Does a len`s F stop change with the crop factor?

[britinjapan]

I know the focal length will increase by the crop factor, but what happens to the F stop?

[streetshooter]

This is a very good question. The crop factor has nothing to do with the fstop but....it does effect the FOV (Field of View), and the DOF (Depth of Field). So, the amount of light theoretically doesn't change with crop factors...what the fstop does besides gathering light does change.
Don

[odklizec]

No, the F stop does not change with the crop factor. The f1.9 is still f1.9, no matter if the sensor is 1/1.7 or FF. For example, the GRD with its 28mm lens set at 2.8 should give you the same exposure as the GXR A12 28mm set at 2.8. Of course, there could be small differences, because of different programming of GRD/GXR. But basically, the difference should not be significant (let's say more than 1/3 stop).

[AlbertTRAL]

I know the focal length will increase by the crop factor, but what happens to the F stop?

Just a small clarification, the focal length of a lens NEVER changes. Is a physical parameter and is only one. When shooting with a smaller sensor, the crop factor appears and this generates the "equivalent" focal length in a FF system that will produce the same Field of View.
When you use a DOF calculator, the focal length of the lens is the physical, native, focal length. In the case of the GXR A12 modules, it will be 18.3 and 33mm (not 28 and 50).
This takes us to the other common error that smaller sensors generate bigger DOF. NO! is the opposite. Smaller sensor generate smaller DOF because the CoC (circle of confusion) is also smaller. What generates bigger DOF is the "minuscule" focal length of the lens we use in these compact systems to generate similar FOV than in a FF system.
I mean smaller sensor>smaller DOF, smaller focal length>bigger DOF, final effect is that in a compact system we have bigger DOF because the effect of the focal length is stronger than the fact that the sensor is smaller.
Hope I have not confused you more but I think that this concepts must be clear.

[rondo]

AlbertTral's explanation is spot on. This is one of the most tiring subjects that come up in forums, which generates even more tiring and most of the time wrong headed debate.
The sensor size has nothing to do with depth of field.
In case of small sensor cameras the focal length need to be extremely short to generate the field of view of a larger sensor camera, or a 35 mm film.
For example, in grd line, equipped with a 1 1/7" sensor, the focal length to generate the field of view equivalent of a 28 mm lens (on a 35 mm camera) needs to be a mere 6 mm lens.
A 6mm lens, regardless of the sensor behind it, generates a lot of depth of field. The longer the focal length the less the depth of field at a given aperture.
Basically, with a small sensor camera, we all get fish eye lenses, with all its optical characteristics, albeit, cropped in center.
This is also tied to another misunderstood subject, the perspective....
All lenses, regardless of their focal length, will generate exactly the same perspective, which is a function of vantage point, not the focal length. That's why your 6 mm lens on a small sensor will give you exactly the same field of view of the 28 mm on film....but, it will have the physical properties of the 6 mm lens.

[olyz]

quivalentblende__.jpg (426.87 KiB) Viewed 786 times

That means:
Big sensor f stop 16 (Kleinbild=24x36mm) is eqivalent to small sensor f stop 2,0 (4,8x6,4mm)
Big sensor f stop 2 (Kleinbild=24x36mm) is eqivalent to small sensor f stop 0,4 (4,8x6,4mm)

Source: LFI 3/2006

Christian

[rondo]

No such thing thing as big sensor f stop, or small sensor f stop....
optics is optics...nothing to do with sensor size.
The source of confusion is somewhat understandable: for equivalent field of view, the focal length increases as the sensor size increases...the depth of field is a function of the focal length, not the sensor size.
The concept needs to be understood correctly.

[olyz]

Hi rondo,
the translation of Tabelle 4 "Mit abnehmender Sensorgröße..." is:
With decreasing sensor size the DOF (Schärfentiefe) is increasing rapidly.

Christian

[AlbertTRAL]

The sensor size has nothing to do with depth of field.

Well, although I think I understand what you mean, it really does. CoC is reduced by the same amount that the focal length is increased to find the "equivalent" focal length, this is, by the crop factor. This is why smaller sensor implies smaller DOF.

Hi rondo, the translation of Tabelle 4 "Mit abnehmender Sensorgröße..." is:
With decreasing sensor size the DOF (Schärfentiefe) is increasing rapidly.

Christian, this phrase is not precise. Should say LENSOR and not SENSOR. Tabelle 4 includes both, the brennweite (focal length) and zerstreuungskreis (circle of confusion), so it is an evaluation of how the DOF increases when we go from a complete 50mm/FF to a 9.2mm/Compact system (which varies focal length and CoC) that is what I call "final effect in a compact system" because the focal length DOF increase is stronger than CoC decrease effect of the DOF.
Christian you should post all the article of LFI because this "crop" is slightly misleading...

[bertalan]

Its too bad that camera companies, like Ricoh, insist on using the "ff equivalent" focal length instead of stating the angular fov for how wide/narrow a lens/sensor combination is. People would get used to it easy enough, but I'm afraid its too late now.
As far as actual "brightness" goes, if I understand correctly, for astrophotography(stars) the actual physical opening of the lens determines the brightness of a point light source. So a 50mm f2 lens will show brighter point sources than a 25mm f2 lens. Back in my film days I used to do a bit of astrophotography, and it takes alot longer exposure and/or faster lens to get bright stars at wide angle than at standard or telephoto.