35mm film has an image area of 24mm by 36mm. These are the exact dimensions of the area on the film to which an image is recorded.

Medium to low-end digital cameras sold today have sensor chips smaller than 24x36mm in size, since producing a 24x36mm image chip is still quite expensive to do. Similarly, APS film records to an area of film 16.7x30.2mm in size.

The upshot of this is that if you use such a digital or APS camera you’ll be taking photos which do not record the same image size as 35mm film. So it’s like taking a photo using 35mm film and then cropping out (snipping off) the edges. Imagine drawing a smaller rectangle within a given 35mm photo and then cutting it out - you’ve got a digital or APS photo.

This cropping factor is often confusingly referred to as a focal length multiplier. This is because the cropping makes, say, a 50mm lens on an APS camera behave rather like a 70mm lens on a 35mm camera. Not because the focal length has actually changed - it hasn’t - but because of this cropping of the image. The cropping factor is sometimes specified as a numeric value - 1.3x or 1.6x, say.

Most lenses provide sharpest results when used in the middle part of their aperture range. Lenses usually have performance problems when used wide-open. Stopping down helps a great deal, but once the aperture becomes too small then an optical phenomenon known as diffraction comes into play and the quality deteriorates once again. So most lenses work best at around f/8 or f/11 or so.