[LFLee]

A beginner question.... I have various lenses with various widest apertures, but they all doesn't seem to have the same opening diameters....
So my question:
- A F2.0 micro-four third lens has the same aperture diameter as a F2.0 FF lens?
- put it another way, a F2.0 aperture will have the same lens opening diameter on FF glass, APSC glass, and micro four third glass?

I suspect the answer is no, as I seen compact camera with stated F2 aperture but the opening is so tiny.... so I guess it is sensor/film size dependent?
In this case, a F2.0 FF-compatible lens will let in more light than a F2.0 crop-sensor-only lens?

Lee

[elliott]

Aperture diameter is dependent on focal length, the reason the f2 aperture on the compact looks tiny is because the lens itself is very short focal length due to crop factor. It could be only 11mm focal length on a 1/1.7" sensor, but have the effective FOV of a 50mm on a 35mm frame. The sensor is irrelevant, a 35mm f2 lens on full frame is a 35mm f2 lens on crop and vice versa.

[mattt]

I'd explain (to make sure I understood correctly) but the wiki does a better job. F-number - Wikipedia, the free encyclopedia

To sum, the Fnumber is a function of the opening size relative to the focal length (irrespective of the sensor size)

Edit: what he said.

[Anvh]

F number is the ratio between focal length and the pupil opening.

So 100mm focal length and 10mm opening gives you f/10
10mm focal length and 1mm also gives you f/10

Don't forget we are talking about the "effective" opening of the pupil here so not directly the physical size of the front glass per se

[vladimiroltean]

In this case, a F2.0 FF-compatible lens will let in more light than a F2.0 crop-sensor-only lens?

NO.
Don't confuse focal lengths with field of view. Field of view (wide angle, normal, telephoto, it's measured in angles) is dependant upon focal length and sensor size. But DOF and brightness are not, they are only dependant on aperture and/or focal length. Sensor size has nothing to do with brightness.
Take it this way: a 55mm lens will produce an image. On a tiny sensor (think compact), only a small fraction of that image will be recorded, so it will look as though it is magnified (which it is). On APS-C, however, 55mm is considered "short telephoto". On 135, it is "normal". On 120 film, it is wide angle. All of these because more of the actual image gets to be captured. Of course, given that the 55mm lens can deliver a large enough image circle to cover 120 film. But take a Pentax 6x7 55mm and mount it on your APS-C DSLR: it will look the same as any other 55mm.
The same is with brightness. f/2 for example says it all about the lens. It's f/2 no matter what format it's used on.

[twitch]

Focal length (mm) / Aperture opening (mm) = Aperture f-stop. f/2 just means the focal length is twice as big as the aperture opening.

[lytrytyr]

Focal length (mm) / Aperture opening (mm) = Aperture f-stop. f/2 just means the focal length is twice as big as the aperture opening.

"Aperture opening" here does not mean the diameter of the hole made by the aperture blades,
it means the diameter of the image of that hole as seen through the front of the lens.
Compare Entrance pupil - Wikipedia, the free encyclopedia,
or the following comment from mattt's reference (Post #3):
"The necessity of clearly distinguishing between effective aperture and diameter of physical stop is strongly insisted upon."