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For all you macro fans out there!

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  • Petrochemist
    replied
    Re: For all you macro fans out there!

    Originally posted by Mark_R2 View Post
    I am not familiar with that Canon lens, but let’s look at some general principals.

    The relation between magnification M, focal length f and image distance S (measured from the second principal plane of the lens) is

    S = f*(M+1)

    (The * means ‘multiply’)

    Thus when you have 5:1 magnification with a 65 mm lens, the image will be 65*(5+1) = 390mm away from the second principal plane of the lens. This is just physics.

    In general, the second principal plane could be anywhere inside the lens or even outside the lens.(As an aside, a lens with the 2nd PP in front of the lens is called a telephoto lens, one with it after the last optical element is called retro-focal or inverse telephoto). However, when the lens is focussed at infinity, the second principal plane is exactly f away from the image, i.e. 65mm in this case.

    So, if the 65mm lens were a simple type with all optical elements in a fixed position relative to each other, then moving from infinity focus to 5x mag would require moving the lens 325mm. This is of course perfectly feasible, if rather unwieldy, but would generally be done with a bellows rather than a helicoid mechanism.

    It is also worth noting that the light intensity of the image is reduced (via the inverse square law) by a factor of (M+1)^2 (where ^2 means ‘squared’). In this case that is a factor of 6^2 =36. This is equivalent to 5.17 stops. The bellows factor is M+1, so the f2.8 lens thus behaves like an f16.8 lens (=2.8*(5+1)). This is the same for any focal length, but it is more practical to make large aperture lenses with shorter focal lengths, excepting when the lens needs to be strongly retro-focal to also allow it to focus at infinity.

    I strongly suspect the Canon lens uses ‘floating’ elements that reduce the focal length substantially at high magnification. (Floating is a term used to describe elements that move along the optical axis relative to other elements, changing the relative separation of elements within the lens.) This will change the field of view as David pointed out. It might also increase the effective f-number of the lens. Anyway, it is a neat bit of optical design. The longer focal length at low magnification gives you the advantage of larger working distance. Then, the focal length is reduced to get to high magnification without needing huge extension.

    It is worth noting that lenses with internal focusing use floating elements and usually (maybe always) give a reduction in the stated focal length when not focussed at infinity. Or, to put it anther way, the stated focal length is only correct when the lens is focussed at infinity.

    Mark
    The MP-E 65 is a zoom lens with no real focusing. The focal length is changed to increase magnification & change working distance. It's for too expensive for me but does give great results.

    I rather like the DIY 'anybrand' alternative, but I've not got round to trying it yet.

    Leave a comment:


  • Phill D
    replied
    Re: For all you macro fans out there!

    Should have thought of it like that myself, thanks Mark that helps a lot.

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  • Mark_R2
    replied
    Re: For all you macro fans out there!

    The optical magnification is the same, since it doesn't depend on the size of the sensor. However, to fill a m4/3 sensor with the image, a lower magnification is required when compared to an APS-C or 35mm film sized sensor. The m4/3 sensor is 17.3 x 13 mm. So at 5x magnification, an object 3.5x2.6mm in size will completely fill the sensor. On an APS-C sensor, the same object will underfill the sensor.

    The working distance depends on the lens design. At 5x magnification, the distance of the object to 1st principal plane of a true 65mm lens would be 78mm. However, the 1st PP could be well inside the lens and so the working distance (the distance of the object to the front of the lens) would be much smaller.

    Since that Canon lens is (I believe) not 65mm at 5:1 mag, the working distance will be much smaller. I would assume the lens specs would give the working distance. It is bound to be small at that sort of magnification.

    DoF will be very small. Focus stacking would be a good idea since stopping down will make an already dim image even dimmer.

    High magnification photography is not trivial.

    Mark

    Leave a comment:


  • Phill D
    replied
    Re: For all you macro fans out there!

    Not sure I quite followed all the physics here so forgive me if these are daft questions but what mag would that lens give on a micro 4/3 system? is it 2x the mag quoted assuming the quoted figure is FF?
    Also what working distances are we talking about here?
    Dof is presumably tiny too so focus stacking would no doubt be the order of the day.

    Leave a comment:


  • Mark_R2
    replied
    Re: For all you macro fans out there!

    Originally posted by alfbranch View Post
    Canon make the 65mm 2.8 MP E that shoots from 1:1 to 5:1 so it possible at longer focal lengths.

    Leave a comment:


  • David M
    replied
    Re: For all you macro fans out there!

    Originally posted by alfbranch View Post
    Canon make the 65mm 2.8 MP E that shoots from 1:1 to 5:1 so it possible at longer focal lengths.
    I seem to recall reading that the Canon has the field of view of a 65mm lens at 1:1 magnification and that the field of view changes as you increase magnification. So it doesn't need a lot of extension to get to 5:1.

    The OM 38mm needs a lot of extension to get to 5:1.

    Leave a comment:


  • alfbranch
    replied
    Re: For all you macro fans out there!

    Originally posted by Mark_R2 View Post
    For high magnification you have to have a short focal length, otherwise the extension required becomes enormous (and the effective aperture becomes very slow). Think of a microscope - very short focal length and tiny working distance, but necessary to get high mag with a decent numerical aperture. This is 4.5x mag, so is not surprising it looks like a microscope objective.

    Mark
    Canon make the 65mm 2.8 MP E that shoots from 1:1 to 5:1 so it possible at longer focal lengths.

    Leave a comment:


  • alfbranch
    replied
    Re: For all you macro fans out there!

    I dont think I will be bothering just now as I can get 3:1 out my 60mm f2.8 or Sigma 105 f2.8 and manage handheld shots.

    This would be great for focus stacking on dead subjects though.

    Leave a comment:


  • David M
    replied
    Re: For all you macro fans out there!

    I never used the OM 20mm but it does look very similar. I still have the 38mm OM macro, I keep thinking I should make more use of it but never seem to find the time.

    Leave a comment:


  • Mark_R2
    replied
    Re: For all you macro fans out there!

    For high magnification you have to have a short focal length, otherwise the extension required becomes enormous (and the effective aperture becomes very slow). Think of a microscope - very short focal length and tiny working distance, but necessary to get high mag with a decent numerical aperture. This is 4.5x mag, so is not surprising it looks like a microscope objective.

    Mark

    Leave a comment:


  • Ross the fiddler
    replied
    Re: For all you macro fans out there!

    Originally posted by Ian View Post
    It's been pointed out this lens looks like an OM Zuiko macro of similar specifications?

    Ian
    It's interesting with such a short focal length, but then again, an OM28mm lens reversed is better than a 50mm lens (reversed) for macro on extension tubes or bellows.

    Leave a comment:


  • Ian
    replied
    Re: For all you macro fans out there!

    It's been pointed out this lens looks like an OM Zuiko macro of similar specifications?

    Ian

    Leave a comment:


  • Ian
    started a topic For all you macro fans out there!

    For all you macro fans out there!

    A 4.5:1 super macro lens for $199:

    http://dpnow.com/20mm-super-macro-lens-mitakon/

    Ian
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