Sensor sizes and f stops

Started 10 months ago | Discussions thread
Tom Axford
Senior MemberPosts: 1,463
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Re: Sensor sizes and f stops
In reply to starlight110, 10 months ago

starlight110 wrote:

Thanks for the answers!

In digitalcameras there´s a great deal of tradeoffs between sensor size, optics and Price. I want to buy a superszoom with a mínimum of 12x to about 420mm eqiv and will use it a lot for videos, in fact, the only reason why I´m considering a digicam instead of a camcorder is the sensor size. After searching I found that the cameras with the proper optics and price usually have 1/2,3´sensors, for instance canon sx510 with 720mm, f/5,8 tele and a smaller but newer sensor. To compare it to my old A630 I wanted to know if the smaller sensor size has a big influence or not, my A630 has f/4,1 tele and the bigger sensor, the sx510 has f/5,8 tele (a whole fstop slower) and the smaller sensor, would in this case be a great difference when using both cameras in tele?

Unfortunately, the camera manufacturers are not particularly helpful when trying to make comparisons between different cameras.

Sensor sizes get quoted by manufacturers in a variety of different ways:

  • DSLRs are generally specified as FF or APS-C
  • Sometimes sensors are quoted as 4/3", 1", 1/1.7", 1/2.3", etc.
  • Sometimes sensors are quoted as something like 8mm x 6mm
  • Sometimes the crop factor is given

Most photographers are not able to instantly convert between these various different measures of the same thing (sensor size).

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.]

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