One of the most annoying problems in digital photography, especially if you intend to sell your work is noise in the image. In the old days, we used to call that “grain.” It originates for many of the same reasons. With film, we got grainy pictures when we used too small an image size, used very high speed film, or tried to “shoot in the dark”. To do those things we had to use high speed film, “over-develop” the film, and, often, enlarge it for publication.
Here’s how you can keep noise to a minimum when you’re shooting digital: Use as large a sensor as is practical and within your budget, shoot at the lowest possible ISO, expose for the shadows, keep your exposures as short as possible, and (if the subject isn’t in motion) shoot multiple exposures that you can combine into one “HDR” image.
If you memorize the above list, you’re way far ahead. I’m going to explain the “why’s and wherefore’s” of each of those things because there’s more to them than you might suspect.
Use a Large Sensor
The biggest difference in noise you’ll see in your shots is if you take one picture with a compact camera and another with a DSLR. DSLR sensors come in two sizes: 35mm and APS-C. APS-C size sensors generally “cram” more individual sensors into a given amount of space…even when the larger sensors have more individual sensors. The more individual sensors (a.k.a. sensor cells), the higher the image resolution. But the closer they’re crowded together, the more noise they make. If you have a compact camera with more than 10 megapixels, you’ll probably get lower actually resolution due to noise than if you had 10 or less megapixels. Now, it’s true that the further the technology advances, the more pixels it’s practical to put in a limited space. However, that’s all relative and it will always be true that larger sensor will give you less noise and much higher resolution.
One of the best ways to judge that is to look at the number of pixels in a square centimeter (pixel density) of a small compact, slightly larger sensor compact, typical APS-C 12MP sensor, and a full frame 35mm sensor of twice the resolution of the APS-C sensor:
Canon Power Shot A100 12MP = 43MP pixel density
Canon Power Shot G11 10MP = 23MP pixel density
Canon Digital Rebel T1i 15MP = 4.5MP pixel density
Sony Full Frame A850 25MP = 2.9MP pixel density
Think of it this way: A DSLR has about 10% of the pixels per square centimeter as its compact small- or large-frame counterpart! To put it another way, you get about the same amount of noise at about ISO 1200 from the highest resolution full-frame camera as at ISO 100 on the larger frame compact cameras.
Shoot in the RAW
Bet that caught your eye, but all kidding aside, a RAW file has about 10 stops of brightness range instead of the approximately 4 stops of a JPEG. So if you lighten your shadows in a RAW processor, you’ll see much less noise and more local area contrast.
So if you lighten your shadows in a RAW processor, you’ll see much less noise and more local area contrast. And when you do expose for the shadows (see below), you’ll have a lot more ability to recover detail from the over-exposed highlights (but not as much as if you’d shot RAW and underexposed for the highlights because they were more important to that particular shot than the shadows).
Shoot at a Low ISO
Just about any camera will keep noise low enough to get an decent 8 X 10-inch print if you’re shooting at ISO 100. Noise acceptability will “max out” on most compacts at about ISO 200 if they’re more than 10MP in resolution. If you have an APS-C or 4/3ds size DSLR, ISO 800 will probably be tops (give or take). Believe it or not, you can double that for a full frame camera. (I want one, dammit)
Expose for the Shadows
Much less of the dynamic range of a digital image is given to the shadows in an image than to the shades above medium (50%) brightness (“gray”), so if you’re shooting an image in which the areas of most interest are in the darker tones (and especially if the brighter highlights aren’t too important), simply over expose the shadows and then darken the image when it’s processed. You’ll be amazed at how much less noise and how much more detail you’ll see in the shadows. You do, however, have to expect that your highlights are going to blow-out, so best if they’re either not in the scene or simply not of any interest or can be cropped out.
Shoot Multiple Exposures
Scenics are often the big problem because we generally want to make larger prints and to stare at them for a long time to study all the details. So shooting for the shadows seldom works well. You should thank heavens for HDR technologies.
Keep Time Exposures Short
If you’re going out to shoot a 30 minute exposure between moonset and sunrise, best to rent a full frame camera if you’re not ready to make the 2-8 thousand dollar investment in one of these. Either that or accept the noise as part of the “artistic feel” of your shot. In fact, the longer your exposure times, the more noise you’ll get. If you’re using a DSLR, though, it’s not likely to be all that objectionable until the exposure gets to be several seconds long.