What exactly is Depth of Field (DOF) - also referred to as Focus Range? It is important to take DOF into consideration while shooting small objects such as in Nature Photography (ie. flowers, insects) or in Jewellery Photography. Small objects can be of the size of a postage stamp.
In photography, DOF is the distance between the nearest and the farthest objects that is judged to be in focus in an image.
Depth of Field is a function of 1) aperture (f-stop), 2) distance between the object and the camera sensor, and 3) the focal length of the lens on your camera.
The higher the aperture (i.e. the lower f-number), the smaller the DOF. The same applies to the focal length of the lens – higher focal lengths (telephoto) produce images with shallower DOF.
Using macro lens to shoot small objects at close range (distancefactor) also produces shallow DOF.
There are many online sites that have depth of field charts to show you all relations between the numbers for different lenses.
Here is another important & helpful information regarding DOF :
Depth of field extends 2 times more behind the focal point than it does in front of it.
At maximum aperture, aberrations (ie, spherical aberration, coma, astigmatism, field curvature & distortion) start to play an important role. All lenses have these aberrations and they are particularly apparent with fast lenses. Stopping down (ie. higher f-numbers) on a lens greatly reduces aberrations. Does this mean higher f-numbers result in better sharpness in our images? Not really. Why? This is because another phenomenon called diffraction sets in and reduces image sharpness.
In general, we aim to strike a balance between decreasing aberrations and increasing diffractions and the rule of thumb is to stop down the lens (usually by 2 to 3 stops from maximum aperture) in order to achieve the optimal image sharpness.
Are you confused? Please call me and I can help you.
Here is the conclusion I took from an article by Bob Atkins:
If you want to keep your images sharp, don't use f/32 with an APS-C DSLR. The effects of diffraction are clearly visible at f/32 and significantly degrade the image. Use f/32 only if you have no choice. Optimal sharpness depends on the lens. For a lens with significant aberrations (e.g. a consumer zoom at maximum focal length and minimum focus distance), stopping down to f/16 may give optimum results. On the other hand, for a lens with less aberrations (e.g. a consumer zoom used at infinity focus), optimum performance is around f/11, though both f/8 and f/16 are very similar. For a really good lens like the EF 300/4L, with well-corrected aberrations, performance may peak at f/5.6, but would be good from f/4 to f/11. f/16 is acceptable, but f/22 and smaller apertures should be avoided.