What is equivalence good for?

Started Jul 25, 2014 | Discussions thread
Just another Canon shooter
Just another Canon shooter Veteran Member • Posts: 4,691
Re: What is equivalence good for?

mostlyboringphotog wrote:

Just another Canon shooter wrote:

olliess wrote:

Just another Canon shooter wrote:

olliess wrote:

Just another Canon shooter wrote:

...the answer is as simple as that: just multiply three numbers (F-stop, FL and ISO) by the crop factor.

I think multiplying the ISO by the crop factor would actually be equivalent to a "screwup," but I don't want to derail this fascinating and original conversation. So please carry on.

It would be a screwup if you don't. GB does not want to include it because he does not want to be criticized on the basis that different sensors have different noise characteristics. Recent sensors have remarkably close QE, so this is not a more important problem than, saying that different lenses with the same f-stop differ by their t-stops.

Umm… the difference between f/2.8 and 2x f/2.8 = f/5.6 is 2 stops. The difference between ISO 100 and 2x ISO 100 = ISO 200 is only 1 stop.

Oops, multiply by the square of the crop factor, my bad.

crop factor for f-stops but square of the crop factor for shutter speed (the other direction) or ISO; and why is that?

Total light: it is the light over the whole sensor. With the same intensity, you get the crop factor squared times the light with the bigger sensor.

To put it differently, FL and f-stop are linear quantities, total light is quadratic, and the ISO compensating for that is of the same type.

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Canon EOS 5D Mark II Canon EF 15mm f/2.8 Fisheye Canon EF 35mm F1.4L USM Canon EF 50mm f/1.2L USM Canon EF 135mm F2L USM +4 more
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