What exactly does it mean to say that a certain film or process 'is tolerant' or 'has latitude'?

Question

Millions of photographs have been taken with simple cameras with no exposure control, under very varied lighting conditions (scenes, that if they were metered for, would entail several stops' difference between them) - and yet millions of acceptable prints have come back from ordinary labs.

This, as I understand it, is a function of the wide tolerance of film and the development process.

What I understand already

(please correct me if this is not right)

• that a film with exposure latitude will allow you to get your exposure settings "wrong" and still end up with a usable image
• that colour negative film (or perhaps C41 in particular) has a great deal of latitude
• that colour slide film has little latitude

What I think that this must imply

Let's say that for reference, the most tolerant film has a dynamic range from nominal values of 0 (darkest) to 100 (brightest). That is, if a scene contains a range of 100 brightness values, this film, when correctly exposed, will capture all of them.

On the other hand if the scene has a range of say only 50 brightness values, then the film can comfortably capture all of them even if the exposure is off target - as long as the brightness range of 50 can be accommodated somewhere within the range of the film.

As for a less tolerant film, say with a range of only 50 brightness values itself, then you could never capture all of the range of the first scene of 100 values - you'd have to choose whether to lose detail in the dark parts or the light parts of the scene. And to capture all of the range of the scene of 50 values, you'd need to have the exposure spot on.

Is this more or less correct?

Things I don't understand

Say I take two photos with my most tolerant film with a dynamic range of 100, of a scene that has a range of 50.

I take one photo that lands in the film's range 0-50, and one that lands in the film's range 51-100 - i.e. the same scene, with different exposure values.

When the film is developed, there won't be a single point in either frame that has the same brightness as any point in the other frame.

So, how do we get good prints from these two completely different negative frames? Does it depend on the expertise of the person doing the printing to make judgements about how the range of 50 brightness values in each frame should be translated to brightness values in the final print?

If the film is sent to a typical lab for printing, what happens - does the printing machine make a guess at the what the average brightness of the scene should be, and expose each print to bring it closer to that average?

Where does this latitude for exposure come from: is it in the film, in the developing process, or in the printing process?

Answer 1

Part of the equation left totally out of the question is that negative film has a wider dynamic range than the vast majority of the papers used to make photographic prints.

For example, in the time when Ansel Adams did his most significant work, the monochrome films he used had around 10-11 stops of dynamic range, but the papers available for him to print on were limited to around 6-7 stops. For modern color film, the dynamic range is a little reduced compared to B&W film, but so is the dynamic range of color papers compared to B&W papers.

Where does this latitude for exposure come from: is it in the film, in the developing process, or in the printing process?

Yes, yes, and yes. Some of it is from all three parts of the process.

Film

Film itself works by chemistry. The rate at which chemicals react to being exposed to light varies based on how much of the chemicals in the emulsion have already reacted to light. As film is exposed, it takes longer and longer to double the amount of silver salts that have reacted to light. This is because the longer the film is exposed, the amount of unreacted chemicals in the emulsion per unit area becomes less and less as the portion of the emulsion that has already undergone a chemical reaction in response to light is no longer able to absorb more photons.

So the closer an area of a film is to full saturation, the more light it takes to push it even closer. This is what is often referred to as the "shoulder" of film response curves. Digital, in contrast, is purely linear all the way to the saturation point, so it is much easier to clip highlights with digital than with film.

One easy way to understand this is to consider the Schwarzschild effect, sometimes referred to as reciprocity failure. For conventional B&W and color films exposed for longer than one second or so, doubling the exposure time does not make the image twice as bright. To do that the exposure time must be extended, sometimes rather significantly. Just how much more time is needed varies from one film to the next. Manufacturers of film usually publish data sheets that, among other things, specify the needed adjustment for long exposures when using a specific film they produce.

Development

Altering exposure times and development times can increase or decrease the overall contrast of an image. If a scene with a wide dynamic range is shot, that dynamic range can be compacted into fewer stops by reducing contrast. If a scene with a limited dynamic range is shot, that limited dynamic range can be stretched by increasing contrast.

Printing

Developing the latent image on an exposed piece of film is only half the process, though. The other half is using that developed negative to produce a print. Making a print from a negative uses light in much the same way that capturing a latent image on a piece of film does. The exposure of the photosensitive paper can be controlled by the amount of light used. The amount of light allowed to reach the paper is controlled by several things:

• The intensity of the light
• The density of the developed negative
• The amount of time the light is on and shining through the negative and striking the surface of the photosensitive paper.

In addition, darkroom techniques such as dodging and burning can be used to develop specific areas of the photo paper for longer or shorter amounts of time than the overall print. This is usually accomplished in the darkroom using masks shaped to block some of the light shining from the enlarger head onto the photo paper. With contact prints, the masks block the light before it reaches the negative.

Answer 2

It is the negative / positive process that grants pictorial film its wide latitude (exposure tolerance). The key to understating how this works is the fact that the negative itself is just a means to an end. The final product is a print on paper. To make this print, the negative is held in contact with or its image is projected on light sensitive photo paper. If you just think about it, you will come to realize that printing exposure is tantamount to re-taking of the original picture. What I am trying to say, we can fine-tune the printing exposure in such a way that it is possible to mitigate an exposure errors that occurred when the film was initially exposed. Thus the printing exposure becomes a powerful tool allowing the salvaging of substandard negatives. Believe it or not, a tolerable print for the amateur market can be made from negatives 3 stops under thru 6 stops over exposed.

I know these facts intuitively after a 55+ year career in photofinishing and photofinishing equipment manufacturing. Before the 1930’s print exposure make using high-speed photofinishing printers were 100% operator controlled. Operators previewed each negative and then pressed an approximate button that modified the exposure. The operators became highly skilled and this upped the percentage of saleable prints.

Next, a photocell was added to preview the negative. The operator simultaneously previewed and made a judgement call and was able to override the automation. By the 1960’s that system was upgraded. Each upgrade reduced the necessity for operator intervention.

The photo papers used by the photofinisher also evolved. In their final version, they were capable of making automatic contrast adjustments prompted by exposure time variations (not optical filters).

I have been describing black& white photofinishing printers. Color negative photofinishing printers, by the 1970’s were even more automated. These printers evolved, they scanned the negatives, identified the various scene types, custom adjusted the color of the exposing light and exposure time.

At the height of my photofinishing career, I was technical manager for 7 labs. Each was sized to process and print 20,000 rolls of film a day. Over my time in the business I saw lots of enhancements, lots of automation. Always, always, quality of film processing and optimization of the prints was our paramount.

Answer 3

The perception of brightness values from 0 to 100 with film doesn't seem like a good analogy.

When one goes to print from a negative, something wildly blown out has caused the grains to all clump together in an area such that it is overly dense and no finer detail remains.

The opposite is true for shadow detail - if not enough light hit the negative, then you won't have any grains and the negative will show as almost or even fully clear. If it is fully clear, then there is no image.

But, the fact remains that even if you have the tiniest bit of detail, as long as it's there, it can be burned into the final print. Likewise, areas lacking in density can be dodged to even them out.

When one says that a film has latitude, they're referring to its ability to record minute details while being severely underexposed while also recording details while not completely blocking up the film and becoming too dense when overexposed.

That being said, I've talked about salvaging an image at time of print. It is also true that, if you know your negs are overexposed you can pull the developing to attempt to compensate. That's where you get the adage, expose for the shadows, develop for the highlights.