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Post deleted by robertj

Discussion in 'Help Team' started by robertj, Sep 22, 2003.

  1. robertj

    robertj Well-Known Member

     
  2. TimF

    TimF With as stony a stare as ever Lord Reith could hav

    Re: Focal length/One for Huw methinks! (nt)
     
  3. huwevans

    huwevans Not Really Here

    Re: Focal length

    according to thin-lens theory, one half of the distance between the film and the optical center of the lens, when the lens is focused on infinity

    Sorry Robert, but I don't know where you get that factor of a half from. Strictly the focal length is the distance from the rear principal point (which some of us sometimes loosely call the 'optical centre' - a notion that makes sense in thin lens theory) to the plane of focus, for an object plane at infinity.

    You might be getting confused with the situation where a lens is focussed at a distance giving 1:1 reproduction, or 1x magnification. In this case for a thin lens the focal length is half the distance between the rear principal point and the plane of focus.

    Huw Evans.

    www.huwevans.freeuk.com
     
  4. robertj

    robertj Well-Known Member

     
  5. huwevans

    huwevans Not Really Here

    Re: Focal length

    Well strictly speaking thin lenses do not exist in the real world. The concept is used for rough calculations and analysis only. Even in a theoretical thin lens the two principal points do not in general coincide, which means that the rear principal point is not normally going to be in any exact sense the 'centre' of the lens. It might be close to the physical centre, but it equally might not - it doesn't even have to lie within the physical boundaries of the lens. But for anything actually approaching the notion of a thin lens in the real world - which would necessarily be quite a weak lens - the rear principal point will inevitably be quite close to the physical centre.

    The half distance is coming from the fact that an image (in the film plane) is created approximately a focal length (as defined in the Gauss theory of lenses) of a lens behind a focal plane (as defined in the Gauss theory of lenses)

    Now, you're really confusing me by the way you are using some of the terminology. The 'plane of focus' term refers to the 'plane' (approximately) where the image comes to a sharp focus. By definition this is also where you want the film to be, so it is also (when the camera is correctly focussed) the film plane. If Gauss defined the plane of focus differently from this then I'm not aware of it, but then I can't say I've studied Gaussian theory in detail. Maybe there's a problem of confused terminology when the work is translated? The only way I can make sense of what you are describing is to understand what you refer to as the 'plane of focus' as what I know as the 'rear principal plane'. Either way an axial object at infinity will be in sharp focus at a distance of one focal length behind the rear principal point - not two focal lengths. That, as far as I understand it, is the universally agreed definition of 'focal length'.

    If you want a treatment of optics specifically in relation to photographic lenses, then I would recommend Rudolf Kingslake's works - particularly 'Lens Design Fundamental' (Academic Press, New York, 1978), which is probably the definitive text on the subject.

    Huw Evans.

    www.huwevans.freeuk.com
     
  6. robertj

    robertj Well-Known Member

     
  7. huwevans

    huwevans Not Really Here

    Re: Focal length

    Robert - briefly, because I'm busy -

    You said "... the rear principal point will inevitably be quite close to the physical (lens) center." Okay, that is what I expected.

    That is only for something that approximates to a thin lens - and whilst photographic lenses can be thought of in that way for many purposes, they are actually very, very far from being thin lenses. Nevertheless, let's continue.

    a plane containing a lens focus (a point to which all light is converged, again approximately).

    There is no such point - certainly all paraxial rays come to a focus at a point on the axis which is one focal length behind the rear principal point, but non-paraxial rays do not pass through that point.

    So if I understood you correctly, in simpler terms: "focal length is the distance from the lens to the film, when a distant object is in focus",

    Then, a distance between the lens and the film for 28mm lens when we have a distant object in focus is 28mm, and not 56mm as I thought.


    Okay so far.

    Please have a look at http://... etc.

    Can't I'm afraid - I don't have Word. But I think I know the document you mean anyway. It's basic thin lens depth of field theory.

    So, to say that the focal length is 'd' when 's = Infinity' is the same as to say that it is 'f'. As a person with some background and interest in physics it looked to me more natural to define focal length of a lens (in general not camera lens) to be 'f', i.e. a distance from lens to the plane of lens focus.

    Focal length is indeed defined as the distance from the rear principal point to the plane of focus. The thin lens equation just tells you something about the relationship between f, u, and v - to use the more usual terms.

    all of the light rays (coming from a distant object and therefore parallel to lens axis) are confined almost to a point

    That's true (allowing for aberrations) - no problem there. However not all infinitely distant objects are on the lens axis - a point source off the axis will form a focussed image which is also off the axis - but also at the same perpendicular distance behind the lens (again, allowing for aberrations, distortion, field curvature, etc). Think of the lens as being replaced by a pinhole, and all rays travelling through the pinhole in a straight line (let's neglect diffraction here). Only the point source that is right in the middle of the field will form an image which is on the lens axis. Everything else is focussed to points on the plane of focus that are directly 'opposite' the source. That's how you get an entire image field rather than just a point. I think that's where you are going wrong.

    Huw Evans.

    www.huwevans.freeuk.com
     
  8. robertj

    robertj Well-Known Member

     
  9. huwevans

    huwevans Not Really Here

    Re: Focal length - and beyond

    Robert - a word of advice: if you want someone's help you should make your questions very, very much more brief.

    I haven't read this lengthy post, but if it's more elementary optics I would just refer you again to any basic text on the subject, and if is photographic optics in particular then Rudolf Kingslake is your man - several works, but the one I mentioned earlier is probably the bast one which is deep enough to cover the theory you want.

    Huw Evans.

    www.huwevans.freeuk.com
     
  10. TimF

    TimF With as stony a stare as ever Lord Reith could hav

    Re: Focal length - and beyond

    Robert,
    Given that Huw has vastly more knowledge of optics and optical theory than anybody else on this site I'd take his advice if I were you. /img/wwwthreads/smile.gif

    Tim BSRIPN
    [​IMG]
     
  11. robertj

    robertj Well-Known Member

     
  12. robbybobby

    robbybobby Well-Known Member

    Re: Focal length - and beyond

    Don't blame you, Huw! I was going to make a comment about how much content there is in the posts in this thread . . . oops, I've gone and done it. Never mind.;-)

    Rob NRIPN

    If something's not worth doing, it's worth forgetting about.
     
  13. Larry Shone

    Larry Shone Well-Known Member

    Re: Focal length/28 is 28

    the focal length of a 28mm lens is 28mm. The physical centre of the lens may be at 56mm or more(it certainly won't be 28mm with an SLR due to the mirror box) but the optical centre is marked at 28mm!
    Look at a fisheye lens with a FL of 6mm, no way you could get the physical centre of that lens 6mm from the film and still focus with an SLR, but its designed so that it has an optical centre that is 6mm from the film plane which is down to somew very clever lens design(which is why ultra wides are rather expensive!)

    I don't do people!
     
  14. TimF

    TimF With as stony a stare as ever Lord Reith could hav

    Re: Focal length/28 is 28

    Quite so Larry. Almost all wides these days, even in rangefinder cameras, are retrofocus designs (basically introducing extra glass behind the image forming elements so as to lengthen the distance from the optical centre to the point of focus). This is as you rightly say to cope with the "flipping mirror"in SLRs (as the Japanese used to say!), and in M Leicas to allow the meter to function correctly. In times past many 20/21mm lenses for SLRs were so-called mirror-up designs as the rear element was right up close to the film plane.

    Tim BSRIPN
    [​IMG]
     
  15. huwevans

    huwevans Not Really Here

    Re: Focal length/28 is 28

    Almost all wides these days, even in rangefinder cameras, are retrofocus designs

    Err, the Contax wides are Biogons, which are not retrofocus, and I thought the Leitz ones were basically Super Angulon clones - also not retrofocus. And most MF rangefinders with wide lenses likewise use Super Angulons (either actual or clones). Which rangefinder wides do you think are retrofocus?

    Huw Evans.

    www.huwevans.freeuk.com
     
  16. Benchista

    Benchista Which Tyler

    Re: Focal length/28 is 28

    Leica wideangles are generally mild retrofocus designs - not as severe as for SLRs, but still retrofocus. Ditto the C/V lenses, I believe.

    Nick BSRIPN
    All I got is a photograph - and it's not enough.
     
  17. huwevans

    huwevans Not Really Here

    Re: Focal length/28 is 28

    So not Super Angulon clones then? Oh well, I guess I was misinformed.

    Huw Evans.

    www.huwevans.freeuk.com
     
  18. Benchista

    Benchista Which Tyler

    Re: Focal length/28 is 28

    Some of the lenses actually WERE SAs at various times.

    Nick BSRIPN
    All I got is a photograph - and it's not enough.
     
  19. TimF

    TimF With as stony a stare as ever Lord Reith could hav

    Re: Focal length/28 is 28

    Indeed they were. I have an old 21mm/3.4 Super Angulon for the M series. The rear element when mounted is only a few mm away from the shutter curtain. As such it interferes with the metering diode in the M6/7/P cameras, and can not be used (except for some slightly altered late examples) in the M5 and CL bodies as the deep rear element can damage the swinging arm with the metering cell on in those cameras.

    There is a way to make it possible to use the meter with this lens, which involves cutting away part of the rear end barrel (!). Jonathan Eastland has one like this, but when I had a chat with him about it he implied that even so altered metering was difficult, and that the deviations from correct exposure were not consistent and therefore predictable beforehand .

    In any case, as the lens can't be fully used on any of my M's (except the M2, which is my reserve body really), I got myself a s/h Voigtlander 21/4 Color-Skopar.

    As an aside, the only other SA lens in the current Leica line up is the 28mm f/2.8 PC Super Angulon-R, which is a rebadged Schneider effort that I believe is available in several different mounts.

    Tim BSRIPN
    [​IMG]
     
  20. robertj

    robertj Well-Known Member

     

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