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I have a Canon G7X II camera, which has a 24-100mm f/1.8-2.8 lens. Why does DPReview's chart show it having an aperture of around f/8 at a focal length of 100mm?

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The actual focal length range of the lens on your G7X II is not 24-100mm, it is 8.8-36mm. Coupled with the smaller sensor (13.2 x 8.8 mm) of the G7X II the 8.8-36mm lens gives the same field of view as a 24-100mm lens does on a full frame (36 x 24mm) camera. Similarly, the aperture of f/1.8-2.8 will behave, in terms of Depth of Field, like an aperture of f/5-7.78 when used with the smaller sensor (this equivalence breaks down at macro distances and distances past the hyperfocal point). In terms of exposure, however, it will still be an f/1.8-2.8 aperture.

For more about why many zoom lenses have variable apertures across their focal length range, please see Why do zoom lenses and compact cameras have varied maximum aperture across the zoom range? and Why does my aperture setting change as I zoom on my DSLR kit lens?

  • On my Nikon D90 (APS-C sensor) i have kit lens i see symbols 18-105mm and 3.5-5.6f. When i would have chart like this from question, what max aperture on chart will be for this lens on max zoom? I would like to compare my kit from Nikon to lens from G7X. – marioosh May 7 '16 at 6:06
  • Your Nikon kit lens on your APS-C camera would behave like a 27-158mm f/5.2-8.4 lens with regard to field of view and depth of field. It would continue to behave as an f/3.5-5.6 lens with regard to exposure. For a more detailed and complete answer please see photo.stackexchange.com/questions/10079/… – Michael C May 7 '16 at 7:14
  • For reference, an APS-C sensor crop factor is around 1.6x, Micro Four Thirds is 2.0x, 1"-type sensor is 2.7x and compact cameras and smartphones are about 6x. You can compare depth of field this way, for example comparing between 1" and Micro Four Thirds, the difference in aperture for the same depth of field is 2.7 / 2.0, or 1.35x. – thomasrutter May 29 '18 at 1:37
  • The answer used 2.727272_X because that is the actual ratio of the diagonal of a 13.2x8.8mm sensor to the diagonal of a 36x24mm frame of reference. 98.181818_mm was rounded up to 100mm, as lens focal lengths are almost always rounded. – Michael C May 29 '18 at 4:29

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