Though certainly not the norm, occasionally some photos can come out grainy.
When an image is very dark, or very low contrast, the potential for graininess rises. In particular, when your camera is forced to boost the low light level that it captures, the image may become grainy. You may not realize this is happening because cameras generally make this boost automatically. And some cameras do this boosting much better than others.
In order to enhance the contrast so that we see the image in a more dynamic and pleasing way, we are essentially amplifying the differences between pixels. If the image contains a good amount of visual noise (unwanted small image artifacts), the noise is amplified along with the image and becomes more noticeable in the form of graininess. There are filtering techniques that can be applied to reduce this, and we use them, but they must be used sparingly lest the image become “cartoony” due to over-smoothing the image.
Also, when an image is sharpened, it tends to bring out the graininess. Sharpening amplifies the differences between pixels, and visual noise or grain is exactly that...differences between nearby pixels. We have chosen a sharpening technique that minimizes this effect by attempting to sharpen only the actual “edges” of the objects in the image, but the technique is not perfect. Also note that the effect shows differently with different image content. For example, you rarely see graininess in the sand, but a “clear” blue background makes it obvious. Grain is also reproduced differently in different size images, with small images like our side by side being more prone to the problem than larger image files.