Forgot Password
Pentax Camera Forums Home

Show Printable Version Search this Thread
08-16-2012, 10:13 AM   #1
New Member
Andrew2208's Avatar

Join Date: Jun 2012
Posts: 14
Cheating a zoom's F-Stop

so i was thinking i big into using old lenses and though if i bought a zoom like say... Vivitar Series 1 70-210mm f/2.8-4.0 Q-DOS Lens Reviews - Vivitar Lenses - Pentax Lens Review Database
Say if i through every thing tho manual and put the Aperture ring on 2.8 and zoomed to 200 would it not be 2.8 at 200mm? or would it be forced to F4??

Thanks for any reply's

08-16-2012, 10:18 AM   #2
Site Supporter
boriscleto's Avatar

Join Date: Dec 2009
Location: Liverpool, NY
Photos: Gallery
Posts: 12,872
An F4 lens is an F4 lens. Nothing can magically make it F2.8.

Zoom lenses typically have a maximum relative aperture (minimum f-number) of f/2.8 to f/6.3 through their range. High-end lenses will have a constant aperture, such as f/2.8 or f/4, which means that the relative aperture will stay the same throughout the zoom range. A more typical consumer zoom will have a variable maximum relative aperture, since it is harder and more expensive to keep the maximum relative aperture proportional to focal length at long focal lengths; f/3.5 to f/5.6 is an example of a common variable aperture range in a consumer zoom lens.
08-16-2012, 10:19 AM   #3
Veteran Member

Join Date: Mar 2010
Photos: Gallery | Albums
Posts: 1,545
It would be forced to F4.
08-16-2012, 10:28 AM   #4
Loyal Site Supporter
pacerr's Avatar

Join Date: Jan 2008
Location: Henry, TN
Photos: Gallery | Albums
Posts: 2,910
In VERY simple terms, an F-stop is simply a ratio of the size of the aperture "window" relative to the focal length of the lens. If the aperture remains fixed (say, at the maximum diameter?) and the focal length increases the ratio changes proportionally.

08-16-2012, 10:29 AM   #5
Veteran Member
Anvh's Avatar

Join Date: Sep 2011
Posts: 4,616
F-number or better yet the F-ratio is an ratio between the focal length en the pupil opening of the lens so physical properties.
08-16-2012, 01:17 PM   #6
Inactive Account

Join Date: Dec 2008
Location: North Carolina
Photos: Albums
Posts: 1,720
I have the Komine version of that lens with the A contacts, the camera will still say f2.8 at 210mm, but it is really f4. If you are in Av mode, you can watch the shutter speed change while you zoom.
08-16-2012, 01:52 PM   #7
Loyal Site Supporter
dadipentak's Avatar

Join Date: Jul 2008
Location: Baltimore, Maryland
Photos: Gallery
Posts: 10,410
Violating the laws of physics is a very serious offense. Also, too, very difficult.
08-16-2012, 01:53 PM   #8
Site Supporter
boriscleto's Avatar

Join Date: Dec 2009
Location: Liverpool, NY
Photos: Gallery
Posts: 12,872
QuoteOriginally posted by dadipentak Quote
Violating the laws of physics is a very serious offense.
Even if there's Klingons on the starboard bow.


  • 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!
k-mount, lens, lenses, pentax lens, slr lens, vivitar
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Similar Threads
Thread Thread Starter Forum Replies Last Post
f stop on a zoom? coreyhkh Troubleshooting and Beginner Help 11 04-03-2012 07:45 AM
Why Post is cheating in my mind... thereturn Photographic Technique 161 12-30-2010 09:46 AM
K-5: Any word on cheating the flash sync speed? thigmo Pentax News and Rumors 20 09-29-2010 06:21 PM
Should I be cheating? Ira Site Suggestions and Help 18 07-14-2010 04:45 PM
I have been cheating on you ;) vievetrick General Talk 3 12-30-2007 03:21 PM

All times are GMT -7. The time now is 07:15 PM. | See also:, 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?
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]