Fastest EFCS shutter speed usable on the a7RII?

Started Aug 11, 2015 | Discussions thread
Antisthenes Regular Member • Posts: 364
Re: Image plane and DOF effects of EFCS

Horshack wrote:

Thanks for the detail write-up and diagrams. I read it several times but can't find any thing you described that would explain the phenomena I see regarding the image distortion

A table looks different depending on whether you look at it while seated at that table, or standing next to it. The difference in view is caused by the vantage points' height difference.

The Mitakon has an entrance pupil diameter of about 5.3cm. The entrance pupil's size is thus far from negligible relative to the subject distance — about 50 cm — in your test pictures. Vantage point height differences must thus be taken into account.

As a counterexample, a 24mm lens at f/8 has an entrance pupil diameter of 24mm/8 = 3mm. If the subject distance is, say, 3 meters, then the entrance pupil would have a diameter that's 1/1000th of the subject distance. Any viewing angle difference between the upper and lower part of the pupil, caused by the vantage point height difference, would then be so tiny as to be essentially invisible and insignificant.

As I mentioned earlier, if the shutter curtains move upwards, then, in EFCS, the 2d curtain will tend to block the tilted light rays coming from the lower part of the entrance pupil. The sensor will therefore capture those lightrays that come mostly from the entrance pupil's upper part, and the resulting picture will be one corresponding to a relatively elevated vantage point.

IOW, the upper part of the entrance pupil "sees" the subject — e.g. the aforementioned table — from a vantage point resembling that of the standing person; the pupil's lower part sees the table from a vantage point resembling that of the seated person.

If the Canon 5DSr's shutter curtains travel in the opposite direction of Sony's cameras, then, in EFCS mode, the 2d curtain will tend to block the lightrays coming from the upper part of the lens' entrance pupil.

The resulting picture, in EFCS mode, will then resemble the seated person's view of the table.

One easy way to check the curtains' direction of travel is to take a picture, in EFCS mode, of a defocused, distant point light source with a high-speed (f/1.4 or faster) lens and a very high shutter speed — one might put a small LED light source, say, 5 meters away, and focus the lens to 45cm.

If the OOF disk's lower part is truncated, then, the curtains travel upwards, as is the case with Sony's A7 series.

OTOH, if it's the upper part of the OOF disk that's truncated, then the curtains must be travelling downwards.

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