What does 'rating' film speed mean?

I always hear how peope 'rate' films. For instance you have say T-

Max 400 but people say they 'rate' it at, say, 320. What does this

mean? Does that mean those people pull it to 320 instead of 400?

Does this affect how you should develop it? And how do you know

to 'rate' it at something other than its marked as? I understand why

you would want to pull or push film for various reasons but dont

understand this business about rating film...

 

Just curious...

Simply, 'rating' film at something other than its marked speed means setting that rated speed (from your example, 320) on their ISO dial and shooting it that way. It's then processed normally. You are in affect over or underexposing the film a bit from what the manufacturer recommended.

 

How do you know to do this? Either by word-of-mouth or your own experimentation. If somebody that you know gets better results by 'rating' a given film differently (and you've seen the results and trust their methods), then give it a shot.

You control cantrast with development: longer time in the soup = more contrast.

 

You control density with exposure. Increasing development WILL increase density, mostly in the high light areas, less in the shadows.

 

Experiment with development to get a negative with a contrst you like, then fine tune your exposure to get the shadow detail you want.

In most cases that will be pretty close to the manufacturer's rating, like Tri-X at 400. But it might be 250 or 320 or 500 or ?

 

It also depends on your meter, how you use your meter, what in the scene you read with your meter

Many people use a lower speed rating than the manufacturer claims because of different standards on how to measure film speed. In addition, the choice of film developer can have some impact on the speed of the film.

 

In the Zone System, film speed is rated by measuring the density of the negative with a densitometer. A Zone I exposure (4 stops less exposure than would be given to an 18% middle gray card) should yield a density of .10 over film base and fog (the "clear" part of the film). Zone I is used because development time has little impact on Zone I density.

 

Keep in mind that the difference between 400 and 320 is only approximately ½ stop.

Rerating a film, up or down, doesn't necessarily involve changing development. Some folks simply prefer the results they get by rerating a film during exposure but processing it at the nominal speed. A common example is Velvia - some folks shoot it at 40 because they believe they get better results, but they have the film processed normally.

 

Sometimes this is due to differences in metering technique or simply because their meter isn't calibrated the same as others. I shoot Kodachrome differently according to the quality of the light. It tends to go greenish under heavy foliage so I'll overexpose slightly to minimize this effect. I don't bother changing the ASA/ISO dial, tho', because I've been using Kodachrome for so long I'm accustomed to compensating in my head.

 

Another example: when I use a green filter with chromogenic monochromes like Kodak T400CN I prefer shooting at 250. Not to compensate for the filter factor - I'm already doing that with TTL metering. I simply prefer the results under normal processing.

 

Rerating film *and* customizing the processing is another matter. For b&w work zonies do it in pursuit of an ideal balance of contrast and tonal range. In color work it's usually done out of necessity when more speed is desired. But some photographers deliberately "push" a film - underexposing it via rerating the speed higher, accompanied by overdevelopment - because they want more contrast or a difference in color saturation.

Peter-as has been said all film manufacturers give us a starting point for setting our light meters through their assigning an ISO number. Remember the words "starting point". If you are shooting B&W your exposure primarily controls the amount of shadow detail in your negatives. If your negatives show adequate shadow detail your prints will show dark rich detail in those areas. If your negatives lack shadow detail they will look very transparent in those areas and your prints will show dark shadowed areas as black without detail. If your negatives lack shadow detail what do you do? You begin lowering the "rating" of your film. For an ISO 400 film you might choose 200. That new number is commonly referred to as your E.I. (exposure index-a personalized version of the ISO). If your shadows show too much density (unlikely but possible) you would choose a higher number. These adjustments are normally made independent of your development times. Don't think of these chages as over or under exposing - think of them as exposing "enough" for the pictures you want. If you are shooting color transparency film (slides) its just the opposite. You look at the highlight(bright) areas. If they are too bright (blown out/without detail) pick a higher E.I. (expose less) and vice versa. There are some pretty elaborate tests for rating B&W films. In fact entire books have been written on the subject but for most us if we take the time to carefully look at our negatives and prints we can get in the ball park very quickly.

Hi everyone, thank you for those helpful explainations. I guess everyone does end up with different (or preferable) results even when using the same film or equipment. This is more helpful to know than I thought! Just another way to calibrate to ones own satisfaction I guess.

Rate film at the speed suggested by the manufacturer. Expose according to the rules in most texts remembering that the camera meters for 18% grey and bright scenes may fool it (open up a stop) and dark may fool it too (reverse the process).

 

The Engineers at the film Manufacturer know more than we do. Trust their information and you will do FINE.

Meters are actually calibrated to a specific luminance (297 footlamberts), but the equivalent reflectance is 12% according to ISO/ANSI specs. The standards allow for some variation, but no where near 18%.

Whether or not the ISO/ANSI standards are 12% or 18% (about 1/2 stop difference), many meters are calibrated by the manufacturer at 18%. This may be becasue there are few (if any) 12% grey cards, while 18% grey cards are the standard. Some meters (such as many Sekonics) have adjustable calibration.

Mark,

 

Not that I have seen. Each manufacturer is required to state the K factor for their meters. Just check the owners manual. Most state a K = 1.16 (depending on the unit of measurement). This equates to a 12% equivalent. Manufacturers are part of the determination of the ISO/ANSI standards. It's for the benefit of all. Why would they not follow them?

 

Exposure is based on the following formula:

 

A^2/T = B*S/K

 

A = Aperture - f/16

 

T = Shutter Speed - 1/125

 

B = luminance = 297 footlamberts

 

S = Film Speed - 125

 

K = Constant - 1.16

 

To balance the equation, K must equal 1.16.

 

Reflected light meters use:

 

A^2/T = I*S/C

 

A = Aperture - f/16

 

T = Shutter Speed - 1/125

 

I = Illuminance - 7680 footcandles

 

S = Film Speed - 125

 

C = Constant - 30

 

Because both of the right hands of the two formulas equal the same left hand side, they also equal each other.

 

B*S/K = I*S/C

 

Equivalent reflectance can be found either by converting B to footcandles and dividing it by I, or dividing K (converted to footcandles) by C.

 

B*pi/I = Reflectance or K*pi / C = Refectance

<i>"Why would they not follow them [the ISO/ASA 12% reflectance standard]?"</i></p>

 

As I already stated, the reason is because there are no 12% grey cards, all the ones sold commercially are 18% grey cards to best of my knowledge. If you aim a meter calibrated for 12% at a 18% grey card you will not get an accurate reading (off about 1/2 stop).</p>

 

However, since most film manufacturers over-rate their film by about 1/2 stop, and a little more exposure rarely hurts, there is not much harm if the meter is calibrated to 12%. Several meter companies recommend using a 18% grey card for accurate exposure (including Sekonic which sells 18% grey cards).</p>

 

In the final analysis, when a photographer does film speed testing to take into account all of the factors involved in the entire end-to-end system (film, developer, meter accuracy, shutter accuracy, etc.), it doesn't really mater.

Mark,

 

Where is this information coming from? Mine is from multiple papers written by scientists and the ISO/ANSI standards that are derived from those papers. We are talking about tone reproduction and exposure theory here. If anyone is interested in reading more than the "popular" tech books, I'd be happy to put a list together.

 

First of all, who ever said 18% is middle grey? It is a visual middle (psychophysics), and not the average reflectance. Read the information that comes with the card. Kodak's card says to open up 1/2 stop when metering off the card. The 18% card is useful as a good reference within a photograph.

 

The speed of any film is exactly what the manufacture says it is based on the ISO conditions. The ISO refers to the standard which contains very specific conditions in which to test. They cannot place ISO on the film if they do not adhere exactly to the standards. Look at any of the "pushing" films - Delta 3200, TMZ, Neopan 1600. None of them say ISO. They all use EI. The old packaging of Kodak's other T-Max films (TMX, TMY) also didn't have ISOs because the films didn't do well in the old ISO developer. Because of this Kodak decided instead to test the T-Max films using a developer that produced speeds reflecting real world use (probably D-76 because the T-Max films were created using D-76), they couldn't use ISO on the packaging. The newer ISO standard now allows for the use of any developer the manufacturer wishes as long as it is stated somewhere.

 

In the final analysis, doing a Zone System calibration will set the film speed rating around 2/3 stop too low causing a 2/3 stop over exposure. This will place the Zone V exposure up around where it needs to be. The extra density in the shadow areas doesn't really hurt anything, besides flare plays such a big part with shadow placement that it really is absurd to think that shadow place can be precise anyway. From my point of view, it's important to understand how it really works so we don't go around claiming something like the film manufacturer are trying to put something over on us.

 

People can achieve good results without knowing anything about photography. My first camera when I was a kid was a pocket instamatic. It had a fix aperture and shutter speed, yet it still made acceptable images most of the time. So why should anyone bother with any of this technical stuff?

<i>"First of all, who ever said 18% is middle grey?</i></p>

Supposedly, Ansel Adams persuaded Kodak to use the 18% standard on their grey card. The reason was that 12% was about Zone 4.5 and it was more convenient to use make it Zone 5.</p>

<i>"The speed of any film is exactly what the manufacture says it is based on the ISO conditions...The newer ISO standard now allows for the use of any developer the manufacturer wishes as long as it is stated somewhere."</i></p>

Very few people use TMAX developers that Kodak uses to inflate its ISO speed rating.</p>

<I>"In the final analysis, doing a Zone System calibration will set the film speed rating around 2/3 stop too low causing a 2/3 stop over exposure. This will place the Zone V exposure up around where it needs to be."</i/></p>

Yes, I agree that proper film speed testing will place Zone V where is should be, regardless of meter calibration standard, developer used, shutter accuracy, etc. But even though the ISO standard may be 12%, I think there are some meters that are calibrated at 18%, at least according to the marketing departments of manufacturers.</p>

 

<i>"People can achieve good results without knowing anything about photography� So why should anyone bother with any of this technical stuff?"</i></p>

These forums are riddled with persons who regularly push film 1-5 stops (either on purpose or accidentally). That is a far cry from the 1/2 stop difference between 12% and 18%. With proper end-to-end testing, all the potential problems (meter calibration, effect of developer on speed, shutter accuracy, etc, will all come out in the wash and yield the correct exposure.</p>

 

But I don�t object to the �rule of thumbs� such as the f/16 rule (1/ASA for film speed in bright sun). This would yield far superior exposures than most amateur photographers use.

The f/16 rule of thumb should read "1/ASA for <b>shutter</b> speed in bright sun."

" Supposedly, Ansel Adams persuaded Kodak to use the 18% standard on their grey card. The reason was that 12% was about Zone 4.5 and it was more convenient to use make it Zone 5."

 

Trust me when I say that Ansel Adams wasn't a motivating source for photography standards. I'd love to know the source where you got this piece of information on the 18% card and Adams, but I believe this is just part of the Ansel Adams mythology. As far as I know Munsell determined sometime between 1900 and 1920 through psychophysical testing that the average perceived middle value was 18%. I believe the spread was from around 11% to 22% or so.

 

The values that meters are calibrated to have been adjusted over the years as measuring devices, optics, and the science of psychophysics have improved. According to Allen Stimson in a 1962 paper appearing in the Journal of Photographic Science and Engineering titled, "An Interpretation of Current Exposure Meter Technology," the 1962 "ratio of K to C is the average scene reflectance for which the meter is calibrated. The mean value is now R = 3.333/20.83 = 16%." So as you can see it has changed over the years.

 

The big question is, was Adams ever aware of this? He wrote the final editions of the Photography Series in 1981. Did he retest for the this edition? Was he still using an older meter? Also, just because the new standard was adopted in the mid 1960s doesn't mean meters changed over night. The current ISO speed standard was adopted in 1993. Kodak has just retested their black and white films and that was only because they changed coating alleys. They may never have done it otherwise.

 

BTW, the Sunny 16 rule is firmly grounded in the exposure formula. You'd be surprised how interconnected it all is.

Here is another way of looking at the equivalent reflectance of a meter. The average scene has a SLR of 2.2. The highlight falls at 100% reflectance (that's reflectance not reflects). If the mid-tone is 12%, and the highlight is three stops above that than 12%, 24%, 48%, 96%. That pretty much fits. If the meter read 18%, then 18%, 36%, 72%, 144%. That doesn't fit.