- When cameras focus, light from the subject shines
through the lens where it's focused on the photo-sensor
in a digital camera or
the film surface in an old-style film camera.
In either case, there's a minimum grain size.
In the case of film, that grain size would be
the particles of light-reacting dye.
In the case of a digital camera,
it's a single photoreceptor.
When a single point of light lands perfectly
within a photoreceptor, it's in focus.
When it doesn't, it's not.
It's as simple as that.
But what if that single point of light could be
even more in focus than a single photoreceptor?
Well, in that case it could never look any more
in focus than a single photoreceptor
and it would look exactly as sharp as it did before,
and that's depth of field.
When you focus a lens there's actually a range
of focal distances that are as sharp as
the light sensor can see.
When you open up the lens by using a lower F-Stop
or T-Stop you widen the area
the light is shone onto on the sensor.
Truly in-focus items will still look in focus
but the items that were only effectively in focus
will begin to smudge over into other photoreceptors.
They look out of focus.
When you close up the lens with a higher F-Stop
or T-Stop you're making the hole smaller
and this means there's less scatter.
The image seems sharper over a greater range
of distances from the camera.
Of course you'll also need more light
because there's less coming through the lens.
The important thing to understand is
that actually only a small amount
of the picture is ever really perfectly in focus.
It's just that there's a range of focus
that's greater than the sharpness of the sensor.