Sensor sizes and f stops

Started 7 months ago | Discussions thread
olliess
Contributing MemberPosts: 888
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Re: Sensor sizes and f stops
In reply to starlight110, 7 months ago

starlight110 wrote:

Tom Axford wrote:

I prefer to use the crop factor relative to FF. To work this out for a sensor specified as 1/2.3", you simply need to know that FF corresponds to 8/3" = 2.7" on this scale. So the crop factor for the 1/2.3" camera is 2.7 x 2.3 = 6.2.

You can use the crop factor to work out the equivalent lens (with respect to field of view, depth of field, background blur, diffraction effects) by simply dividing the focal length and the f-number by the crop factor.

So a 50mm f/11 lens on FF is equivalent to a 8.1mm f/1.8 lens on a camera with a 1/2.3" sensor (with respect to field of view, depth of field, background blur, diffraction effects).

[Of course, the exposure is still determined by the actual f-number, you do not multiply by the crop factor.]

Interesting formula, that means a smaller sensor will always produce more depth of field at a given aperture?, let´s say a 1/5,8 sensor in a camcorder at f3,4 will have more depth of field than a 1/2,3 sensor at f/5,6?? sorry if comparing digicam and camcorder is wrong but I guess the optics and system would be in some aspects be similar

What the formula tells you is that the 6.2x crop camera will have more depth of field than the 1x crop camera whenever you use the equivalent focal length (50mm FF vs. 8.1mm on the 1/2.3" sensor) and the exact same f-number (e.g., f/11). If you use those lenses with the "equivalent" apertures of f/11 on the FF sensor and f/1.8 on the 1/2.3" sensor, the DOF will be exactly the same.

You have to keep in mind is that people usually talk about the aperture in terms of "f-number," which is the focal length divided by the (effective) aperture diameter. When you use a smaller sensor to take the same picture, it's assumed that you'll also use a shorter focal length. What this means is that the aperture diameter will actually be smaller for a given f-number. As it turns out, the depth of field will be the same between a larger and smaller sensor for the same aperture diameter! (But nobody ever sets their camera according to aperture diameter).

By the way, to get full "equivalence" between pictures taken on two different sensors, what you do is set the same shutter speed, "equivalent" apertures, and raise the ISO on the larger sensor by the appropriate number of stops. This is useful for understanding relative sensor performance, but probably not so useful for everyday use.

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