Calculating aperture width

[Sherkin]

When calculating aperture widths using the formula Focal Length/F Stop, should the focal length actually be the flange plate to focal plane distance?

The reason I ask is that I'm doing a Nimslo panoramic conversion, using a Bronica 40mm lens, and the flange plate to focal plane distance is 76.5mm. I used 40mm in the aperture width calculation and have ended up with a roll of underexposed film.

It would seem that I should use 76.5mm as the focal length (possibly?), but I've not been able to find any information about this.

Anybody had any experience of this?

[John Koehrer]

The 40 is a retro focus lens so the effective FL is 40mm. Flange to FP is to clear the mirror in the Bronica camera. What kind of linkage are you using to control the aperture? Stopping it down or opening it?

[Q.G.]

Why calculate the aperture width?
For exposure, only the f-stop matters.

If you used the results of, say, 40/8 = 5 to set your exposure, i.e. chose a shutterspeed to match 5, you would indeed be more than 1 stop (1.36 in fact) stop off (underexposed) from what it should be.

If you set the lens to f/8, use the shutterspeed to match f/8. Nothing else.

[RalphLambrecht]

When calculating aperture widths using the formula Focal Length/F Stop, should the focal length actually be the flange plate to focal plane distance?

The reason I ask is that I'm doing a Nimslo panoramic conversion, using a Bronica 40mm lens, and the flange plate to focal plane distance is 76.5mm. I used 40mm in the aperture width calculation and have ended up with a roll of underexposed film.

It would seem that I should use 76.5mm as the focal length (possibly?), but I've not been able to find any information about this.

Anybody had any experience of this?

What John said. Also, if that would be true all your lenses would have the same focal length. The focal length is the distance from the rear nodal point to the focal plane at the lens' infinity setting. Finding the rear nodal point is complex and depends on lens design.

[Sherkin]

I've taken the lens elements out of the Bronica lens housing and fitted them onto a compur rapid shutter, which is positioned to use hyperfocal distance rather than manual focussing. Now of course, I have to recalculate the aperture widths and re-mark the shutter, which I did using 40mm in the calculation. Since I've ended up with underexposed frames, I'm guessing that I should have used the measured 76.5mm flange plate to focal plane distance, but I can't find any information to support this assumption.

[Bob-D659]

If your iris is in the same position as the original, can you measure the factory one? Is the bore of the new shutter iris assembly the same as the original iris?

Instead of measuring diameters, try putting a piece of white paper on the film plane and measure the brightness fully open, then close the iris and mark the full stop increments. May not be perfect, but it should be closer than what you have now.

[Q.G.]

You should indeed not use the flange to film plane distance.
What you should use however is the pupil diameter, i.e. how big the physical hole size appears when magnified by the glass.

[ic-racer]

Focal length.

[David A. Goldfarb]

I'd follow Q.G.'s suggestion with a retrofocus lens and just measure the entrance pupil--the size of the aperture as it appears at the front of the lens. Put a ruler or calipers across the front filter threads of the lens and measure it, and you'll be close enough, and then use that value to calculate f:stops using the formula, focal length/entrance pupil diameter=f:stop (or more likely, you'll want to know the normal stops and use the formula to determine what the entrance pupil diameter will be for each stop).

If you hunt around on photo.net, Noah Schwartz used to have some illustrations demonstrating how to do this. It may still be there.

[Sherkin]

Thanks everybody. David, I found the Noah Shwartz explanation http://photo.net/large-format-photography-forum/00FILX and measuring as he, you and Q.G. suggest indicates that I've been underexposing by about 1 1/3 stop. Time to put a film through again... I'll let you know the result. Many thanks.