C-41 overexposure - What is effect on resolution and sharpness?

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camera critter - 17 Mar 2005 17:57 GMT

Does anyone have any numbers (RMS or MTF) to show how much the
resolution and sharpness of color negative film (print film)
deteriorates when the film is over-exposed by 1 or 2 stops?

For example, exposing 100 speed film at EI=50, exposing 400 speed film
at EI=100 and EI=200, exposing 800 speed film at EI=400 and EI=200.

Overexposure of color negative film (but not b&w negative film) is
reputed to decrease the perceived graininess because its dye clouds
overlap more, but I suspect that some tradeoff probably is paid in
decreased resolution and sharpness. I understand that negative films
give better sharpness when slightly underexposed.

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Bill Tuthill - 18 Mar 2005 18:47 GMT

> Does anyone have any numbers (RMS or MTF) to show how much the
> resolution and sharpness of color negative film (print film)
> deteriorates when the film is over-exposed by 1 or 2 stops?

Datasheets of movie film graph both grainyness and sharpness
against ISO exposure value, but I've never seen that for still film.

> For example, exposing 100 speed film at EI=50, exposing 400 speed film
> at EI=100 and EI=200, exposing 800 speed film at EI=400 and EI=200.

It depends on the film. Steve Dunn did a study with Supra 400
(IIRC) and decided it lost sharpness at 100 but not much at 200.
When testing a film I try bracketing to see the grain/sharpness
tradeoff. Grain often improves at +.3 EV but sharpness doesn't
usually suffer until way beyond that.

> Overexposure of color negative film (but not b&w negative film) is
> reputed to decrease the perceived graininess because its dye clouds
> overlap more, but I suspect that some tradeoff probably is paid in
> decreased resolution and sharpness. I understand that negative films
> give better sharpness when slightly underexposed.

I'd say that most negative films attain optimal sharpness when
correctly exposed. When underexposed, they grain up (especially
in shadows) thus reducing acutance.

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