Just in case any one isnt sure of the Depth of field concept, this short article may help.

If you want a shallow depth of field: to achieve a shallow depth of field, you use a large aperture opening, which corresponds to the smaller f stop numbers, such as f2.8. this will cause what you focus on to be sharp, and anything closer to or farther away from your subject to be blurred.

If you want a broad depth of field: you want a broad depth of field, so you use a small aperture, which would correspond to f16 or f22. this makes everything from very close to infinity appear in sharp focus.

for the third instance, where you neither need to isolate one small aspect nor have a very broad range of sharpness, use a medium aperture, either f8 or f11, as these apertures make the best use of your lens's 'sweet spot,' and you will ensure that your subject is absolutely tack sharp, with little to no blurring outside of your subject range, but without the need for extreme depth of field, which does not take advantage of the lens's entire sweet spot.

Large aperture = big hole = small number i.e f2 etc

Small aperture = small hole = large number i.e f22

Good depth of field seperation i.e background very blurred, subject sharp = Larger aperture

Everything in focus, subject and background = Small aperture

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