I think this image illustrates quite well the issue macro photographers have with depth of field.
If you bear in mind that this fly was no more than 15mm in length you can see the sharp plane of focus curving from the left corner of the frame, across the wooden fence rail, across the fly's head and out of the frame to the right. Its not very wide is it!!!
It was taken at 1:1 magnification at an aperture of F11, which is a middle of the road aperture for macro, but shows how precise you have to be in getting plane of focus critically on the eyes of the beast. It shows why you should not give up if you are disappointed with early results when you begin macro shooting. You will find out what techniques suit you when getting the camera and lens steady enough to shoot.
You can brace the lens on your spare hand, which is in turn resting on a post or the gound, you can rest the lens on the fence, or carry a bamboo cane as a simple monopod to slide your lens-gripping hand up and down. When free holding its best to set the magnification and rock gently back and forth a few millimetres so the plane of focus passes over the subject. Relaxation and breath holding are desireable.
And this is for 1:1 shots. Any attempts to go to greater magnification will see the depth of field reduce still further, which requires more practice and technique.
For insects I find tripods useless. Any attempt to set one up will disturb the animals and lets face it, they often don't hang around...sometmes you get but a few seconds to compose, focus, shoot! Part of the fun is walking around and stalking what you find...a tripod would hinder this process so its not for me. Others may have a use for one.
Be patient, you may not get many keepers at first but you certainly will as you get a little experience.
A while ago I mentioned how you should choose your macro lens with a bit of thought as to what you want it for and your experience. This shot was taken on the Sigma 150mm, which is an excellent lens with razor sharp rendition. I can compare it directly to the Sigma 105mm, as I use both. If my understanding of optics is correct, at 1:1 with both lenses giving the same field of view and magnification, the depth of field rendered in focus should be identical. However in practice the 150mm is significantly harder to get a good image from as the depth of field seems much shallower. I can only presume this is because the plane of focus falls away into a visible blur at a faster rate.
Personally I would recommend the 105mm length to a photographer starting in macro because its easier to get the critical parts sharp and because the reduced focal length makes being steady slightly less critical than with the longer lens. If anything the 150mm lens probably has an optical edge in quality (a small one) but for an easy life the shorter lens gets more keepers. Obviously there are other things to consider like the 150mm gives you more working distance to your subject, has a hypersonic motor for better focusing (out of macro range), has a tripod collar and is compatible with Sigma's teleconverters.
I thought I may sell the 105mm after getting the 150mm, but I haven't. Why? I love its light weight and compact size for smaller camera bodies and travel abroad. Its easy to manage with extension tubes on it and...as stated above, its a bit easier to get a higher keeper rate.
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