How does a minilab get the exposure right?

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Question:

How does it know how long to expose a negative for!? Does it have some kind of sensor!? And does it always print to around 18 percent grey (or whatever value it is)!? If this is the case I guess this means there is not a lot of latitude for creativity with regard to exposures!. e!.g!. you couldn't take a photo and over or underexpose for artist reasons because the machine automatically prints it at 18 percent grey!?

Is this also the reason why film negative is said to have a lot of exposure latitude!? or is this a property of the film!.!. or of the development process!.!. or a bit of both!?

Thanks for any advice!.Www@QuestionHome@Com

Best Answer - Chosen by Asker:

The mini labs do indeed attempt to make a 18% gray of all the film run thru them for an exposure value!. While the operators have a fair degree of control in exposure and color, they are not paid enuf to do so, especially at the fast 1 hour joints, Cosco, Walgreen's or Wal Mart!.

Many pro labs use these fast machines too, but they ARE more trained and paid to 'read the negs' before stomping the foot control, making your print!. An attempt is made to understand what the picture was meant to be, and make an exposure from these assumptions!. Not always right, but at least more care and thought than the "fast places"!.

You are also correct in your assumption that the creativeness you gave your exposure will most likely NOT come out in the print from any of these fast 1 hour joints!. They are geared for the flat lighting and picture type taken from your standard vacations, b-day parties and of the like!.

C-41 (color neg film) is far superior in exposure latitude than any slide film is and has many stops more in exposure latitude than slide film!.!. However, when exposed correctly, slide film will have better colors than a neg print, but even then, the dynamic range is still missing, and most slide film has only about 1 stop either side of middle to make or break the image!. Kodachrome 25 was the exception!. Long live the exposed Kodachrome's for Kodak is cutting this too from it's stock pile and devoting more time and $$ into digital!.

But back to your question tho as to how the machine does it!. When it looks at a neg there is a ground glass like filter that the RGB sensors look thru!. The ground glass mixes all the colors of the neg to that OF 18% gray, and not only is the exposure derived from this (by it's brightness) but color exposure values as well!. It tries to turn every pic into a perfectly exposed - 18% gray print, IF the diffusion filter was kept in place!. However, as the actual paper is exposed, this ground glass filter is removed and the neg film is now projected up into the cabinet, onto the paper!.

All this happens in the blink of an eye with servos and solenoids flipping filters in and out of the lights and papers path!. A 36 exposure roll can be done in less than 60 seconds!.!.!. while it may have taken you hours or days to work on it!.!.!.

Bob - Tucson

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http://en!.wikipedia!.org/wiki/C-41_proces!.!.!.
35 mm negative has more latitude for exposure than positive slide film does!. Your best bet is to take your film to a regular pro or custom lab and tell them what you did and they will know how to develop the film to get the results that you want!. If you have pushed or pulled the film let the know so they adjust the develop time accordingly!.Www@QuestionHome@Com