# ISO as a factor in exposure?

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• 01-08-2014, 06:05 PM
RalphLambrecht
Quote:

film speed is certainly a factor in determining exposure

I think of it as the two sides of an equation...on the one side you have film speed and light on the other side you have aperture and shutter open time.

S + L = A + T

for a given film speed and light, you need to select an appropriate aperture and shutter speed

Yes, but H=I*t
sorry.:D
• 01-08-2014, 08:28 PM
Chan Tran
Quote:

Originally Posted by RalphLambrecht
the analog analogy equivalent of the exposure triangle is the exposure waterbucket.the aperture is the size of the valve;time is the filling time and ISOis the size of the bucket. the object is to fill the bucketto the rim. too much waterand the bucket overflows=over exposure;not enough water and the bucket is not full=under exposure;filled to the rim is perfect=perfect exposure;so whatever you change (aperture,shutter speed orISO)just make sure to fill the bucket;anything goes,but with analog,the bucket has a fixed size.:whistling:

Exactly but by the meaning of the word exposure only the amount of water is the exposure. The size of the bucket doesn't matter. It does make the water run over or too low in the bucket BUT that's the results of a specific exposure (a specific amount of water) on a different film emulsions (different buckets) have the different results yes but the exposure (the amount of water allowed to the bucket) is the same given the same subject brightness (water supply pressure), aperture (size of valve opening) and exposure time (filling time) you get the same exposure (same amount of water going to the bucket). My argument is that the term exposure in and by itself does not involve sensitivity.

The exposure which is measured in Lux.Sec is only intensity x duration. The sensitity isn't the exposure.
• 01-09-2014, 05:01 AM
markbarendt
The bucket analogy is very useful for beginners just learning to set an exposure. It can get unwieldy though when we start talking about more advanced possibilities.

For example the bucket analogy suggests that "a single perfect camera exposure setting exists" where the bucket is full to the rim, no more, no less; when in reality for most negative films "there is a significant range of possible exposure settings" (EI's) that can be used to get excellent prints.

Part of this "perfect exposure" myth comes from Adams et al, in that they quantified perfect and people like firm right/wrong definitions. The problem is that the west coast school definition of perfect is based on a set of criteria (an artistic ideal) that doesn't necessarily matter to others. (Specifically sharp focus, typically small apertures, no visible grain, minimizing exposure time...) To those who shoot short DOF, who view grain as part of the expression, or who have camera shutters that top out at 1/400 or 1/100 and who purposefully use fast film mid-day, the calculus is very different.

The other problem that the bucket analogy has, is that it suggests to the person receiving the analogy that "everything in the bucket" (everything on the negative), will or should naturally straight print.
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