Exposure and using light source clipping as a baseline

I’ve been experimenting with an exposure technique that Nate has suggested before, i.e., taking a reading from the pure light source (without holder or film in place), setting the shutter speed at the point where the light source just clips, and then using that as a baseline exposure, since once the negative is in place, the film emulsion’s least dense areas will come in under the clipping limit. This has given good results so far using a Sony a6700.

One thing, though: I haven’t been using the camera’s histogram for checking clipping because the histogram is a representation of the in-camera JPG preview, not of the RAW file. So I use zebras instead. [EDIT: I think they’re based on the JPG preview too, but you can set the clipping limit higher.] I’m currently using “lower limit 109+”, which makes use of Sony cameras’ 9% headroom above the histogram clipping point. In other words, it’s my understanding that the histogram’s clipping point is set to 100, whereas the camera sensor can go to 109 without clipping when shooting RAW.

So here’s my question: Is there any reason at all to set the zebras a bit lower, above 100 but less that 109? Or is 100 (same as the histogram) preferred for some reason, maybe because of something to do with the NLP conversion process? Or should I just keep going with 109 since it seems to be working out so far? Thanks!

[EDIT: I know there’s no NLP profile as of yet for the a6700 and that I’ll have to redo the conversions later, but I’ve been testing this exposure technique anyway, just to see what the ballpark results are like].

For me basing the exposure off the point when the light source starts to clip is just that, a consistent repeatable way to arrive at a base level exposure from which you can then decide the optimum exposure. For that reason I don’t see that it matters too much if you use 100+ or 109+ because whichever you use you should probably experiment to arrive at your optimum exposure for NLP conversions. Obviously setting to clip at 109+ will give you a slightly longer exposure than if you set for clipping at 100+ but only by 9% or so.

Many suggest over-exposing for colour negative so why not experiment with half a stop more than ‘109+ base level’, then a full stop more.

You may at the end of the day decide that your 109+ exposure is right, but you may decide to give it a bit more because it gives better conversions, at the end of the day it is your choice.

Yes, quite right, the question of using one’s best judgement is fundamental. And my question was about whether there’s a theoretical ‘right’ answer or best practice because the situation with the histogram is a bit complicated. But as you’ve indirectly (and rightly) pointed out, it would be difficult (in this case anyway) to separate a theoretical right answer from the question of good judgement.

Nevertheless, regarding the histogram and zebras, I’ll just mention something that I’ve read elsewhere: the histogram clipping point (100+) seems to be set to R235, G235, B235. And if you bump 235 (clipped) up by 9% you get 256, or pure white + 1 (i.e., clipped). Hence the 109+ zebra setting, I guess. In other words, I take it that the RAW format allows for recovery of highlights in that specific zone between 235 and 255. (And of course that’s a bit of a simplification but let’s just go with that for the moment.)

My theoretical question was (to reformulate it) whether pushing the clipping threshold up to 255+1 might have any negative impact on the conversion process itself (as a question of math or science).

But maybe the answer will be functionally the same. For example, maybe the conversion process just doesn’t care where the clipping point is set, in which case we’re thrown back on our best judgement based on results.

I would suggest that NLP doesn’t care, it just cares about the exposure. As you know Nate suggests keeping the exposure constant but others will adjust it if they have particularly dense or particularly thin negs. Quite a few devekop their own colour negs in which case I’d think you’d have to allow for under and over development as well since it’s difficult to keep the necessary consistency when home developing.

I don’t have a Sony but I had to look into their ‘Zebra’ settings for something else. A video that I watched stated that the Zebras only work off the green channel. Absolutely no idea if that is true.

Yes, I’ve also read that the zebras are based on the green channel only. If so, it makes sense because, on my understanding, the green channel will be the first to clip in most bright-ish lighting conditions or when using a daylight white balance (see here). Green channel zebras should therefore be fine for negative scanning, where the light source is supposed to be stable somewhere in the 5000-5500K range.