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Apparent Depth of field with Canon 20D

 
 
W
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      12-22-2006
Interesting and thanks. I had always thought that when I used my lens
wide or close to wide open that the depth of field in the captured
image seemed less than what I percieved when I shot the image. I had
always attributed that to the fact that I would examine the image at
larger magnification and with more scrutiny than when I shot it (let's
say quickly for a street shot). When I shot the image I described
(glasses above text), I did so very carefully. It was then that I
realized that there was more than meets the eye (literally in this
case!).
Thanks for the enlighting explanation(s)!

Kennedy McEwen wrote:
> In article <(E-Mail Removed). com>, W
> <(E-Mail Removed)> writes
> >Folks,
> >
> >I recently did a shot where I bracketed f-stops to tailor depth of
> >field. The shot was of eye glasses sitting on a page of print. I was
> >surprised to see that the viewfinder image appears to have way more
> >depth of field than the actual photographed image (20D with 50mm f
> >1.4). With the lens wide open (f/1.4), I could almost read the print on
> >the page (focus was above the plane of the page on the eyeglasses) when
> >looking through the viewfinder. On the photographed image, the page was
> >a total blur. It seems the viewfinder appeared to have significantly
> >greater DOF than the actual image. Any ideas why this would be?
> >

>
> That is a very common, but rarely discussed, disadvantage of "bright"
> focussing screens which prevents the SLR from actually achieving one of
> its main objectives - showing the viewfinder image "as is".
>
> An ideal focus screen would accept all of the rays forming the image
> from the lens and project them equally into the eyepiece so that you
> could see the image exactly as it was formed on the sensor.
> Unfortunately it is only possible to achieve a rather crude
> approximation of that ideal where the focus screen scatters the image
> forming rays equally in all directions. Some of these rays then reach
> the eyepiece and hence the eye. Without such scattering, rays from the
> periphery of the lens would miss the eyepiece and hence would not form
> any of the image that you see in the viewfinder. Only rays coming from
> the central portion of the lens would reach the eyepiece lens and hence
> form the viewfinder image. Since the screen has scattered all of the
> image forming rays, you see a relatively faithful reproduction of the
> image from all parts of the objective lens - you see light from the
> peripheral parts of the lens just as much as light from the central
> parts and hence get a view of the depth of field of image. The downside
> of this is, of course, that the highly scattering focus screen means
> that most of the light doesn't reach the eyepiece and hence although the
> image is reasonably accurate, it is fairly dark.
>
> Brighter focus screens scatter less of the image, so that more of the
> light reaches the eyepiece directly, however this is at the expense of
> more of that light coming from the centre of the lens and less coming
> from edges, which must be deflected to reach the eyepiece. The downside
> is that the image in the viewfinder, despite being brighter, actually
> corresponds more to the image from the stopped down lens than it does to
> the wide open aperture that the lens uses in normal viewing. You will
> still see a darkening of the screen when the lens is stopped down,
> because some of those peripheral rays are still scattered towards the
> eyepiece, but the darkening is much less than would be produced from an
> older, darker, conventional ground glass screen.
>
> If you remove the focus screen entirely, you get a very bright image
> indeed, but completely lose the ability to assess DOF in the image. In
> addition, the image will not darken as you stop the lens down until you
> reach very high f/#s.
>
> The image in the viewfinder of the SLR is only an approximation of what
> the sensor sees. How good an approximation depends - advantages in some
> aspects can be traded off against disadvantages in others. People
> generally value the brighter viewfinder at the expense of DOF accuracy
> for AF cameras. However these focus screens are almost useless for
> manual focus lenses because they only show a "stopped down" DOF. Hence,
> for many of their interchangeable SLR cameras, Canon also offer manual
> focus screens (which scatter more of the peripheral rays into the
> viewfinder hence producing an image more representative of the open
> aperture). These manual focus screens usually have the caveat that the
> viewfinder image is darker than the standard screen.
> --
> Kennedy
> Yes, Socrates himself is particularly missed;
> A lovely little thinker, but a bugger when he's ****ed.
> Python Philosophers (replace 'nospam' with 'kennedym' when replying)


 
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Dave
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      12-29-2006
<snip>

>" IAE, tho, I suspect it's the proper working of the human eye in play
>here....."
>
> --
> John McWilliams


That's an interesting idea, John, you may be on to
something there. As you know each human eye is
different, with a unique sweet spot of focus. Too far off
and you need glasses or a dipodic correction to bring
images into ideal focus. For example binoculars have
a built-in adjustment for individual eye focus. Of course
the human eye also has it's own focus mechanism that can
physically squeeze the lens of the eye for ideal focus.
Perhaps when looking through the viewfinder with
abnormal vision the ideal focus was shifted off a little,
resulting in a plane of focus that was different than
perceived by the eye at the eyepiece... I can't say
I have ever noticed this but it seems possible.

Dave


 
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