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As an inspector, people ask me how to get more images accepted at iStockphoto. One of the most common reasons why I (and my fellow image inspectors) might reject a photo is due to noise issues. In this case noise refers to compression artifacts, blotchiness and banding. Here are 5 quick tips to help you avoid this problem and get more photos safely through to your portfolio.

1. Shoot at the Lowest Possible ISO

Some cameras start at 50, most at 100 and a few at 200. Whatever the lowest ISO setting on your camera is, that's what you should be using for stock. Higher ISO allows you to photograph under less than perfect lighting conditions, but most cameras tend to generate too much "digital grain" beyond ISO 200. A few models are better at high ISO shooting (such as the Canon 5D), and in those instances you might get away with going as high as 400.

2. Beware of Dark Gradients

Noise first appears in the dark areas of your images, because these are the parts that contain the least amount of data. If you brighten your image or tweak the curves too hard, noise could appear in the shadows and shady spots. If you think, your image might have issues, check there first. The easiest way to avoid this is to use masks that avoid tampering so much with the darker areas.

3. Blotchy Sky

Blue sky is very hard for digital cameras to reproduce, and for that reason it is very easy to end up with noise in parts of the sky. Sometimes this is almost impossible to avoid without heavy post-processing, but an easy fix that sometimes work, is to first blur the sky (using gaussian blur) and then add a little noise afterwards. Adding the noise is important, because it smoothens out what blotchiness there might be left and keeps your sky from looking too heavily filtered.

4. Avoid Underexposure

This is actually something to thing about during the shooting itself. If you underexpose your images, you will have less wiggleroom before noise appears. Again, this is because less data is stored in darker pixels, which forces Photoshop (or whatever you are using) to have to estimate what might go where before there was nothing. Thus, noise is made. Use a lightmeter or shoot in one of the semi-automatic modes on your camera to avoid underexposure.

5. Shoot/Process in RAW

RAW files are much more forgiving than jpg. With jpg you actually lose quality each time the file is saved, where a RAW file never loses quality at all. To get the full potential of RAW, I highly recommend you do as much correction to the RAW file as possible, before opening it up in your image editor. Most good RAW converters will let you fix white balance, do color correction, add/remove vignetting and even do spot healing and cloning in a completely lossless way. Just remember that tweaking in RAW can create noise as well, you just have a little more room to play in before that happens.

So, if you're a budding microstock photographer with noise issues, hopefully this little checklist can help you along.

Rasmus Rasmussen is on loan from iStockphoto, where he is part of the image inspection team.