On 2015-06-24 10:01 , Darren Addy wrote:
For a normal SLR you apparently can't use
much of an extention tube with anything wider than 16mm
perhaps that's because you don't need much of an extension tube?
i just tried my DA 15mm Ltd with a 12mm extension tube (Kenko auto, since
the DA 15 doesn't have an aperture ring)
it worked; focus was extremely close, but workable; had to photograph at
oblique angle to get enough light on the subject; if this calculation
(top-rated answer) is correct, i got 0.95x:
<http://photo.stackexchange.com/questions/5603/how-can-i-calculate-what-the-effect-of-an-extension-tube-will-be>
doing a ruler test, i got an image 28-29mm across the width of the frame
when the center was in focus; the K-5 sensor is 23.7mm wide, which would
give a magnification of 0.83
(the DA 15 is not flat field at these distances, so the actual image width
might be somewhat less if i had a curved ruler that was at a consistent
distance from the front nodal point of the lens...)
however the depth of field was so tiny that it puts the lie to Venus' sample
images — short of stacking a large number of shots, i don't think you can
get a meaningful background in something like a garden scene with 1:1 at
15mm; also the farthest i could focus was barely longer than the closest, so
to get a range of magnification possibilities one would need a set of very
thin (perhaps unobtainable) extension tubes — that might be the real win of
the Venus design
i tried 20mm extension too (by above calc, 1.48x); focus was basically at
the glass of my filter (meaning tiny dust on the filter was in focus too)
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