What is image grain or noise?
Film grain is caused by the grains of silver present in the film. ISO noise is caused in the digital sensor and is pixel based.
Image noise is random variation of brightness or color information in images, and is usually an aspect of electronic noise. It can be produced by the image sensor and circuitry of a scanner or digital camera. Image noise can also originate in film grain and in the unavoidable shot noise of an ideal photon detector. Image noise is an undesirable by-product of image capture that obscures the desired information. Blah, Blah, Blah!
If you want to know more you can Google it, The bottom line is it makes pictures look grainy/noisy!
What causes it?
In brief it is caused by using a high ISO setting on the camera, typically 3200 and up.
(Click on the images to enlarge them)
The examples above indicate the noise on the left of each image and after its been removed on the right of each image.
In the first image the whale was moving and there wasnt a lot of light so in order to get a reasonable shutter speed (didnt want it blurred) I had to increase the ISO to 3200 (shutter 1/640 second), hence the noise.
In the second image it was close to sunset and the light was fading, I didnt have a tripod (hate carrying it) In order to get a shutter speed that I could hand hold I used an ISO of 1600.
How to reduce it
There are many ways to reduce Noise, photoshop and lightroom can to a point. I use a product called Topaz noise AI, Its a blanket filter (Does all the image not parts of it) but I've been impressed with the results.
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