Calculating exposure with no lens

Well, the size of the aperture and the distance to the film still define an f-number, with or without a lens. If the diameter of the aperture is the same as its distance from the film then it will be F/1.
You can approximately ignore light absorbed by a lens -- it's not very much. Without the focusing of a lens, you'd get about medium gray all over your film, and no image.

Welcome to photrio!

 

The f/1.0 is not a good assumption! That merely identifies the marginal ray angle to the image plane, or the concentration of light per unit area. Outside of a pinhole arrangement with a defined marginal ray, it is not appropriate to think in terms of f/# for a lensless system!

OP, what you’re asking for is a radiometric calculation determined by the geometry of the imaging system including the light source as well as the irradiance at the lensless hole. This will give you the amount of energy falling on the film.

From there, you need to convert the irradiance determined above to an exposure value and know where that value is on the film’s characteristic curve.

It is not an easy problem to wrap your head around, but omce you have then you can solve it.

Look up radiometry and start reading. . In particular you would be interested in the equations/geometry for a “simple radiometer” setup, which is what you are describing.

-Jason

 

A sensitometer exposes film without a lens. Since the ISO is equal to 0.8 over the exposure (in Lux-sec) needed to obtain a 0.1logD density on the film, you can get a handle on how much light in Lux-sec is needed to just get some density on the film.

 

ic-racer is correct. If you then need to convert the results of a sensitometry test to the exposure on some lensless arrangement that you are trying to build, then radiometric modeling of your setup is required to tell you things like how far away and how bright a light source needs to be.

A faster approach is to simply do a series of exposure tests when you set up your lensless camera.

 

like a pinhole camera???
a known "aperture" to start, and a known measure of the light sensitive material (asa/iso), then you back into the "speed" of exposure...
... or i don't really know, but that's how i'd start!


welcome!