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I don't quite understand why most lenses seem to have approximately the same diameter aperture regardless of focal length.

F-stop is, by definition, a ratio of focal length to diameter, so a 20mm lens with a 40mm aperture should be f/0.5 and a 400mm lens with a 40mm aperture should be f/10.

However, what we actually see in lenses is roughly the same distribution of f-stops for both wide angles and teles, yet the actual diameters aren't largely different. So is my understanding of f-stop flawed?

Note: The question has been substantially reworded as the point I was trying to get at is not covered in the answers to the 'duplicated' question. I hope my reworded question will be reconsidered. Thanks.

Octopus
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  • Related: Why do wide angle prime lenses have relatively small apertures? – coneslayer Mar 26 '13 at 20:40
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    I voted to close it as a duplicate (which will keep it around and point to the other question). Other users with enough reputation can also vote to close, or I think a moderator could do it directly. That's probably better than deleting it. – coneslayer Mar 26 '13 at 20:54
  • Thanks. I reworded the entire structure. I see someone actually modified the wording of the Question itself as well which I was concerned about. – Octopus Mar 28 '13 at 19:58
  • That was me, when I voted to reopen it I also changed the title since that really is what you are asking. If you aren't, please clarify it. – Paul Cezanne Mar 28 '13 at 20:28
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    Your understanding is basically correct. I'll add some more information that may help explain it once this is properly reopened. In the meantime, look at http://photo.stackexchange.com/questions/7365/how-do-constant-aperture-lenses-work, about constant-aperture zoom lenses (and why they don't just have super-fast apertures at the wide end) — I think that clears up a lot of the puzzle. – mattdm Mar 28 '13 at 20:39
  • By orifice, do you mean the aperture, or front element? There's no actual orifices in a lens in the literal sense and I'm not sure exactly what you mean by it metaphorically. – mattdm Apr 08 '13 at 17:57
  • @mattdm, well okay, I clarified by editting the question and I inserted 'aperture' wherever I said 'orifice' or 'opening', but that's a good point. I thought f/stop was a ratio of aperture to focal length. does the size of the front element also factor in somehow? – Octopus Apr 08 '13 at 18:39
  • No, it doesn't, except that the front element has to be large enough to handle the widest aperture of the lens. – mattdm Apr 08 '13 at 20:44
  • I think we're back to needing a clarifying question: how are you measuring or observing the diameters of these various lenses? Do you have any real examples in mind? (Clearly not a 20mm f/0.5.) Because the basic fact is that your simple understanding is right, so we can work backwards from there to find out where the confusion is. Maybe it's about the size of the physical aperture (if you dissect the lens) vs. the virtual (effective) aperture (which depends on exactly how the aperture relates to the other lens elements)? – mattdm Apr 08 '13 at 20:49
  • @mattdm, f/0.5 seems unreasonable, yes. but 40mm diameter aperture doesn't. does it? (considering the fact that the threads for filters on my lenses are typically 58mm or 77mm and also by direct visual estimation) – Octopus Apr 08 '13 at 21:49
  • @mattdm, or consider my 17-40mm zoom which at the 17mm end is supposed to go as large as f/4, but that only equates to 4.25mm. surely my aperture is much larger than that. I can see that it isn't so tiny. – Octopus Apr 08 '13 at 21:59
  • Aha! I think I've got it. :) It isn't the physical size that's important; it's the effective size given the light path. Look through the front of the lens and it will look appropriately tiny. I just tested this with my 15mm lens – mattdm Apr 09 '13 at 11:43
  • I'm going to ask a followup question on this, because I don't see it explored adequately elsewhere.... – mattdm Apr 09 '13 at 11:43
  • http://photo.stackexchange.com/questions/37796/whats-the-difference-between-real-and-effective-aperture -- does that clear it up? – mattdm Apr 09 '13 at 11:48
  • @mattdm, finally getting at the meat of what it is i'm trying to understand – Octopus Apr 09 '13 at 16:13
  • @Octopus So, does that make sense? – mattdm Apr 09 '13 at 16:39
  • @mattdm, well youve asked the right question. i just hope there will be more answers to better clarify. – Octopus Apr 10 '13 at 03:21

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Your reasoning is simply but incorrect since it is only based on the size of elements. What is easier to make is a bright normal lens.

Any lens wider than the flange distance of the camera requires more complex designs and the wide aperture has to be considered throughout all the elements.

Itai
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    Re your statement about flange distance versus focal length: It is true for SLR cameras, because you need to move the mirror out of the way before taking the photo and thus the mirror-box needs to be free of obstructions. For mirrorless systems it is not necessarily so; I have an old Russian, Zeiss-designed 35mm/2.8 for Leica that pokes so far into the camera body that it almost touches the film plane. (That said, on digital cameras retrofocal wide-angles generally seem to be the way to go, just as on SLR cameras, to avoid corner pixel shading and other problems.) – Staale S Mar 27 '13 at 12:20
  • @Itai, the entire wording of this question has changed dramatically (with guidance from admins) to more accurately address the gist of my misunderstandings. Just thought you should know, in case you want to alter your answer. – Octopus Mar 28 '13 at 20:03