I have had a Canon A540. A compact digital camera with widest aperture at F2.6. That was the reason I bought it anyway. During the 10 months period of using it, I tried and tried and tried taking close up picture (so called macro on the camera setting) at the widest aperture but I just couldn't get nice blurry background. I thought I need wider than that to get it there. But after a lot of close up shots with my SLR plus my 50mm lens set at F3.5, I realized that I can get much better blurred background than at F2.8 with my compact.
In fact, at F2.8, the depth of field is so shallow that I got too much blurred area!
F2.8 with my SLR....
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F2.6 with my compact....
On top is F2.8 with my SLR while below is at F2.6 with my A540. See the difference?
While the aperture rating is almost the same, the depth of field is entirely different. Why? Back to the basic, the lens size on a compact is very small, the size of a peanut, while the lens size of an SLR, are much bigger. Hence the amount of light entering the sensor is more in SLR than a compact. Reason is, the sensor size is much much smaller on a compact hence the lens size.
The conclusion is, the aperture rating between an SLR and a Compact by any mean is not the same. So one should not expect to get equivalent result at the same F number to an SLR by using a compact.
It doesn't matter whether you have the top notch compact digital camera which may be more expensive than an entry level SLR. As long as you lens is small, your sensor is small, you just can't get the same result.
Hope this'll help!
