ISO vs Under-Exposure – A Noisy Battle
Imagine yourself in a tiger reserve at the fag end of the day with light going low rapidly. You are struggling to keep up the shutter speeds (cranked the ISO to 1600 already) and a tiger decides to pay you a visit.
You have a full grown male tiger walking head on to you and you are getting shutter speeds of 1/100th. Just not good enough for a moving subject. What do you do?
Well firstly you would probably take a look at the aperture because you might want to go full open with it. What next?
Do you reach out to the ISO or do you under-expose the image a little to get that extra shutter speed?
Those are the two options you have. Let me explain if its unclear.
- What does increasing the ISO do? – It increases the sensitivity of your sensor and to obtain the same exposure as earlier, you would now need light coming in for a lesser duration of time, thus giving a faster shutter speed (and probably a sharper images)
- What does under-exposing do? – It tells the camera that you do not need as much light as it is asking for, so the camera gives you a faster shutter speed (and probably a sharper image) and also a darker image than earlier because you have not allowed as much light as the camera said.
I have seen a lot of people shift to reducing the exposure to half and trying to achieve 1/200th of a shutter speed in the above scenario. With the improving performances of the digital Sensor I go the other route. I personally am very scared of massively under-exposed images. They generate a lot of noise in the shadow areas and might render my image un-usable. I would rather go and crank up that ISO to a higher value and get a correctly exposed image. Lesser shadows would always mean lesser noise so please go for the ISO in such situations
Try it in the field or at home and see the comparison.
Here is an image that I made on a pretty High ISO on a rainy foggy day in Thailand. Thick Forests are always a challenge to your camera and it important in such situations to know your camera well.