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Back-focus or Iris issue...?

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Matt OrfaleaBack-focus or Iris issue...?
by on Aug 25, 2012 at 6:34:57 pm

my Sony EX-1 camera produces a soft/out of focus image whenever I close the iris around or passed f16? the more I close the softer it gets.

at 1st I thought it was a back focus prob, EXCEPT increasing the shutter speed and opening the iris makes a wide shot sharper. ( which is a little confusing since opening the iris is supposed to decrease DOF...)


HELP appreciated. PLEASE ANYBODY.

ps i had firmware upgraded so it is NOT a firmware issue.

thanks!
Matt

http://www.youtube.com/orf
https://sites.google.com/site/orfvideo/list


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Matt OrfaleaRe: Back-focus or Iris issue...?
by on Aug 25, 2012 at 6:41:08 pm

also did the auto back-focus adjust procedure several times.

http://www.youtube.com/orf
https://sites.google.com/site/orfvideo/list


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Robin ProbynRe: Back-focus or Iris issue...?
by on Aug 26, 2012 at 3:37:03 am

Its a problem of all lenses .. dont know the science but up around F16 you,ll see this focus problem.. its not your camera.. use ND filters ..



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Robin ProbynRe: Back-focus or Iris issue...?
by on Aug 26, 2012 at 4:27:34 am

Called Diffraction.. ? or something like that.. anyway looking at an image through a very small hole.. if you squint your eyes its the same thing.. or a mesh screen..



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Bruce RawlingsRe: Back-focus or Iris issue...?
by on Aug 26, 2012 at 8:10:12 pm

The small chip cameras need to shoot at around f4 or wider to avoid the diffraction. I put an extra ND.6 on the camera to make sure of not stopping down beyond f5.6 at most.


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Rafael AmadorRe: Back-focus or Iris issue...?
by on Aug 27, 2012 at 6:11:45 am

[Bruce Rawlings] "The small chip cameras need to shoot at around f4 or wider to avoid the diffraction. I put an extra ND.6 on the camera to make sure of not stopping down beyond f5.6 at most."
Right.
There is a lot of literature that shows why this effect happens on small sensors.
rafael

http://www.nagavideo.com


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Michael SloweRe: Back-focus or Iris issue...?
by on Aug 27, 2012 at 7:41:57 am

This is not just small sensors it's a standard photographic principle. We learnt this at our mother's knee, always try and expose with an aperture around f5.6 or f4. That's why Sony have given us two ND filters, if you need more (must be a hell of a bright light) then fit them.

Michael Slowe


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Rafael AmadorRe: Back-focus or Iris issue...?
by on Aug 27, 2012 at 12:39:51 pm

[Michael Slowe] "This is not just small sensors it's a standard photographic principle. We learnt this at our mother's knee, always try and expose with an aperture around f5.6 or f4. That's why Sony have given us two ND filters, if you need more (must be a hell of a bright light) then fit them."
i thought that small sensors somehow made things worst.
Thanks for clearing this, Michael.
rafael

http://www.nagavideo.com


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Michael SloweRe: Back-focus or Iris issue...?
by on Aug 28, 2012 at 10:00:51 am

I'm not sure whether small sensors make it worse or not, they may well do so, but the standard photographic principles still apply. Similarly with depth of field. Less depth the wider the aperture and less depth the longer the focal length of the lens (referred to as 'zoom' in these modern times).

Michael Slowe


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Rafael AmadorRe: Back-focus or Iris issue...?
by on Aug 29, 2012 at 2:40:13 am

[Michael Slowe] "I'm not sure whether small sensors make it worse or not, they may well do so, but the standard photographic principles still apply. "
Whatever, but I've always read the experts saying about the EX-1 never to go over f5.6.
rafael

http://www.nagavideo.com


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Bruce RawlingsRe: Back-focus or Iris issue...?
by on Aug 29, 2012 at 7:45:58 am

I also have a Panasonic HPX250 and use the same ND6 principle of never shooting above f4-5.6. Alan Roberts explains all in his BBC report for the HPX371 amongst others. As mentioned it is physics not the manufacturers that causes the softness.


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