Fast lenses, and High ISO

Started Jul 18, 2014 | Discussions thread
bobn2 Forum Pro • Posts: 56,079
Re: You missed the question.

jrtrent wrote:

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.

You are pretty firmly wedded to the film emulation methodology, particularly if using in-camera JPEGs, because that is what they are based on. That means effectively that you preselect the exposure that you want to use with your film or ISO selection. Once you do that, what you're aiming to do when you set the exposure is ensure that the output tonality is 'correct'. However, that way of working is based on the idea that once you make the film and processing decision, that decision is fixed for the roll. Simply not true for digital, so you can choose to say that the fine tuning of tonality will be done in processing, given that we have tools that allow just that, and you'll set exposure to give you the most information for the processing to work on. This means minimising SNR which means maximising exposure (the real exposure, not 'brightness' or ISO). One method, but not the only one, is ETTR. The other is simply to set your exposure to the maximum subject to any other constraints that you are giving yourself (DOF, motion blur, camera FWC) and set the ISO control so you don't lose the highlights.

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