Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
12-08-2014, 03:36 PM   #16
Site Supporter
dakight's Avatar

Join Date: Sep 2014
Location: Phoenix, AZ
Photos: Albums
Posts: 1,178
Since the f/stop is a ratio between the entrance pupil diameter (what the OP is calling "aperture opening") and the focal length what the designers have done is vary the entrance pupil diameter with the focal length so that the ratio, or F/stop remains constant. The common usage is to refer to the f/stop as the aperture but that is technically incorrect and leads to confusion. In a "constant aperture" zoom lens, the aperture is not in fact constant but rather varies with the focal length to maintain a constant f/stop number just as soycory has observed.

12-08-2014, 03:42 PM   #17
Pentaxian
emalvick's Avatar

Join Date: Feb 2008
Location: Davis, CA
Photos: Gallery
Posts: 1,475
QuoteOriginally posted by Stone G. Quote
Unfortunately, wide angle lens designs are much more complicated than that. In particular when we talk about zooms: As you reduce the focal length you need a lot of elements and lens groups to "project" the image over a constant flange/registration distance of some 45mm. Just look at fast, ultrawide primes: They are enormous, bulky and heavy beasts. Now, try to build such design into a reasonably compact 17-50mm zoom with constant focal ratio...........

The entrance pupil is very rarely the same as the diameter of the lens front element of a prime and it never is neither for any wide angle prime nor for any zoom.
Your comment just further supports that having the entrance pupil stay at a fixed diameter would be even more detrimental to the image quality (which I am aware of). Early, when I was learning about photography and optics, my own thoughts about it were more in reference to other zoom lens where the aperture is variable. In many of those lenses, the range in apertures at the extreme zoom levels are tied directly to the largest diameter possible for the entrance pupil being fixed and the variable aperture (f-stop) resulting from the focal length range and that fixed opening.

As for maximum opening sizes, I've never owned a lens where the entrance pupil is large enough to be the same diameter as the largest elements. Heck with the 17-50 lens I own and an f 2.8, The 18 mm opening is not nearly the same as the nearly 60 mm diameter at the front of the lens. I'd probably have to have rather large and fast telephoto lens to see a case where the pupil diameter would rival the maximum lens diameter, but that is digressing from the topic of aperture anyway.

I think the OP was really just trying to understand exactly what constant aperture (f-stop) means and why it's called that. I think it's a reasonable question. I know aperture was one of the tougher items for me to understand, even more than what focal length meant, minimum focus distance, etc. I think part of that difficulty is the fact that we often talk about it in terms of opening size, yet once a zoom comes into play, there is a bit of ambiguity between the physical aperture size (i.e. diameter) and the constant f-stop, which essentially normalizes the physical aperture diameter for the sake of consistent exposures.
12-08-2014, 06:31 PM   #18
Forum Member




Join Date: Sep 2014
Posts: 69
Aperture means opening, but the opening (abstract attribute rather than physical measurement) of something is usually understood to be relative, in this case to focal length. It doesn't generally mean diameter of the pupil, as has been used here.
12-09-2014, 11:38 AM   #19
Pentaxian
emalvick's Avatar

Join Date: Feb 2008
Location: Davis, CA
Photos: Gallery
Posts: 1,475
QuoteOriginally posted by Antonio Marques Quote
Aperture means opening, but the opening (abstract attribute rather than physical measurement) of something is usually understood to be relative, in this case to focal length. It doesn't generally mean diameter of the pupil, as has been used here.
The key is that understanding is typically understood to be relative in photography (and perhaps with items like telescopes, etc if it applies)... However, when one is newer to photography or at least that concept, many books etc start with the actual opening size. Heck coming from an engineering background, opening size is often used for whatever the term aperture is referring to. Of course, it's understood that f-stops are the mechanism for presenting the relative aperture in photography, but many books don't get into the detail of the why it's there. I'm sure there are some good basic photography books that do actually, but it's surprising how many mainstream books keep certain concepts pretty basic. Aside, a similar problem exists in trying to understand what ISO is and how it works in either film or digital photography.

In reality, it is quite useful to know why things are the way they are and not just how to use them. Thus, this thread is great for discussing aperture as it relates to the OP's question. From a mechanical standpoint and at the most basic level, it's a reasonable question to ask. The answer is quite useful, and even I've learned a bit more or at least have learned considerations that didn't necessarily occur to me.

Photography is actually quite an educational hobby, not just from the point of view of art, but even with regard to physics (probably no-duh). I've known people that when I explain that a lot of items related to photography are basic physics end up surprised how easy basic physics can be. Aperture and Depth of Field are really spectacular for how things work.

12-09-2014, 04:56 PM   #20
Site Supporter
stevebrot's Avatar

Join Date: Mar 2007
Location: Vancouver (USA)
Photos: Gallery | Albums
Posts: 27,428
QuoteOriginally posted by Stone G. Quote
The entrance pupil is very rarely the same as the diameter of the lens front element of a prime and it never is neither for any wide angle prime nor for any zoom.
Thank you for fielding questions here. I have been doing a poor job attempting the same on another thread posted today. The technical terms end up being ambiguous when used in simple descriptions and sometimes I think that the only people who really understand them are astronomers and macrophotographers.


Steve

---------- Post added 12-09-14 at 04:13 PM ----------

QuoteOriginally posted by emalvick Quote
As for maximum opening sizes, I've never owned a lens where the entrance pupil is large enough to be the same diameter as the largest elements.
Yes, for a small format SLR camera, that would be exceptional. I believe my 50mm f/2.0 rangefinder lenses are the only ones in my collection where the calculated entrance pupil (25mm) is the same as the measured front element diameter or the iris maximum diameter. I think my 150mm f/5.6 view camera lens may also come close. Edit: Yes, my view camera lens front element also measures the same as the calculated entrance pupil.


Steve

Last edited by stevebrot; 12-09-2014 at 05:21 PM.
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, camera, constant, diameter, element, entrance, f4, front, lens, pentax help, photography, steve, troubleshooting, variable
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Variable aperture continous exposure mikeodial Pentax K-3 20 10-19-2014 05:10 PM
Variable aperture zooms DOF and Light. D1N0 Pentax SLR Lens Discussion 4 09-16-2014 08:40 PM
why constant aperture zooms? wildman Pentax SLR Lens Discussion 2 03-03-2014 08:33 AM
Constant Aperture vs Variable Aperture Netsoft Pentax SLR Lens Discussion 13 01-11-2010 07:29 AM
Variable vs. fixed aperture zooms: am I missing something here? Finn Pentax SLR Lens Discussion 11 05-15-2007 05:05 AM



All times are GMT -7. The time now is 12:43 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