[Newtophotos]

I read somewhere that the f value (effective value?) of a lens drops as the sensor size decreases. Is this true? Will a FA 31 f1.8 have an even lower f value on a APS-C sensor camera?

[Adam]

I read somewhere that the f value (effective value?) of a lens drops as the sensor size decreases. Is this true? Will a FA 31 f1.8 have an even lower f value on a APS-C sensor camera?

No, it doesn't change. On smaller sensors you get the same amount of light, but the sensor doesn't "see" the entire frame.

The f number is simply a ratio of focal length of the diameter of the front element.

[Wired]

The depth of field changes, but not the light. And really the purists and math guys will tell you the depth of field doesn't change, it's the same image as you would see on 35mm format, just cropped.

A image taken at 1/160" f18 iso100 on any camera film, nikon, canon, apsc, ff, pentax, m4/3, compact should have pretty close to the same exposure.

[bdery]

The f number is simply a ratio of focal length of the diameter of the front element.

For the sake of accuracy : not the front element, but the "aperture stop", which is usually the iris.

[ElJamoquio]

I read somewhere that the f value (effective value?) of a lens drops as the sensor size decreases. Is this true? Will a FA 31 f1.8 have an even lower f value on a APS-C sensor camera?

Different people use this f-value for different things.


1) Correct Definition, but not particularly useful: The f/number is the focal length of the lens divided by the aperture diameter.
This number is solely a property of the lens.

2) User ramification A: The user-selected aperture (f/number), along with the shutter speed and sensor/film sensitivity, determines the exposure of the image to available light.
This uses the f/number number as solely a property of the lens, i.e., the same number as the one given in 1).

3) User ramification B: The user selected aperture (f/number), determines the depth of field of the image.
This number is a property of both the lens and the sensor size.
To alleviate the sensor-size dependency, people use 'equivalent' F-stops.

So, as used right now, ramification A and B aren't separated, and some people use 'actual' F-stops and 'equivalent' F-stops.

Confusing enough? There's an easier method, but it involves equivalent F/stop and equivalent sensitivity, but no one actually uses this system, so if you're just starting it's better not to learn it.