film base + fog

[Sean Bauer]

Hello and thanks for all the good info I've gotten off this site. I'm trying to find my film speed and I think I understand ever thing else. I know how to use a densitometer coming from a printing background (non-photo). My question is film base + fog the same as zeroing out the densitometer?
Thanks for your HELP.

[d.s.]

Subtract the base + fog reading from the other readings you take with the densitometer. That will give you your NET density. Some densitometers allow you to zero the machine after taking the base + fog reading and then every reading you take after that will be a NETdensity reading.

dee

[d.s.]

I should add that by zeroing the densitometer after reading the base + fog will save time with the calculater, but it is only accurate with the actual film you zeroed it with. The next time you use that same type of film , there may be differences and therefore the base + fog should be read on the new film. I zero my unit and subtract the current b+f from any other readings I take.

dee

[Witold Grabiec]

No or sort of.

Film base + fog is the density of the clear film developed as intended (or Zone 0, Zone 1 being around 0.1 plus this value). In other words, the minimum density you can possibly have for a given film. It will differ from film to film, and, at times, even with different batches of the same type of film. The latter usually not being an issue. Usually you should have enough clear film around the negative to measure it whenever necessary.

My "sort of" applies to the fact, that you would subtract this measured value from your normal density readings to get the actual density of the negative in question.

[Brian Ellis]

Film base plus fog represents the density of the film base itself plus the fog added by chemical processing (i.e. the density of the processed film without any exposure to light). To make a minor correction to one of the previous messages, you wouldn't subtract it from the "normal density readings" to get the "actual density" of the negative in question. The actual density of whatever area of the negative you're measuring includes film base plus fog. You would subtract it only if the density number in which you're interested is the number produced solely by exposure of the film to light .

For example, when making film speed tests you don't care about the density produced by film base plus fog, that will remain constant regardless of exposure as long as the film and development times remain the same. You're interested in the density produced by exposing the film. So when making these tests you first determine film base plus fog (by measuring an the border of the negative or perhaps by separately processing a sheet of unexposed film) Then measure the target area that you photographed and subtract the film base plus fog number from the latter number. But if you're simply interested in the density of a particular area of a negative you don't subtract film base plus fog.

Not only does film base plus fog change from one film to another, it also changes when development times of the same film change.

[Sean Bauer]

Thanks for all the answers. So when you zero out your densitomerter its saying that your film base + fog is zero and giving you the actual density. Which is what you want for finding your film speed. Right???

[Richard Ryerson]

Sean,

As I have just gotten my first densitometer recently please take this with a grain of salt.

Zeroing the densitometer does just that, zero's the reading. This is based upon what ever the machine is seeing at that particular time.

So if you have a zero exposure + normal developed negative in the machine. You are zeroing according to the FB+F.

You zero a weight scale the same way. Placing a container on the scale and hit the zero button, even though the container has weight, the scale reads zero. Now place something in the container, the scale will show you the NET weight. Same deal. Without zeroing the scale with the container the result will show you the GROSS weight.

So if you dont zero to your FB+F the reading you get back will be the GROSS density (sum of the FB+F density plus exposure density). If you do zero it, you will see the NET density or only the exposure density value.

Personally I don't think one way or the other is anymore useful than the other. I believe the real value in all this is being able to communicate with others. Knowing the difference allows me to talk about negative densities with someone else.

As far as what I do, I don't zero the densitometer to FB+F. But then I have to subtract the FB+F density from the total density to result in the exposure density. You need to know all this for records (if you choose to record your findings) so I just do zero to FB+F.

Best regards,

Rick

[Richard Ryerson]

***Thanks for all the answers. So when you zero out your densitomerter its saying that your film base + fog is zero and giving you the actual density. Which is what you want for finding your film speed. Right???***

Sorry I missed this while posting the previous message.

Generally yes, that's what you are looking. I say generally because it's possible that someone may say that a good start is for Zone I density is .14 total.... That's usually not the case. Usually someone will say to look for .1 over FB+F. (.1 OVER FB+F = .1 + FB+F)

Hope this helps,

Rick