Q - I would like to know the steps in making a double exposure with a 35-mm camera.
A - There are two ways to make double exposures of transparencies. One is in your camera and the other is by recopying two slides by placing one on top of the other.
To make a double exposure in your camera you need a feature on it that will let you set the shutter release without advancing the film. On some models all you have to do is to push in the rewind button on the bottom and recock the shutter without engaging the film advance mechanism. On others, you can set the ring around your shutter button to R (rewind) and rewind the film one full button and the ring edge so that you have a reference mark). Then, after you've rewound the film back into the cassette, set the film advance to A and recock the shutter to make another shot over the previous picture.
The easiest way to make a double exposure is by recopying two transparencies, if in color, or projecting two different negatives on the same piece of paper, if it is in black-and-white.
The advantage of recopying and projection over shooting in the camera is that you can see the effect you're getting, whereas camera technique calls for pre-planning of the position of the image and the right amount of exposure on each picture so the two images will complement each other, not cancel each other out.
A rule of thumb for making a double exposure on color transparencies is to overexpose each by one stop. The reverse is true for negative color and black-and-white: under-expose each one stop.
Q - Do pictures at the same speed and stop, and with the same film, turn out to be identical whether taken with a fast or slow lens?
A - The stickler here is the word "identical." Alike, yes, but when one thinks of identical the assumption is that both images are interchangeable.
I have been taking photographs all my life, and except for some copying and lab photos I have yet to come up with an absolutely identical picture. Somehow each lens has a particular characteristic that it imparts to the image. Other differences are that the elements in the picture may change slightly due to change in light, the position of the subject or camera viewpoint.
There are also variables in the film and processing, even between rolls of "identical" film.