andreasdegner
Hi everyone!
Ever since I started enlarging medium-format images with my Opemus 6 (using an Anaret 4.5/80), I’ve noticed some quite bright concentric rings appearing alongside the projected image. In my opinion, these can only be caused by reflections from the inside of the bellows. I took the circuit board out and had a look inside...the bellows is quite shiny, and I imagine that this causes a fair amount of stray light on the image, which reduces contrast and distorts the gradient in the highlights. Does anyone recognise this problem? Is there perhaps some sort of matt spray paint that adheres to plastic? Or are there other bellows made of fabric?
Best regards
Andreas
Wolfgg
Hi Andreas,
Just as an experiment, try wrapping a tube of black fabric (e.g. felt) around the inside of the bellows to see if the bellows really does reflect sound.
Best regards, Wolfgang
piu58
The next best option is velour, which is available as a self-adhesive sheet in DIY stores. It does tend to shed a little: just brush it off first and then peel it off with adhesive tape at the end, and it’ll be fine. I’ve lined several MF cameras with it, and it works well – much better than any black paint, which tends to reflect light again when there is a light leak.
Black velvet is best, but it’s difficult to work with.
andreasdegner
Thanks for the tips so far!
The only problem is how to get the velour neatly stuck to the inside of the bellows... It’s going to be quite a fiddly job, though it is doable. Perhaps I’ll do Wolfgang’s test first to see if the bellows really has such a big impact.
The reason for my suspicion regarding internal reflections is that I wanted to measure the densities of a grey card (ranging up to 1.95) in the projection using my newly acquired Hauck Trialux. In doing so, I noticed significant differences in the measured values for the high-density steps, depending on whether or not I used the negative mask strips to limit the section of the corresponding density field. The measured values for the density range of the wedge fluctuated between 1.5 and 1.9 (the latter value would be acceptable to me). This leads me to conclude that the stray light is definitely coming from the lens, as I have prevented reflections from the base board to the ceiling and walls and back by using molton fabric. So there are only two possibilities: either the lens itself has quite low contrast, or the stray light is coming from reflections on the bellows. As I have a diffuser head, a lot of undirectional light enters the bellows, so I imagine this to be the most likely source of error.
I also repeated the test with my KB El Nikkor 2.8/50... same effect, even though this lens should be beyond reproach and is also in very good condition.
Best regards
Andreas
TR
Why not just line the bellows with a piece of cardboard for the time being? Then you'll know for sure.
bernhardmangelsgmxde
I don’t really quite understand what you’re doing there. You have a grey scale as a negative with a maximum density of 1.95, and you project it onto the base plate, where you measure the density of the projection, which is then supposed to correspond to the density of the grey scale when measured under reflected light?
andreasdegner
Hello,
I think I’ve found the reason for the discrepancy: on closer inspection, the Anaret had a film on the glass (including on the inner side of the rear lens). I removed the lens and cleaned it, and lo and behold: suddenly the projection is much higher in contrast.
A second reason was the initial lack of masking of the empty areas within the negative plane (the cut-up grey wedge didn’t quite fill the 6x6 format). If you don’t do this, you end up with too much stray light, even with the best lens.
With these findings, I now measure a density difference of 1.9, which I’m happy with. So for the time being, I’ll do without lining the bellows (especially as the ring-shaped reflections lie well outside the illuminated format). Well, in any case, I’ve learnt something new.
@namir: I just wanted to test my newly acquired Trialux against the grey wedge out of curiosity; the actual reason was to calibrate my colour mixing head to my photographic paper. However, I’m not taking any incident light measurements anywhere. The Trialux only measures the incident light intensity of the projection at a single point. If I measure the brightest and darkest areas of the projection, and the displayed exposure times are T1 and T2, then log(T1/T2) must equal 1.95. However, my first measurement only gave a result of 1.5. Reason enough to investigate the error.
Best regards
Andreas
