Film Speeds

[William Rivelli <rivelli@xxxxxxxxxxxxxxxxxxx>]

Patrick Weaver asked about setting his camera for the right film speed when
using IR film.

> I want to know what film speed I should set Kodak B&W High Speed Infrared
> Film to. The reason i ask is that i set it as 400 ISO as recommended in a
> magazine however when I asked my lab to process it at 400 they said it was
> off the scale and should be rated 50 ISO. I am now confused , do I rate it
> at 400 and have it processed at another speed?

I wrote about this a couple of weeks ago to someone else who had the same
problem. The information is in the FAQ, but here it is again:

Patrick, the reason you are confused is because so many other people are
confused when it comes to using IR and they pass on mis-information.

The recommended speed of Black & White Kodak IR films (HIE and HSI) is ISO
80 without a filter. This is the speed at which you would set a HAND-HELD
meter (NOT an in-camera meter). When you use the # 25 filter with HIE/HSI
you need to give 2/3 of a stop more exposure; therefore the film speed
would be set at 50 on your hand-held meter. (All these speeds are
approximate; they may vary a bit due to equipment and processing).

Now the confusion:

Many people are using 35mm cameras with through-the-lens meters and since
the meter has to read through the # 25 filter, they set their cameras to a
film speed of around 200 to 400.

The reason is this: with regular B&W film such as 400 speed Tri-X, you have
to give 3 stops more exposure when using the # 25 filter, since the filter
cuts off a large amount of the light to which the film is sensitive. If you
keep your camera meter set at 400, the meter-which is looking through the
filter- will compensate, and indicate that you have to give 3 stops more
exposure and the photos will be properly exposed.

When you load the camera with HIE film, set the film speed correctly at ISO
80 and have the meter look through the # 25 filter, the camera meter
doesn't know what kind of film is loaded and it still says that you have to
give 3 stops more exposure. BUT THAT'S WRONG!

With HIE film you need to give only 2/3 of a stop more exposure. Therefore
your negatives will be overexposed by 2 1/3 stops! YOU CANNOT SET THE METER
FOR THE TRUE FILM SPEED. You have to fool it so that it will indicate the
right exposure. That's why a lot of people set their in-camera meters to
400.

You have to experiment to find out where to set YOUR camera in order to get
the same exposure as you would with a hand-held meter set at 50 when using
the # 25 filter. If you own, or have a friend with, a hand-held meter, take
a reading and then change the film speed settings on your camera so that it
indicates the same exposure. Most people find that a setting of between 200
and 400 works out about right.

But please, remember: no matter where you set your camera, the FILM SPEED
is not 200 or 400; it is still 80 with no filter and 50 with a # 25 filter.

Happy photographing,

Bill

    William Rivelli

 WILLIAM RIVELLI PHOTOGRAPHY     212 254-0990
 303 Park Ave South              212 254-0922  FAX
 New York, NY 10010

    rivelli@xxxxxxxxxxxxxxxxxxx
        www.rivelli-william.com


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