Hi,
Yesterday I came across a set of three Hama auto extension tubes and since
the set only cost $20 I bought them, so yesterday was my first experience
with extension rings and I was quite impressed at the magnification level
they allow with the 50/1.8 lens (I haven't really tested them with other
lenses yet).
I asked Hans for a table of magnifications and he recommended me to focus on
a ruler and then use these measurements (in relation to the 36x24mm
framesize) to determine the magnification level.
To do this pretty accurately, I mounted the rings on my OM-40 (as I didn't
adapt the screwmounts yet so the rings can safely be used on the 4Ti without
shearing off the reset button) on a tripod and vertically clipped a ruler on
a bookstand. Then for each combination of rings I first moved the focusing
ring of the lens all the way to the min. focusing distance, and then moved
the bookstand (to and away from the camera) until the ruler was focused
properly (because of the incredible small d.o.f. this wasn't a
straightforward task at times), then I carefully moved the ruler
(vertically) so that the 0 cm mark was aligned with the top of the frame
(the viewfinder frame, that is), and then I wrote down whatever amount of
milimeters the bottom of the frame was aligned with. Then I performed the
same procedure with the lens' focusing ring turned all the way to the max.
focusing distance (i.e. "infinity"). By doing this I obtained the following
table of ranges (expressed in mm.):
Tubes: Max.: Min.:
-----------------------------
13 : 90 <-> 54
21 : 56 <-> 41
31 : 37 <-> 30,5
13 + 21 : 33,5 <-> 27
13 + 31 : 26 <-> 22
21 + 31 : 22,5 <-> 19
13 + 21 + 31 : 18 <-> 16
Now, (incorrectly) assuming the viewfinder gives a 100% view of the actual
picture, I took the number 24 (i.e. the frame height) and divided that by
the max. resp. min. amount of milimeters to obtain the following "viewfinder
magnification graph":
Tubes: Viewfinder:
------------------------------
13 : 0,27x <-> 0,44x
21 : 0,43x <-> 0,59x
31 : 0,65x <-> 0,79x
13 + 21 : 0,72x <-> 0,89x
13 + 31 : 0,92x <-> 1,09x
21 + 31 : 1,06x <-> 1,26x
13 + 21 + 31 : 1,33x <-> 1,50x
Then the final step for obtaining the actual magnification (assuming the
OM-40 viewfinder has a 93 overage of the actual frame) was obtained by
multiplying the above ranges with 93/100, this then yields the actual
multiplication table (all multiplication factors are expressed as "... times
lifesize"), which is:
Tubes: Frame:
-------------------------------
13 : 0,25x <-> 0,41x
21 : 0,40x <-> 0,55x
31 : 0,60x <-> 0,74x
13 + 21 : 0,67x <-> 0,83x
13 + 31 : 0,86x <-> 1,01x
21 + 31 : 0,93x <-> 1,17x
13 + 21 + 31 : 1,24x <-> 1,40x
Now, can anyone verify if my reasoning (and hence, the calculations) is
correct? AFAIK there should be no flaws in it (not taking small inaccuracies
while measuring into account).
On a sidetrack, taking the averages of: min. (resp. max.) magnification
factor divided by the corresponding amount of mm.s extension used, the
following two (rough!) constants can be deduced:
Max. focusing distance: 0,0199 -> 0,020x per mm
Min. focusing distance: 0,0247 -> 0,025x per mm
Using these constants, I can more or less estimate the magnification ranges
of the other off-brand extension tubes that have the sizes 12mm, 20mm and
36mm (can anyone verify this against the actual results?):
Tubes: Frame:
-------------------------------
12 : 0,24x <-> 0,30x
20 : 0,40x <-> 0,50x
12 + 20 : 0,64x <-> 0,80x
36 : 0,72x <-> 0,90x
12 + 36 : 0,96x <-> 1,20x
20 + 36 : 1,12x <-> 1,40x
12 + 20 + 36 : 1,36x <-> 1,70x
Finally, for comparing it against the Oly tubes, the calculations would come
down to:
Tubes: Frame:
-------------------------------
7 : 0,14x <-> 0,18x
14 : 0,28x <-> 0,35x
7 + 14 : 0,42x <-> 0,53x
25 : 0,50x <-> 0,63x
7 + 25 : 0,64x <-> 0,80x
14 + 25 : 0,78x <-> 0,98x
7 + 14 + 25 : 0,92x <-> 1,15x
Hmmmm, I just checked these numbers against the table that appears on Hans'
e-sif page and it turns out that the left side numbers (i.e. focused at
"infinity") are pretty accurate, but the min. distance focusing numbers are
quite off. It seems that the min. focusing distance multiplication factor
would actually be something like 0,023...
Oh well, this should probably be attributed to inaccurate measuring on my
side, although I thought the measurements were quite accurate...
Either way, the magnification range is pretty impressive, almost the
complete range between 0.25x and 1.40x lifesize is covered by something as
cheap as three off-brand extension rings and a regular 50/1.8 Zuiko!
Hope this is of use to anyone...
Cheers!
Olaf
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