Re: Technical question about image scaling
Jay Kneese
shifting the sensor data to the buffer or memory card. Obviously, a digital
zoom from an already degraded image will be even worse. Think about it.
"Peabody" <waybackNO...@yahoo.com> wrote in message
news:KM7zl.173551$xK6....@newsfe12.iad...
> As will be obvious, I'm new at this, but just want to
> understand.
>
> So everybody says optical zoom is good and digital zoom is
> bad, and in general I understand why. But my Canon A590 has
> something called "Digital Tele-Converter" mode, which simply
> fixes the amount of digital zoom at 1.6x or 2.0x, and you
> can add whatever optical zoom you want, if any, on top of
> that. I think this means the camera only uses a smaller
> central segment of the sensor as its "raw" data, and
> produces the final jpeg from that.
>
> If the final jpeg size is bigger than that central segment,
> then the camera would have to upscale the raw image to get
> the final jpeg, and that's where the "digital zoom is bad"
> thing comes from because there's going to be a loss of image
> quality.
>
> But suppose you're shooting in one of the small jpeg sizes
> anyway. In that case, the use of the digital tele-converter
> mode means that you're just doing a more modest downscaling
> than you would otherwise do.
>
> So in my mind this raises the theoretical question of
> whether this modest downscaling should produce any worse
> result than downscaling from the full sensor raw data. Is
> there any way to generalize about that?
>
> It does seem to me that there might be good reason to use
> the digital tele-converter mode for smaller jpegs if there's
> no quality penalty, and that would be that you can "zoom in"
> without changing the effective aperture as would happen when
> using optical zoom. Well, for example:
>
> If I can ever find an artist to participate, my next project
> is a time-lapse video of a painting being painted. Since
> there are going to be thousands of individual images, and
> since it's going to end up as a video anyway, I'm probably
> going to be shooting in 2MP mode - 1600x1200. Well, it
> just seems that the digital tele-converter mode might
> actually be better than optical zoom. Using some form of
> zoom, the camera could be farther away from the artist, and
> therefore less audible or in the way, but using digital
> tele-converter mode I could avoid the reduction of aperture
> that optical zoom would produce. There would be no need to
> change shutter speed or ISO to compensate. I'm not sure
> about the effect on DOF of the two zoom options.
>
> Anyway, bascially you have two situations:
>
> 1. 2x optical zoom, no digital zoom, and the full raw image is
> downsized from 8mp to 2mp.
>
> 2. no optical zoom, fixed 2x digital zoom, and that smaller
> raw image is also downsized to 2mp.
>
> The question is - can you say anything in general about the
> relative quality of the final jpeg between these two
> situations? Is one always better, or does it depend? On
> what?
>
> Thanks very much.
>
Jürgen Exner
>
>If the final jpeg size is bigger [...] small jpeg sizes
Those are two totally different animals. Bigger JPEG and smaller JPEG
have nothing to do with zoom or focal length, be it digital or optical.
JPEG supports different compression rates and bigger JPEG simply means
less compression and thus less information loss and better picture
quality.
jue
Don Stauffer
> As will be obvious, I'm new at this, but just want to
> understand.
>
One reason many of us do not think much of digital zoom for serious work
is that whatever the camera does as digital zoom, you can do in an image
editing program, and often do it better.
Many of the editing programs give you choices of the mathematical
algorithm you use if you do have to downsample.
And, doing it in your camera is essentially "reversible" as long as you
save the downsampled image as a different filename.
Digital zoom is for those folks who do not use computers with their
digital camera, and even so, optical zoom is generally better.