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Over exposure and highlights


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#1 Rob Featherstone

Rob Featherstone
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Posted 17 November 2009 - 10:55 AM

Hello:

I am wondering about peoples' working practices when they rate a stock at a lower ASA than the manufacturer recommends.

Specifically in regards to to the shoulder and toe of the characteristic curve.

Say the stock is rated at 500 ASA and the characteristic curve says the stock approaches full density at 5 stops over middle gray.

I decide to rate the stock at 250 ASA. Normal processing.

When I point my spot meter at a window and it is 5 stops over middle grey would you consider this full density/ no detail or would you think there is an extra stop there because you have slid down the
curve by re-rating the film at 250?

In other words do you disregard the manufacturer's characteristic curve or consider that you
are moving down it?

I know the real answer is to shoot comprehensive tests and judge for myself.

I am just curious about other dp's working practices.

Thank you!
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#2 David Mullen ASC

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Posted 17 November 2009 - 11:52 AM

Unless you alter the curve by push or pull-processing, it stays the same shape no matter how you rate the film, so all you are doing by giving it a different rating is placing information at a different point on the same curve, so yes, if you rate it a stop slower, you lose a stop of overexposure information and gain a stop of underexposure information. More or less. The overall dynamic range stays the same.

On the other hand, since the curve is flatter at the top and bottom, the response to light at either end is no longer linear, so it may not exactly be a stop at the ends when you are measuring stops in the middle.

Most of us rate a stock 1/3 to 2/3 of a stop slower than recommended, which is not significant compared to the overall curve or in terms of printer lights. It just means that our midtones are slightly higher on the straight-line portion of the curve.

I don't use a spot meter for measurement, not too often, but generally you aren't placing important detail at the extreme ends of the curve, you are measuring midtones and then measuring bright and dark areas that you hope to keep within what you consider a basic black and white point. In other words, you may decide that 5-stops over 18% grey is "white" and 4-stops under is "black" even though the stock itself will register small amounts of detail beyond that 9-stop range. So if you actually rated the stock one-stop slower, it's up to you whether you want to keep saying that white is 5-stops over and black is 4-stops under -- the latitude of negative stock can handle that (actual dynamic range is more like 14-stops), it's just that more of your brightest highlights are going to be in the flatter, muddier part of the shoulder, whereas more of your darkest details will be lifted out of the flatter, muddier part of the toe.
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#3 Rob Featherstone

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Posted 17 November 2009 - 12:37 PM

thank you!
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