rcholas
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 p.24 #13 · A7/A7r - performance with WA RF lenses | |
theSuede wrote:
If you remove the plates entirely, you have to shift the sensor mount since they (the plates) change the optical path length. This means two mount plates and two different mount settings for the machine that places the parts - much harder to shift production then.
It seems the A7 and A7R have different body frame parts (which the D800/D800E doesn't). And additionally - they're not the same sensor plate assembly... So there's no reason to add expensive materials that needn't be there (for Sony).
Er, no.
The reason the D800E uses two birefringent plates is because one of the birefringent plates is bonded to the sensor's ceramic package itself.
The filter stack (IR filter and AA filter) is located between the focal plane shutter and the sensor, and must be kept as thin as possible. Since the flangeback distance of Nikon's lenses is constant, a thick stack would reduce the room available for the swinging reflex mirror.
The addition of a vibration-based dust removal system to the sensor stack, and the resulting need for space, thus presumably drove the decision to replace the sensor package's hitherto plain optical glass cover plate with a birefringent crystal cover plate implementing the second half of an X-Y birefringent AA system.
Considering that the sensor is probably the most expensive part in a full-frame DSLR, it probably didn't make sense from a design and production standardization, as well as spare part inventory management, to have separate, non-compatible sensor packages and sensor mounting frame designs for the D800 and the (niche) D800E.
Nikon therefore decided to use the D800's sensor package covered with a birefringent plate, even on the "lowpass-less" D800E, and just modified the structure of the front part of the D800's AA filter (front birefringent crystal and wave plate) so that the net effect is a cancellation of the birefringence, and hence cancellation of the AA effect.
The A7R's sensor, being a 36MP version, is different from the A7's 24MP sensor anyway, production standardization issues are moot.
The A7R's 36MP sensor is thus probably, unlike the D800E's 36MP sensor, just covered with plain glass.
This, however, in no way implies that the optical stack in front of the A7R's sensor is any thinner than the A7's.
To account for the AA stack's refraction effect, the lenses must be designed accordingly. Lenses that didn't account for the refraction caused by the filter stack and sensor cover glass would suffer, among others, from astigmatism in the sagittal direction and field curvature.
To maintain compatibility with the lenses designed for other E mount cameras, the A7R must thus be equipped with an optical stack that has the same refraction properties as the AA-equipped NEX and A7 cameras, minus the birefringence part.
This means that the A7R probably has a fairly thick glass plate in front of the sensor.
With non-telecentric wide-angle rangefinder lenses, the glass plates will presumably induce levels of aberrations — i.e. smearing — that are comparable to the A7's, if one excepts some tiny, incident angle-dependent AA amplitude effects that will be present only on the AA-equipped A7.
Of course, if the old rangefinder lens has, by chance, a field curvature characteristic that's in the reverse direction of the one induced by a digital sensor's optical stack, then, using it on a digital sensor might actually improve the IQ in the corners. I have no idea how frequent such fortunate coincidences would be.
Edited on Oct 21, 2013 at 01:39 AM · View previous versions
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I have no desire to re invest in a new lens lineup.
