Exposure correction for bellows, filter, reciprocity

[Laszlo]

I have developed a Palm software, called LF Exp (http://www.vassar.info/LF/), that I am using in the field to adjust the exposure for the bellows draw, filter factor and reciprocity failure.

The central idea is that the light meter info is converted using the given film's EI, and the shot's chosen aperture. As I am using several filters, each film-filter pair is factored in as well. Next, the bellows draw for the shot is entered in the calculation. Finally, if the shutter speed exceeds one second, reciprocity correction is applied.

The exposure data can be stored in the Memo pad to be printed later.

I have developed this software mainly to fit my needs, but I am happy to share it with the LF community as a donationware. Please see more (including the disclaimers) here (http://www.vassar.info/LF/).

I hope the moderators will be OK with this announcement.

[JW Dewdney]

Do you have the reciprocity failure characteristics for EACH film? Keep in mind also - that the actual focal length of, let's say, a 150mm lens is RARELY that... normally up to 5% off... though I suppose it wouldn't make enough of a difference for any but the most exacting applications...

[Laszlo]

I use 3 BW films (TMX, Pola 55 and Maco infraref) and 2 color films (Provia and Velvia) and have obtained the manufacturer info on them -- usually a pdf document which includes reciprocity and filter factors.

[Laszlo]

... and also, re: the 150mm focal length. The precise focal length is not really important. I decided to set the software at 1/3 stop precision, so each adjustment step is on 1/3 stops. Theoretically it is possible that such errors accumulate to become significant but in practice this has not (yet) been a problem.

For a 150mm lens the first bellows extension of interest (which leads to a 1/3 stop drop off of light) is 168mm, the next one is 188 and then 212 etc. A few mm here and there of focal length or standard displacement are not really important, nor are they easily measured in the field on a wood camera. If such precision were important, bracketing or a digital back on a monorail would be needed.