Film processing theory - rating film speed, speed loss developers etc!

[Subfrequenz]

Hey all,

Just to say I've enjoyed reading all the posts on this excellent forum. I've recently got into 35mm black and white photography and have invested in a home processing kit and also an enlarger. I've successfully developed about 20 films using ilfosol 3, Rodinal and caffenol variants. I've mainly stuck with HP5+ film. I've printed using Ilford VC paper and have loved the results!

The thing is....
I'm just trying to get my head around some of the processing technicalities!
Is there a good resource on the the following?:

Rating film speed: I've read that some people rate film speed at higher or lower than the manufacturers stated speed. I'm guessing this is to do with ensuring correct exposure of shadow detail etc. But would you then develop the film at the Normal time specified for the box speed of the film? So shoot at ISO 200 but process it with 400 Dev times?

Developers: Speed enhancing or Reducing- again is this mainly to do with shadow detail etc? If so would you remedy processing a film in a speed reducing developer by shooting at a lower than box speed ISO?

These are the kind of things I'd like to learn so I can gain more control over the developing process! Any thoughts and recommended resources welcome. Thanks for reading

 

[flavio81]

Hi,

Welcome to APUG!

"Rating" a film different to "box speed" (the speed printed in the box) can be because of different factors.

One is because people have different ways to use a meter. Another is that all meters are not exactly going to report the same figure. Yet another: CdS meters have different spectral (color) response than other meters so this is a factor as well.

Yet another factor is the actual speed brought by the choice of developer, there are some developers that enhance speed (i.e. Microphen). Finally, there are films whose box speed is not the real speed, i.e Fomapan 100 is about ISO 64-80 in D76.

All this is different than "pushing" film, which is underexposing film and then increasing development to compensate (to increase density of what would otherwise be a thin negative).

So for example, i could buy Fomapan 100 but be "rating" it to ISO 80. This means i would set my meter to 80 and processing normally.

Or perhaps i'd want to experiment, pushing it to 320 (+2 stop) and increase development time to compensate.

 

[Mr Bill]

Another welcome to APUG, and a comment that you have some good questions.

Rating film speed: I've read that some people rate film speed at higher or lower than the manufacturers stated speed. I'm guessing this is to do with ensuring correct exposure of shadow detail etc.

Yes, this is pretty much it. The general rule is that changes in DEVELOPMENT (time or temperature) have a large effect on the brighter parts of the scene, but very little effect on the shadow detail. So to affect the shadow detail, changing exposure is the main method.

Developers: Speed enhancing or Reducing- again is this mainly to do with shadow detail etc? If so would you remedy processing a film in a speed reducing developer by shooting at a lower than box speed ISO?

Yes and yes, basically. But unless you want to do all the testing yourself, you would probably want to get your basic information - speed rating and development time/temperature - from the manufacturer's data sheets.

As a note, the ISO speed rating method (for B&W negative film) is based on the amount of exposure needed to register a small effect on the film (roughly a small amount of shadow detail) when the film is developed to a specific amount of contrast. Any other method of rating speed is not strictly ISO, so is usually just called EI, for exposure index. So when you read about a different speed for film, in the same developer, this usually means it is being developed to a different contrast. Just a fine point to pay attention to when reading data sheets.

I don't have a specific online reference, but if you want to learn these things on a firm technical basis, you would want to learn how to "read" a "characteristic curve." Once you can do this, then you'll be able to understand articles about "sensitometry," which is the technical metodology to study the things which you have asked about in a general way. Of course, many people have been very successful in photography with almost no knowledge of this, so it's mainly a matter of how YOU want to learn things. Best of luck to you.

 

[MattKing]

Welcome to APUG.
Mr. Bill's post is on point - "speed" is basically about exposure, and development is basically about contrast.
With the exception of some special purpose developers, one can generally use the ISO speed to achieve usable shadow detail, and adjust development to achieve desired contrast.
That being said, many people like increased shadow detail, so they choose more exposure - they meter using an exposure index (EI) that is less than the ISO rating for the film. In some cases, they will also reduce development, in order to preserve highlight detail, but that isn't as necessary with a lot of modern films.
"Pushing" doesn't actually give you (significant) speed increases. Your shadow detail is still lost, but much of the rest of the scene looks better, because the contrast is boosted. Highlights may suffer as well.

 

[Subfrequenz]

Thanks all for your detailed suggestions and answers. The questions I had arose because I started to use non ilford developers with ilford films and read comments about e.g Rodinal introducing speed loss so I wanted to know how to compensate for that. I'm happy to experiment but at the same time don't want to be in the dark about certain developing and shooting principles.

 

[Sirius Glass]

Sometimes you should be in the dark with film. For example, when loading the film onto reels and loading the reels into the tank.

 

[Subfrequenz]

Sometimes you should be in the dark with film. For example, when loading the film onto reels and loading the reels into the tank.

Ah now I know why nothing ever came out!!! ;-)

 

[Bill Burk]

Welcome Subfrequenz,

When a certain amount of light hits film and the film gets developed, the resulting negative will get a certain density. The graph of the density versus exposure is called a film's characteristic curve. When you have a set of these curves for different development times, you have a curve family.

The curve family for a film-developer combination is rock-solid information. If you develop some film, it will have one of these curves. And any amount of light that hits that film will put a density on that film along that curve. It might not be what you are thinking, but your goal might be to make sure you give enough exposure so that your whole picture is on that curve ... with just a little of the shadow falling off (if that fits your taste). People who rate their film at a lower speed (like me, I shoot 400TMAX at 250), usually do that because they don't want the shadows to fall off. You might shoot HP5+ at 200 when developing in caffeinol because it might be a speed losing developer (I don't know...) and that would be a different reason. You really might need to shoot at 100 if you want fully-exposed shadows in a speed losing developer.


While I can't show you HP5+ in caffeinol (because I like Kodak), I would expect it to look different than HP5+ in a standard developer like ID-11. Speed-losing? If you look at the curve family it would tell you (assuming you start looking at curves and begin to understand them). A developer that gives less density in the thin part of the shadows might be considered speed-losing. But you might not care about speed, and you might pick that developer anyway for different qualities that it offers. Here jnanian often talks about and uses caffeinol, so you may get some real straight answers.

Speed-enhancing developers work a couple different ways. POTA is a special kind of developer which makes a very flat negative that you can print on very hard paper and sure enough... you can shoot at higher than the film's normal rated speed and get fine pictures. But. Most other speed enhancing developers only give you 2/3 stop more speed than standard (I am taking a stab in the dark here... can someone confirm my assertion?). Really, unless you hit the film with enough light to show a bit of density, you cannot get a picture out of it.

 

[tedr1]

Here are some simple principals for 35mm B&W that I found helpful while learning (some of this has been mentioned already):

great pictures can be made using the box speed and normal development

when the subject is very contrasty (bright light with deep shadows) and the film is exposed at box speed and given normal development negatives may be tricky to print, either the shadows turn solid black or the highlights wash-out all white, reduced development can be used to reduce film contrast (for the whole roll unfortunately) and retain in the print either more of the shadows or more of the highlights using normal grade paper.

Some people find normal development gives negatives that are too contrasty even with normal subject contrast, the printed look is too "hard". In this situation the development can be reduced to lower the contrast and give "softer" prints. Underdevelopment causes some loss of shadow detail, this can be compensated for by increasing exposure, which is done by using a slower film speed. If it is desired to reduce the development by one stop to lower contrast then the film should be over-exposed one stop to retain shadows, this may be thought of as a change of film speed by one stop down.

Some history. Originally film was exposed one sheet (originally a glass plate) at a time and developed by inspection under red light (early film was blue sensitive only), the development was stopped when the highlight density reached the desired quantity judged by eye. The concept of film "speed" was primitive. Development by inspection became impossible with panchromatic film (all B&W film is now panchromatic and has to be developed in darkness). Things became even more complicated when roll films were introduced it became impossible to develop frames individually.

There is a motto for exposure and development that comes from the very early days of photography and is still relevant today "expose for the shadows, develop for the highlights". Highlights create a lot of silver density in the negative, sometimes too much. Shadows create very little silver density, sometimes not enough. Highlights are strongly affected by development duration, shadows less so. The exposure should be enough to capture the shadows. Longer development will emphasize the highlights and should be stopped when they give the desired contrast.

Here is an example of "speed re-rating". Films can be underexposed and over developed to give results that may be useful in some situations. For example in a theater we may chose to use ISO800 film at ISO3200 to get very short shutter times so that motion blur does not ruin the shot. In order to get a useable density on the negative the development has to be increased, a lot, so that the underexposed face highlights are printable. However there is no shadow detail, the exposure was too short to capture shadows. This loss does not matter much in this situation, the print of the face is what matters. This is called "pushing" a film to increase "speed".

Ilford makes a range of developers and publishes helpful information sheets that give details of their use.

Some special film developers are chosen because of characteristics that influence the shape of the film tonal curve (toe slope and shoulder) for special effects that are preferred by some advanced photographers, this may be distracting for those learning the fundamentals of the process.

I believe Rodinal was invented to give increased sharpness and not for its exposure/speed/contrast/tonal curve properties.

 

[Subfrequenz]

Superb, thanks for the info. It's great that it's possible to have so much creative control over the look of the negative.