On 9/15/2013 8:21 PM, Chuck Norcutt wrote:
> Basically what Moose said. But I didn't answer this question quickly
> because I wasn't sure exactly how it would work out. Now I've had a
> chance to do some calculations and can quantify it a bit. I say "a bit"
> because the only calculations I can do is from the optical center of the
> lens.
More likely, two nodes, one as seen from the subject end, and another seen from
the camera end. The front one is used
for panoramas.
I think the space between them adds length to the simple lens calculation,
which makes simple calculation from image
plane impossible. If the front node location and focal length at the repro
ratio were known, working distance could
easily be calculated.
> As Moose points out some lenses move their elements
> independently, some change focal length and none of them may have the
> optical center of the lens at the physical center. Then, even if you
> know where the optical center is located the working distance is
> measured from the end of the lens, not the optical center. So take what
> you're about to read as an approximation which will be somewhat
> different from lens to lens.
>
> First, let me accept that for a 1:1 output from a 4/3 sensor we really
> only need to make the image 1/2 life size on the sensor. There are
> probably some valid quibbles about doing this but if we reject it we
> really can't talk about equivalent focal lengths for even non-macro use.
>
> So, what I've done is calculate the object/image distances for a 120mm
> lens on full frame shooting at 1:1 (life size)
This is completely impractical, as there aren't any such lenses for 35 mm FF.
The flagship FF macros from C&N are
60/2.8s. I believe I've read that the Canon is actually about 40 mm @ 1:1.
Tamron makes a 90/2.8 and I believe the Pentax is the Tamron rebranded. All the
contemporary 120 mm macros lenses are
for MF or LF. I have the Tamron, and it's as good or better in the macro range
than OM 50/3.5, Tamron MF 90/2.5 and
Kiron MF 105/2.8. I tested them all side by side.
> and the same for a 60mm lens on 4/3 shooting at 1:2 (half size).
This is OK, as there are at least three macro lenses for 4/3 format in that
general range, PanLeica 45/2.8, ZD 50/2 and
M.Z 60/2.8
> For the full frame 120mm case at 1:1 the object distance (center of lens
> to object being photographed) and the image distance (center of lens to
> the film/sensor) are both 240mm.
But total distance from image plane to subject may be greater, if front and
rear nodes aren't the same. A clearer
example of widely differing nodes may be telephotos, where the front node is in
front of the front element, and the
reverse for SLR lenses wider than about 38 mm.
> ...
>
> For the 4/3 60mm lens at 1:2 the object distance is 180mm and the image
> distance is 90mm. Note that the image distance is only 1/2 of the
> object distance. While the 120mm lens does have a working distance
> advantage over the 60mm lens the difference is not double. That's
> because we allowed the 60mm lens to get away with a 1/2 life size image.
> Note that the object distance of 180mm is 3 times the focal length
> when set for 1/2 life size instead of only twice the focal length as
> required for life size.
>
> One advantage for the shorter lens that I hadn't thought of at first
> (but which clearly falls out of the calculations) is that, although the
> full frame 120mm lens has a working distance advantage the extension
> required for the 60mm lens is only 90mm from optical center instead of
> 240mm for the full frame lens. That's a lot of extension which brings
> its own problems beyond working distance.
But there is no such lens for this format. The Zuiko 135/4.5 is unique, as far
as I know, and it only goes to 1:2 with
the 65-116 Auto Tube.
> Finally, while doing this little exercise I referenced John Shaw's book
> "Closeups in Nature" wherein he speaks of the "fear circle"*. When
> you're outside the radius of the fear circle little critters ignore you.
> When you get inside the fear circle they tend to get out of your way
> quickly. Long focal length is still the answer...
Which is why so many of my small critter shots are taken with a tele at 300 mm
(which is probably less at closest
focus.)
<http://galleries.moosemystic.net/MooseFoto/index.php?gallery=California/Sugarloaf_Ridge_SP&image=_MG_2819cria80.jpg>
Today's spider shot was right on the edge of that circle. After two successful
shots at about 1:2, I tried moving in
closer, and it felt compelled to move away.
Bottom line; I think the M.Z 60/2.8 has as much or more working distance as its
actual competitors. It also has a far
more practical hood design for close work than the others.
> but is not without its own set of problems.
YeahBut - What's the alternative?
The Short and Long of Moose
* Related, no doubt, to the dreaded Circle of Confusion. :-)
--
What if the Hokey Pokey *IS* what it's all about?
--
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