Variable EIS Margins on Sony A1U

[J. Stephen McDonald]

The PDF manual for the Sony A1U describes that the width of the margins of pixels used for electronic image stabilization (EIS) on its image sensor varies, according to the position of the zoom. At lower zoom positions, the EIS margins are narrower and so more pixels are available for the image. Also, the larger sensing area reduces the magnification effect, which would give it a wider angle of view at low zoom. At higher zoom positions, the EIS margins are wider, increasing both the amount of stabilization and the magnification effect. This seems like a useful idea, if enough pixels remain active for full picture quality at higher zoom positions.

The A1U also has a manually-selected "full-scan" mode, where many of the pixels normally used for the EIS are used instead for the image.

Have any A1U owners noticed the effects of these two features? There doesn't seem to be a full-scan mode for the HC1 or the upcoming HC3, but I'm not sure about that. Also, I haven't been able to find any references regarding variable EIS margins on the HC1/HC3. From the greatly increased focal lengths that are stated in the specs for those two models for the video mode, as compared to the still picture mode, it would seem that they may have variable margins. The EIS margins are normally engaged for video mode, but are always off for still mode, increasing the size of the active pixel zone. Anyone who has more details about this issue, might add to what I've said.

[Steve Mullen]

Originally Posted by J. Stephen McDonald

Anyone who has more details about this issue, might add to what I've said.

You've explained it very well.

What I find strange is WHY Sony did all this and then doesn't really document it very clearly. And, really provides nothing about how much image quality may or may not change?

Perhaps the engineers simply wanted to try out the concept. It would be useful for future CMOS camcorders with lower rez.

[Evan Donn]

Originally Posted by J. Stephen McDonald

Have any A1U owners noticed the effects of these two features? There doesn't seem to be a full-scan mode for the HC1 or the upcoming HC3, but I'm not sure about that. Also, I haven't been able to find any references regarding variable EIS margins on the HC1/HC3.

I'm putting together a page with sample shots, etc for the HC1 - there is no official full-scan mode for the HC1. However, when you switch into shot-transition mode the EIS is disabled & the camera goes full-scan (this doesn't happen if you just disable the EIS). It's most noticeable at full zoom, the EIS margin seems to vary from almost nothing at wide to maybe 20-30%. At full zoom there is a significant jump as you switch into and back out of shot transition - this mode also enables the much slower zoom speed, so I've theorized that the faster default zoom speed is neccessary to avoid some sort of image artifacting which occurs as a result of a conflict between the optical zoom and the digital zoom from the EIS.

[John Jay]

Yes, if you wish to shoot at maximum resolution with the HC1, it is recommended to make all your manual exposure/shutter/wb settings and then enter SHOT TRANS mode.

[Steve Mullen]

Originally Posted by J. Stephen McDonald

At lower zoom positions, the EIS margins are narrower and so more pixels are available for the image. Also, the larger sensing area reduces the magnification effect, which would give it a wider angle of view.

At higher zoom positions, the EIS margins are wider, increasing both the amount of stabilization and the magnification effect, which would give it a narrow angle of view.

Since when you zoom Wide you expect a wider field of view and when you zoom In you expect a narrower angle of view -- how would you know what is "zoom" and what is "magnification effect?"

Have you tried the same zoom on the same subject with Full Scan OFF and ON? In the latter case, there will be no "magnification effect" and the camera should record less wide at Full Wide and more wide at Full Tele.

Does it? Or, is the zoom coverage the same?

Either could be true! The following I think is true:

According to Sony, both the A1 and HC1 have a single, 1920x1440 element CMOS sensor that has a 4:3 native resolution. (The effective number of pixels is 2.76 million, while the gross number is 2.97 million.) Therefore, each CMOS element is taller than it is wide. Specifically, the pixel-aspect ratio (width to height) is 0.75:1.

To obtain HD video, a 16:9 window into the sensor is used capture the 16:9 image projected by the lens onto the sensor. At full Wide, the need for pixels used by EIS is minimized and so the 16:9 capture window has almost 1920x1440 pixels.

As the lens zooms to full Tele, the 16:9 capture window also proportionally shrinks to 1440x810.

The image that is recorded to tape must, of course, have 1440x1080 pixels. Therefore, the Sony Enhanced Imaging Processor (EIP) chip performs a transform that normalizes image width to 1440-pixels and image height to 1080-pixels.

[J. Stephen McDonald]

Originally Posted by Steve Mullen

Since when you zoom Wide you expect a wider field of view and when you zoom In you expect a narrower angle of view -- how would you know what is "zoom" and what is "magnification effect?"

Have you tried the same zoom on the same subject with Full Scan OFF and ON? In the latter case, there will be no "magnification effect" and the camera should record less wide at Full Wide and more wide at Full Tele.

Does it? Or, is the zoom coverage the same?

Either could be true!

Yes, either could be true, as you would be hard pressed to detect it by simple observation. However, the effect of optical zoom, as opposed to the magnification effect of a shrinking active pixel zone, should reveal itself, by comparing the image with Full Scan On and then when it's in the Off position. I don't have one of these critters yet, so I can't do any experimenting. If I do get ahold of one, I'll measure the differences in magnification and wideness of view in those two modes. All my tedious speculations could then be quickly resolved by some hands-on work.

I'd expect that with the stabilizer off and the full CMOS being used in Full Scan, that the image would be wider at both ends of the zoom range. But, there'd be a lesser increase in wideness at the wide end when Full Scan was used, as the narrower EIS margin wouldn't add as much width, when it was removed (I'm not sure even I could follow what I just wrote, if I read it again tomorrow).

[Rodolphe Pellerin]

Originally Posted by Evan Donn

...when you switch into shot-transition mode the EIS is disabled & the camera goes full-scan (this doesn't happen if you just disable the EIS). It's most noticeable at full zoom, the EIS margin seems to vary from almost nothing at wide to maybe 20-30%. At full zoom there is a significant jump as you switch into and back out of shot transition...

A1 users, is it the same thing for you ?

[Steve Mullen]

[QUOTE=Evan Donn: When you switch into shot-transition mode, the EIS is disabled & the camera goes full-scan.[/QUOTE]

This is indeed interesting. I wonder why? Perhaps since you could create 2 zoom settings very close in time -- a very fast zoom could be required.

IF both the lens is zooming AND it is adjusting the image size on the CMOS -- it simply can't do both at high-speed. So it switches to Full Scan to eliminate the need to simultaneously adjust image size during the fast zoom.

ALTERNATELY -- as you suggest, a very slow fast zoom could be required.

IF both the lens is zooming AND it is adjusting the image size on the CMOS -- at very slow speed the image might "shudder" as both the optical zoom and the digital zoom from the EIS are executed. (Perhaps one adjustment follows the other.) So it switches to Full Scan to eliminate the need to "simultaneously" adjust image size during the slow zoom.

I think we have an answer to the optical zoom verses the digital zoom from the EIS question. Both occur.

[Evan Donn]

Originally Posted by Steve Mullen

IF both the lens is zooming AND it is adjusting the image size on the CMOS -- at very slow speed the image might "shudder" as both the optical zoom and the digital zoom from the EIS are executed. (Perhaps one adjustment follows the other.) So it switches to Full Scan to eliminate the need to "simultaneously" adjust image size during the slow zoom.

Exactly - I'm sure this is why it's disabled in shot transition mode. The camera needs to be able to do potentially very slow zooms and having to do so with both an optical & digital zoom simultaneously would probably cause a shudder or flicker.

This is probably also why the camera can't do a very slow zoom. I'd bet it's some combination of engineering and marketing people making a decision - allow very slow speeds only with EIS off, which then also would require changing the digital zoom(and the zoom multiplier), or set a relatively fast minimum zoom speed across the board to avoid confusing the zoom specs and usage.

[J. Stephen McDonald]

It would seem that if the camera shifted to Full Scan during a fast zoom in video mode, meaning that the EIS was momentarily turned off and afterwards went back to EIS, that disruptive jumps in image size would occur. There'd have to be a smart processor that would taper the EIS out and back in, to smooth the transition points. But, this would tend to delay the onset of the zoom effect and would extend the zooming effect after the end of the zoom range had been reached. All these things need to be determined by playing with the three models.