"My images are dark, will increasing ISO help ? " one of the most often asked questions.
For this photograph of a Belly dancer in Dubai, ISO was increased to 5000 to achieve a shutter speed of 1/40 at f4. The Image was further underexposed by one stop, both to get correct exposure as well as not to increase the ISO too much. (Canon EOS 5D Mark II Focal length:81 mm)
If you are shooting at Aperture priority mode, increasing the ISO will only let you get a higher shutter speed at the set aperture. Higher ISO does not let in more light to enter but allows you to shoot a scene in lesser light. Increasing the ISO lets you manage to get the same exposure with less light but at the cost of NOISE. Check if there is any other reason of under exposure, maybe the exposure compensation dial is rotated towards under exposure ( towards minus ) In general, at aperture priority the exposure, if not absolutely correct is closer to the desired exposure.
This may vary based on the metering mode set on the camera. In other words, one may say that increasing the ISO lets you have the same shutter speed at a given aperture as one would have got if the light was more. In bright daylight, lets say, if you get a shutter speed of 1/8000th of a second at f2.8, the shutter speed starts dropping towards 1/60th sec or even lower as it starts getting darker. with shutter speed dropping bellow 1/60th sec it gets difficult / impossible to shoot hand held with a camera shake recorded. To counter this, by increasing the ISO one can achieve a higher shutter speed. You are making the sensor behave differently. It amplifies the signals and achieves the same exposure as it would have achieved with higher light.
Shot with Canon eos 5D mark III with a 50mm f1.4 lens, the ISO was increased to 2000 to achieve a shutter speed of 1/60th sec at f 1.4 aperture. Notice that the higher ISO performance of mark III is better than mark II
The Photograph above was simply not possible handheld without increasing the ISO to such a higher figure. A fast lens like this one also helps in such situations.
Increasing the ISO is simply letting us get the right exposure and is not a substitute to making a photograph bright if it is darker. That will be achieved through exposure compensation and not by increasing the ISO. If the above photograph was shot at a relatively lower ISO, the difference would have been in terms of getting a slower shutter speed but the brightness of the image would have still be the same.
Photographs- munish khanna