Basic exposure question

[ymc226]

Reading threads regarding B&W film development, I came across several comments that during bright sunny/contrasty scenes, some down rate film speeds while under cloudy days, the film speed rating is increased.

Why is this? I would have thought it is the opposite.

 

[Christopher Walrath]

No, that's right. Let's say for example sake that ISO 400 requires less photon activity to get the same exposure level as ISO100 (four times as much). With an older camera (and personally with most any camera) my exposure would be based on metering and exposure settings, not ISO adjustment. Of course, I don't own a camera that don't shoot fully manual.

 

[MartinP]

The idea you read about refers to the Exposure Index used for shooting (with the development adjusted to suit) NOT to the ISO. ISO film-speed is defined and measured under a particular set of controls, and each film has only one ISO rating (yeah yeah, someone is going to come up with different colour temperatures and changed speed rating, but lets talk about just daylight).

Changing the Exposure Index which you set on a meter, or use for estimation, will change the amount of light hitting the film, which in turn means that the development of the film is usually change to get a similar result for some particular tone (say a mid-grey).

The useful difference comes in the contrast of the film when pushed or pulled like this. When the light is bright and contrasty many people like to move the E.I lower than the ISO filmspeed in order to lower the contrast of the image on the film, and vice-versa on a flat dull day. This is what you have read about.

Unless you are using sheet film or, similarly, shooting the whole roll in the same light then there is no need to worry too much until you have really explored that film/developer combination in all the different circumstances you are interested in. At that point you will be able to make a judgment based on your experience of your film, rather than gossip like this !
;-)

 

[KenR]

Contrast control

This has to do with contrast control. On a sunny day, the contrasts can be extreme and so by down rating the film, you ensure that the shadows have some depth rather than being rendered as blank, black areas. On a cloudy day, the scenes are usually rather flat, so by underexposing a bit, you tend to increase the contrast and so render the scene "normally".

 

[Sirius Glass]

Reading threads regarding B&W film development, I came across several comments that during bright sunny/contrasty scenes, some down rate film speeds while under cloudy days, the film speed rating is increased.

Why is this? I would have thought it is the opposite.

Wildi, in his Hasselblad book recommends this. [I do not have the book with me in Virginia, otherwise I would put in the passage.] I tried it both ways. On a cloudy day [read:flat light] the photographs looked flat, opening by one stop boosted the contrast and added brightness. Therefore I shoot box speed using a meter and I compensate only when necessary.

By the way, Wildi says open one to two stops for fog and two stops for snow. It does work.

Steve