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When I first started using a DSLR camera, I thought the camera focuses on a single point. However, after multiple camera bodies later, I understood a truth that the camera focuses on a plane rather than the point. This plane is referred to as a ‘Focal Plane’.

Focal Plane: a plane that is perpendicular to the axis of a lens or mirror and passes through the focus [1]

Hence, if you click a photo of a perfectly flat surface perpendicular to a camera lens, in theory, it should be tack sharp from top to bottom. Having said that, nothing is perfect in this world; since, a camera lens is curved (this curvature is referred to as Petzval Curvature), there is less sharpness on the edges of the focal plane.

Further, there is a small area in front and back of the focal plane that should also be somewhat in focus. This small area is referred to as ‘Depth of field (DOF)’. The area outside the DOF is less sharp out of focus area and is simply called as blurred area or bokeh.

The size of the DOF depends on the aperture, the focal length of a lens and the distance to a subject.

• Smaller the aperture (here, in lens universe, smaller means bigger number) wider will be the DOF, and vice versa.
• Longer the focal length of a lens, shallower will be the DOF, and vice versa.
• The closer a camera is to a subject it is focusing on, shallower will be the DOF, and vice versa.

Now the question is: What is the aperture & the focal length?

Aperture: The aperture is the opening created by a diaphragm inside a lens. This opening controls the amount of light entering through the lens. The opening of the diaphragm is measured in F/Stops where, a stop of light is either double or half the amount of light. The F/Stop is represented in fractions such as, F/1, F/1.4, F/2, F/2.8 because lenses are circular, so if you want to double an area of a circle, you increase the diameter by 1.4. High School Math baby!

Focal Length: The focal length is distance between the point of convergence in a lens and a 35mm full frame camera sensor. Focal length is measured in mm such as 24mm, 35mm, 50mm. In simple words, focal length tells us how zoomed-in (magnification) will be the image and its angle of view.

To summarize this convoluted concept:

If you want a photo with a lot of stuff in focus, chose a wide lens with a small aperture, and if you want a bokehlicious photo, chose a long lens with a larger aperture.