Depth of Focus
Bee - a case study in compromise.
The depth of field increases as the aperture number increases, that is as the diameter decreases.
However the depth of focus is also less the longer the focal length. So in 35mm camera terms, the depth of focus achieved with a 50mm standard lens at f8 will be less than for a 300mm lens at f8.
This also appears to work in reverse. Macro lens have a very shallow depth of focus.
This is a good example of how we have to compromise. Consider taking a photo of say a bee. I take my standard 35mm lens on my digital SLR set it to f8 and focus on bee and press shutter. With luck I might get the bees eye and most of his face in focus. More typically I have a swaying flower and no bee. I have frightened her off.
What I need is a macro telephoto. Fortunately I have a Sigma 105mm 1:1 f2.8 lens, which on my Nikon D90 is equivalent to approx. 150mm. So I set this on my camera, set f8 and try again. This time I manage to get more of the bee in shot, but focus is poor as now depth of field is much less. Ok lets up the f number to f16. This means only a quarter f the light hits the sensor. No worries just decrease shutter speed. If it was previously 1/125 sec. it will now be 1/30th sec.
Results are no better. Now this Sigma lens is good but has no stabilisation, and hold handing a lens at 1/30th sec. breaks the rule of thumb that shutter speed should be no less than reciprocal of the shutter speed, i.e. 1/150th. The lack of focus is probably due to hand shake. OK lets find the tripod and try again.
Results a bit better but no means good. Apart from having to wield camera and tripod around flower bed chasing after the bee there is only one photo I would consider sharp. OK camera is still but wretched bee keeps moving. I need faster shutter speed to freeze motion, but I want to keep depth of focus. So I increase sensitivity by raising the ISO to 800. Now this is where your initial investment is rewarded. Typically the more expensive the camera the bigger the sensor and you may well be able to use ISO 800 with little loss of quality. However if you need to use ISO 1600 or more than you are probably going to lose quality unless you have a full frame camera (mega bucks).
So what about using flash. Now we can have low ISO, high f number and high shutter speed. Now we catch the bee on the photo, all is sharp but the photo is flat. There is more to this technique than just raising the flash.
We could always revert to the old pro trick of catching the bee, putting it in the deep freeze for a couple of minutes, pop it back on the flower and photograph it before it revives and flies off in a huff!