Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
03-04-2013, 09:32 AM   #1
Inactive Account




Join Date: Feb 2013
Photos: Gallery | Albums
Posts: 150
Lens Focal Length

Hi,

When I install an old Manual lens and the camera ask for the focal length, should I put per say 50 mm for a SMC 50 mm 1.4 or shoul I put 75 mm aplying the APS-C 1.5 crop factor?
Also when I get lets say a new 40 mm 2.8 XS is it 40 mm FF or APS-C 1.5?

Thanks for your help

03-04-2013, 09:40 AM   #2
Site Supporter
jatrax's Avatar

Join Date: May 2010
Location: Oregon
Photos: Gallery | Albums
Posts: 10,770
Focal length is always focal length. So if you have a 50mm lens it is always a 50mm lens. The change between APS-C and FF (or any other format) is one of perspective not physical characteristics. A 50mm 'looks' different on APS-C but it is physically the same lens.
03-04-2013, 09:42 AM   #3
Loyal Site Supporter
TER-OR's Avatar

Join Date: Apr 2009
Location: Dundee, IL
Photos: Gallery | Albums
Posts: 5,828
50mm - the actual register distance.

It's 40mm. I'm not sure if the 40XS covers a FF sensor, if so the FOV would be larger.

Just use the lens and see if you like it.
03-04-2013, 10:31 AM   #4
Inactive Account




Join Date: Feb 2013
Photos: Gallery | Albums
Posts: 150
Original Poster
Thanks for the clarification

03-04-2013, 10:51 AM   #5
Pentaxian
Na Horuk's Avatar

Join Date: Feb 2012
Location: Slovenia, probably
Photos: Gallery | Albums
Posts: 10,861
QuoteOriginally posted by dltorre Quote
When I install an old Manual lens and the camera ask for the focal length, should I put per say 50 mm for a SMC 50 mm 1.4 or shoul I put 75 mm aplying the APS-C 1.5 crop factor?
You input the number on the lens. Or closest one.

QuoteOriginally posted by dltorre Quote
Also when I get lets say a new 40 mm 2.8 XS is it 40 mm FF or APS-C 1.5?
It is 40mm on both, regardless of whether you use film, aps-c, or ff. But! Since it is made for aps-c, it might have poor edge performance on ff or film. (vignetting, not as sharp). But only the parts that aren't visible on aps-c. The lens is always the same, crop just means the actual sensor is smaller in size and therefore cuts off some edges. Its only a problem is you strive for "equivalence" across mediums, but that is kind of pointless to do. There is a thread on these forums where people tested out the edge performance of DA (crop) lenses and some have great edge performance even on film, while others don't bother. The 40mm XS is a good lens, its tiny, fast AF, very sharp, and I think it can even be used on FF without vignetting. Just make sure you don't lose the lens cap! Or you can buy some generic pinch caps online
03-04-2013, 11:29 AM   #6
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,400
QuoteOriginally posted by dltorre Quote
Hi,

When I install an old Manual lens and the camera ask for the focal length, should I put per say 50 mm for a SMC 50 mm 1.4 or shoul I put 75 mm aplying the APS-C 1.5 crop factor?
Also when I get lets say a new 40 mm 2.8 XS is it 40 mm FF or APS-C 1.5?

Thanks for your help
Focal length is a fundamental lens characteristic that does not change with sensor size. You always enter focal length. It is based upon the equivalent magnification, to a simple (single element lens) which has a distance from the lens to the focal plane when focused on an infinitely distant object.

QuoteOriginally posted by TER-OR Quote
50mm - the actual register distance.
no 50 is not the regestry distance . The regestry distance is the distance from the lens mount to the focal plane, which is 45.46mm for a Pentax camera. Regestry distance is only relevant whe trying to adapt a lens from one maker to a different camera body because you will not get the lens' full range of focus if the regestry distance is not the same
QuoteQuote:

It's 40mm. I'm not sure if the 40XS covers a FF sensor, if so the FOV would be larger.
don't mix FOV with focal length. Adding a comment about FOV here does not help, the OP is asking what to put into the focal length setting
QuoteQuote:

Just use the lens and see if you like it.
Best advise on the page
03-05-2013, 06:21 AM   #7
Loyal Site Supporter
TER-OR's Avatar

Join Date: Apr 2009
Location: Dundee, IL
Photos: Gallery | Albums
Posts: 5,828
I stand corrected.

FOV is relevant because that's what the "75mm equivalent" means - and why it's so confusing to people. It's a useful reminder that the crop sensors do just that - crop the image. It's also irrelevant because this cannot be changed, but it's useful to remember the same lens will have different FOV on FF vs. crop sensors and if you're very accustomed to older 35mm lenses, you'll have to shift your thinking a bit.
Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
aps-c, camera, length, lens, mm, pentax help, photography
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Manual Focal lens length setting stevbike Pentax DSLR Discussion 21 04-02-2013 02:38 AM
lens aperture and focal length markings sylvanite Pentax SLR Lens Discussion 3 11-13-2011 12:45 PM
Lens Focal Length Rip Off. normhead Pentax SLR Lens Discussion 48 08-10-2011 05:41 PM
Variation in true focal length between copies of the same lens dgaies Pentax SLR Lens Discussion 2 05-22-2011 10:46 AM
Suggestion List 3rd Party Lens Reviews by Focal Length Youngster Site Suggestions and Help 7 01-16-2009 02:11 AM



All times are GMT -7. The time now is 04:23 PM. | See also: NikonForums.com, CanonForums.com part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top