Need help with exposure compensation for bellows extension

[cowanw]

Nice to know that I can ignore it when using a macro lens or extension tubes.

I didn't say that.
My understanding is that the OP understands the math, he has the theory down pat. His video is clear. He gets "exposure compensation"
He just wants to understand why the measurements don't work for his Vivitar 55mm macro lens.

[Doremus Scudder]

You are right with the TTL but the paper is around ISO 3 or less. So I'm going off the scale of the camera there. Then when I get to use b/w colour filters I'm completely off the scale. But again I want to get the knowledge to apply this to a M645 +/- ext tubes (maybe a 124G too) before I'm happy to start burning whole sheets on the Sinar. Which I can't afford to do.

You do realize that your paper is, at best, "orthochromatic," i.e., only sensitive to blue and green light. However, it's not like orthochromatic film, since the proportion of blue to green determines the contrast with VC paper.

If you're using graded paper, it's only sensitive to blue light.

In either case, using standard B&W color "contrast" filters won't work like they do with panchromatic film. A yellow filter will make your paper negative on VC paper less contrasty in addition to maybe making that blue sky a little darker. On graded paper, a yellow filter will effectively block any light the paper is sensitive to, giving you practically no exposure. Red (and orange) filters are effectively "black-out" filters for both kinds of paper. A blue filter will make your VC-paper negative a bit more contrasty and give it a blue-sensitive look (and make absolutely no difference at all on graded paper). Green filters will act similarly to yellow filters. You may want to reconsider using filters at all.

As far as bellows extension goes. If you've got a TTL metering system on your camera, just use that. If the E.I. of your paper is too low to set on the camera, just use a different setting to take a reading and then make an adjustment with exposure compensation. Example: if your paper is ISO 6, then set your meter to ISO 50 and then overexpose from that by three stops (50 - 25 - 12 - 6 is the ISO scale). You should be able to dial that in on your exposure compensation.

Trying to measure bellows extension on a 35mm camera is an exercise in futility. With your macro lens, there is likely some internal focusing that makes measurements useless. Just don't bother. If you don't have a TTL meter, use magnification ratio to find your exposure compensation. Just set up a scene, find 1-to-1 (if you can, most lenses won't focus that close without extension tubes, etc.) or 1-to-2, or whatever and mark that on your lens. Mark other reproduction ratios. Then work backward from the magnification to find your exposure compensation.

And, if I may ask, what the heck are you trying to do taking macro photographs with 35mm paper negatives anyway?

Best,

Doremus