ISO as a factor in exposure?

[Chan Tran]

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.

 

[thegman]

The ISO does not change how much exposure the film gets, but it changes how much exposure the film needs.

I certainly consider ISO a factor. I like very slow film for some things, as I like long exposures, with blurred crashing waves, that kind of thing.

I'd love to see a forgiving C41 film like Portra 400, but with an ISO of 25 or something, would be really interesting to use. I could get an ND filter I guess.

 

[Pioneer]

In one respect it is. But since the ISO is selected in advance by your choice of film it is a passive part of exposure. In a way it becomes the platform off which your other exposure decisions are based. Obviously it is more dynamic in the d***l camp.

But, if you shoot large format, you can easily use ISO just as dynamically.

 

[kintatsu]

It is part of the triangle. ISO or EI determine the amount of exposure as stated above. Then, given that, the other sides can be determined based on desired outcomes. Change 1, and the others must change to keep the same image properties.

 

[MattKing]

In the digital world, you can adjust the light sensitivity by changing a setting on the camera.

In the film world, in some cases, we can switch film backs.

 

[Christopher Walrath]

In the film world, in some cases, we can switch film backs.

Or just keepclicking away until the film advances no more, rewind and then load in a different ISO.

 

[ntenny]

The Zone System and its kin use sort of the same point of view, in which the (effective) speed of the recording medium becomes the third variable. Where the Zone System folks say "N+1", the digital shooters are saying "increase the ISO setting", but it comes to much the same thing: a stop less exposure.

-NT

 

[Tom1956]

By the way, ISO stands for American Standards Association. HNY

 

[markbarendt]

Anybody shot with a disposable?

Essentially what changes/adjusts with each shot in these cameras is the EI. The only way to adjust exposure with these cameras is by switching films or adjusting your print process.

I shoot this way with my Holga, drop in a roll of Portra 400 or 800, go shoot, and then adjust the enlarger to suit the frame. Works surprisingly well.

 

[Christopher Walrath]

And ASA stands for the International Standards Organization.

Or is that the other way around...

 

[Christopher Walrath]

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.

 

[gone]

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.

 

[BradS]

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

 

[Pioneer]

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.

 

[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.

 

[ic-racer]

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.

 

[markbarendt]

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.

 

[ic-racer]

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.

 

[Chan Tran]

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.

 

[markbarendt]

Chan,

Even in the digital world there is a native rating rating for any given camera, all the other settings are essentially the processing being adapted. People tend to think of the setting as a real change but it is not, the sensor does not change.

It is like switching channels on your radio, some stations are louder than others, we use the volume control to adjust. Digital shooters use the ISO knob to similarly dial up or down the gain.

Our analog equivalent is our enlarger exposure settings.

 

[E. von Hoegh]

And ASA stands for the International Standards Organization.

Or is that the other way around...

You're both wrong. It's Deutsche Industrie Norm that ISO stands for. Sheesh.

 

[TimFox]

I once went to the trouble to look up the actual definition of the ASA/ISO value for photographic materials (I think it was in the collection of ANSI documents at Chicago Public Library). Since the true ISO value is a function only of the film, it relates to the "toe" of the exposure curve, since the slope of the curve above that depends on development. Ansel Adams' careful description of the Zone System is consistent with that definition. "EI" generally relates to a midpoint up the curve (where one would normally expect 18% grey), and therefore depends on development. 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).

 

[Chan Tran]

You all missed my point! The amount of exposure doesn't change by changing the film sensitivity. The film will have different densities depending on its ISO (or sensitivity) but if you don't change the aperture or shutter speed or the subject brightness the film is receiving the same amount of exposure no matter what. Although for the same amount of exposure one film may be underexposed and the other overexposed as they have different sensitivity or ISO rating.

 

[markbarendt]

I actually understand your point and I don't disagree that there will be a difference in density between films of different sensitivities.

But, comparing Film ISO and Digital ISO is an apples and oranges comparison.

The digital ISO standard doesn't care what the sensitivity level of the sensor is, the "developing" process is included in the standard. The ISO standard for digital cameras uses the output of the camera, specifically a JPEG image having specific qualities (essentially a digital print), to define the camera setting that created it.

The film ISO standard doesn't care about the print. (edit: yes the original standards were based on judging prints, but it is now a mathematic formula.)

If I use the idea behind the digital ISO standard and include my printing process in my film ratings (my EI) I can better compare the mediums.

I can, and regularly do, adjust my printing process with Delta 400 (to do the equivalent of spinning the digital ISO dial) from about 1250 to about 25 (first excellent print to last excellent print). FP4 roughly from 250 to 8.

 

[bdial]

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?