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# Thread: ISO as a factor in exposure?

1. If one wishes to get really tech-nickle about the whole matter, ISO is key in determining a film's key stop which is the square root of a film's speed rating (ie. square root of 400 is 20, key stop = f/22). Along with determining a starting shutter speed going from your meter reading of a middle value (1/(middle reading's candles per square foot) gives you a base exposure from which to begin to determine where you want your negative to go.

Of course, that's REALLY tech-nickle.

2. You summed it up Chan in your last sentence. It has nothing to do in the real world w/ your exposure. Like others said, everything has to change together. With film, it's usually an indicator for what the film's grain will likely be like, as low ISO films tend to be more fine grained than high ISO films, but it escapes me as to how that would influence exposures. I'm not sure why it would be different over there in the D forum, but as I only shoot film I'll leave that one for others.

3. 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

4. Originally Posted by momus
You summed it up Chan in your last sentence. It has nothing to do in the real world w/ your exposure. Like others said, everything has to change together. With film, it's usually an indicator for what the film's grain will likely be like, as low ISO films tend to be more fine grained than high ISO films, but it escapes me as to how that would influence exposures. I'm not sure why it would be different over there in the D forum, but as I only shoot film I'll leave that one for others.
Makes absolutely no difference whether you shoot film or digital, it has everything to do with your and my world if you are actually taking pictures.

5. I think many times we get to hung up on thinking that "appropriate" means "exact" or "one right exposure setting", for example to match densities with the standard Ansel set.

IMO appropriate camera exposures fall inside a range, not at a given point.

6. Originally Posted by markbarendt
I think many times we get to hung up on thinking that "appropriate" means "exact" or "one right exposure setting", for example to match densities with the standard Ansel set.

IMO appropriate camera exposures fall inside a range, not at a given point.
Yes, in Jones et al they demonstrated a plateau in picture quality with increasing exposure when negatives are used for contact prints. However, smaller negatives that would be used for enlarged prints, there was usually an optimum exposure. This is because the film tends to lose resolution as silver density builds up. Though, you could consider it a plateau with a peak value.

7. Originally Posted by ic-racer
Yes, in Jones et al they demonstrated a plateau in picture quality with increasing exposure when negatives are used for contact prints. However, smaller negatives that would be used for enlarged prints, there was usually an optimum exposure. This is because the film tends to lose resolution as silver density builds up. Though, you could consider it a plateau with a peak value.
I do agree that there're limits and that staying within a tested range is important to get decent quality.

I also believe that it's not the same for everybody, it's highly dependent upon the subject matter in question. Those for whom high detail in both shadow and highlight is important, have a much smaller range to play in, than those of us that are just worried about landing the mid tones.

As with everything photographic, there are trade-offs. For me, and my subjects, allowing the EI to float is a way of making shooting simpler and even getting people to take the camera less seriously (say with a Holga, disposable, or 35mm P&S instead of an F5). It's also a way to focus more on content and timing and worry less about the technicalities.

8. Originally Posted by Christopher Walrath
And ASA stands for the International Standards Organization.

Or is that the other way around...
Actually ISO stands for International Organization for Standardization so the acronym is a little strange.

9. Originally Posted by markbarendt
I also believe that it's not the same for everybody, .
Exactly! The Jones et al papers used a panel of viewers to rate the images. You (and most advanced film users) don't need a panel of viewers to judge the exposure of the negatives used for your prints. Though most beginners and casual users benefit highly from the information contained in the ISO number and it is good for everyone to know how the ISO numbers come to be.

10. What I really meant is that if someone with light skin and I who is asian an have relatively dark skin go out to the beach together and sun bath at the same spot for the same amount of time. Both of us would receive the same exposure to UV light but the effect is different as each has different tolerance from UV exposure. This is the different in ISO but the exposure is the same.

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