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"rating" film

Karencs

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35mm
I've read interviews with film photographers on the Web who talk about "rating" a 400 film at 320 and I'm not clear on what that means. Does it meant that they put in a 400 film and set the ISO on the camera to 320 then let the meter think the film is set at 400? Something else?
 
I think it means with some peoples exposure and development then get the results they are after by rating a film higher or lower than the box says. But I think it depends all on how you do everything and testing, I have been told its best to shoot as the box says and then after a while change one thing at a time to see what results you get back and fine tune from there.

Paul
 

It simply means that they don't think the advertised film speed is correct for them. So yes, they take a ASA 400 film and tell the camera or meter that it is an ASA 320. This will expose the meter by an additional 1/3 stop over the box speed.
 
But is it not all about the equipment you are using, say your meter is off my 1/3 stop then you adjust the film speed, so say I use a different camera meter setup would you not change the speed again if needed?
 

The idea is to load a roll of 400 speed film, set your meter to 320 and then let the developer think that it was exposed at 400. What you've done is slightly overexposed the film by 1/3 of a stop, and then developed the film normally - by normally, I mean by following the developing procedures as if you have exposed the film at 400.

This is a fairly simple way to gain a little bit more shadow detail without getting into more complicated techniques in metering and developing. There are many threads on APUG devoted to this topic.

Tim
 
Shaggysk8 said:
But is it not all about the equipment you are using, say your meter is off my 1/3 stop then you adjust the film speed, so say I use a different camera meter setup would you not change the speed again if needed?
Click to expand...

Correct, that's part of 'the advertised film speed is not correct for them'. It is the difference between how the manufacturer rates this film and how it behaves for the photographer who uses it. That's why it's called personal film speed or exposure index 'EI'.
 

Rating a film at a different film speed has nothing to do with development. In fact, every exposure index has it's own development. Rating a 400 box speed at 320 will most likely require that the development is shortened a bit to control the overexposed highlights.
 
Here's a response on this general topic of rating and the nonlinearity of the tone scale that I gave in (there was a url link here which no longer exists); perhaps it will be helpful:

~~~
The thickest portions of the neg become the highlights in print; the thinnest parts of the neg become the shadows. The difference in density is the contrast index.

The important thing to remember is that the tone curve (neg density versus exposure) is nonlinear... it has a quite linear central part, but then it has a "knee" and at the other end a "toe." If the tone curve were linear then tones would just shift proportionally when you under/over expose. But the nonlinearity affects the tone separation when you under/over expose. So under/over exposure has different effects on details falling on different parts of the tone curve.

~~~

It is important to think a bit about what the slope of the tone curve means to how the tones will be rendered in print. Where the tone curve is very sloped, there will be more separation between adjacent tones. Where the tone curve is flatter (i.e. above the knee and below the toe) the tones will tend to clump up (=posterize). The latter situation is a major benefit of film (wrt/ digital) because there is a tendency for highlights not to blow in a specular way but rather to have a slow, smooth transition. What's so nice about film is how smoothly the tones ease into the knee and toe regions.

Now, how you expose and how you develop will determine where the tones of your scene fall on the tone curve. People may tell you to rate the film at some number different from the box speed; sometimes that is because they prefer more shadow detail or more highlight detail... or similar effects. It's worthwhile to listen to those opinions, but first and foremost, you should learn how to get full range and optimal detail (=good tone separation) across the tones! And usually that means placing the centremost midtones smack dab in the middle of the tone curve and developing normally. Simple.

You can also develop the film in such a way that there is less overall range or more overall range. You'd typically do this to build contrast or to reduce contrast.

~~~

Short version:

(1) In a well-exposed neg, the midtones will be somewhere around the middle of the tone curve;

(2) If you overexpose, the resulting neg will be too thick and the highlights will tend to clump together at the knee of the tone curve;

(3) if you underexpose, the resulting neg will be too thin and the shadows will tend to clump together at the toe of the tone curve.
 
Thanks for all of the fast responses. Think it's time to go out and give this a try.
 
Anon Ymous said:
Contrast Index would be the appropriate term I suppose. If you develop your film for a given CI, that affects the obtainable speed as well doesn't it?
Click to expand...

Film speed depends on material and process characteristics (inputs), not on desired shadow detail or contrast index (outputs).
 
RalphLambrecht said:
Film speed depends on material and process characteristics (inputs), not on desired shadow detail or contrast index (outputs).
Click to expand...

I guess that's true from the manufacturer's point of view (ISO standards?). The user's point of view might be quite different, and that's why we all have our personal preferences.
 
When you choose a personal EI to use with a film, you aren't "rating" the film alone.

Instead, you are "rating" your combination of the film, your equipment, your approach to metering, the light conditions you work in and, to some extent, your development process.

If your technique or the conditions are inconsistent, your EI won't be totally reliable.

That's one of the reasons why it's nice to have some latitude built in!

Matt
 
There is a simple test you can run to establish the effective iso for your equipment. It works best if You are developing and printing your own film.
 
Anon Ymous said:
I guess that's true from the manufacturer's point of view (ISO standards?). The user's point of view might be quite different, and that's why we all have our personal preferences.
Click to expand...

Well, the user needs to decide what outputs are required and select the inputs accordingly. What is the contrast index and desired shadow detail? and then... What is the exposure and development to get us there?
 
jeffreyg said:
There is a simple test you can run to establish the effective iso for your equipment. It works best if You are developing and printing your own film.
Click to expand...

Forgive me, I need to nitpick again. There is no such thing as an 'effective ISO'. There is only one ISO speed, and it's written on the box! What you mean is your 'personal film speed', or as it is usually called 'exposure index' (EI). I'm sure that's what you meant anyway, right?
 
I rate B&W film in part for the development I get using my particular developer and developing methods, and in part for the properties of the negative I desire for my particular intention. In my case, for most subjects I prefer dense negatives with good contrast, so my film speeds are usually under rated. Exactly how much depends on my experience with a particular emulsion, the subject at hand, and also how and on what I intend to print it. Most photographers that are going to do their own printing and developing can't go too far wrong by adding a third of a stop (many developers/developing regimens don't give full speed and a bit of overexposure beats under exposure hands down, and that third is good insurance), but beyond that it takes experience or testing with a particular stock and developing regimen to find a personal film speed for a given emulsion, and even then, it may change with experience or other factors.
 
Ralph,

This is the way I understand it:

The ISO film speed depends on the material, the developer, and a standardized process i.e. in ID-11 Delta 400, processed per Ilford instructions exactly, is ISO rated at 400. If Microphen were the standard developer Delta 400 might have been ISO rated at 500 and called Delta 500. If Perceptol were the standard it may have been called Delta 250.

Once the film/developer/process combo is defined, modifying the process; time, temp, or agitation, re-defines the contrast index the film is developed to, n-1, n+1, n+2. More developing, more contrast, less exposure latitude, and vice versa.

The big effect is on the highlight end of the curve but the shadow end changes a bit too. An n+1 process might bump the film rating to a 1/3 of a stop, say from 400 to 500. Using n+2 might bump the rating from 400 to 640.

Where shadow details pick up and highlight details end is defined by exposure placement within the contrast range a specific film/developer/process combo can create.

RalphLambrecht said:
Film speed depends on material and process characteristics (inputs), not on desired shadow detail or contrast index (outputs).
Click to expand...
 
"Film Rating" is a synonym for "Exposure Index" or "EI"
 


Mark

That's correct! Only one thing, Ilford doesn't rate their film speeds according to ISO standards, and they say that in their spec sheets, but otherwise, your statements are the way I understand it too.
 
Ralph, I agree with your "nitpick" but I was referring to his particular camera as the shutter speed may not actually be what it says on the dial, his developer ,etc. It's the end result that counts in the long run.
 
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