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Iris - Affect on sharpness


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#1 Daniel Smith

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Posted 15 December 2010 - 02:51 PM

Hi,

I overheard once that opening the iris to levels like f1.4 and 1.8 (wide open) can soften the image, and I'm not talking about shallow depth of field, I'm talking about an overall image softness. Operators like to stick to around f2.4, 2.8 at least.

I was filming a gig last night with a HPX371 and I was slightly disappointed with the sharpness of the image, however I noticed it was slightly sharper in some parts more than others. I don't recall moving the focus around much, however I did alternate between f1.8 and f2.8 every now and again. Perhaps the iris adjustments were making it sharper/softer? I was already on +3db so I had to keep it fairly open.

I can't see how it could have been a depth of field issue, I was on a wide shot for most of it and the depth of field should have covered it even at f1.0.

Has anyone here experienced this?

Thanks.
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#2 Adrian Sierkowski

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Posted 15 December 2010 - 03:02 PM

That's correct inasmuch as both ends of the lenses tend to screw with the travel of light waves hence softening the image up a bit. It's one of the reasons why a lot of DoPs like to stick in the middle 2.8~8 range, with 4~5.6 being preferable. A lot will depend on the lens as well as most modern lenses are a lot better opened up than previous generations.
I'm sure there's a lot of technical reasons for which this happens, but to keep it simple in my own mind, I jut thought of the iris blades interfering too much (stopped down) or not enough (wide open) to adequately place the light on the imager.
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#3 Daniel Smith

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Posted 15 December 2010 - 03:18 PM

I thought it had to to be something like that. I don't think it was the best lens either.

I wish I'd pushed it to f4 and set the gain to 6db's now, but you live and learn.
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#4 Leo Anthony Vale

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Posted 15 December 2010 - 04:40 PM

Spherical aberration.


Light rays coming through the edges of a lens don't quite hit the focal point, blurring the image slightly. Stopping down the iris cuts out the edge rays.

http://en.wikipedia....ical_aberration

As the lens is stopped down to very small apertures, light rays passing the along the edges of the iris are bent once more blurring the image. Edge diffraction.

http://en.wikipedia....dge_diffraction
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#5 John Sprung

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Posted 15 December 2010 - 05:52 PM

A lot will depend on the lens as well as most modern lenses are a lot better opened up than previous generations.


Yes, it was about 10 - 20 years ago that the cost of making aspherical surfaces came down to the point that such elements became cost effective for the vast majority of lenses. Typically they accumulate all the corrections into one aspherical surface.




-- J.S.
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