Aperture theory question

Discussion in 'Beyond the Basics' started by bigfatbadger, Jul 27, 2006.

  1. bigfatbadger

    bigfatbadger TPF Noob!

    Joined:
    Mar 19, 2006
    Messages:
    464
    Likes Received:
    0
    Location:
    Nottingham, UK
    Ok, so thanks to Michael Langfords Basic Photography, I am getting more of an idea about all the theory of photography, which is a good thing. It's also makig me realise how little I actually know, which must also be a good thing.

    Therefore, I'm putting a question to the collective:

    I understand that F number= focal length / effective aperture. Hence f2 on a 50mm lens has an effective aperture of 25mm. This much I get (I hope that's right).

    This explains why those 400mm f2 (or whatever) lenses have such massive fron elements, because the effective aperture size must be 200mm, whereas the effective aperture on a 50mm at f2 only needs to be 25mm.

    So here's my question: Why do lenses like the 17-55 f2.8 have such massive front elements? Surely if the effective aperture is smaller then the front element doesn't need to be as large?

    I know I don't need to know this, but I want to! Is it going to hurt my brain?

    TIA!
    Jon
     
  2. Tiberius

    Tiberius TPF Noob!

    Joined:
    Jun 19, 2006
    Messages:
    328
    Likes Received:
    0
    Location:
    Michigan, USA
    There's no reason the aperture can't be larger than required. Plus it allows Nikon to stick to one standard filter size (77mm) for their Pro lenses.
     
  3. Unimaxium

    Unimaxium TPF Noob!

    Joined:
    Nov 13, 2004
    Messages:
    2,689
    Likes Received:
    22
    Location:
    Northeastern University, originally from Philly
    Can others edit my Photos:
    Photos OK to edit
    I don't think the size of the front element is necessarily related directly to the aperture size. When dealing with large-apertured telephotos, you're right that they do need a big front element to collect all the light, but I would imagine that lenses that go wide like yours (or like my 17-35 f2.8-4) have large glass in the front in order to be able to see the full angle that you get at 17mm. I'm not sure of this, and I could be very wrong, but that's my guess anyway.
     
  4. Torus34

    Torus34 No longer a newbie, moving up!

    Joined:
    Jan 26, 2006
    Messages:
    2,117
    Likes Received:
    37
    Location:
    Tottenville, Staten Island, NYC USA
    Can others edit my Photos:
    Photos OK to edit
    For short fl lenses the problem is to avoid vignetting. Thus the large front element.

    There, now. That didn't hurt, did it? [concerned expression . . .]
     
  5. bigfatbadger

    bigfatbadger TPF Noob!

    Joined:
    Mar 19, 2006
    Messages:
    464
    Likes Received:
    0
    Location:
    Nottingham, UK
    Aha! Thank you!

    @unimaximum - I wish I had one, I was just thinking about it!
     
  6. journeyman

    journeyman TPF Noob!

    Joined:
    Jun 14, 2006
    Messages:
    51
    Likes Received:
    0
    Location:
    Pittsburgh, Pa
    You can have vignetting in 35mm cameras?
     
  7. Unimaxium

    Unimaxium TPF Noob!

    Joined:
    Nov 13, 2004
    Messages:
    2,689
    Likes Received:
    22
    Location:
    Northeastern University, originally from Philly
    Can others edit my Photos:
    Photos OK to edit
    Of course you can.
     
  8. Torus34

    Torus34 No longer a newbie, moving up!

    Joined:
    Jan 26, 2006
    Messages:
    2,117
    Likes Received:
    37
    Location:
    Tottenville, Staten Island, NYC USA
    Can others edit my Photos:
    Photos OK to edit
    Journeyman;

    Just take a glim at some of the kooky lensshades for the extreme 35mm WA's and then reconsider your question!
     
  9. Luke

    Luke TPF Noob!

    Joined:
    May 9, 2006
    Messages:
    159
    Likes Received:
    0
    i don't know how all of it works but basic theory you said is right.
    thats why zoom lenses have f 32
     
  10. ksmattfish

    ksmattfish Now 100% DC - not as cool as I once was, but still

    Joined:
    Aug 25, 2003
    Messages:
    7,021
    Likes Received:
    34
    Location:
    Lawrence, KS
    Can others edit my Photos:
    Photos NOT OK to edit
    Besides vignetting, they may make the lens larger, with more coverage, so that they don't have to use the edges of the lens. Thereby using more of the "sweet spot", and not having to deal with edge/corner abberations.

    Final appearance of the lens will be influenced by many factors, of which maximum aperture is just one. Making a zoom lens that is decent over all focal lengths and apertures is extremely complicated.

    I have the Sigma 28mm f/1.8. It's huge. Bigger than my zooms. 77mm filter size, but it should only need an aperture of a little over 14mm right? I had no idea when I ordered it. It's more than twice the size of the Canon 50mm f/1.8. Why? The only thing I can tell is that it can focus very closely (about 4 cm).
     

Share This Page

Search tags for this page
aparture theory
,

aperture theory

,
aperture theory photography
,
apeture theory
,
lens+diaphragm theory
,
oes aperature theory
,
perture theory
,
theory of aperture
,
theory of camera aperture
,
theory of double aperture