Why do macro lenses extend when getting closer?

Discussion in 'Macro' started by mark_stephan|2, Dec 12, 2017.

  1. Picked up a used Tamron SP 90 f2.8 Di recently and I've been getting used to it. Why does the lens extend when getting closer to subjects?
     
  2. Vincent Peri

    Vincent Peri Metairie, LA

    I assume it doesn't have internal focusing. I don't shoot autofocus lenses, so I don't know if they usually have internal focusing or not. My manual focusing lenses don't have internal focusing and extend except my Nikkor 200 macro lens, which has IF.

    See: Internal focusing - Wikipedia
     
  3. 1/u+1/v=1/f
     
  4. Traditionally, all lenses extended as they focused closer. At infinity, the optical center of the lens is the same distance from the focal plane(film or sensor) as the focal length of the lens. Focusing closer involved moving the entirety of the optics("unit focusing") further away from the focal plane. This is most noticeable at macro distances. For example, a 35mm format 50/55mm manual focus macro(regardless of make) traditionally focuses to a reproduction ratio of 1:2. This means that the optics must move 25 or 27.5mm, or a little more than one inch, away from their infinity position to focus. Double this distance on a 100/105mm lens that only goes to 1:2. These lenses would also typically include a 25 or 27.5mm extension tube that brought the lens that distance from the focal plane and would allow focusing in the 1:2 to 1:1 range.

    Beyond 1:1, it use to be common to use tools like bellows to bring the lens even further out, although longer extension tubes also have their place(and are a lot easier to use). You can even do things like use an enlarger lens, which typically won't have any focusing provisions built on, on a set of bellows.

    Modern lenses employ a lot of optical tricks to enable close focusing, but many still extend the lens barrel to some extent or another. My AF Micro-Nikkor 105mm 2.8D, for example, focuses to 1:1(lifesize) but doesn't quite have to extend a full 105mm to do so. My newer AF-S Micro Nikkor 105mm 2.8 does not change length at all up to 1:1, although it can drive you crazy as one of the "tricks" used is changing focal lengths at close distances so the framing can change as the focus changes.
     
  5. Really? - I thought in the beginning they were brass housed optical elements and all the focusing was done via the cameras' bellows extension? - Better cameras used to offer "double -" or even "triple extension" i.e. bellows and beds that permitted focusing to 1:1 or beyond with a standard lens.
    My Leica M macro works only extension based, my Pentax AF 50 & 100 mm at least partially too.
    Hard to tell which concept might be better. If a zoom is built to not extend for close focusing it tends to eat focal length of the long end for that purpose; i.e. you bought a x-200mm that behaves like a 135mm at minimum focusing distance. I assume if you wanted to kiss your subject with your front element, you wouldn't have picked a 90 mm macro? - OTOH you do loose lens speed when you get closer with an extending one due to the bellows factor.
     
  6. I guess I didn't really articulate what I was thinking very well-I was at least partially picturing a view camera in my head when I was describing the above. I have a B&J field camera that came with a 7.5" lens, and I have a project on the back burner to make a lens board for it that will allow me to fit a Pacemaker Graphic board to it(I've put all my other LF lenses on Graphic boards). In any case, the camera bellows will extend about 15", or enough to go to 1:1 with the lens that came on it.

    Still, the principle is the same whether we're talking about a view camera, an old folder, a Rolleiflex, a Mamiya RB67, or any unit focusing lens with a helical. Move the lens further away from the film/sensor and you focus on closer objects.
     
  7. My honest envy for the B&J! - Yes, I had to look them up, but still: I only have a 1910 era wooden 13x18 cm tourist camera, without any rear or tilt movements, crudely converted to take tin plate holders, in that range.
     
  8. Normally to focus closer distance the lens is moved away from the film/sensor plane. Lens with internal focusing can focus without moving but generally shorten its focal length as it focuses closer.
     
  9. Good info - Thanks
     
  10. steve_g|2

    steve_g|2 Posting to strangers is just a hobby of mine.

    Funny thing is, unlike super zooms which give a specified range of F stop vs. focal length, ,the advertised F stop of the macro lens stays constant.
    Which is technically OK but don't count on it for calculating exposure when focusing close. Extending that front element group really knocks down the light available back at the focal plane.
     
    Last edited: Jan 4, 2018
  11. When calculating exposure, I work in equivalent apertures.

    In LF and even in MF to some extent, we frequently have to calculate the "bellows factor" even for things that would not traditionally be considered macro subjects. A tight headshot is around 1:5 in 4x5, which I think works out to about 1/3 to 1/2 stop of compensation. Move up to 8x10 and you're not too far off 1:1-or a full 2 stops of compensation. Folks who do this sort of stuff routinely will often put a paper scale on the bed of their camera as a "cheat sheet" for the bellows factor.

    In MF, Mamiya was thoughtful enough to put a scale on the RB67 that tells you the compensation needed for all the lenses in the system. It's rigidly fixed to the front lens board and one of the focusing rails, so pulls out of the camera body as the bellows are extended and you just find the line for the lens you're using and mark it off.

    All of that is the faithful inverse square law for light coming into play. As you move the lens out from the focal plane, the image circle necessarily becomes larger. Since the same amount of light is going through the lens as at infinity, but it's spread out over a greater distance, you lose intensity at the actual film/sensor.

    At least with my Nikon 105 2.8D and a camera that can read CPU lenses, the camera reflects the effective aperture as it's focused close. Since I'm often using strobes powerful enough that only aperture and sensitivity affect the exposure, and consequently am shooting in full manual mode, that makes my life easy. If my flash meter tells me f/22, for example, I can just keep the lens set there(assuming I'm controlling it through the camera body) and the exposure will be correct regardless of the magnification ratio.
     
  12. "Miniature" (i.e., non large format) cameras use a focusing helix in lieu of a bellows.Either way you must extend the distance between the film plane and the lens to focus closer. I can focus using the rear standard on my view camera, keeping the lens in a fixed position, but the same principal applies.

    The basic lens formula is 1/f = 1/d', where f is the focal length, l and l' are the distances from the center of the lens to the subject and film plane respectively. This only applies to a simple lens, but it's a fairly good approximation to what happens in a compound lens too.

    AFIK, all macro lenses undergo some extension as you focus closer, whether AF or MF. Theoretically you would need an extension twice the focal length to magnify the subject 1:1. To accomplish this without extreme lengthening, most AF lens also focus internally. This is done by changing the focal length, which leads to other issues like focus breathing, and reduced "working" distance (between the front element and the subject.

    The effective aperture decreases as you focus closer, calculated using the l' divided by the diameter of the entrance pupil The further the distance, the less light reaches the film plane. At 1:1 magnification, the relative aperture is half that at infinity. There are tables and on-line calculators for this value, or you can use auto exposure.
     
  13. Mr. Ingold wrote
    The 200/4 MicroNikkor AI/AIS has internal focusing and doesn't change extension as it focuses closer. It focuses closer by reducing focal length.
     
  14. Which I don't know if it's a good or bad thing though. I think by shorten the focal length it reduces the working distance that someone needs by buying the 200mm instead of the 105. However, doing so it can focus to 1:1 without the use of extension tube and the effective aperture doesn't get too small at 1:1. A macro lens that focused only by extension at 1:1 you lose about 2 stops.
     
  15. The IF 1:1 macro lenses are great for going down to 1:1 in my experience, but I've also found that they start to fall apart when you start taking them past there. There's also the annoying thing of how they change they change the composition relative to unit focusing lenses as you approach 1:1.

    For anything beyond 1:1, give me a good flat field unit focusing lens. I don't even want CRC or any other sort of floating element. Lately, I've been getting what are(IMO) stunning results using a pre-AI 55mm Micro-Nikkor in the 5:1 range on my D800.
     
  16. I have to try that as I have the pre AI 55mm micro Nikkor and the PB-6 bellow.
     
  17. If I recall correctly, the 200 AIS only reaches 1:2 magnification without an extension tube.
     
    Vincent Peri likes this.
  18. Vincent Peri

    Vincent Peri Metairie, LA

    That's right. All 3 of the Nikkor manual focus macro lenses only go to 1:2 magnification.
     
  19. I've posted this a few times and some folks are probably getting tired of it, but this is a 100% D800 crop on a set of Novoflex bellows at about 1:6. The lens was normal, not reversed. This was taken at a marked f/5.6 with light coming from two strobes in shoot-through umbrellas and a third in a reflector firing against the wall/ceiling behind it. I need to go up to my next sized power pack to be able to close down a bit more.

    balance wheel copy-100 crop.jpg
     
  20. You're thinking of the autofocus 200/4 MicroNikkor. The manual focus ones (AI, AIS) go to 1:2 on their own mounts.

    The 105/2.8 MicroNikkor AI and AIS increase magnification by a combination of adding extension and reducing focal length. They go to 1:2 on their mounts, to 0.88:1 on their PN-11 (I think that's it, could be mistaken) extension tube.

    I can't speak to the AF versions, don't have any. I do have 55/2.8, 105/2.8, 200/4 MicroNikkors, all AIS. Why don't you look up the AF 105 MicroNikkor's properties and share with us?
     

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