Forgot Password
Pentax Camera Forums Home

Show Printable Version Search this Thread
06-26-2014, 11:46 AM   #1
Site Supporter
Dewman's Avatar

Join Date: May 2012
Location: Idaho
Photos: Albums
Posts: 3,228
Question about lens nomenclature.

Why are lenses called out as: f:3.5-5.6? Is it a 3.5 or a 5.6? I'm confused.

06-26-2014, 11:52 AM   #2
Site Supporter

Join Date: Aug 2013
Posts: 58
The aperture number changes through the zoom range.
06-26-2014, 11:53 AM   #3
Site Webmaster
Adam's Avatar

Join Date: Sep 2006
Location: Arizona
Photos: Gallery | Albums
Posts: 43,124
That means f3.5 at the wide end through f5.6 at the long end; in order words, less light passes through the lens the more you zoom in. Most consumer lenses have variable aperture as this allows them to be cheaper and more compact.

Adam Webmaster (Site Usage Guide | Site Help | My Photography)'s high server and development costs are user-supported. You can help cover those costs by donating. Or, buy your photo gear from our affiliates, Adorama, B&H Photo, or Topaz Labs, and get FREE Marketplace access - click here to see how! Trusted Pentax retailers:

06-26-2014, 12:31 PM - 2 Likes   #4
Na Horuk's Avatar

Join Date: Feb 2012
Location: Slovenia, probably
Photos: Gallery | Albums
Posts: 10,863
Hey, welcome! Okay, so lets start at the beginning.
50mm f1.7 means the maximum aperture (lowest f-number) is 1.7 at 50mm focal length. There is only one focal length listed, so the lens has a fixed focal length. It has no zoom, the frame is fixed. Its called a prime. Maximum aperture depends on the size of the front element and the focal length. More on this later
17-50mm f2.8 means the lens has a zoom range from 17mm to 50mm (so it goes from wide angle to tele), and maximum aperture of f2.8 throughout. Its a zoom lens with constant aperture. Zoom lenses tend to have slightly lower image quality than primes, but they have a convenient, flexible zoom range. Modern lens designs are also so good, that the image quality of certain zoom lenses matches prime lenses. These tend to be higher-end zoom lenses.
18-135mm f3.5-5.6 means the lens zooms from 18mm up to 135mm, and the maximum aperture is f3.5 at 18mm and f5.6 at 135mm. It also changes throughout, so at 50mm it might be f4, for example. So if you zoom to 135mm, you cannot choose f3.5. It is a variable aperture zoom lens, because the aperture varies depending on the focal length. This lens design is slightly easier and more compact than a fixed aperture zoom, but it is not quite as useful since you get a lower aperture at the tele end. So these tend to be zoom lenses that put convenience over image quality, though some variable aperture zoom lenses are still quite great.

Maximum aperture is the lowest f-number, which is also the brightest photo and the shallowest Depth of Field. All lenses with an aperture mechanism can be stopped down, this means you choose a higher f-number, and the aperture blades inside the lens close to make the hole smaller. Less light passes and the DoF in the photo increases. Pretty much all lenses can be stopped down to f16, some to f22 or even f32. But on the latest Pentax DSLRs the high resolution of the sensor means that if you go above f8 or f11, the photo becomes slightly fuzzy (not quite as sharp). At f22 it is quite noticeable. Cameras with a higher resolution (usually this means smaller sensor) this happens at even lower number.

Last edited by Na Horuk; 06-26-2014 at 12:39 PM.
06-26-2014, 02:54 PM - 1 Like   #5
Site Supporter

Join Date: Jun 2011
Location: Texas
Photos: Gallery
Posts: 1,149
Back before through-the-lens exposure meters zoom lenses had to be made with constant aperture throughout the zoom range. Otherwise the exposure would vary. TTL metering and modern electronics can compensate - so the lenses can be simpler and cheaper. Back when variable aperture zooms first came out I was sure I'd never buy one.
06-26-2014, 03:08 PM   #6
Na Horuk's Avatar

Join Date: Feb 2012
Location: Slovenia, probably
Photos: Gallery | Albums
Posts: 10,863
I'll also add that there are some other ways of writing the aperture. For example 3.5/30 means the lens is 30mm and f3.5. And 1:1.2 50mm means it is 50mm f1.2. This can be confusing when search algorithms mix up 1:1.2 and 1:2.0 - one is f1.2, the other is f2.0, big difference! Also, 1:1.4 f=50mm is a 50mm lens with f1.4 aperture.
And the latest line of Pentax HD lenses simply say 4/15 on the top of the barrel, which means 15mm f4.0 (they still have a regular text near the top of the lens).
The aperture is also written as f/4. It gets called f-number, f-stop and others. On some older, exotic lenses the focal length is even written as centimeters (cm instead of mm). Don't let these things confuse you

All of this is because aperture, written as f-stop, is actually a fraction of the focal length divided by the size (diameter) of the entrance pupil

Last edited by Na Horuk; 06-26-2014 at 03:24 PM.
06-27-2014, 09:20 AM   #7
Site Supporter
Dewman's Avatar

Join Date: May 2012
Location: Idaho
Photos: Albums
Posts: 3,228
Original Poster
Thanks for all the great input. You guys (and gals) are the greatest.

  • 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!
camera, pentax help, photography, question about lens
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Similar Threads
Thread Thread Starter Forum Replies Last Post
Genuine Pentax Lens Hoods Nomenclature Vranx Pentax Lens Articles 4 01-15-2014 03:45 PM
Question about DA35 2.4 lens Richjac Pentax SLR Lens Discussion 7 09-08-2013 05:53 AM
Question about SMC Pentax-A 35-70mm F4 lens pattygs Troubleshooting and Beginner Help 8 08-21-2013 06:42 PM
question about the 01 vented lens hood volosong Pentax Q 14 01-05-2013 03:20 PM
Lens nomenclature rmlewis Pentax Medium Format 2 12-19-2011 06:05 AM

All times are GMT -7. The time now is 12:25 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]