Changing shutters?

[E. von Hoegh]

Ed... Any chance you can do a comparison of the two lenses regarding *gasp* bokeh? Then swap the cells and duplicate the test? I'd really like to see the difference between the round vs. penta iris shape. Ill bet others are interested too. BTW, I'll be doing similar comparisons one of these days.

It won't make a bit of difference in the character of the lens, only a little difference in the shape of the OOFHs.

[Old-N-Feeble]

It won't make a bit of difference in the character of the lens, only a little difference in the shape of the OOFHs.

True, thank you, and that's precisely what I'm curious about.

[Leigh]

In real life, there aren't that many fundamentally different lens designs across modern LF lenses. Indeed you often have no choice among design types if the focal length, maximum aperture and shutter size are to be the same. And within a design type, that constant does not vary that much, usually by much less than a third stop - things only start to get difficult once you switch design types...

My statement was meant to be correct for photographic lenses in general, and I believe it is.

If you use enough assumptions and constraints, one lens and shutter could be used for everything.

- Leigh

[Andrew]

sorry to drag this back to my level, but I'm gonna beat my head against this wall until I see the light

I want to ignore issues of cell spacing because I really only care about the aperture scales here
if I swap lens cell between shutters...
how will I confirm that the aperture scales are still correct OR how do I quantify the error so I can make new scales ???

[Leigh]

Hi Andrew,

It's really a question of how accurate you expect your scales to be.

If you want the scale to be right, order one from S K Grimes at www.skgrimes.com
They have all the info to make one for your lens/shutter combination.

An alternative is to shoot some test scenes with a lens having a known accurate aperture,
and the same scenes from the same spot with the new lens (acknowledging a different field of view).

Bracket the test exposures with the new lens in 1/3 stop (or finer) increments, if possible.

A perhaps superior methodology would be to use the lens in its original shutter as the standard, then move
the cells to the new shutter and repeat the exposure series. This provides a consistent field of view.

In this case you would use the existing scale and determine a correction factor based on comparison of the negatives.
You could apply that correction in your head, or you could work out a change in film speed for the same result.

- Leigh

[Andrew]

Thank Heaven for good ol' fashioned empiricism... I can cope with that

[Andrew]

further to above post
rather than wasting time, film and chemicals
as I have a good quality light meter that reads incriments of 0.1EV can I jerry-rig that to make a measurement device to confirm/calibrate aperture scales?

[Leigh]

Yes.

If you have a way to measure the light on the ground glass and exclude all other light sources,
that would work quite well.

The methodology would be the same. Use the original lens/shutter as your standard.
Take a reading of some uniformly gray subject. A gray card or the side of a building or anything similar.
Then take the same reading using the lens in the new shutter and make the comparison.

Take readings at a couple of different mid-range apertures, like f/11 and f/32.

This would be more accurate than the film test, provided you really exclude other light sources.

A point regarding accuracy when using any digital instrument for comparison:
The greatest repeatability will be achieved when the reading is bouncing back and forth between adjacent values,
e.g. 3.4 and 3.5 EV or some such. (See note below)

This can be used to advantage for the type of tests under discussion.

Set the reference lens at your favorite mid-range aperture (f/22 or whatever) and note the EV reading.
Adjust the aperture ever so slightly until you get the jumping values. This is a very fine adjustment.
Note the aperture setting, then move the cells to the new shutter.
Do the same thing with the new lens, setting the aperture for the exact same pair of values.
This is a very accurate measurement.


- Leigh

Note: If you're unable to achieve the bouncing value setting with your meter, disregard the method.
Some meters are designed to suppress the bouncing value display. There's no way to defeat that.

[Dan Fromm]

Hmm. Photometric methods measure transmission. What does that have to do with geometric f/ stops?

[Leigh]

The meter is being used as a luminance comparator.

When the illumination on the GG is the same for both shutters, then the actual aperture is the same.

- Leigh