The full frame DSLR legacy compromise explained

Here is a reminder why converting a full frame film SLR system to digital is a fundamental technical compromise

Four Thirds and Micro Four Thirds were designed from the start to be digital systems and have the required over-sized lens mount to avoid image quality problems in the corner of the frame

I was recently asked to remind readers why designed-for-digital interchangeable lens camera systems. like Four Thirds and Micro Four Thirds do not share fundamental compromises exhibited by so-called full frame DSLRs that are adapted from a film legacy. You could argue there are several topics to discuss but I am going to concentrate on the problem surrounding the relative size of the lens mount to the sensor size and accompanying technical problems when the lens mount is too small, as it is in full frame legacy system DSLRs.

When the sensor is wider than than the exit pupil of the lens not all of the light can be collected

With designed-for-digital systems the lens mount is over-sized to accommodate a large enough exit-pupil that minimises any light loss at the extreme corners

(Diagrams courtesy Olympus Europe)

Four Thirds and Micro Four Thirds lens mounts are almost the same diameter as, say, Nikon's F-mount used in all its main SLRs and DSLRs since the 1960s. Olympus (Four Thirds) and Panasonic (Micro Four Thirds) developed lens mounts that are over-size in relation to the sensor. A Micro Four Thirds sensor is a quarter of the area of a 24x36mm full frame sensor. So why did they do that?

In the days of film lens designers had a lot more flexibility. Detail in an image is projected from the scene via the lens as a cone of light with the sharp end focused on the film plane. With film, the angle of incidence of the rays, which was often quite acute at the corners of the frame, was not a problem because of the characteristics of light absorption with film. That's not the case with digital sensors. A sensor captures light in microscopic pits called photosites. Think of them as wells. If the light is directly overhead then then the interior of the well is illuminated fully. This is an ideal that is sometimes referred to as 'telecentric'. But if the sun is not directly overhead then you only get a fraction of the light inside the well. Sensor designers alleviated the problem to a degree by introducing microlenses above each photosite. A microlens gathers more light when the light source is not perpendicular, but it is not a 100% fix.


This is why a lens needs to have a large exit pupil (the final lens element at the rear of a lens) to cover the sensor area, including the corners of the sensor frame. SLR systems adapted from 35mm film for digital, and rangefinder systems like that from Leica, for example, don't have wide enough lens mounts to enable designers to furnish their designs with the optimal size of exit pupil. This means the corners of the sensor receive light rays at a relatively acute angle and some of that light is not collected by photosites in the corners. Not only does this mean the corners will be progressively darker, which is called vignetting or corner-shading, but if you attempt to compensate for that loss either in the camera or through post-processing you will introduce noise. Contrast, dynamic range and detail will also be compromised in the corners. Some professional photographers using full frame DSLRs are known to avoid tight framing in order to avoid incorporating the extreme corners of the frame in their images because they aren't satisfied with the image quality there. The problem is usually worst with pre-digital era lenses, especially wide angle lenses, but the problem can still be seen with new lenses and new full frame camera bodies.

It's actually quite difficult to avoid some corner shading, even with digital systems like Four Thirds and Micro Four Thirds if the priority of the lens design is, say, compactness or cost, compared to ultimate image quality. But in general I see a lot less of the effects that are related to under-size lens mounts with designed-for-digital systems than with legacy full frame digital cameras.


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