ISO as a factor in exposure?

TimFox said:

I have no idea how the digital camera manufacturers define the "ISO" value on their cameras, but I suspect it is closer to "EI". If you change the "ISO" setting on a film camera, it only effects the setting on the internal light meter (with or without auto exposure based on that meter).

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They do the same as Ilford: they evaluate "ISO" on a practical, rather than ISO standard form. Most (all?) Ilford films have disclaimer like this:
"It should be noted that the exposure index (EI) range recommended for 100 DELTA Professional is based on a practical evaluation of film speed and is not based on foot speed, as is the ISO standard."

Edit: actually I was wrong, there is ISO for digicams: http://www.dpreview.com/forums/thread/2838786

I wonder how accurate my digital metering for film is then... :-/

bdial said:

It sounds like you are speaking of "exposure" in terms of the amount of light the subject is exposed to, where in still photography, most of us think of exposure in terms of the amount of light allowed to strike the film.

Still photographers have a lot of flexibility in adapting to the light hitting the subject, we have both duration of exposure (to the film/sensor) and intensity to work with. OTH, cinema photographers have a fixed shutter speed, and so control exposure by the amount of light placed on the subject, which is why even when shooting in daylight they have truckloads of lights, reflectors, and shading panels to exercise that control.

In both cases, the ISO of the film is a "tweak", that is, we can choose a faster or slower film, which then influences the other choices. A faster film will allow the use of small lens apertures, or shorter shutter speeds, to yield increased depth of field or reduced movement blurring, for example.

Does this address the point you are making?

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No, not my point, nor I think Chan's.

I think both of us are assuming that the scene EV would be the same for any specific comparison.

I think Chan is suggesting that in the digital realm the ISO setting is not necessarily considered a tweak, that it has become a primary adjustment that can be used in place of/instead of time and aperture.

I'm suggesting that we can do essentially the same thing with film, as long as we consider the adjustments that can be made in the darkroom.

XP2 is a great example, the following was copied from the Ilford spec sheet:

CHOOSING THE RIGHT FILM SPEED FOR THE JOB
Best overall quality EI 400/27
Finer grain (with easy printing) EI 200/24
Finest grain (but with denser negatives) EI 50/18

Note
No matter which film speed is chosen, standard C41 processing is recommended.

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In this example essentially only the exposure of the paper need be adjusted and the ISO/EI can be allowed vary from frame to frame. Using ISO/EI as a primary adjustment allows a very different way of shooting than Adams used and taught. Yes the technical look of the print may change with the change in exposure but, the creative use; the ability to maintain a given DOF and given amount of motion blur across a range of EV's is very freeing.

This same principle (ISO/EI being adjusted) is used in disposable cameras, Holga's, blah, blah, blah...

In my experience normal B&W films can be used the same way.

Yes Mark! I saw posted by those folks who claim they use manual mode but with auto ISO so basically they can set the shutter speed and aperture any where they want and yet it's the meter that automatically gets the right exposure (or not). Since many times the exposure is not correct automatically the standard control for exposure is Exposure Compensation control. Therefore on their cameras it must be an easy to use control not one of those on our camera where it locks and located in place hard to use with the camera to the eye.

Chan Tran said:

I don't hear anyone in apug saying so but folks in the D forum consider ISO as one factor in exposure. They talked about the exposure triangle. But however, one control the amount of exposure by changing the subject brightness (if that is possible), changing the aperture or changing the shutter speed. The ISO doesn't change the amount of exposure the film gets.

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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.

BradS said:

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

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Yes, but H=I*t
sorry.

RalphLambrecht said:

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.

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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.

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.