View Full Version : about f/value

Jitendra Katre
24-03-2012, 08:23 AM
hello everybody,
if f/value changes with focal length, shouldn't f/5.6 of my 400mm prime, which is full frame lens, change to f/9, when attached to crop sensor body? i know there is no actual increase in focal length, but light available for full frame is also reduced for crop sensor and that's what happens when focal length is increased. please help.

Sabyasachi Patra
24-03-2012, 08:47 AM
1) Aperture is the lens opening.

2) Aperture is not a function of the size of your sensor.

3) f value ie. aperture doesn't change with focal length. (in a variable aperture zoom lens the maximum aperture changes with focal length, however, that is due to lens design)

Only in case when you attach a teleconverter the aperture changes ie. the widest aperture changes. So if you attach a 1.4x teleconverter you lose one stop of light ie. your f5.6 400mm lens becomes max f8 ie. the lens opening reduces by one stop. So less light passes through it. Ofcourse the focal length gets multiplied by 1.4x, so your 400mm lens becomes 560mm.

In a crop sensor ie. a smaller sensor, the angle gets even narrow, so the effective focal length increases. However, optically there is no change in the lens.

Bibhav Behera
24-03-2012, 10:30 AM
Hi Dr. Katre,

The f number is calculated with the following formula:

f number = Focal Length/ Diameter of the Lens opening

Since the size of the sensor does not figure in the formula, the f number will not be affected by it.

However what does change with the sensor size will be the Depth of Field. The smaller the sensor size, more will be the depth of field.