You want to take a photo in a dark place, but do not want to use flash for many reasons: Maybe use of flash is prohibited there. Camera flash whitewashes the photo and ruins the image. Flash has a limited range, it cannot cover long distances.... There are many reasons for not using the flash in poorly lit locations. ISO comes into the picture here.
ISO (International Organisation for Standardisation) refers to the sensitivity of the film, in earlier days of the film camera. Now in digital cameras, it refers to the sensitivity of the image sensor. In simple words, it tells how sensitive the film/sensor is to light.
Photography has three main settings – ISO, Aperture and Shutter Speed. This is called the Exposure Triangle. All three are interrelated to each other
Camera used is Canon t5i (50D). Time of capture is 10:24 pm. Two aspects for such a clear image at night without a flash or any other lighting was the ISO setting of 800 and long exposure duration of 30 seconds. Exposure is the shutter speed, one of the sides in the Exposure Triangle. Obviously, a lower shutter speed, in terms of seconds needs a tripod or some support to steady the camera for long duration such as 30 seconds.
How are ISO and Brightness related?
ISO affects the brightness of the image. The ISO settings in the camera start from 100 and this number doubles up in the range of 200, 400, 800. Lower the number, lower is the sensitivity.
This means that higher the ISO, brighter is the image, lower the ISO, darker is the image.
ISO 800 brightness is twice the brightness at 400.
With aperture (F-stop) and shutter speed (Exposure time) kept constant, for each jump of the ISO to its next higher value, there is an increase in brightness of the image.
Time of capture: After 10:50 pm
In the combo image, we can see the relation between ISO and shutter speed.
Compare 1st and 2nd images: Same exposure time of 30 seconds. 2nd image has half the ISO of the 1st, so it is darker.
Compare 2nd and 3rd images: Same ISO of 800. 3rd image has exposure (shutter speed) of 25 seconds, 5 less than the 30 sec of the 2nd. So it is darker than the 2nd image.
The balance between ISO, Shutter speed and F-stop
The eight images above are almost same in brightness, though ISO is different for all. This balance of brightness is maintained by varying the shutter speed (Exposure time). The increase in brightness due to a higher ISO is compensated by a lower exposure time. Shutter speed of 1/125 th of a second lets in lesser light than a shutter speed of 1/60 th of a second. This is to compensate for higher sensitivity due to a higher ISO.
Noise
No writeup about ISO is complete without the use of the word “noise”.
When sensitivity of a microphone increases, audible noise too increases. In the same way, increasing the sensitivity of the sensor/film increases its ability to amplify visual noise, or grains. A high ISO image is brighter, but more grainy or dotty to look.
In this combo image, the upper image is noisy or grainy because of high ISO of 6400 compared to the 1600 of the lower image. The upper one is darker only because of higher F-stop. Aperture of 5.6 is wider than that of 16. More about aperture or f-stop in another article.
How to reduce noise? As seen from the above combo image, setting a wider aperture is one of the methods. Post processing using Adobe Photoshop, Lightroom are the out-of-camera methods. Latest cameras have noise reduction features inbuilt in them.
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