The mirror is down as you say during focus. That means the light is
being reflected up to the prism to your eye at the viewfinder during
focus. There is no light then to illuminate the sensor. And the
shutter is closed at that point. No you cannot focus with a DSLR LCD.
Or evaluate the exposure before the shot with the histogram. No
preview. It reviews what you have already captured.
I suppose that you could engineer a camera that would allow you to
optionally lock the mirror up, open up the shutter, and use a sensor
that could drive an LCD that would give you a preview and a focus
too, but it would no longer be acting as a DSLR. It would be like a
digicam mode. It isn't going to happen though. I can't imagine anyone
wanting it until it gets better than a mirror/prism system. And if it
does it will just replace the expensive optical system. There is talk
of providing an auxiliary sensor illuminated by a secondary mirror
that would provide a pre-exposure histogram.
A digicam works differently. As I understand it the little shutter is
open so that you get the preview, and metering information. Tripping
the shutter closes it, dumps the preview pixels in the sensor, opens
the shutter, times the exposure, and shuts off the sensor, and dumps
the image pixels to the processing chip.
Winsor
Long Beach, California, USA
On Oct 26, 2005, at 11:45 AM, Willie Wonka wrote:
> I am not sure if I understand, Winsor. Isn't the mirror down
> during focusing? And here is an even dumber question: Can't you
> use the LCD on a DSLR to focus, before taking the picture? And if
> you can, I don't see any problem implementing the feature I have on
> mine.
> Be patient, you are explaining this to someone who takes pictures
> of static subjects only, I see now how this method of focusing
> could be a little problematic with stuff that moves.
> Thanks
> Boris
>
> Date: Tue, 25 Oct 2005 08:05:23 -0700
> From: Winsor Crosby <wincros@xxxxxxxxxxx>
> Subject: [OM] Re: Shots again the E-500
>
>
> A DSLR has a mirror that prevents the LCD from being used as a
> focusing device.
>
>
>
> Winsor
> Long Beach, California, USA
>
>
>
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