Front element swapping

[Greg Erker <erker@xxxxxxxxxxxxxx>]

Hi all,

  I did some additional lens FL testing on
my five Rikenon TLR lenses.

  This time I kept the rear lens group the
same and swapped lens fronts.

  I started out with #5 in front and in rear
(and left #5 rear in for the rest).

  The results were interesting:

- Testing #5 front with #5 rear duplicated the
previous measurement.

- With #4 front installed the projected cross
hairs were to blurry to measure! I use a Maglight
(focusing) flash light as my light source with
some frosted mylar taped to the front to diffuse
it slightly. When an image is badly out of focus
the cross hair will move around as you move the
light since the light comes though different parts
of the lens. When the image is in focus or close
to it you don't get this swimming effect.

- With #3 it was less blurry but still to blurry
to measure.

- With #2 and #1 it was sharp enough to measure.
I'll put up the numbers later.

But basically the two blurry ones show the lens
FL changes quite a bit when you change the front
element only. A lot more than the 0.5% and 1% changes
I measured among my 5 Rikenons when used in proper sets.

  The last thing I did was put #4 front back in and
refocus to get the cross hairs sharp. The difference
in distance between the lens and wall would tell me
something about the FL change.

  With the #5/5 combo the best focus distance was
757 mm to the wall. With #4/5 combo it increased
to about 1000mm.

  If we assume the 5/5 combo is exactly 80mm FL
then we can calculate the distance the test slide
is from the lens using 1/80 - 1/757 = 1/dist.
and dist works out to be 89.45 mm.

  For the 4/5 combo dist is still 89.45mm so we
can calculate what the lens FL must be if the image
is sharp at 1000mm.  So 1/FL - 1/1000 = 1/89.45
This works out to be 82.1mm FL.

  Thus the FL changed by about 2.6%.  As you may
recall my #4 lens was the oldest one and was about
1% different than the other four when used in its
proper set. So exchanging only the front make a big
difference.

  I don't know if this means the resulting lens
wouldn't be sharp (due to not being a matched set)
but there does appear to be some evidence that the
fronts and rears are matched to give a tighter FL
tolerance.

More later in anyone cares - Greg