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Bright spot in center of image


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#1 Jason Hinkle (RIP)

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Posted 15 October 2009 - 10:27 PM

I got some footage back and a lot of my exterior shots have a bright hot spot in the center of the image. It's sorta like vignetting except the outside looks good and the spot in the center is overexposed and washed out. This is the second time I've seen this but I can't figure out for sure why it's happening so that I can avoid it. I'm using Nikon 35mm lenses on my super-16 camera (but the same thing happened on my r-16 camera too). It seems to be some combination of lenses 50mm or wider plus bright sun and stopping down f8 or more. It's not 100% consistent either, some shots look perfectly fine.

I'm wondering if using 35mm lenses on super-16 are just unpredictable in the bright sun, or is having a hot-spot something that happens with any lens? I don't want to blame the lenses if it's some other thing I'm doing. As for my setup, I had a matte box with a french flag and an 85 filter and it didn't seem as though the sun was shining directly into the lens or anything. Some shots look fine and others vary as to the degree of the hot spot in the center. I can't seem to find a pattern in my camera report.

thanks for any ideas or suggestions.
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#2 Charles MacDonald

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Posted 15 October 2009 - 10:35 PM

Thanks for any ideas or suggestions.


Well, one idea that comes to mind is to grab a nikon and see if the lens in qusetion works in its "native mode"

are you getting too much Image light in the center? or white liight that is overlaying the image?
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#3 Jason Hinkle (RIP)

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Posted 15 October 2009 - 11:48 PM

Well, one idea that comes to mind is to grab a nikon and see if the lens in qusetion works in its "native mode"


I use the lenses all the time with my old Nikon w/o any issues & they even look fine on the 16mm camera as long as it isn't in the bright sun. It must have something to do with the aperture and/or light shining onto the glass..? It seems so random though, I can't figure it out.
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#4 Satsuki Murashige

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Posted 16 October 2009 - 12:05 AM

Silly question, but you're not leaving the viewfinder uncovered while rolling the camera in these bright exteriors are you?
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#5 Jason Hinkle (RIP)

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Posted 16 October 2009 - 12:13 AM

Silly question, but you're not leaving the viewfinder uncovered while rolling the camera in these bright exteriors are you?


That is not silly at all actually, I suspected that problem and it is a possibility that happened on a shot or two. But the weird thing is that I had this problem even when I was using another camera w/o video assist - so my eye was always against the viewfinder.

So, it sounds like having a bright spot is not a normal problem - in other words it is probably not a typical problem where I need to shade the lens more..?
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#6 Paul Korver

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Posted 16 October 2009 - 01:40 AM

Hi Jason,
Another possibly dumb question... but how are you viewing the footage? If digitally how was it transferred? There are some ghetto transfers out there that can create bright-spots on film in the center when they actually aren't there (usually from a projector with a non-diffuse light source).

If you're viewing it on a projector then I'd check to make sure you have some diffusion in front of the lamp.

-Paul
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#7 Bruce McNaughton

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Posted 16 October 2009 - 02:59 AM

Sometimes the light bounces between the lens and the film. If your lens has a silver rear or just a large rear element (like Nikons) the image can reflect beck into the lens and back again into the film. I noticed this on our VistaVision cameras before we blackened the rear of the lenses.

Bruce
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#8 John Sprung

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Posted 16 October 2009 - 12:47 PM

Can you post an example here?

Edit: Maybe also pix of the adapter setup? Try using the Nikon lens in hand to form an image on white paper, then do it again with the adapter on the lens.



-- J.S.
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#9 Jason Hinkle (RIP)

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Posted 17 October 2009 - 12:35 PM

Thanks so much for the feedback. Bruce's idea of the reflection actually makes some sense, the rear element on the nikons is huge compared to my 16mm lenses. I think it is safe to rule out the transfer, I was there at the lab and we were noticing the problem shots which are only a select number of shots - what they all have in common is that they were shot in fairly bright sun at around f11. I am starting to suspect that using 35mm lenses on my 16mm camera just doesn't work in bright light, which is depressing. I would rather there was some operator error that I could correct.

The camera is an ACL and the lens adapter is custom made by Les Bosher, basically the best adapter made for this setup. It looks exactly like a stock intermediate adapter for an ACL.

Here's a screen grab of one of the bad shots in case it helps. (btw, I've managed to salvage all of these shots using a an inverted vignette plugin, but it's annoying and tedious)

Posted Image

Edited by Jason Hinkle, 17 October 2009 - 12:35 PM.

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