Effect of DOF:
Further elaborate the importance of DOF(depth of field)…. An image DOF quality produced at a given f-numbers dependent on factors such as focal length(lens mm),subject distance(camera to subject),film sensor(iso),format(full frame or half frame).If intended to acquire shallow D.O.F,you need to have a smaller f-numbers of a larger aperture or vice versa….
Modern lens when stop down to f7 or f8 ,will normally produce full sharpness,While for older lens like (Tessar formula,German made),only with 4 elements lens group built in,it will need to stopping down to at least f11 to obtain the same result.
The overall sharpness D.o.F of any given f-numbers is still constrained by “diffraction”, diffraction occur especially occur at high f-number, when a light wave lenght encounters an obstacle, the bending of waves around small obstacles and the spreading out of waves past small opening.,and creating lights dispersion
Where at low f-number, limitation of the lens constrained by “aberration”, and this occurs when light from one point of an
object after transmission through the system does not converge into a single point.
After all, the sharpness of any desire focal point not just determine by lens focus, it also depends on the f-numbers that effect the sharpness of the image, professional photographer tends to use this technique vividly telling the viewer what his/her intention….this even harder in street photography, as things occurs in disarray and confusions, sometimes just to tell this story alone it take hundred of an second with no hesitation…not to mentioned a photographer can instantaneous captured the benevolent irony, a humanistic sensibility, traditional to the spirit sympathy for the creatures of dogs deeper than he had imagined and the iconic photographer, he is Elliot Erwitt,I guess all these DOF means nothing to him…