Should NEX go full frame?

Sonyalpharumours has posted a firm rumour that there will be a full-frame NEX. Anyone who understands the design of camera bodies, and the geometry of optical projection, has known from the start that this was possible given the design of the E-mount.

Those who have made uninformed comments (all over web forums!) to the effect that the rear register is ‘too short’ or would cause problems simply don’t have their brains switched on. There is no such thing as a camera body which is too thin or a mount to focal plane register which is too short.

Sony already designs lenses for NEX APS-C which incorporate what amounts to an extension tube to push the rear nodal point and exit pupil positions as far forward as needed for optimum illumination and matching of off-axis ray bundles to the sensor’s microlenses, filter array and cover glasses. That’s why the 30mm Macro is not so very different in size from putting a 30mm f/2.8 Alpha SAM macro on an adaptor (and that’s why we do exactly that here!).

What matters for full frame is a correlation between the clear lens throat diameter (the bayonet mount inside width), the sensor size, and the register.

The NEX system uses an 18mm register. That is the term of the distance between the front surface of the bayonet mount on the body (or rear surface of the mount on the lens) to the sensel layer surface of the sensor. There is a very small adjustment made in all digital cameras for the four physical layers which usually go between the sensor and the lens: the RGB filter layer (so thin it has no effect), the microlens layer (again, no effect), the low-pass or anti-aliasing filter and the infra-red cut and protective self-cleaning outermost glass. These filters may be combined into a single glass but in NEX they remain separate.

The sensor requires an image circle of just under 30mm allowing for assembly tolerances of 0.5mm overall in positioning and conformance of the lens axis. If in-body stabilisation is used, this diameter must be increased to 35mm at least; some Konica Minolta documentation suggests that a clear diameter closer to 40mm was needed for the original AS/SSS.

For full-frame, the required image circle is 44mm and the maximum diameter for in-body stabilisation could be between 49mm and 54mm depending on how Sony’s statements about sensor travel are understood (“5mm in any direction” could mean the total scope is 5mm, or that 5mm travel from axis might be possible). A good idea of the actual travel of an APS-C sensor with IBIS is given by the Pentax K-5, which has a manual sensor shift function allowing the user to move the sensor off-axis for a slight rising or cross front effect – but only by 1.5mm. This sounds more realistic though it would, of course, be a great feature if you could shift a sensor 2.5mm or an extreme 5mm – every lens with enough coverage would become a PC lens!

Now let’s compare these image circles with the lens throat diameter and its distance from the focal plane.

The Alpha system has a register of 44.5mm and a clear lens throat internal diameter around 42mm. Allowing for the orientation of the film plane and the position of control connections and the electronic contact array, the Alpha mount ends up providing an almost exact fit for the optical projection path if a 45mm diameter image circle is needed. Even if a 2000mm non-telephoto lens was attached, its image would cover the sensor without physical shadow vignetting caused by the mount.

It is important to remember that a 44.5mm register allows space for the reflex mirror, but also allows the image forming light to spread out from a rear element positioned over a wide range of possible distances, from just inside the camera body (by about 5mm before the mirror would hit it) to a good three or four centimetres in front of the mount. This allows a larger overall image circle and indeed most SLR lenses produce one greater than 50mm diameter. That’s how the Alpha 900/850 is able to offer in-body SSS, and also why certain lenses show vignetted corners occasionally when SSS is active because their image circle is only just sufficient to cover the sensor in its central, parked position.

This is a very accurate representation of 24 x 36mm sensor areas excluding any of the surrounding assembly, showing that the internals of the E-mount could be modified to fit FF. The white line shows the 24 x 36mm in the sensor plane, relative to the APS-C sensor. The red line shows the 24 x 36mm sensor as it relates to the mount, in the flange plane. The two rectangles are needed to show the size accounting for the perspective of the macro lens used to take this shot.

The NEX E-mount appears to have not much smaller a diameter, but this is deceptive. You need to look INSIDE the mount and study the position of the electronic contact pins. Once these are considered, the actual clear diameter is not 46mm at 18mm from the sensor (apparently bigger than the A-mount) it is 39mm at 12mm from the sensor. The contacts are located 5mm into the camera body and occupy a clear 3mm zone. The outermost contacts in the array are positioned  sufficiently far from the extreme corner of a theoretical 24 x 36mm sensor to allow a full frame design, assuming most of the other detail of the darkchamber design is changed to ensure the widest possible clear area.

Ideas about putting SSS and full frame into such a NEX are wishful thinking, unless the camera was to be huge and the possibility of the lens mount innards shadowing part of the image was accepted. There’s just enough room to fit a full frame sensor, and no more.

It is more likely that Sony would introduce a range of new full-frame lenses with OSS, or non-stabilised adaptors for Alpha lenses designed to allow an unrestricted light path. It is also possible that a stabilising optical adaptor could be made, but we’d put that as the least likely option and one which would probably cost over $1000. Sony would more likely to put an electronic pixel-shift IS into a full frame NEX, though at present this only works with video and imposes a slight crop (1.87X) on what would be the maximum 16:9 frame area.

As for suggestions that existing NEX lenses might cover full frame, that is also uninformed speculation. They won’t and don’t. The same applies to the LA-EA1 and LA-EA2 adpators – neither of these would allow Alpha lenses to be used on the NEX and cover full frame, though there are a few wide angle lenses which might just squeeze their image through the small internal baffles. A few of the third party adaptors for lenses such as Leica M might allow coverage, some would not, depending on exactly how the light baffles and inner mount components have been designed. For APS-C NEX, it pays to add baffles which prevent stray light and flare; for FF NEX, the same baffles might vignette the image.

The question of sensor design is another matter, but it can be assumed many owners would use a full frame NEX to host legacy full frame lenses from a wide range of systems. Most of these have a degree of telecentricity which removes corner colour shifts. New full frame NEX lenses would be designed to match the new sensor, so that would not be an issue. Old rangefinder lenses would have no more problems on full frame NEX than they do on, say, a Leica M9. Sure, there would be issues, and weak combinations alongside strong ones.

These would not over-ride the value of a full frame NEX body. Good photographers can use any format well. They do not demand ‘the full frame look’ or believe that any one format is going to change their work in some way to make it superior to others. There’s a place for every format and choice is a good thing. End of story.

– David Kilpatrick

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