Best Photo Quality System – Full Frame, APS-C or 3/4 ?

I read on a photography forum that members discuss which camera system is the best. Someone argue that Four-Third system is a new design that is completely optimize for digital cameras, meanwhile he also point out that FF is old and designed for frame cameras.

Well, that forum thread gets really long, over 100 posts. However, I am not agree to both side, I think APS-C size is the best, base on the Lens to Sensor size ratio.

What do people talk about Full Frame and Four-Third?

According to the inventor of the Four-Third system, digital cameras should be different from the frame cameras, digital cameras should use up the lens focal area. So, he invented the 3/4 size sensor that height to width ration of the sensor is 3:4. He believed that this sensor design best use the lens focal area; which can produce the highest quality photos with lighter and smaller lenses.

However, other forum members point out the fact that Canon and Nikon own 90% of the D-SLR market share. They also think that Canon, Nikon and Sony all rush into Full Frame market, it may indicate that Full Frame is the best camera system for high quality photos. They further point out that full frame sensor can use up the whole focal area that they are finally use the “original” 35mm standard focal length. Nevertheless, the also point out that full frame sensors from Canon and Nikon are better in noise control than 3/4 sensors.

Lens to Sensor size ratio

One of the forum member points out a key for producing high quality photos: lens to sensor size ratio. He points out that Leica’s new S2 system use a larger sensor and larger lenses, and the height to width ratio of the sensor is close to Four-Third system design (30 x 45mm).

After reading his post, I have another opinion; if the lens to sensor size ratio is important, so APS-C sensor should produce the best quality pictures.

sensor lens ratio

From physics point of view, focal plane of a lens is not flat; just like our eyes, they are “eye ball”, not “eye box”. Our retina is on a sphere inner surface.

What I want to say is, since camera sensors are flat surfaces, only part of it right on the focal plane. So, usually the center part of a photo has highest resolution / sharpness / contrast / best color etc.

What is really important is, in order to product the best quality photo, the focal plane sphere should be as big as possible, so that the focal plane is more “flat” and more area of the sensor would be right onto the focal plane.

sensor focal plane

From above example, you may easily see that lenses which produce big focal plane (focal plane 1) would produce far better pictures than lenses which produce small focal plane (focal plane 2); because focal plane 1 focus sharply on the sensor, meanwhile focal plane 2 focus partly onto the sensor only.

Back-focus is a trick, not a solution

So, how to get a better picture with a small lens-to-sensor-size ratio? One easy way that commonly used by camera manufacturers would be “back-focus”.

sensor focal plane back-focus

The above graph show you that, by back-focus a little bit, it could “average out” the focus… so the whole picture looks more “equal” … prevent that the center part of the picture is completely sharp, but the rest area getting less and less sharp / bright…. The result is a “average” sharp / brightness picture. Or I would say, no “sharp point”.

Larger lens to sensor size ratio, higher photo quality

So, if you are looking for a really sharp picture (corner to corner sharp), large lens to sensor size ratio would be your answer.

To fulfill a large lens to sensor size ratio, we need a system which uses large lenses with a reasonable size sensor. It is the case of using full frame lenses on APS-C size sensors.

Since the full frame lenses are designed for 35mm frames, their focal area are well bigger than the APS-C sensors. Their focal plane is also comparatively large for APS-C sensor.

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