I was asked by a friend this morning about aperture/f stop and depth of field, as he was confused by some of the more generic articles on the Internet.. I thought it would be a great idea to post it here, as it may help someone else!

If you are struggling with shutter speed click here for more info..

Ok F stop, such as f1.8 of f22 is the measurement of the size of the little ‘eye’ of your camera, this is the aperture, the hole in which light is let through the lens and into the camera. The numbers, such as f1.8 or f22 etc corresponds to the size of the hole, the SMALLER the f stop number (IE f1.8) the BIGGER the ‘eye’/aperture…and vice versa a LARGER f stop value such as f22 is a physically SMALLER size..

So when you have your aperture set at it’s ‘widest’ setting (for instance f1.8) you are trying to let in as much light as possible.

So how does f stop relate to depth of field (DOF)?
Ok so when you are reading small print, or trying to focus on something small you find yourself squinting, yeah? The reason you are squinting is because you are making your ‘eye’ / aperture as SMALL as possible..because when the aperture is small you can focus much further.
Shallow depth of field
On the contrary, if you have a LARGE aperture such as f1.8 your depth of field becomes very limited, you’ve seen photo’s where the item is in focus and the background is blurred? This is exactly what is happening, because the aperture is LARGE, the depth of field is SMALL, or limited.

In a model photoshoot example, if you had your aperture set to f1.8, you may find that if you focus on the end of her nose, that even just a small way back such as in line with her ears, will be slightly out of focus.. this is becuase the aperture is too BIG (the f stop number is SMALL remember!).. so to get more of her in focus you would increase the aperture…. the smaller you make the aperture (the bigger the fstop number) the MORE will be in focus.

If you had 7 people standing one behind the other and you wanted to get all the faces in focus, you would have to have a SMALL aperture (High f stop number such as f22) to get it all in focus… whereas if you wanted and arty picture you might choose a big aperture such as f1.8 to focus on one face at the front and the the others would blur.

If you set a lens to an f stop of f1.4 this will allow twice as much light through (to hit the sensor) than f/2. Each stop from that is a square root of 2 (Well, actually 1.4).. so the next number in the list allows twice as much light through:

(Wide ‘eye’ – ‘fast’ – shallow DOF) f/1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, etc. (Small ‘eye’ – ‘slow’ – large DOP)

So I hope that has helped someone who didn’t quite understand before!

Just remember the SMALLER the f stop number IE f1.8, the BIGGER the size of the ‘eye’…which results in a SMALLER depth of field…
and vice versa.
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