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I use my Kodak portrait lens wide open (f4.8) and sometimes I get a real nice glow in the out of focus areas and other times it is too sharp all over with no glow. It seems that when I use a slow shutter speed, 1 second I get the glow, when it is short say 1/5 second I don't. Is this the likely explanation or is there something I'm missing?
The glow is probably caused by light scatter. The more light (longer exposure) the more scatter. The more glow.
I think.
I believe that if you eliminate any variables beyond the shutter speed, you will see no difference in the negatives so long as the exposure duration is under the film's reciprocity departure threshold.
To prove this to yourself, set up the camera and lens and focus it on a stationary subject with a background that is likely to give you some good out of focus material to compare. Set your light intensity for a proper exposure at say f5.6 and 1/15 sec. Put a 3-stop ND filter on the lens (that's an ND .9) and make an exposure at f5.6 and 1/2 second. Replace the ND with a 2 stop ND .6 and expose another sheet of film at 5.6 and 1/4 second. Replace the ND with a 1 stop ND .3 and expose a third sheet at 5.6 and 1/8th second and finally remove the filter and make one last exposure at 5.6 and 1/15th second. You will have made 4 exposures that barring defects in your filters, should yield identical negatives. The aperture is constant, the film is constant, the light is constant and hopefully your development will be constant. The only variables are the shutter speed and the presence of filters, which in this test should not influence the results in a way that would affect the look of the out of focus parts of the subject.
I hope this helps.
I have found that in using these lenses, the glow comes from lighting that is a little harder than you would normally use in a portrait setup. I have found I like mine at about f-6. Wanna flair it out blow it out.