Exposure Q for LF without f-stop

[Nick Appelmans]

Hi: I am starting to photograph 4x5 using a microscope with a photo objective and no discernable f-stop. I have no idea how to determine what my exposure time shoul d be. I can try to place a meter in the opening where the ground glass goes but wouldn't know what to do with the reading anyway. Can someone point me in the ri ght direction so that I could calculate approximate exposure times knowing the l ight intensity at the film plane and the type of film used? I wouldn't have to d o this that frequently since I would be taking nearly similar pictures every tim e. Thanks for any help that you can provide. Nick

[fw]

You could try a metering back, either one with a spot metering probe, or one with a general meter, such as the Horseman. Not cheap though. You might also want to choose a film with rather more exposure latitude (perhaps colour negative?), as microscope images are generally of high contrast.

[N Dhananjay]

Metering backs, maybe. Calculite makes an accessory fibre optic probe for their meters for taking readings off ground glass surfaces - you can sample quite small areas, so it makes for a cheap densitometer and a cheap TTL spot meter as well. You calibrate it once by using a grey card. Good luck. DJ

[Patrick Raymore]

All the above Ideas are good. In addition you may want to get a voltage stabilizer for the microscope so you don't get light fluxes between meter readings and exposure.

[Nick Appelmans]

HI Everyone: Thanks for the input. I feel asthough I should clarify my question which will illustrate my ignorance. If I am able to successfully guage the incident light at the film plane and know the ASA of the film how do I calculate the exposure time? Sorry if this is elementary but I haven't found an answer in the archives. Nick

[N Dhananjay]

Don't know if I've understood you correctly. I presume you're asking how to calculate exposure time since there is no aperture scale available i.e., even though you measure the light at the film plane, what you get are a series of f-stop/shutter speed combinations but you're stuck without a marked f stop.

Without a variable aperture, what you have is a constant aperture lens. The only thing you have to figure out is what aperture that is. I would think what you need to do is calculate the diameter of the lens opening. Divide the focal length of the lens (can be determined by focusing it at infinity and measuring the distance from nodal point to film plane, alternatively if that's messy, calculate the magnification or the subject distance and image distance and use the lens formula to arrive at the focal length of the lens - 1/v+1/u=1/f)) by the diameter and you should have the f stop of your lens. Now that that's been determined, your light reading at the film plane should allow you to fix the exposure time. This procedure should get you into the ball park.

Alternatively, just run some tests. Burn some film and bracket vigorously and check your results. Might be the easier way to do it if you're going to use the same standard setup subsequently.

DJ

[Nick Appelmans]

This answers my question. I'll look up the "lens formula" to make sure I'm entering the correct values then I'll do some bracketting around the exposure time determined from that f-stop. Thanks everyone, Nick