@chris : Good idea, because lightning could be more homogenous. Will take some weeks until I have time to try it.
With respect to the aperture vs. lamp I should have been more precise: as a (analog) slide never is truely flat I have to stop down aperture to at least f8, better f11 to get the full slide area in my depth of field. On the other hand this added depth of field on the slide also produces a “better” image of the glowing lamp and thus is not wanted: you would get optimum “blur” of the lamp when opening aperture as much as possible - with the drawback of loss of focus on the slide. That’s why I have to stop down aperture and need the diffusor. Maybe the collimator will help to get the diffusor more equally illuminated.
Thanks for the interesting idea!
I’m glad I read your post. I saw this effect yesterday by accident when comparing corner sharpness between lenses - one lens was generally better but I was puzzled why some details appeared sharper with the ‘less sharp’ lens setup.
Having dug further through my tests shots I concluded that the difference was caused by the distance between the diffuser and the slide having varied between the two shots. I also found two reasonably ‘like for like’ shots (same lamp, lens etc, just with the diffuser position changed). Look at the almost horizontal black line. I think it is a telephone wire.
Diffuser at circa 50mm
Diffuser at circa 250mm
My conclusion is that moving the diffuser further back means that the light is more collimnated when it reaches the slide. Gettin consistent illumination is tricky and my test shots failed to achieve this.
I was using a slide projector as the light source for both slides with some 3mm opal acrylic as the diffuser.
To me it seems that having a more diffused source is ‘losing information’ from the image. I’m puzzled why that would be a good thing. The real image is composed of grains so if they appear in the raw output (I don’t think my setup can resolve that far BTW) then surely that is an accurate reproduction? Grain and dust can then optionally be taken out in post processing.
I tried (briefly) using my slide projector light source with no diffuser. The result was horrible - a big flare in the middle of the frame. From what I have read elsewhere, slide projector condenser optics are not good enough to produce an even illumination for backlighting purposes.