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Following is from Film speed

Film speed is the measure of a photographic film's sensitivity to light, determined by sensitometry and measured on various numerical scales, the most recent being the ISO system. A closely related ISO system is used to measure the sensitivity of digital imaging systems.

Relatively insensitive film, with a correspondingly lower speed index, requires more exposure to light to produce the same image density as a more sensitive film, and is thus commonly termed a slow film. Highly sensitive films are correspondingly termed fast films. In both digital and film photography, the reduction of exposure corresponding to use of higher sensitivities generally leads to reduced image quality (via coarser film grain or higher image noise of other types). In short, the higher the sensitivity, the grainier the image will be. Ultimately sensitivity is limited by the quantum efficiency of the film or sensor.

It says "reduction of exposure corresponding to use of higher sensitivities".

Questions

  1. What are the parameters (of exposure) reduced at high sensitivity?
  2. Is there a way to keep the exposure parameters high even at high sensitivity?
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I think what's happening here is that you are reading "the reduction of exposure corresponding to use of higher sensitivities generally leads to reduced image quality", and then, very logically, thinking: okay, so, reduced exposure = reduced image quality, so clearly I need increased exposure....

This is, in fact, completely right: see this answer on digital ISO for some more explanation.

That link and the Wikipedia article are both saying the same thing. There are three factors that you can change when photoraphing given scene (with a set amount of light): aperture, shutter speed, and ISO (sensitivity or gain). Increasing the first two increases the actual light that reaches the sensor or film: a longer shutter speed lets light in for more time, and a wider aperture lets more light in at once.

The ISO, however, is how quickly the sensor or film responds to that light. If you increase it but keep the other factors the same, the too sensitive ISO means that the image is overexposed. So, that's why you must reduce them. And, because physics can't be beat, less light means less data, and less data means reduced image quality. (Fortunately, with modern equipment, this reduction might be hard to perceive until the ISO gets very high.)

This is all extra confusing because people use the word "exposure" in multiple different ways, all slightly contradictory. See this answer where I go into detail, but, basically: often people include aperture, shutter time, and ISO, and possibly even post-processing into "exposure factors" — with "exposure" basically meaning the resulting overall brightness. This is a perfectly fine and common definition, but another equally correct* definition only considers the amount of light allowed to hit the sensor — just the shutter time and aperture, and that's the definition used in the Wikipedia quote.

So, when I say that you're completely right, I mean that you need to have increased exposure in the latter sense: just more light. Unfortunately, what the Wikipedia article is saying is also completely right: you can't have that with a higher ISO without changing the exposure in the other sense, rendering your output too bright.

(The only way out is to add more light: change the time of day, or add flash or other artificial light. But that's a whole different story.)


* (possibly, pedantically more correct, but... English is inherently a sloppy language)

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mattdm is right but I feel when reading it, I understand it only because I know this answer all too well and also that if I was reading it fresh, I wouldn't understand it...

1. What are the parameters (of exposure) reduced at high sensitivity?

Parameters of exposure, for any photograph or camera, are always:

  • Available light (Sun, lamp, flash)
  • Shutter speed (Longer the curtain is open for, the more light hits the film/sensor)
  • Aperture (size of pupil entrance. Bigger the size, the more light hits the film/sensor)
  • ISO (Sensitivity of film/sensor. Increasing it does introduce more noise!)

Changing any of these can over or under expose your image. However, giving and taking from each parameter can produce equally exposed images without over or under exposing your image.

2. Is there a way to keep the exposure parameters high even at high sensitivity?

There's a Sunny 16 rule in photography that says on a sunny day, to get an ideal exposure, use ƒ/16 and the ISO can match the reciprocal of the shutter speed.

Eg.

ƒ/16, ISO100, 1/125
ƒ/16, ISO200, 1/250
ƒ/16, ISO400, 1/500

Rough guide and not set in stone. Rounded off to the closest typical shutter speeds

Increasing the ISO has increased the shutter speed. But if you also wanted to increase the aperture, you will have to reduce shutter speed or ISO to keep the exposure "correct", otherwise you will over expose your image. So, increasing the aperture by 2 stops to:

ƒ/5.6, ISO400, 1/500

Will likely to over expose your image. But, reducing ISO by 2 stops (as well as increasing aperture by 2 stops) will result in a more pleasant exposure:

ƒ/5.6, ISO100, 1/500

Don't take Matt Grum's answer as gospel. Personally, I wouldn't go shooting at ISO1600 or above in the bright sunlight (Not that Matt was suggesting that...).

However, lack of light is the main factor that introduces noise to images. When there is lack of light (indoors or outside at night), increasing ISO is your best option.

Raising exposure in post processing will introduce more noise compared to increasing ISO.

If you can control the available light (eg. a studio setup) then yes, you can keep exposure parameters high, even at high ISOes/sensitivity, but it's more common to use low ISOes in such situations (umbrellas, flashes, diffuses...).

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  1. They simply mean that when using a higher ISO, you can use a higher shutter speed, smaller aperture, or both. There is no such thing as "reduced exposure parameters."

  2. It seems like you don't have a good grasp of exposure. There's a good primer at http://www.cambridgeincolour.com/tutorials/camera-exposure.htm

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Sensitivity to light of a digital camera is constant. It can't be changed. ISO sensitivity is basically changed through an amplifier. Data coming out of the photodiode in the pixel can be changed only with the amount of light hitting the diode during the exposure. The rest is conversion factor, gain, and multiplication. The stronger is the amplification the more "to the right" the shadows are moved. Highlights also move to the right and start to overflow (hitting the right wall). So if the combination of shutter speed, aperture, and light available in the scene are not reduced to allow less light onto the sensor, the highlights will be blown out with the increase of ISO in camera (given the exposure was "to the right" for the lower ISO).

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