ND Filter

BrianShaw said:

Is the “uncoated” really that flat of a curve, or is that notional?

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Petrochemist said:

It probably is that flat.

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No and yes....
Both, refraction and reflection of glass is dependant on wavelength. For refraction this nonlinearity (dispersion) is most important, for reflection though considered negligible. And thus omitted in the respective graphics I know.

AgX said:

No and yes....
Both, refraction and reflection of glass is dependant on wavelength. For refraction this nonlinearity (dispersion) is most important, for reflection though considered negligible. And thus omitted in the respective graphics I know.

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Considering the graphs specifically refer to reflection any dispersion would not be relevant, in any event it is fairly minimal for thin pieces of clear glass.
So as you say wavelengths effects are negligible on uncoated glass - i.e. Yes it is apparently flat.
Some minor variations do show in all the filter spectra I've measured (a few hundred using kit at work), but I tend to go for relatively quick scans & my filters are all old & battered (and have dyes etc rather than plain glass...)

Petrochemist said:

Considering the graphs specifically refer to reflection any dispersion would not be relevant,...

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But reflectivity variation over the spectrum and dispersion are related to each other and that was my point.

Hi guys
Reading here some recommend using variable ND filters, how would one calculate exposure for a (non-TTL) say Hasselblad V camera?

Make markings at the variable filter, then meter the transmission wth a TTL camera or hendheld meter per marking.

AgX said:

Make markings at the variable filter, then meter the transmission wth a TTL camera or hendheld meter per marking.

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Thanks! Still pondering between an ND200/ND400/ND1000 and a variable ND since I already have an NDX8

catacore said:

Thanks! Still pondering between an ND200/ND400/ND1000 and a variable ND since I already have an NDX8

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Get some experience with the ND filter you have before spending more money on more filters. Go shoot film.

I just got another thread going on same subject without realizing this was on. To me ND filters are mostly special case filters. While can be used to fit shutter/film used limitations for given available light, I only see them as when very slow shutter is called for special effects. As it appears VD NDs are inferior to fixed ones, I don't think I will entertain convenience for obvious additional layer of effect uncertainty. And exposure through a fixed one will at least be more repeatable. VD can be set on a mark, but this is just a layer prone to making mistakes. And on film each frame counts.

I was guessing some sorts of exposures (like 30-60 seconds or more) would require more than an NDX8 filter.