"Equivalence" demonstrated: Canon 5D and Panasonic GX1

Started Apr 27, 2013 | Discussions thread
JosephScha Veteran Member • Posts: 5,198
Re: Are you kidding me?!

The goal (as I wrote above, having learned this yesterday) is to get the same total light on the sensor. To do that, if we start with 50mm f/4 = 12.5mm aperture, on m43 we need 25mm f/2 = 12.5mm aperture.  Those pictures will have the same DOF.   If the shutter is open for the same amount of time, then since the 4/3 sensor is 1/4 the area of FF, that means the light intensity (which is "exposure") will be 4x higher on m43.  To make up for that, you can "brighten" (ie, raise the ISO) on the FF camera by 2 stops so it can use the same shutter speed and look the same. This reduces what I would consider to be the advantage of FF, which is that you COULD go to 50mm f/2 and not have to raise the ISO.  The goal is to see how the sensors perform given exactly the same total amount of light, with same DOF (so same size aperture, not same f ratio) and same angle of view and framing and display size.  If we keep shutter speed the same then we must do something (ie, raise ISO on the larger sensor) to compensate.

As I wrote, if you don't think this is useful - and I still reserve my own judgement - that's just fine.  I'm just trying to explain that the way you use the word "exposure", ie, keep the ISO the same and keep the apparent brightness the same, is not what they are aiming at.

Certainly when I shoot I will be thinking of aperture, shutter speed and ISO.  I will consider DOF and if necessary go to aperture priority to get more or less of it.  But since when I'm shooting I'm not comparing my 4/3 sensor to any other size, I don't care about "equivalence" and you don't have to either.

For the people who want to compare efficiency of conversion of photons to electrons by the sensor, they worry about equivalence as they define it.  Let them happily do that.  Is is not a threat to the exposure triangle that I'm sure we all use when actually doing photography.

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