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Keeping the Sweet Spot zone and FOV of a 35mm lens.


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#1 Michael Maier

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Posted 07 September 2008 - 08:32 AM

In this day and age of HD cinematography, I find myself more and more being asked to shoot projects with the Mini35 or Pro35 or other 35mm adapters. RED may end that soon with its 35mm sized sensor, but for now folks who shoot digital and want a shallow DOF have to put up with the adapters.
One thing that has always bugged me is the field of view matter. Those adapters, especially the P+S ones are supposedly designed to get you the exact FOV of a 35m lens as if it was being used in a 35mm camera. But given that when using the 35mm adapters you have to zoom in on the image to retro photograph it, you can pretty much get any FOV you want, depending on how much you zoom in. Now this wouldn?t bother me that much if it was for the FOV alone. But the FOV comes with a ?sweet spot? zone. The way I normally do it and I think everybody else who uses one of those adapters, is to zoom in until I can no longer see dark corners/vignette. The thing is there?s no telling if this is getting me the right FOV for the given lens I?m using. I normally have the feeling I?m not zoomed in enough as objects on the edge of the frame look very soft and are sometimes impossible to focus on. This tells me these objects are out of the ?sweet spot? circle of that lens and for that reason makes me feel like I have not zoomed in enough and the FOV is then wider than it should be. But if I try to zoom in more the Director will normally complain the lens is then getting too tight and we should use as much of it as possible, also saying that it will probably mess the FOV and no longer have the right FOV for that lens in 35mm. It?s a little confusing, but basically what I?m looking for is a way to make sure I have the right FOV for that lens and a way to make sure I?m in the sweet spot and not really much wider. Apart from putting the lenses in a 35mm camera and comparing the frames which is normally not practical and nearly impossible for most projects I shoot, is there another way?
Sorry for the long post but I felt it would be a good idea to explain it all in detail since it?s already confusing as it is. Thanks for the input
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#2 Allen Achterberg

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Posted 08 September 2008 - 12:19 AM

are you using cine lenses or stills lenses? Here's a way you can save yourself the confusion.

if using the redrock m2Take a piece of 35mm film, 4 perferations in height, and stick it to your cine screen (without scratching the screen) and then use the camcorder and zoom in until you no longer see the perferations. it might be easier for you to use a razor blade and remove what would be the picture area and zoom in until you have a zoom length set. then remove the film, put a lens on and turn the screen on. voila, you see exactly what 4-perf 35 sees.

for the PS technik, or movie tube, once you get past the vignette you should be all set. if your director wants wider...then throw a wider lens on or move the camera further away from the subject. don't allow a vignette! if he wants a vignette the sure, do it. I'd play with the idea of doing that in post, but would I allow a vignette because we need to be a bit wider? not a chance in hell.

refer to the field of view tables in the American Cinematographer manual. that will help you too.
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#3 Chris Keth

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Posted 08 September 2008 - 01:54 AM

if using the redrock m2Take a piece of 35mm film, 4 perferations in height, and stick it to your cine screen (without scratching the screen)


Hehe. It doesn't really matter if you scratch it. What's a groundglass but a very, very scratched piece of glass.

Anyway, to my point. When I set up those adapters I look up the vertical and horizontal fields of view for a lens I have. Then, at a given distance I can quickly calculate the size rectangle I need to exactly fill a 35mm frame. I'll tape that to a wall and it does double duty.

1.) It allows me to find the zoom setting that exactly gives me a 35mm frame size.

2.)It also allows me to check all of my lenses for coverage. I can just zoom out and see the coverage of the lens in relation to that rectangle. If any lenses are very close to not covering (the wide lenses will be the likely candidates), I can check them for soft corners due to lack of image circle.

I've had lenses that don't vignette with dark corners but they focus vignette by not focusing properly in the corners because you're trying to cover a too-large frame with them. Just because the corners aren't dark doesn't mean that the lens is properly and sharply covering the frame.

See this as an example of a lens whose coverage has been exceeded, but it still illuminates the frame:
Posted Image
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#4 Allen Achterberg

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Posted 08 September 2008 - 06:32 PM

good job Chris, nice example.

But there is a big difference between Micro scratches and a big noticable scratch on the screen, a big scratch will limit your shutter speeds. I've got away with higher shutter speeds on a GG with no large scratch, but not the otherway around.
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