Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
10-24-2009, 04:15 PM   #1
Inactive Account




Join Date: Oct 2009
Location: Prairie County, Montana
Posts: 4
f stop calculation with 67 lenses on dslr

I have a k10d and k20d with a multitude of pentax manual lenses. The other day I purchased a convertion mount for 67 lenses to k mount. I am under the understanding that the lens focal length will be increased by about 3x on a dslr and about 2x on my ME Super and MX. What about the f-stop? Does it remain constant?

10-24-2009, 05:28 PM   #2
Veteran Member




Join Date: Feb 2008
Location: Toronto
Posts: 3,915
the focal length is the same and the aperture is the same as before
10-24-2009, 08:59 PM   #3
Site Supporter




Join Date: Feb 2008
Location: MT
Posts: 1,075
I bought one and have done the comparisons...the VIEW of a C crop digi is about 1/6 of the VIEW you get through that same lens on 6x7 format. However, a 6x7 (or 67) lens that says 100mm on the barrel, shoots the EXACT VIEW of any other 100mm lens on your digi. Same for 300mm or any other length. So the digital image is much cropped vs the 67 image shot through the same lens, but all 100mm lenses give the same IMAGE field of view regardless of which format 100mm lens you use. I've tried it on 45mm clear up to 300 and I even have a 400mm lens for my 67 format. They all MATCH the view I get with lenses from the 35mm format. So a 100mm is a 100mm is a 100mm regardless of the format of the lens.

Regarding aperture, I accept the theory of no difference, however if the smaller format only uses the middle sweet spot of the lens, could an argument be made that the aperture opening is relatively a larger percentage of that smaller window than it is of the entire front element, thus changing the effective f stop? I could probably disprove this thought playing with lenses in the front yard tomorrow, but since that's opening day of hunting season in Montana (and you are forceably removed from the state if you don't hunt), I'll be afield without a camera tomorrow.
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!
dslr, k-mount, lenses, pentax lens, slr lens
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Zeiss to stop ZK lenses for Pentax Zav Pentax News and Rumors 64 12-20-2010 03:18 PM
f/stop in Raw or EXIF (using MF lenses) yusuf Pentax DSLR Discussion 13 09-13-2010 11:18 AM
f stop input for manual lenses? axl Digital Processing, Software, and Printing 6 06-30-2010 05:09 AM
K-x Stop-down metering with m-42 lenses Bazwell Pentax DSLR Discussion 8 11-20-2009 12:57 PM
M42 lens on a DSLR: how to stop down? Syb Pentax SLR Lens Discussion 2 01-19-2009 09:18 AM



All times are GMT -7. The time now is 08:34 AM. | See also: NikonForums.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