Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
01-18-2010, 08:40 AM   #1
Senior Member




Join Date: Mar 2008
Location: Québec
Posts: 117
Question about the pancakes lens

How come the aperture in the 3 pancakes is bigger when the focal length is higher, but in zooms, it's the inverse?
It's 21mm/3.2, 40mm/2.8 and 70mm/2.4 but the kit lens has 18mm/3.5 to 55mm/5.6.
In fact almost all zoom lenses are that way(or fixed aperture, I never seen any inverted aperture in zooms but I guess it doesn't mean it doesn't exist), so why is it inverted in prime pancakes?

01-18-2010, 09:45 AM   #2
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,400
I would be willing to bet that the optical corrections needed to make larger apertures in a "pancake format" lens would be impossible at larger apertures.

Lens design is a trade off.

note in variable aperture zooms, you have different limits, and part of the design is a function of where in the optical path the aperture is placed, but another part is that eventually, the diameter of the front element can be a limiting factor, as a result, as you zoom the aperture increases because focal length gets longer but the diameter of the element is fixed.
01-18-2010, 10:31 AM   #3
Veteran Member




Join Date: Nov 2008
Location: The Untied States
Photos: Albums
Posts: 1,882
I don't think it's for any particular reason, it's just the lens design Pentax thought would best fit each of those limited lenses.
01-19-2010, 01:26 PM   #4
Pentaxian
Marc Sabatella's Avatar

Join Date: Sep 2006
Location: Denver, CO
Photos: Gallery
Posts: 10,686
A zoom is *always* a lower f-number aperture at the wide end, a f-number is a ratio: actual size of aperture in mm divided into focal length. Sure, constant aperture zooms perform optical trickery to get around that, but there's no getting around the fact that since the physical aperture of the lens stays constant, a zoom is always going to "want" to have a lower maximum number at short focal lengths. It's simple math, and is completely unrelated to what might happen when comparing different primes. If Pentax happens to make the 40 a lower maximum f-number than their 21, that their business - they can make whatever pertures they want. It's only zooms where simple math requires lower f-numbers at the wide end without the optical trickery necessary to achieve constant aperture. BTW, I'm not using "optical trickery" in a negative sense here - just observing that the natural order of things is for the the wide end to have a lower max f-number, because the physical aperture is constant, and it takes effort to defat this (just as it takes "aerodynamic trickery" to make a heavier-than-air object like an airplane fly).

As far as the primes go, I'd say the general truism is that 50mm lenses tend to be the fastest for a given size or price point, and then maximum aperture tend to get smaller (f-numbers higher) as you go to larger of smaller focal lengths. If we imagined the DA* 55/1.4 was available in a smaller "DA Limited" form that was, say, f/2, then we could see this progression happening with the DA series too.

01-29-2010, 06:56 PM   #5
Pentaxian
Clarkey's Avatar

Join Date: Jan 2008
Location: Brampton, ON, Canada
Photos: Gallery
Posts: 1,766
QuoteOriginally posted by Marc Sabatella Quote
....Snip...
It's only zooms where simple math requires lower f-numbers at the wide end without the optical trickery necessary to achieve constant aperture. BTW, I'm not using "optical trickery" in a negative sense here - just observing that the natural order of things is for the the wide end to have a lower max f-number, because the physical aperture is constant, and it takes effort to defat this (just as it takes "aerodynamic trickery" to make a heavier-than-air object like an airplane fly).
I'm curious though as to why the wide end of a constant zoom doesn't have a larger aperture value. Is it all about the positioning of the aperture in the lens?
01-29-2010, 07:28 PM   #6
Veteran Member
Pentaxor's Avatar

Join Date: May 2009
Location: Vancouver, B.C.
Photos: Gallery
Posts: 6,513
QuoteOriginally posted by Clarkey Quote
I'm curious though as to why the wide end of a constant zoom doesn't have a larger aperture value. Is it all about the positioning of the aperture in the lens?
you would need a bigger glass, which makes it more expensive and a burden to carry.
01-29-2010, 08:24 PM   #7
Veteran Member




Join Date: Jun 2008
Location: Borås, Sweden
Photos: Gallery
Posts: 3,165
QuoteOriginally posted by Marc Sabatella Quote
A zoom is *always* a lower f-number aperture at the wide end, a f-number is a ratio: actual size of aperture in mm divided into focal length.
The f-number is the ratio of the focal length divided by size of the entrance pupil.

Here: Entrance pupil - Wikipedia, the free encyclopedia

Basically, it's the image of the aperture stop as seen through the front of the lens. Not the actual aperture.
01-29-2010, 09:01 PM   #8
Veteran Member
mattdm's Avatar

Join Date: Feb 2007
Location: Boston, MA
Photos: Gallery
Posts: 2,964
QuoteOriginally posted by Pentaxor Quote
you would need a bigger glass, which makes it more expensive and a burden to carry.
Yeah, but if you've got, say, 135mm / 2.8, you get a 48mm actual physical aperture. (Or would, if the lens was simple instead of many elements.) That implies that if the same aperture were open all the way at 50mm, you'd have an f/1.03 lens.

So there's two possibilities: one (which goes to what you're saying), if the lens were really f/1, the light falloff and distortion at the corner would be unreasonably ugly, so the aperture is physically limited opening less at 50mm to keep the image quality decent. Or two, the design is such that the aperture really is only able to open physically to what works out to be 17.9mm when zoomed out.

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!
aperture, k-mount, lens, pancakes, pentax lens, slr lens
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Getting rid of the 16-50 to go to pancakes. Thoughts? herzzreh Pentax SLR Lens Discussion 33 01-05-2010 11:52 AM
Two pancakes; hold the syrup krt1 Pentax SLR Lens Discussion 16 01-15-2009 07:46 PM
Politically correct pancakes! ftpaddict Post Your Photos! 9 06-16-2008 08:49 AM
How to carry DSLR and 3 pancakes? Ben Rubinstein Pentax Camera and Field Accessories 10 04-11-2007 10:18 AM



All times are GMT -7. The time now is 08:44 AM. | 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