Fog? Film base? Density?

[tom_patrick]

<p>I am reading through the film developers cookbook and finding out lots of excellent info. But these three terms keep arising and i do not understand them in the context they are used?</p>

<p>I have looked at lots of info on the web intended to be describing them in simple terms but they make no sense, and seem to just dissolve into numbers and equations.</p>

<p>I just need to know what density means in regard to film? It seems to be like saying contrast but i know thats not what it means because contrast is clearly explained and defined as separate in the book.</p>

<p>Fog and film base has come up a few times and seems to be key in my understanding of a lot of things i want to understand!</p>

<p>As always, thank you everyone for brilliant information and taking the time to help!</p>

[thirteenthumbs]

<p><strong>Film Base:</strong> the material that the silver salts and sensitizing dyes are coated onto. It will be transparent so that light will go thru it easily with little or no light loss. The film base may have a slight tint to it depending on the film and manufacturer.<br>

<strong>Fog:</strong> the byproduct of the chemicals interacting with the film base and coatings, similar to applying a light tinting to a window.<br>

<strong>Density</strong><strong>:</strong> the opaqueness of the silver salts and sensitizing dyes once they have been developed by the developer.</p>

[ellis_vener_photography]

<p>Usually Film base + development fog are combined into one measurement: It combines the light sensitive emulsion on coated on a piece of legible film (base) and once the film is developed the chemical reaction of the developer and fixer with the emulsion creates a slight density even in areas of negatives that are completely unexposed (development fog). the amount of light blocked is considered the film base + fog density. Film base + fog is also referred to as dMin and for a negative will print as a completely detail free black. </p>

<p>Density (with regard to negatives is a measurement of how much light is blocked at specific spot in the developed and fixed film: for a negative, the greater the exposure the higher the density reading. You will also come across the term dMax. dMax is a measurement of the maximum density a film is capable of up. dMax in a negative is reached when exposing exposing a piece of film to greater amounts of light results in no change of density in the negative, in other words that area would print only as a details white.</p>

<p>The dynamic range of the film is the difference between the completely unexposed areas base + fog or dMin) and areas where the light sensitive areas of the negative are completely saturated (dMax).</p>

<p>Both dMin and dMax are logarithmic measurements. </p>

[tom_patrick]

<p>Thanks</p>

<p>So, dMin is totally unexposed so no detail is recorded and with no density, and dMax is completely exposed so that no detail is recorded?</p>

<p>I still do not get base+fog. I understand that base refers to the emulsion (???) and fog refers to the tint of the film after development (???) but i do not understand why they are written as 'base+fog'. Why is it not just called fog, if it refers to absolute black and no exposure or detail?</p>

<p>Have i totally misunderstood?</p>

<p>Also - this obviously relates to contrast, as if you scanned/printed a frame to include all from dMin to dMax it would be high contrast???</p>

[Alan Marcus]

<p>Hi Tom:</p>

<p>Fog - Film darker than normal or discolored, this darkening can be uniform or blotchy. Outdated or film poorly stored will likely show an overall abnormal darkening. This darkening particularly shows itself along the edges and between the frames. Several things can cause fog; and it can be product of two or more of these. Exposure to non-image forming light, either in the camera and in the film handling area. As an example, film can receive<br />unwanted exposure during loading or unloading particularly true if the film box and/or wrapping are faulty. Handling film in a darkroom that is leaking stray light. Exposing the film to a safelight during development that is too bright or the wrong color or both. Developing using faulty chemicals such as contaminated developer. Over-developing i.e. too much time in the developer or the developer being too strong. Again, fog is an abnormal darkening either even or irregular.</p>

<p>Density - Exposed film after developing displays an image made up of clear film, dark film and intermediate shades. The amount of darkening makes up the image. Thus density defines<br />the blackness of an area in a negative or print. The intensity of this blackness determines how much light can pass through the film or reflect off the print.<br>

Base density - The amount of blackening present in un-exposed regions. All film display some density in unexposed areas however light fog or ageing or improper procedures will elevate this base (natural) density. Elevated base density is a flag that the film has undergone something that altered its normal characteristics.</p>

<p>Contrast - The density range of a negative or print or slide. Additionally the way we light a subject has a great effect on the contrast of the finished photograph. Thus we also talk about scene brightness range, the difference between light and shadow, falling of the subject calling this the scene brightness range or scene contrast.</p>

[john_shriver]

<p>Most films have deliberate density in the support layer to provide anti-halation. This prevents light that passes all the way through the emulsion from reflecting back in.<br>

If you fix the film (without developing), and then measure density, you get base density. This isn't very useful, since there's no purpose in this use of film.<br>

If you normally develop and fix unexposed film, and then measure density, you get film base plus fog. <br>

In real world usage, there's no distinction between film base density and fog density, so they are measured "as one", and described "as one".</p>

 

[thirteenthumbs]

<p>The film base, however transparent, will block some light trying to go thru it. This can only be measured before any coating is applied. Once a coating is applied it will change the film base light transmission characteristics due to the chemical interaction of the two materials. The change to the base transparency by interaction of the coatings and the interaction of chemicals used in processing is fog. It is called fog as that is what it looks like plain old low cloud water vapor fog. The film base and the fog are two separate things adding up to block light transmission when trying to view, print, or scan the image built up on the top of the base materials. <br>

Excessive fog can reduce contrast by blocking the black or near black tones. If you expose a low contrast scene it will print or scan as a low contrast scene if you process normally. If you expose a high contrast scene it will print or scan as a high contrast scene if you process it normally. You have to manipulate the exposure/development to make either of these appear as normal or the other.</p>

[User_5888660]

<p>Basically, the base is clear plastic. Think of a sheet of glass. Even though you can see through it, it still holds some light back. If you put a sheet of glass on top of photo paper and expose under an enlarger, you will get a bit less light through the glass to the paper than to an area of the paper not covered by glass.</p>

<p>If you develop a piece of unexposed film, the chemicals will leave a chemical stain on the base. The film should be clear, but it's not. Any residue left on the film is fog. Measured together, the film base plus the fog (FB+Fog) is as clear as you can get a sheet of film. There is no exposure, i.e. no light has hit the film, so you have no build up of silver on the film. But, you have a bit of light held back, from the film base (the light held back because it is not truly clear) and the fog (the chemical stain). Think again of the sheet of glass. If you breathe on the glass, you will leave a bit of fog on the glass. Chemicals do this when you process the film. </p>

<p>The FB+F is your stating point for measuring density. The FB+F is as clear as you can get the film. For most purposes, density is measured above the FB+Fog. The film base is coated with emulsion--silver. When exposed, the silver will get thicker the more light that hits it. The measurement of the thickness of the silver is the density. </p>

[ian_gordon_bilson]

<p>A very interesting thread.<br>

Anyone who wants to know more could head over to "the online photographer",where a chap called Ctein (I think the C is silent) explains why even fogged film can make excellent photographs,and goes on to explain why fogged printing paper does not.</p>

[lex_jenkins]

<blockquote>

<p>"...Ctein (I think the C is silent)..."</p>

</blockquote>

<p>If I'm recalling correctly from ye olden dayes of yore on CompuServe, he described the pronunciation as <em>kuh-tine</em>.</p>

<p>And if I'm correctly recalling a reviewer's comment on one of Ctein's books years ago, <em>"I think he's having us on with that name."</em></p>

<p>I've gotten some interesting results with old fogged paper, but not in any conventional sense. The paper was useful for Ray-o-gram style prints, sunprints of silhouetted objects, that sort of thing.</p>

[ellis_vener_photography]

<blockquote>

<p><em>So, dMin is totally unexposed so no detail is recorded and with no density?</em></p>

</blockquote>

<p>Almost. There is density (base + fog) but it is the effective zero point for the negative developed the way it was .</p>

[larrydressler]

<p>It is also good if you want to print an X-ray... I never understood why maybe it is the heavy blue of the X-Ray film but then ... That is almost a dead science too. All dead papers have some use as do dead films...</p>

[Alan Marcus]

<p>Base density is defined as: a reading taken from an area of the film that is not expected to have received exposing energy. Likely, this Dmax reading will be taken from an area such as clear film at the edge or between frames. The classification as to what base density is can be expanded, however, generally this is the customary definition.</p>

<p>Base density is a valuable measure. When compared to a reading taken from an area of maximum density (Dmax) the scale (dynamic range) of the material is determined.</p>

<p>Comparing Dmin with Dmax together with readings taken from other points on the film's scale are the tools needed when films are designed as well as the chemicals of the process, the papers used to print images, and the machinery that develop and print.</p>

<p>These are the twin sciences of Sensitometry and Densitometry. </p>

[profhlynnjones]

<p>Here is something you need to consider. 35mm b/w films when developed normally (GBar .49-.55) have a FBF (film base plus fog) of from .27 to .29, almost 1 full f stop of NEUTRAL density (no imaging density). This relates to anti halation. So if you are doing sensitometry on identical films from 35, MF, and LF, you must subtract the FBF from each of them in order to find the true information regarding contrast and speed. FBF in MF will be about .05 to .06, occasionally less, LF will run about .01 higher just because the differences between .0045 and .007 inches base thickness. The anti-halation in MF and LF are dyes in the base which wash off in processing. The strange color in each different film represents the color of least sensitivity to the emulsion.<br>

Lynn</p>

[photo-gear]

<p>Thanks for this thread. Now I know about the fog.<br>

I now understand how come some of my films have this dark density all over their surface sometimes. Most of the time, it is from old films or films bought on websites without being sure of their conservation methods...<br>

Mainwhile, I would like to make another observation regarding fog effect. I did develop some foggy frames and, even though it takes more time to expose under the enlarger, the final print result is quite acceptable.</p>