Fast lenses, and High ISO

Started Jul 18, 2014 | Discussions thread
jrtrent Veteran Member • Posts: 4,474
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.

This is true; for example, in dark conditions I would often shoot 400 speed Tri-X at ISO 800, then have the film push-processed.  However, I would set my meter for 800, knowing the very specific process that would follow, and my exposures therefore were still based on a particular ISO.

Precisely as is the case for digital processing og raw files.

I don't currently shoot raw, preferring out-of-camera JPEG's, but when I did, I still set my meter for a particular ISO value and exposed accordingly.  Perhaps you can expand on the role ISO plays, if any, in your shooting and processing procedures.

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.

On my digital cameras, I have typically found one ISO setting that best balances highlight detail retention and low noise, and I leave my cameras at those respective "best" settings.  On my GX-1S, I like ISO 400 best, so that's what I always shoot that camera at; for an E-450, I liked ISO 200 best; the SD14 was kept at ISO 100 (for color) or ISO 400 (for black and white under low light conditions).  Since the ISO is established, and constant, exposure is based on that.

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