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There are plenty of comparisons between DSLR cameras and mirrorless/EVIL/system cameras, and the overall picture is that the two systems target different users and different uses. But I'd like to focus on another aspect that I couldn't still figure out.

The main difference of course is the presence/absence of the mirror, which allows for smaller size but - I guess - also a smaller minimum distance between sensor and lens. Can it have any significant advantage in term of wider angle, different optical designs, adn ultimately better potential quality? Can it allow larger sensors (compared to FF) with less complications?

For this sake I'd like to avoid comparing sensor sizes (there's already plenty of that), also given that APS-C and FF mirrorless are being made. So let's compare APS-C vs APS-c and full frame with full frame.

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I wouldn't say that DSLRs compared to mirrorless target different users necessarily, but different use cases - yes. Canon would love for you to own both a 1DX for professional use and a EOS M for the family for example. –  dpollitt Dec 3 '13 at 19:17
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Here's a related, specific question about Fuji's X-Pro system: How does a short flange back distance improve image quality? –  mattdm Dec 3 '13 at 21:44
    
@mattdm very nice point: but I'm curious, would it be easier to make very wide lenses, which are quite expensive in the tranditional aps-c format? –  clabacchio Dec 3 '13 at 21:59

2 Answers 2

up vote 7 down vote accepted

It provides a benefit as it allows for more potential designs. If you think about it, a mirrorless lens mount can take any design for a reflex camera and adapt it to it with a simple extension of the lens tube, so anything that is possible for a DSLR is for a mirrorless.

The more flexible design is very real in fact because one can mount the lens much closer to the sensor and therefore allow for shorter focal-lengths without a retro-focus design. This lets wide angle lenses be simpler and those with larger apertures.

Of course lens designers have to make lots of compromises and one of them is the incidence of rays. As a lens is mounted closer to the sensor, the angle of incidence varies more which causes more vignetting. Designers can correct for this at the sensor-level with micro lenses or simply extend the tube to a point where they determine the issue to be minimal.

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This question and this question confirmed my suspicion about the advantage of having a shorter flange focal distance. As Itai and the linked questions mention, having a focal distance shorter than the flange distance requires a complicate retro-focus design, which means additional lenses, ND-filters with obvious consequences on price, quality and speed (in terms of aperture).

For most DSLR cameras this complication will arise for any focal length smaller than 46 mm, while mirrorless cameras the limit is way lower (the Sony E-Mount use by the NEX series has a flange distance of 18 mm with APS-C sensor).

The main implication of this is that it would be possible to realize 18 mm lenses for APS-C format (27 mm equivalent) without retro-focus complications. Therefore mirrorless cameras could potentially provide higher image quality on wide angle lenses.

Update: I believe that this lens might be one of the signs that mirrorless cameras have something more to offer than just a compact size.

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