### Calibrating f/stops with lens in a new shutter

greenspun.com : LUSENET : Large format photography : One Thread

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?

-- Dan Smith (shooter@brigham.net), February 20, 2002

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.

-- N Dhananjay (dhananjay-nayakankuppam@uiowa.edu), February 21, 2002.

Two things need to be done.

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.

-- Bob Salomon (bob@hpmarketingcorp.com), February 21, 2002.

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.

See: my page about iris scales (The picture at the top is a compressing scale type shutter and the others are more modern evenly spaced types.)

In all cases the actual iris diameters of the original should be replicated in the new installation.

-- Steve Grimes (skgrimes@skgrimes.com), February 22, 2002.

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.

-- Ron Shaw (shaw9@llnl.gov), February 27, 2002.