I have been examining the films I have developed over the last couple of years, and have started to wonder if I am getting an acceptable level of base fog. I know a small amount of this is unavoidable, and can be "printed through", but I am obviously keen to minimise this. I don't have much to compare my results with, as I have only been developing for a couple of years.

I have not tested this in any scientific way. What I have done is hold up a piece of developed and fixed film with no images on it, and tried to look through it. What I see is by no means clear. There is a distinct fog.

Firstly, is there any easy way of telling if this is at an unusual level? Secondly, any recommendations for how it can be minimised?

For the record: I mostly use new Ilford Delta 400 120, developed in Ilfotec-DDX. Ilfostop stop bath for 2 minutes. Ilford Hypam Rapid fixer used for 5 minutes. All done in Patterson Super System 4 tanks (I have 1-reel, 2-reel and 3-reel tanks). Agitation 4/5 inversions for 10 seconds in every minute. Film loaded in absolute darkness of course!

-- Ed Hurst (BullMoo@hotmail.com), December 05, 2001

Answers

The 'text book' level of allowable base fog is 0.2D; about 2/3rds of a stop density. This is reckoned to be the level consistent with the fullest film speed, and much above that, it's just plain fog. That's emulsion density, not including the film base, which may also have a slight tint to it.

-- Pete Andrews (p.l.andrews@bham.ac.uk), December 05, 2001.

Fog is not harmless.

Ed, I always believed that the fog is a constant density over the image and therefore it can be "printed through" with no harm to the image. But recently I found an opposite note in James & Mees "Theory of photographic process" (4th edition): the fog level does depend on exposure, unexposed area has more fog, exposed -- less fog. Thus the effect of this variable level of fog is equivalent to presence of a slight positive image, which of course eats shadow details.

A sort of "good news" is that if we process a test roll and then use the same processing for important negatives, we have the same characteristic curve for test and for real roll, the same shadows etc, thus the contrast as it could be without the fog, may seem to be an imaginary benefit: we cannot achieve it. But it in no way follows that other developer/time/agitation combination cannot give less fog with the given film speed.

Measuring the fog: you have to compare a clear area of developed and fixed piece of film (film base plus fog density) and a piece of only fixed film (film base only). At least if we speak about the chemical fog the film receives in developer.

I consider the fog level as normal if film base plus fog together have no more density than 1 stop. Most my films fit in the range 0.7- 1. The pleasant exception is pulled TechPan: ca .3 stops.

Unfortunately I cannot tell how to decrease the fog. Adding inhibitors (KBr, benzortriazole) decrease the fog at expense of decreasing film speed also.

Please let us know if you find something. I wish you luck.

-- Andrey Vorobyov (AndreyVorobyov@yahoo.com), December 05, 2001.

I think you will find some films have a lot more fog than others. Some will show more fog in different developers. A few friends who print a lot of platinum will not use HP5plus due to the base density, way too much in their estimation for the type of prints they like. Using FP4plus orTriX solves the problem for them. I guess the trick is to try a few developers & see if one gives you a level you can easily live with using the film you want. If not, change films & try again.

-- Dan Smith (shooter@brigham.net), December 05, 2001.

Some films use a denser base than others to prevent halation and light piping. The light striking the protruding leader of 35 mm film is "piped" along the film. A base density of 0.1 becomes a very high density very quickly along the film and is sufficient to prevent piping. Films with this low base density use other means of preventing halation. HP5+ will never be as clear as TMX for this reason. If you remove the emulsion from a piece of HP5+ by soaking it in Clorox you will see a density of about 0.3, 1 stop. This density can be printed through and has no effect on the density range of the negative or its graininess. It is no different than stopping down one stop. Chemical fog is all you have to worry about. Develop an unexposed piece of film, fix it and compare it with a piece cleared in Clorox. The difference shows the minimum chemical fog you will get in that developer. There is also the appearance of fog caused by flare in camera or enlarger. This fog does change the apparent characteristic curve of the film from what you would get in lab tests without flare, generally changing a short toe film to a long toe.

-- Patrick A. Gainer (pgainer@rtol.net), December 06, 2001.