"Equivalence" demonstrated: Canon 5D and Panasonic GX1

Started Apr 27, 2013 | Discussions thread
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Great Bustard Forum Pro • Posts: 36,253
An answer:

Olymore wrote:

GB, a question.....


Noise is always mentioned as an advantage of 4x light on a FF sensor over an identical except for smaller sized M43 sensor.

For a given exposure.  It is important to understand that the way FF gains these 2 stops more light is by either using a larger aperture (entrance pupil) for the same shutter speed, which will also result in a concomitantly more shallow DOF, or by using a longer shutter speed, which will only happen at base ISO when motion blur is not an issue (since the larger sensor can absorb more total light than the smaller sensor).

And the amount of extra noise that an M43 sensor would produce for the final image viewed at the same size etc etc. can be calculated mathematically.

Correct, after we figure in the sensor efficiency.

But what about the other aspects of the sensor like DR for instance or colour depth ?

Both are initimately related to noise.

Would a FF frame sensor, given the above scenario, always have 2 more stops of DR and can this be calculated or is the gain from these other components of IQ harder to define scientifically ?

It depends greatly on the sensor efficiency, and sensor efficiency is a function of the ISO setting for many sensors.  For example, the D800 sensor is significantly more efficient than the EM5 sensor at base ISO, and thus enjoys a huge DR advantage.  However, in lower light at higher ISOs that DR advantage vanishes.

But let's simplify things.  Let's say that we had a FF sensor that was the same design as the EM5 sensor, and even had the same size pixels (i.e. 64 MP, ignoring the differences in aspect ratio, which is trivial for 4:3 vs 3:2 in terms of comparing noise and DR).

Then, for a given exposure, the FF photo would have one more stop of DR than the EM5 photo, but for Equivalent photos (same DOF and shutter speed, thus the same total amount of light falling on the sensor), the DR would be one stop less.

The reason is that for the same exposure, the FF sensor would collect 2 stops more light, but the read noise for four pixels would be twice the read noise for one pixel (read noise does not sum linearly), thus, the DR, which is the ratio of the saturation limit to the noise floor, would be twice as great, and thus a one stop increase in DR.

On the other hand, for Equivalent photos, the FF sensor would collect the same amount of light, but the read noise would still be twice as much, so the ratio of the saturation limit to the noise floor would be have as much, and thus one stop less DR.

Hopefully, this explanation isn't overly technical and confusing.  Let me know if it is, and I'll try to explain better.

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