Sony's new curved sensor - this is a big deal

Started 6 months ago | Discussions thread
alanr0
Senior MemberPosts: 1,261
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Re: How curved?
In reply to stevo23, 6 months ago

stevo23 wrote:

alanr0 wrote:

stevo23 wrote:

eyeswideshut wrote:

Afraid not. Curvature of the sensor will allow simpler lens designs but wreak havoc with legacy lenses designed for a flat field sensor.

I don't know why - legacy lenses are anything but flat. We don't know how much curve is in this new sensor and we don't know how flat lenses really are other than the fact that they aren't flat. Flat is the goal, but flat they are not.

If we refer back to the photoblographer article you linked to earlier we can make an estimate.

One of the touted advantages is that rays from the centre of the lens strike the sensor at close to normal incidence. For a simple lens, that puts the centre of curvature at the rear nodal point of the lens. The radius of curvature of the sensor must be close to 35 mm for the 35 mm f:1.8 lens mentioned.

? How is this known?

One rationale for the curved sensor is that it makes possible a simpler lens design with low aberrations at relatively wide aperture. A feature of such lenses is that the rear nodal point and exit pupil are both reasonably close to the physical centre of the lens.

A second advantage of the curved focal surface is that light from the lens strikes the sensor at close to normal incidence even at the edges.

So, to a first approximation, light from the lens appears to radiate from a region on-axis and at a distance roughly equal to the focal length of the lens. For light to strike the sensor at normal incidence, the centre of curvature of the sensor must be located at the same point. It follows that for a 35 mm lens, the radius of curvature of the sensor must also be approximately 35 mm.

This will not be exact, but this image from the photoblographer article is broadly consistent with a radius of curvature comparable with the focal length of the lens

Sony 35mm f1.8 lens for curved sensor patent illustration via Egami

At the edge of the 24 x 36 mm frame, 18 mm off-axis, the height of the sensor will be 3.9 mm above the centre.

? What am I missing?

Simple trigonometry tells us that at the edge of the field, curvature lifts the sensor surface 3.9 mm above a flat plane through the centre of the sensor if the radius of curvature is 35 mm.

Alternatively, if the figure above is to scale (which it may not be), the sensor height at the edge of the field is about 2.6 mm.

With a conventional flat field f/1.8 lens focused at the centre of the field, the diameter of the circle of confusion at the edge of the sensor will be (2.6 mm / 1.8) = 1.4 mm.

Even with this lower curvature estimate, it would be completely impractical to use a conventional lens with such a strongly curved sensor. For an 'acceptable' circle of confusion of 0.036 mm, only a small central area of less than 3 mm radius will be in focus.

HTH

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Alan Robinson

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