A smaller senor better than APS-C on brighter lens ?

Started May 16, 2012 | Discussions thread
Great Bustard
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In reply to Graystar, May 18, 2012

Graystar wrote:

The maximum aperture of a lens has no effect on noise. Only sensor size, and then ISO, affect the amount of noise.

The primary factors for the amount of noise in a photo are:

  • The total amount of light that falls on the sensor during the exposure

  • The proportion of that light that the sensor records (QE -- Quantum Efficiency)

  • The additional noise added by the sensor and supporting hardware (Read Noise)

Note that ISO has squat to do with noise, except inasmuch as some sensors have less read noise at higher ISOs.

The total amount of light that falls on the sensor is a function of the aperture diameter of the lens and the shutter speed, where the aperture diameter is the quotient of the focal length and f-ratio.

So, for the same AOV, the crop factor between the XZ-1 (sensor diagonal = 9.5mm) and Canon APS-C (sensor diagonal = 26.8mm) is 26.8mm / 9.5mm = 2.8. Thus, f/1.8 on the XZ-1 puts the same amount of light on the sensor as f/5.1 on 1.6x for the same shutter speed. Not coincidentally, by the way, this also results in the same DOF for the same perspective and framing.

That said...if your comparison also includes a shutter speed limitation where the shutter speeds must be the same, then it's a little closer. The XZ-1 has a lens that's two stops faster, but the sensor has a crop factor of about 2.8. So the XZ-1 still ends up having nearly 1 stop more noise than the APS-C sensor.

Anything faster than f/5.1 on APS-C will put more light on the sensor than f/1.8 on the XZ-1 for a given shutter speed, but it will also result in a more shallow DOF, which may or may not be desirable.

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