Imagon's speed or how to expose

[pasiasty]

Imagon manual says that H values correspond to aperture values of a standard lens, so with the softest disk set to H=5.8 exposition should be calculated as for f/5.8. But... in case of Imagon 250, the older one, with Compound 3/VII, maximum shutter aperture is 40mm; no magnifying lens in front, so the lens speed is f/(250/40) or f/6.3. How it is possible that after inserting a disk, which is 50% or more opaque, we get equivalent of f/5.8? How do you expose without disks?

[Mark Woods]

I expose them with the F/ value engraved on them. I haven't had a problem shooting without a disk, in fact I rarely use the disks. YMMV

[pasiasty]

I've just compared my Imagon with some other lens (old Xenar 240/3.5) and results are as follows:

no disk - T 5.9
H 5.8 - T 7.1
H 7.7 - T 8
H 9.5 - T 9.5
H 11.5 - T 11.7

One explanation comes to my mind: the speed is actually lower than H value, but a soft focus lens diffuses much of the light and makes shadows less dense, so the difference between H and T values is to compensate this.

[Mark Woods]

That sounds good to me. The lens is very "flary" and that's going to add to the neg density. As I said, I shoot with them wide open all of the time. The results are a fairly low contrast image. One could change that with development/HC paper. I love them. I have the 200mm & the 300mm. I also shoot with an old 1909 Kodak lens that I've modified. It's even softer. The only "regular" lens I take out with me is a TR triple convertible: 12" 21" 28" Not coated. The 12" & 28" are good, but the 21" sucks.