Calibrating f/stops with lens in a new shutter

[dan_smith]

When one puts a barrel lens in a new shutter, how do you calibrate the f/stops? Is there a set angle the aperture dial goes that equates to fixed stops once the first one is known? Or is it a matter of measuring the iris opening? I know on most of my existing lenses the progression is even across the board, so wonder if the set number of degrees after the first f/stop is known is the way to go?

[dhananjay_n]

I would measure the iris instead of using a standard angle - I've

seen lenses where the angle varies considerably (typically f/8 to

f/11 will be a larger angular turn than f/45 to f/64). These

differences are probably related to the fact that the area of the

aperture is reducing as a function of the square of the radius - on

small lenses, it may not matter significantly, but on some larger

lenses I've seen substantial differences. If someone like SKG put it

into the shutter, he should be able to engrave an fstop scale for

you. If it's front mounted, I think the best solution might be to use

the barrel iris. Cheers, DJ.

[bob_salomon]

Two things need to be done.

 

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First the spacing between the front and rear cells has to be

adjusted and may require the use of shims.

Then the aperture scale needs to be engraved for the lens and

shutter you have.

If the first isn't done performance suffers. If the second isn't done

your exposures may not be consistent or agree with your meter.

[s.k. grimes inc]

Its necessary to replicate the diameters of the original lensbarrel

in the new shutter. Modern shutters have a logarithmic progression

to the iris control scale which makes the f-numbers evenly spaced.

You can, for this type, measure the diameter of a large stop and a

small stop then divide the spacing evenly to fill in the rest. Older

shutters have a non-logarithmic progression in which the spacing of

the numbers on the shutter grow closer together at the smaller iris

openings. On this type you need to measure the diameter of each stop

opening in the old barrel and transfer those diameters to the shutter.

 

<p>

 

See: <a href="http://www.skgrimes.com/iris/index.htm">my page about

iris scales</a> (The picture at the top is a compressing scale type

shutter and the others are more modern evenly spaced types.)

 

<p>

 

In all cases the actual iris diameters of the original should be

replicated in the new installation.

[ron_shaw]

To calibrate a newly reshuttered lens, first mount a (different) lens

that you know is correct. Set your camera up on a tripod, aimed at a

brightly lit wall. Set the reference lens to a given F stop, and using

your meter (on reflective setting), measure the center of the ground

glass. Now replace the reference lens with your new unknown lens.

While measuring the center of the ground glass with your meter, adjust

the aperture control until you get the same reading as your reference

lens. This position of the aperture control is now the same F stop as

the reference lens. Mark the position. It is now easy to calibrate, as

each larger F stop number (smaller aperture), is a 2:1 ratio.

[brit]

wrt to the last post...I have a reference 150mm but want to calibrate another lens but of focal lenght 80mm. Should the meter reading be taken with each lens focused at the wall or at infinity (ie GG back to lens distance = to focal lenght of lens in use?)