RustyBug
Offline
• • • • • • •
Upload & Sell: On
|
 p.59 #10 · Pre-order: Fujifilm GFX 50S Medium Format body ($6,499) | |
+1 @ choose your poison(s)
Shorter flange distance itself is not the holy grail to the projected image as some advocate ... it has it's pro's & cons too. Nice presentation of some of those yin-yang attributes.
cmarker wrote:
Sator photo has a great comparison of the GFX50 and the X1D. Very interesting discussion of the flange distance and how it will affect vintage lenses, lens size, and corner performance.
https://plus.google.com/116458677975033889029/posts/NdVHsqGuKJa
"The flange distance of the G mount is 26.7mm. By comparison, the X mount has a flange distance of approximately 20mm. This difference of around 6.7mm is very striking indeed. The shorter the flange distance, the closer the sensor is to the rear exit pupil, and the better the image should be. So far it may seem that Hasselblad is ahead.
The interesting thing about the G mount flange distance is that we would predict, based on the flange distance of the Fujifilm APS-C X mount, that the G mount should have a flange distance of 34mm:
Fuji X mount flange distance = 17.7mm
Multiply x 1.52 to get 135 format equivalence = 26.9mm
Multiply x 1.27 again to get 4433 format equivalence = 34mm
Yet at 26.7mm, the new Fuji G mount has a much shorter flange distance than the predicated 34mm distance based on these calculations. The G mount flange distance of 26.7mm is the almost the same as that of the 135 format Leica M mount at 27.8mm, which is also a "mirrorless" mount because rangefinders have no mirror box in the optical pathway. The Fuji G mount has a flange distance one format "too small". Even more astonishingly, the Hasselblad X mount has a flange distance even shorter than the Fuji G mount—at 20mm it is so ridiculously short it is almost the same as that of the micro 4/3 mount flange distance of 19mm.
This follows a trend towards increasingly short flange distances started when mirrorless APS-C mounts flange distances were all made shorter than that of the M4/3 mount. This trend continued with the Sony FE mount, which was originally an APS-C mount (NEX mount), but which was retrospectively converted into a 135 format mount. That means that the FE mount is a 135 format mount with APS-C dimensions, including an APS-C flange distance of 18mm (comparable to the Fuji X mount 17.7mm). Sony thus furthered a trend of creating mounts with a flange distance deliberately set to be one format "too short" compared to previous generation mirrorless mounts.
As already stated, shorter flange distances permit great acutance, and that is the reason for this new trend in mount design. The shorter the flange distance, the better the image quality, and both the G mount and X mount have dramatically shorter flange distances than the 70.87 mm flange distance of the Pentax 645 mount. The Hasselblad X mount flange distance is 6.7mm shorter than the Fuji G mount, and is therefore theoretically able to provide better image quality. The shorter the better—it seems so simple, doesn't it ? In fact, why didn't Hasselblad go in for the kill by making the flange distance almost zero...meaning that the light would hit the sensor corners almost horizontally.
Corner Angle of Light Incidence
Here is the catch with making flange distance "too short". The shorter the flange distance, the steeper the corner angle of light incidence. This creates a problem with modern digital sensors because if you look at a digital sensor under a microscope it looks like a city with lots of high-rise buildings. At midday, with the sun straight above, it illuminates the city well. However, at sunset or sunrise, the light hits the "buildings" at a steep angle, casting deep shadows. That's what it's like at the corners of the sensor when the flange distance is too short: you get shadows cast. Digital sensors only fully register light when it hits the sensor nearly square on, at close to right angles. If the angle of light incidence is too steep, as it can get at the corners, the sensor fails to register light information. The result is loss of light intensity (shading) and loss of resolution.
So there is your answer to why Hasselblad couldn't just reduce the flange distance to near zero. It would mean that light hits the sensor corners at too steep an angle. The result would be dreadful corner performance, however stellar the centre acutance.
Although the problem of angle of light incidence on digital sensors is poorly appreciated amongst photographers, it is universally regarded as a critical issue amongst optical engineers, making it a major area of photonics research. The ultimate solution that would allow ultra short flange distances, while allowing light to hit the corners at optimal angles, would be to make curved sensors to reduce the angle of incidence of light in the corners:
https://www.dpreview.com/articles/6229436014/sony-s-curved-sensors-may-allow-for-simpler-lenses-and-better-images
Curved sensors are still in the experimental phase, and it does not look like they will become a feasible option for some years to come. In the meanwhile, sensors look like their vertical structure is going to become more built up and complex with the advent of stacked sensors. That means, for now, ultra short flange distances look set to remain a major headache for optical engineers for some time to come. Meanwhile the lay public seem to be in gross denial over the very existence of this issue.
Offsetting Steep Angle of Corner Light Incidence
Fujifilm has dug into their deeper R&D pockets to develop novel strategies to overcome the issue associated with the steep angle of light incidence in the sensor corners. They have a new microlens and a novel silicone process to allow the sensor to tolerate a steeper angle of light incidence. We have seen before that Sony's makeshift solution to the problem was to add BSI to their sensors, even if that means sacrificing dynamic range.
Fuji are doing something similar to what researchers in a recent Nature Photonics paper described about the use of a novel black silicon sensor to improve the tolerance to steeper angles of light incidence:
http://www.nature.com/nphoton/journal/v10/n12/abs/nphoton.2016.226.html
What Fujifilm offer in the GFX 50s is a customised sensor based on similar optical engineering principles allowing a sensor with a modified silicon process to tolerate steeper incident angles without having to go as far as to find a way to manufacture curved sensors.
What have Hasselblad done by comparison? Nothing. Absolutely nothing whatsoever. But they do offer a drastically shorter flange distance, resulting in an even steeper corner light incident angle than the Fuji. The bigger the problem the less you do about it.
Redesigning Lenses to Offset the Steep Corner Angle of Light Incidence
There is a way of achieving this, and it is to make the lens design more telecentric. The steeper the angle of corner light incidence, the more telecentric the lens has to be. A couple of factors increase the corner angle of light incidence. Firstly, the wider the field of view, the steeper the angle of light incidence. Secondly, the wider the maximum aperture, the steeper the angle of light incidence at the corners. So that means, the wider the maximum aperture and the wider the field of view, the more telecentric the lens has to be to compensate.
Problem solved. It doesn't matter how short the flange distance, you can always offset the problem this causes by making the lens more telecentric in design.
But Houston we have a problem! We have replaced one problem with another one, because making lenses more telecentric makes them really big. It's all very well making the body smaller and more portable with a shorter flange distance, but if it makes the lens dimensions blow out in size, it just negates the size benefits from a smaller body (hmmm...maybe it might be better to compromise on the body size to reduce your lens size by increasing the flange distance). Never mind, you can always get your advertising department to just show how amazingly small the body is...you can then get buyers to just forget about how elephantine the lenses are. Out of sight, out of mind. Fanboys will be eating out of your hand.
On the other hand, the fact that mirrorless ultra wide lenses can omit retrofocal elements helps to offset the size increase. This means that lenses for ultra short flange distance mounts favour slower moderately wide angle lenses i.e. pancake or semi-pancake style lenses. This tendency will be more marked on the Hasselblad X mount than on the G mount, making the Hasselblad the ideal pancake or semi-pancake lens system for walkabout casual shooting."...Show more →
|
