Fast lenses, and High ISO

Started Jul 18, 2014 | Discussions thread
bobn2 Forum Pro • Posts: 54,529
Re: You missed the question.

jonas ar wrote:

jrtrent wrote:

jonas ar wrote:

EinsteinsGhost wrote:

Because a competent photographer knows exposure is not dependent on format. It is dependent on: ISO, Aperture and Shutter values for a given scene brightness. That is it.

According to wikipedia photographic exposure is the amount of light (the image plane illuminance times the exposure time) reaching a photographic film (and you may substitute film with "sensor" if you like).

I can see how the aperture and shutterspeed (and the scene brightness) affect the exposure in this defintion, but please do explain how ISO affects the amount of light reaching the sensor.

The light falling on the scene and the speed of the film are conditions that help us determine what exposure settings (aperture and shutter speed combinations) are needed. The sunny f/16 rule tells us that for a front-lit subject on a bright, sunny day, f/16 plus a shutter speed that is the reciprocal of the film speed should result in a good exposure. ISO therefore affects the amount of light we want to reach the film (or sensor). For Kodachrome 200, you might use f/16 and 1/200 (or some equivalent pair such as f/8 and 1/800); for Fuji Astia, f/16 and 1/100 would be correct (or, again, some other equivalent pair such as f/8 and 1/400).

The ISO rating of a film was only specified for a very specific proces. If you changed the processing you would change the ISO. Precisely as is the case for digital processing og raw files.

As bobn2 has patiently explained it imay not be a very good practice to let the exposure follow the ISO (instead of letting the ISO follow the exposure) because it will often lead to a lower exposure (and hence more noise) than strictkly required.

I'm glad there's someone here who gets it.

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