General Photography

Aperture and DOF for beginners

Posted by: davex on Fri Nov 18, 2011 10:37
A brief explanation of one the most important aspects of photography that any beginner needs to grasp.
So, what is aperture?
In photographic terms the aperture of a lens is the ratio of the area of the front element compared to the effective area of the rear element, the effective area of the rear element is usually controlled by the aperture ring. It is referred to in terms of f stops, and just to make it slightly more confusing it is a logarithmic measure; f1.4 is twice as big (in surface area) as f2. So, if a lens is set to f1.4 it allows twice as much light through as the same lens set to f2 and 8 times as much light as the lens set to f 4!, remember it is logarithmic, A range of f stops follows this order: 1.4, 2, 2.8, 4, 5.6, 8, The higher the f number the smaller the aperture.

The first is f 1.4 second is f 8
Strictly speaking it should be f 1:1.4 and f 1:8 as this shows the full ratio.

Aperture and Depth Of Field (DOF)

DOF is the amount of front to back “sharpness” obtained when a lens is focussed on a given point.

Large DOF (high f number) notice the difference in the background. Small DOF (low f number)

The first shot was taken at f8, the second at f2
The smaller the aperture, the greater the DOF increases as the aperture closes (bigger f number) Remember f1.4 is twice the size of f 2.
A DOF calculator can be found here

But why does DOF increase with aperture?

A non physical explanation; not strictly accurate but close enough.
Light travels in a straight line, but enters a lens at all angles. With a small aperture most of the light hitting the sensor (or film) is relatively straight in relation to each other, there is less blur (DOF), with a large aperture light enters the lens from all angles, this creates blur (small DOF).
For a more detailed discussion on this see here

That’s about it for aperture and DOF, next Focal Length, FOV, and Format Size.

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