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Newbie Lens format question


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#1 Mark.Smith

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Posted 12 May 2009 - 02:11 PM

Hello,

Student question. First year film student.

I would like to know what focal length formula is used to convert full frame SLR lenses to s16mm,16mm, 35mm etc.

Or perhaps a better question would be to be to explain why they are not the same. i.e why is the field of view not the same on my full frame SLR as it is on a Arri 35mm? Are they not both full frame?


Im just going over the fundementals and trygin to grasp what the different focal lengths equate to. When someone says 8mm in SLR world that means ultra wide. When I put the 8mm lens on a Arri SR1 at the school, well it wasn't as wide as I'd assumed it would be. It was closer to a 50mm in SLR terms.

I searched but could not find any specific info.

grateful for any info
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#2 Adrian Sierkowski

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Posted 12 May 2009 - 02:38 PM

Has to do with the format size, e.g. how big the negative is. S16mm is smaller negative than a 35mm frame so it just uses the center of the lens's projected "image circle."
For SLRS v 35mm motion picture, the negtives use differant #s of perforations to record an image. Motion picture is 4 perf, or 3 perf or 2 perf, with aspect ratios of 1.33:1, 1.85:1, or 2.39:1, again using a different portion of the projected image surface.
My simple way of working is is that for Motion Picture 35mm a "normal" lens is around 50mm (for me) so on 16mm a normal lens is approximately 25mm (negative is about 1/2 the size, so you 1/2 the lenses). It's really about 45% the size, but you know, rounding helps.
With time you'll start to know the lenses/focal lengths v formats pretty well. Learn it on 1 format and you can translate for other formats.
Also, directors viewfinders often can help for comparisons.
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#3 David Mullen ASC

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Posted 12 May 2009 - 04:02 PM

Some people use the film's diagonal measurement to figure out equivalent views for different formats & focal lengths, but since the aspect ratio affects the diagonal measurement, I find it simpler to just compare horizontal view. That way, all you need to know is the horizontal dimensions of the format.

For example, if a Super-35 cine frame has a horizontal measurement of 24mm, and a Full Frame 35mm still (aka VistaVision 8-perf 35mm) has a horizontal measurement of 36mm, then the difference is 1.5X. So to match field of view, you have to use a 75mm lens on a FF35 camera to match the view of a 50mm lens on a Super-35 camera.

And the equivalent of a 50mm lens on a FF35 camera in terms of horizontal view would be a 33.33mm lens on a Super-35 camera (divide 50 by 1.5.) In practical terms, it would be a 35mm prime lens.

Super-16 is approx. 12mm wide, so the difference between that and Super-35 is 2X, and between S16 and FF35 would be 3X. So you'd need to use a 2.66mm lens on a Super-16 camera to get the same view as an 8mm lens on a FF35 camera -- or in other words, that 8mm lens on the Super-16 camera looks like the view of a 24mm lens on the FF35 camera.

Basically the lens projects a circular image and the format / sensor crops that image, so a smaller format sees a smaller area of the projected image and thus has a narrower view.
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#4 Chris Keth

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Posted 12 May 2009 - 04:07 PM

If you'd like more information, we've talked about this subject about once a month for as long as I've been posting and reading this forum. Just use the search function! (for search terms, try "lens, 16mm, 35mm" as it most often comes up with people using lenses designed for 35mm on a 16mm camera)
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#5 Michael B McGee

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Posted 12 May 2009 - 04:44 PM

check out Panavision site:
http://www.panavisio...ocalLengths.htm

they have a lot of great cinema calculators.
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#6 David Mullen ASC

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Posted 12 May 2009 - 04:52 PM

"fourcolors", just because you're a student doesn't excuse you from following the first rule when registering to use this forum, which is to sign up using a real first and last name. Did you miss that instruction or just skip it?

Go to My Controls and edit your Display Name into a real first and last name. Thanks.
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#7 Mark.Smith

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Posted 12 May 2009 - 08:57 PM

Great information. Thanks very much for the background on focal lengths relations to film size. Things are clearing up ! The Panavision calculator is superb!

Where would I find a good directors viewfinder to get started with?






To David,

My name is already taken (Mark Smith). It doesn't allow for any other characters then A-Z otherwise I would try to make a variation. Any suggestions?

Mark

Edited by fourcolors, 12 May 2009 - 08:57 PM.

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#8 David Mullen ASC

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Posted 12 May 2009 - 09:09 PM

My name is already taken (Mark Smith). It doesn't allow for any other characters then A-Z otherwise I would try to make a variation. Any suggestions?

Mark


I'd ask Tim Tyler, the administrator.

In the meanwhile, I'd add your name Mark Smith as a Signature to your posts. I think you go to My Controls and over to Edit Signature. Doesn't hurt to mention what city you're in under the name.
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Media Blackout - Custom Cables and AKS

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Broadcast Solutions Inc

Ritter Battery

Technodolly

Aerial Filmworks

Abel Cine

Tai Audio

Glidecam