What is fast film? If you have a photographic situation where you can't use the shutter speed and aperture combination you want, all is not lost. You may be able to solve the problem with a faster film. As mentioned in issue four and five of The Art of Photography, shutter speeds and aperture values vary in increments of one stop. But users of amateur level Canon EOS cameras are able to use their cameras with half-step increments and one-third increments with the EOS-1N or the new EOS-3 cameras in addition to the traditional 1-step increments. 1/250 sec. is twice as fast as 1/125 sec., f/5.6 lets in twice as much light as f/8. Both differences are said to be one step apart. Films, too, vary in the same way. With all else being the same, an ISO 100 film requires twice as much light as an ISO 200 film ñ a one-step difference. Thus, shooting with an ISO 200 film means you might be able to achieve slightly more depth-of-field or use a slightly faster shutter speed than you could with a slower film. An ISO 400 film will enable you to shoot at two stops higher shutter speeds or smaller apertures. << Back to Main . 1 . 2 . 3 . 4 . 5 . 6 . 7 . 8 . 9 . 10 . 11 . 12 .