Joined on Aug 4, 2012


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On article Review: Nokia 808 PureView (349 comments in total)
In reply to:

justyntime: Who the hell wrote that review?
"What it can't do, of course, is provide one of the other benefits of zoom in a conventional optical system - background blur. Even on a cheap small-sensor compact, you can achieve a degree of subject and background separation by zooming in, and reducing depth of field."

Fundamental misunderstanding of optical principles here!
Take a portrait with a given distance to a person with a 500mm lens.
Take the same portrait from the same distance with a 17 mm lens.
Then "digitally zoom into" the 17mm pic on your monitor so that the size of the face matches on both pics side by side. What you´ll find is that the amount of background blur is exactly the same on both pics! So digital zooming won`t be any different from optical zomming - concerning depth of field...

I'm a bit surprised, as a posting newcomer to dpreview - easily the best photo review site out there - to see such a fundamental discussion point cause an argument. Seems straightforward to me: For constant subject distance, sensor size and no diffraction effects, DOF (defined by overlap of point detail on the target) decreases as f increases *provided that* N (the f/ number) stays constant. However, if D (the actual physical aperture diameter) stays constant, then the DOF *doesn't* vary with focal length. So, say, a 5~25mm f/2 lens (unusually fast at tele) will behave as the first case, while a 5~25mm f/2-f/10 lens (unusually slow at tele) will behave as the second. Most actual lenses seem to be somewhere in between as all the elements joggle about during the zoom, hence the behaviour is a bit hard to predict. The idea of a super high res sensor plus a wide, bright, fixed focus lens and great digital downsampling, is a good one. My ~1000 characters worth, anyway!

Link | Posted on Aug 4, 2012 at 00:35 UTC
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