Wide-open iris vs stopped down on prime lens for Letus 35XL at DVinfo.net

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Old December 4th, 2006, 02:32 AM   #1
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Wide-open iris vs stopped down on prime lens for Letus 35XL

I'm seeing what appears to me to be conflicting advice regarding how to manage the prime lens on a Letus.

On the one hand, I've seen it said a number of times that it's best to set the prime lens at the widest possible f-stop (or a stop or so down), to get the best image quality, least light loss, reduce the chance of getting the so-called film of grain problem, & so on. To control light, neutral density filters are prescribed.

On the other hand, there's also a need to get every ounce of sharpness out of the Letus, & a wide aperture makes it tricky. So others have suggested using the iris to gain control over depth of field. But then one falls prey to the problems mentioned just now.

So perhaps this thread can be a place where the more knowledgeable members can hash out in a central location the problems of rationalising these conflicting demands.

My question is, Is there a single protocol for how to use the prime lens with the Letus that addresses all these issues, & if so, what is it? And perhaps Letus users can describe what works for them & what doesn't, & some commonalities can be found between them.

H.
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Old December 11th, 2006, 04:40 PM   #2
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Nobody wants to take a gander at this one?

H.
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Old December 11th, 2006, 05:20 PM   #3
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P+S Technik recommend working no tighter than f5.6 iris on the prime lenses.

Individual lens types can behave differently on a grounglass based image relay device. Some can provoke an artifact earlier than others as the apertures are tightened.

There seems to be no characteristic set in stone across brands of lenses which otherwise might have the same published specification.

I recommend you look up the P+S Technik website and download their .pdf manuals for the Mini35 and Pro35 and other articles. Their rules pretty much apply to many other alternative devices and I found them really helpful.

By comparison to the Mini35, the Letus35 seems to have a lower threshold of aperture setting where groundglass artifacts become evident.

My interpretation of the P+S Technik published recommendation is that f5.6 is the highest f-stop number where consistently reliable performance is to be expected across all conditions for their appliances.

So far in my experience, the Letus35 seems to require f3.5 or lower numbers with a preference for lower numbers in some extreme contrast conditions or where pans or tilts across large areas of bright non-textured background are to be undertaken or similar objects are expected to move through the frame.

There are exceptions where particular lenses and conditions will permit higher f-stop numbers without artifacts becoming evident.

Please note I am not an industry professional, my comments are not research based but speculative, therefore should not be accorded any authoritative merit
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Old December 11th, 2006, 08:02 PM   #4
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Thanks for the information, Bob.

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Old December 11th, 2006, 10:14 PM   #5
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On a few tests of my own, I haven't noticed much of a sharpness difference in in-focus areas before or after I close down the iris. I don't know what all this "use the sharper part of the lens" hooplah is about, but I have a feeling it only applies to higher-resolution photography such as the SLR lens was originally intended. The sharpness difference seems to disappear after MiniDV compression and the diffusion of the ground glass. I say "seems to" as it could be the way I use my lenses, the lenses I have in the first place, etc.
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Old December 12th, 2006, 03:26 AM   #6
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Ben.

Your comment is valid but there are a couple of riders to go on it before I endorse it.

Most SLR lenses in my possession, by the time they peer through a 24mm x 18mm patch on an AO5 groundglass only just come up to 850TV lines. In aerial image direct-to-camera, they do little or no better so the groundglass is not necessarily the only culprit when it comes to resoluton loss. The SLR lens and the relay system to camera come in for their share.

Many SLR camera lenses are not sharpest at wide open aperture. With groundglass devices, sharpness in part equates to better contrast separation across edge extremes which happen to be on the plane of focus, purely a subjective aesthetic something I can observe when using f1.8 v/s f3.5.

I'm short-sighted so I am probably more compulsive obsessive excessive about these things.

But as you correctly observe, this is largely moot by the time MiniDV or mpeg compression schemes have had their way with the images.
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Old December 17th, 2006, 10:34 PM   #7
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The worst thing about shooting wide open with a lens of any quality (like Cooke, Zeiss or just Canon or Nikon) is that it will make it much more difficult to get a sharp focus because the depth of field becomes extremely small. ie. A subject can lean in and out of your focus simply by turning their head. As a result follow focusing becomes harder.

If anyone has said it is best to shoot wide open it's not because the image quality is best. It's only because you need to compensate for the amount of light loss when using an adapter. Sometimes for night shots or interior shots however you have no choice but to shoot wide open to get as much light in as possible.

Welcome to the wonderful world of light eating 35mm adapters...
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