----- Original Message ----- From: "Jim Thyer" : If you compensate by opening the lens 2 stops, you avoid reciprocity : failure, but presume that the aperture settings of your enlarging lens are : themselves accurate. that's a good point, one of three really - I found that opening up to an aperture on many lenses produced a different apperture size than closing down to an aperture, ie if I was at f16 and opened to f8 and compared the light levels to that when I was at f2.8 and closed to f8, the size of the aperture is definately different. Having seen how lots of lenses are constructed this is no great surprise given the levers and blades are not a critically tight fit. Another thing is as you say, rarely is what is maked as f11 actually f11 (or 11.2 :-) Lastly, for enlarging lenses produce the *sharpest* prints possible you have to find what that sharpest aperture is by testing and print with it. deviating away from that aperture will yield progressively softer images. Of course if depth of focus is needed for a saggy neg then stopping down to f16 may be necessary unless you can get a glass neg carrier.. : For the laser beam, all lasers have an angle of divergence of the beam. : While we may not be able to assume the centre of the laser as equivalent to : a point source, from the diameter of the spot and known convergence angle we : can determine an effective point source, and from that the inverse square : law will be obeyed. only if we convert the actual distance to relative units or ratios to allow it to comply ;-) Inverse square law application: for a distance at 1m = value of 1x, for a distance of 2m = 1/4x, for a distance of 3 = 1/9x if we were to posed a student quiz and state that a light emitting body produces a value of light at 2m and find it to be 100 lx then ask them to use the inverse square law to determine the value at 4m, the student should logically expect to conclude the value at 4m to be 1/2 this value and be 50 lx. However, if we'd stated a relationship OTHER than one where the inverse square law applied and told them a laser was in use and suggested a relationship relative to the angle then they might stand a chance of getting the answer correct (it may be 1m = 200 lx, 100m = 180 lx) I know what you mean though, light does fall off and it can be calculated - it's just that the inverse square law is often stated as a firm fact when in fact it's practical application can only be a guide, and instead some complex trigonometric equation should be used. k
