Wednesday, January 30, 2013

Is It Time To Go Full-Frame?

By Mike Stensvold Published in SLRs
Is It Time To Go Full-Frame?


The more light a sensor can collect, the better the image quality it can produce. That's because photonic noise (the noise carried by light itself, and the main source of noise in most digital images with current cameras) increases as the square root of the signal. In other words, if you have 4 photons of signal, you'll get 2 photons of noise (2 being the square root of 4). The resulting S/N ratio is 2:1. If you have 100 photons of signal, noise would be 10 photons, and the S/N ratio would be 10:1. If you have 10,000 photons of signal, you'd have 100 photons of noise, for a photonic S/N ratio of 100:1.
All other things being equal, a larger sensor can collect more light (photons) than a smaller one when you use a given shutter speed and aperture combination. So, the image made with the larger sensor will have a better S/N ratio. A better S/N ratio means fine details aren't lost to noise, nor are steps of dynamic range; that means better image quality.

Of course, things aren't that simple. Given the same exposure, a full-frame sensor will collect more photons than a smaller sensor, due to its larger area, and produce a better S/N ratio, and better image quality. But to get the same field of view with the full-frame camera, you have to use a lens with a focal length 50% longer than the lens on the APS-C camera.
Using a longer lens reduces depth of field. To get an image with the same field of view and the same depth of field with a full-frame camera as with an APS-C camera, you'd have to use a lens 50% longer and stop down 1.3 stops (to ƒ/12, in this case).

If you stop down and shoot at the same shutter speed, the sensor will receive less light (1/500 sec. at ƒ/8 delivers more light to the sensor than 1/500 sec. at ƒ/12), and the S/N ratio will thus go down, negating much of the larger sensor's S/N ratio advantage. In order to maintain the same image brightness and the same depth of field, you'd have to shoot at a longer shutter speed with the full-frame camera, which could be a problem when shooting moving subjects or handheld.
The bottom line is, if depth of field isn't a concern, bigger is better. If you need equivalent depth of field with a given angle of view, you'll have to stop the lens down with the larger sensor, and either increase the shutter speed to maintain the same image brightness, or increase the ISO setting and suffer the resulting loss of S/N ratio due to the resulting smaller amount of light delivered to the sensor.

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