Sony A7S High shutter vignette

Ryan Patrick O'Hara

Ryan Patrick O'Hara

Veteran
Anybody else witness this?

1) Take your A7S outside
2) Find a clear sky with very even illumination
3) put your camera to 1/8000th of a second shutter and properly expose the sky
4) Take photo and witness a built in Grad ND filter.


Doing tests, I find the vignette gradient slowly declines down to 1/2500th of a second and is gone at 1/2000th. At 1/3200th, it is tolerable.

I'm thinking this is an interaction between the rolling shutter and very fast moving shutter blades. Anyone else confirm?
 
In physical shutter mode the scan should happen after the sensor has been exposed, so rolling shutter should not come into play.

Is there a chance you have electronic first curtain shutter mode enabled? I'm not 100% on how it works but it may be something to do with it. Which part of the image is darker, the top or the bottom?
 
Ah Good! It seems they don't fully understand it in that thread either, but as long as there's a way around the issue.
 
The electronic first curtain is the first half of a true electronic global shutter: the mechanical curtain is open and all photosites start to receive light at the same time.
The mechanical curtain is very fast, but not THAT fast, so it takes some time to move across the sensor.

At such high shutter speeds, that speed difference will be noticeable in the image. Better to use mechanical first and second curtains, which will move at the same speed (the first one revealing the sensor, the second one hiding it) (actually, the second curtain starts chasing the first one before it's completely up, so it's like a mechanical rolling shutter, but it's so fast you'll never have motion artifacts).



Related topic: with its much bigger photosites, the a7s will have a lot less corner vignetting than the a7 or a7r, at least with some lenses: http://www.bhphotovideo.com/explora...iew/sony-a7-series-which-model-suits-you-best
 
Samuel H said:
The electronic first curtain is the first half of a true electronic global shutter: the mechanical curtain is open and all photosites start to receive light at the same time.
The mechanical curtain is very fast, but not THAT fast, so it takes some time to move across the sensor.

At such high shutter speeds, that speed difference will be noticeable in the image.
Click to expand...

That't the explanation I thought of, but on second reflection it doesn't seem to hold up. Correct me if I'm wrong, but the curtain moves from the top of the sensor downwards. But since a camera lens flips the image, the top of the sensor actually represents the bottom of the image. This would mean that the top of the image would be exposed longer and the bottom would be darker. And this is the opposite of what the op and the people in the DPReview thread have seen. So the issue seems a little weirder than that.
 
It depends on the camera. There are all kinds of designs, in the old times there were even some that moved sideways...

Usually there are two mechanical curtains, one that is closed by moving up, one that is closed by moving down (typical diagram here). When an electronic first curtain is used, one of those two mechanical curtains is disabled. It can be the one that goes upwards when it's closing, or the one that goes downwards when it's closing. You can guess which one Sony decided to use by looking at this "grad-ND" issue :)
(actually, if I was the engineer, I'd alternate between the two curtains, since they have a limited life)
 
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