Focal Distance - Min./Max.

Discussion in 'Macro' started by Erik-Christensen, Feb 22, 2021.

  1. Unfortunately, I have to limit my extensive travel, and I am therefore considering getting into macro photography.

    I have recently seen some impressive insect images, where a Nikon 105mm f/2.8 and a reversed 50mm f/1.4 in front. How do I calculate the minimum and maximum focal distance? Or if there is a table somewhere, then I prefer a link.
  2. Focusing distance for this combination will be nearly identical (around the reversed lenses flange distance - that's 46mm from the flange) however the focusing rings are positioned. with both at infinity magnification on the sensor will be just over 2x
    Adjusting the non reversed lens will bring focus slightly closer but this is usually only by a millimeter or two
    With unit focusing lenses the reversed lenses focusing ring will not change the optics at all merely moving the mount out to block more light.
    Fuller details of the technique can be found at Coupling lenses for extreme macro

    In use as with most macro work focusing is done by changing the subject-camera distance.

    FWIW The actual focal length of the combination should be around 35mm (calculated using the lenses diopters but this ignores the reversal which might change things) I don't know ant reason why it would be relevant.
  3. Check out the "PhotoPills" app for smart phones and tablets. It has a depth of field table for input focal length, aperture, and focus distance that gives near and far limits of depth of field. Also tables for field of view and hyperfocal distances, plus many other features for planning photographs.
    Last edited: Feb 22, 2021
  4. Thank you very much. It seems no reason to try and calculated the distance, easier to change the distance between subject and camera/front lens or sensor? I may even have a focus-rail somewhere in a box.

    I do have the PhotoPills, which I use for planning landscape/sunrise/sunset photography, and also basic DoF, but I cannot find what I am after in the version I have.nothing with 2 lenses combined.
  5. If anyone not using automatic exposure, here is a useful scale from the wonderful Life Library of Photography series (13+ volumes)

  6. On the other hand, there's a lot to be said for simple close-up lenses/'filters'. For some subjects, good ones can be 'good enough'
  7. double lens formulas.jpg

    For your case of using a 105mm lens combined with a 50mm (reversed) the combined focal length will be approximately 34mm. This is only (very) approximate because the formula is from thin lens theory, which gives,
    1/fT = 1/f1 + 1/f2
    Where fT = combined focal length
    f1 = 105mm
    f2 = 50mm

    The distance from a position between the lenses to the subject being photographed (s2') as a function of combined focal length and distance from the sensor or film plane (s1) is (see figure),
    s2' = fT*s1/(s1-fT)
    The magnification is, M = s1/s2'
    For your case M approx. = 105mm/50mm = 2.1

    This assumes that the distance (d in the figure) between the lenses is zero, and that the 105mm lens is focused (without the addition of the 50mm lens) at infinity.

    Again, all this is dependent on thin lens theory, which for combining two camera lenses is only a very rough approximation. A more accurate thick lens calculation would require knowing things like front and rear nodal point positions for both lenses, the spacing between the lenses, etc. If the 105mm lens uses internal focusing, that would further complicate the calculation.

    The best way to determine the distance from the combined lens to subject is to try it. You can get a rough idea of what you are dealing with by just holding up the reversed 50mm lens in front of the mounted 105mm lens and moving back and forth until a close subject is in focus. Once you have the reversed lens mounted, you can measure the distances and make a table.
    Last edited: Feb 22, 2021
  8. Focusing rails are ideal, for precision, but simply handholding & rocking backards & forwards works as well - if there's enough light which usually means flash.
    My estimate of distance was from the subject to the reversed flange. Basically the reversed lens takes the rapidly diverging rays from the subject & bends them till they are parallel (something it does with the subject at the point it's sensor would normally be) Then the second lens behaves as if these parallel rays are from a distant subject.
    When focusing if you're moving the camera & both lenses together it doesn't much matter which part of the system you are measuring to the change in measurement will be the same. I only gave an indication as many photographers don't realise just how close you need to be to the subject 46mm is less than 2inches.

    As I'm sure Johan mentioned on the page I linked to this technique can be very simple & give great results if you pick a good combination of lenses. Other combinations can give rise to vignetting and it's difficult to predict how a pair of lenses will work together.
    I've played with the technique on & off for many years (despite Flickrs quoted EXIF - I know it wasn't a 600mm/32 macro) I think this shot was taken using staked lenses nearly ten years ago:
    [​IMG]Watch Macro by Mike Kanssen, on Flickr
    This older (october 2010) one used a 50mm prime reversed on the kit lens and shows a little vignetting
    [​IMG]Detail of 50p coin by Mike Kanssen, on Flickr

    Both where handheld using off camera TTL flash. With a focusing rail you ought to be able to do considerable better!

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