Lens coverage?

Does anybody know how to determine the coverage a lens will provide

besides the obvious of looking up the specifications. I guess I am

looking for a simple test I could do for an older lens that I have

acquired. My idea is to mount the lens on a plain board and attach

this to the front of a large box. I would then shine a flahlight in

the front of the lens (from some distance?) in a dim room and have a

translucent material at the back of the box to record the diameter of

the lens coverage.

 

Now I got to giving it a bit more thought and believe that the

distance from the lens to the translucent material would be

important. Am I correct? I also want to witness first hand how the

circle of light changes as the aperature is changed. I want to see

how dramatic it is or is not.

 

Lastly I would like to see if the circle changes size if I move the

lens closer to or further from the translucent material. I realize

that I can do these tests without asking but I would like to know if

anybody can tell me if there is any validity to what I will see and

deduct or am I just having a good time on a cold winter's evening?

 

Thanks

The image circle is usually measured when the lens is focused at infinity, so you would need a large focusable box (two nested boxes might do) with a viewing screen of some sort on the back. Point it off in the distance and see what you see. You might not really be able to see the change in image circle as you stop down very well, because of the hotspot on the viewing screen and such. It shows up most clearly on color transparency film.

you ca

put the lens in the camera, focus on what you want to shoot. and look thru the back without the fresnell. you have full coverage looking from the film corners out the front of the lens, when you can see a round apeture.. larger apetures, filters, and front lense movements will bring you to the edges of the coverage at some point causing vigyetting.. dave. some fresnells have the corners cut off at a 45 degree angle for such viewiing.. . so you odnt need to go thru allot, just set up the shot look thru the corners and stop down until apeture is round and hope you have enough light or time to cover the situation. this can get you into some long timed shots, check out the reciprocity of your film your shooting if the time gets very long... reciprocity is the maxumum time the film can be shot without corrections.. good luck, dave...

Make yourself a large box camera with adjustable focus.

Load it with a big sheet of photo paper - at least 8x10, bigger is better.

 

Expose this, assuming paper sensitivity to be around ISO 5 or so.

 

Develop paper negative.

 

I use a 5x7 camera for testing 4x5 lenses. works great!

Even simpler, have someone hold the lens in front of a window and place a piece of cardboard or white sheet to the back of the lens equal to a distance equal to the focal lenght, take out your tape measure and measure the image circle.

Take a light box and go into a dark room. Set the front of the lens on the light box and cover the rest of the box with your dark cloth so that no light escapes except through the lens. Now hold a large white card above the lens at a distance equal to the focal length and measure the circle.

Just don't forget that the circle of illumination may well be larger than the usable image circle.

Thanks everybody for the help. I especially like Jorge's and Butch's methods. Something for me to test this weekend.

 

Cheers