpower10 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.