smh@mhuxl.UUCP (henning) (01/11/86)
**** **** From the keys of Steve Henning, AT&T Bell Labs, Reading, PA mhuxl!smh > What about cameras with leaf-type shutters, as opposed to focal-plane? > Since the total time that the shutter is open is considerably less, > it would seem that you could get away with a slower speed and have > less vibration than with a focal-plane shutter. > > Another suggestion: if your focal-plane shutter moves vertically > across the film (as in my Nikkormat), you can probably use 1/125 and > get stable results. At least, you can most likely use one stop > slower speed than with an equivalent horizontal moving shutter. > > A final question: With a leaf shutter, why isn't the center of the > negative overexposed, and the edges underexposed, since the shutter > leaves the center of the negative open for a longer time than the > edges? All three statements are wrong. 1) A grain of film, or any feature on the film receives light from the lens for the same time with a focal-plane shutter as with a leaf shutter set at the same speed. The difference is that all grains receive the light at the same time with a leaf shutter, but with a focal-plane shutter the film on one edge receives its light about 1/80th of a second before the other edge of the film. 2) Again, a vertical shutter exposes the light from the lens to the film for the same length of time as a horizontal shutter set at the same speed so the blur is the same for the same shutter speed. 3) The leaf shutter is placed at a point where the rays of light do not correspond to their position in a picture. That happens to be the place where your focal-plane-shutter lenses have their aperture. Otherwise as you closed down your lens, the outside of your picture would disappear rather than dimming the entire picture.
jer@peora.UUCP (J. Eric Roskos) (01/14/86)
> > Since the total time that the shutter is open is considerably less, > > it would seem that you could get away with a slower speed and have > > less vibration than with a focal-plane shutter. > > Another suggestion: if your focal-plane shutter moves vertically > > across the film (as in my Nikkormat), you can probably use 1/125 and > > get stable results. > All three statements are wrong. No, I think he's just confusing various causes of shutter-related image problems. In the first case, there *is* a difference, since if there is motion of the camera (or the image) with a focal plane shutter, the interval over which *the whole of the frame* is exposed is indeed longer: any one point on the frame will be exposed for the same amount of time as for a leaf shutter, but the frame as a whole will be exposed for possibly much longer than that, i.e., the time it takes for the slit between the two shutter curtains to move across the entire frame. As a result, assuming the thing being photographed is big enough to fill the frame, the total motion recorded on the film will cover a larger distance than it would with a leaf shutter. As a concrete example, suppose it takes a constant 1/90 second for the front curtain to move across the frame. Then the total motion recorded on the film will be the amount of motion that occurred in 1/90 second, eventhough the slit moving across was sufficiently small that any one point was only exposed for, say, 1/500 second. This is why, for example, if you photograph a fast-moving car with a horizontal focal plane shutter, the wheels will come out looking oval shaped, eventhough they may be relatively sharp: the motion during the exposure of any one arbitrarily small set of points on the film is relatively small, but the motion across the film plane may be much larger, causing the wheel to be stretched out into an oval. In the second case... I've also seen arguments in favor of vertical focal plane shutters that run along these lines, but I've forgotten the basis of them. As I recall, it had to do with vibration induced by the shutter motion, rather than unsteadiness of the hand. -- UUCP: Ofc: jer@peora.UUCP Home: jer@jerpc.CCC.UUCP CCC DNS: peora, pesnta US Mail: MS 795; CONCURRENT Computer Corp. SDC; (A Perkin-Elmer Company) 2486 Sand Lake Road, Orlando, FL 32809-7642 "A people without history is not redeemed from time, For history is a pattern of timeless moments." --TSE