In past we might have interchangeably used the word grain and noise. Actually they are very different. The 'grain' came from film/slides days(film grain). The noise came from digital world (sensor noise). What is the difference? To illustrate this I went back to my slide collections. I took a scanned slide and cropped a 1000x1000 pixel plain crop from slide and another 1000x1000 pixel crop from a digital file. Here they go!
Please see this on a large display (preferably a calibrated display (matt preferred), not on your cell phone!) to appreciate the subtle differences.
First, Slide/Film Grain
Now, Digital (CCD/CMOS sensor) Noise
Do you see the difference?
The point to note here is -
1. Grain has larger random structures - this is caused by lumps of silver halide deposits on the film/slide, which gets unevenly exposed causing granular tiny lumps. Different films/slides had/have different structures. Some films are very fine (like the one I shared above, which is Velvia 50 ISO slide film crop) and some a little more coarser. While excessive grain is not desirable a little bit of it often results in very pleasing image and gives depth to an image. In the digital world it is tough to get this subtle grains which brings life to the final output, mostly to prints (and as we have seen here at CNP it works for web images too).
2. In contrast, sensor noise is uniform and is made up of finer specs which are dimensionless! Sensor noise is not very desirable. We often try to get rid of it (using different noise removal softwares).
Let us try an experiment here. What I will try to do next is, overlaying slide grain and digital noise on an otherwise clean digital image. Let me share the clean image first.
A cleaner sample image (1000x1000 pixels)
Now let me overlay the slide grain on this clean image. Let us see how it looks,
Image with Slide Grain (digital image + slide grain)
Now, the same image with digital noise overlaid (digital image + digital noise)
Which one you prefer?
I definitely prefer slide grain! I love the subtle (emotional) nuances/depth the random grains create.
Now, let us look at the same image with software simulated grain (using Nik's simulated grain)
I think this is better than digital noise but the not as beautiful as slide grain! What you think?
Here is another emulated grains (of Ilford HP5 plus film)
Hope this is useful, for those who never used slide/film.
Next time when we talk about 'noise' or 'grain' we have a reference!!