35mm spherical lenses and their effective focal length on S16

Brad Greenspan · March 7, 2006

Hi,

I'm going to be 1st AC-ing a project in the near future where we will be using an Arri SR-3 with 35mm Zeiss UltraPrimes. The DP has stated his interest in doing this and I would liek to know a few things.

First: If I place a 25mm on the camera, I know what a 25 designed for 16 will look like down the lens. what will a 25 designed for 35 look like? Will it look like a relative 50-ish, or will it still look like a 25 with only the 'sweet spot' of the lens being used?

Second: Is there a change in the size of the circle of confusion when calculating the depth of field, and what should it be?

Thank you all so much in advance.

--Brad

Freya Black · March 7, 2006

A 25mm lens on 35mm film will look wideangle. On 16mm film the same exact lens won't look wideangle.

David Mullen ASC · March 7, 2006

The only difference between a 25mm designed for 16mm shooting versus a 25mm designed for 35mm shooting is that the 25mm lens designed for 35mm shooting covers a larger frame, whereas a 25mm designed for 16mm shooting would probably vignette if put onto a 35mm camera.

Otherwise, the angle of view is the same -- it's a 25mm lens either way.

Stephen Williams · March 7, 2006

Otherwise, the angle of view is the same -- it's a 25mm lens either way.

David,

Surely the angle of view is connected with the image size on the film, so on a larger format the angle of view is wider.

Stephen

Thomas Worth · March 7, 2006

Focal length is focal length, regardless of the film size. A 25mm lens designed to cover 35mm will give the same field of view as one designed to cover S16 when used on a 16mm or S16 camera. This rule applies to video cameras and any other camera as well.

Of course, due to the fact that 16mm or S16 is "cropped" toward the center of the frame, a 25mm lens will have the appearance of being "wider" when used on a 35mm camera. The truth, however, is that you're just seeing more of the frame.

So, in response to your question, yes, it will "still look like a 25 with only the 'sweet spot' of the lens being used."

Stephen Williams · March 7, 2006

TW Focal length is focal length, regardless of the film size.

SW Exactly

TW A 25mm lens designed to cover 35mm will give the same field of view as one designed to cover S16 when used on a 16mm or S16 camera. This rule applies to video cameras and any other camera as well.

SW YES

Thomas,

However the angle of view of a 25mm lens will be different when used on a 16mm, S16mm, 35mm or S35mm the image size as you said is cropped,

http://www.panavision.co.nz/main/kbase/ref...lenseqvform.asp

http://www.panavision.co.nz/main/kbase/ref...calcFOVform.asp

Stephen

March 7, 2006

I think the trick is to use a little wide angle solution when you clean the lens. Of course, you could always shoot on wide angle film.

Honestly, though, hasn't this been discussed a dozen times before?

<<apologizing in advance for being such a snot. . .>>

Thomas Worth · March 7, 2006

However the angle of view of a 25mm lens will be different when used on a 16mm, S16mm, 35mm or S35mm the image size as you said is cropped

Yup. Every time the size of the film changes, you'll get a different field of view even though the focal length is identical.

Here's some related information:

http://www.cinematography.com/forum2004/in...topic=11647&hl=

Leo Anthony Vale · March 7, 2006

I think the trick is to use a little wide angle solution when you clean the lens. Of course, you could always shoot on wide angle film.

Honestly, though, hasn't this been discussed a dozen times before?

<<apologizing in advance for being such a snot. . .>>

---Only a dozen? & something so basic.

I'm ready to punch out the next person who says 'sweet spot'

& won't apologize for it.

---LV

Rik Andino · March 7, 2006

I'm ready to punch out the next person who says 'sweet spot'

& won't apologize for it.

---LV

What the hellm is the sweet spot...?

Is this a made-up term or does it actually exist?

Adam Frisch FSF · March 7, 2006

Otherwise, the angle of view is the same -- it's a 25mm lens either way.

No, the focal lenght is the same - the angle of view changes for every format.

Laurent Andrieux · March 7, 2006

A 25 mm is a 25 mm whatever format you work with. The focal length doesn't depend on the format, whatever format a specific lens was built for.

At a given focal length, the field of view will depend on the image size, therefore on the format.

Relationships : http://www.cinematography.com/forum2004/in...?showtopic=5207

Adam Frisch FSF · March 7, 2006

Don't know who's post you were answering, Laurent, but that's exactly what I said and meant.

Laurent Andrieux · March 8, 2006

No problem. I read the thread quickly and thought there was some good as well as some confusion in it. Mainly posted the link to the formulas I wrote in the FAQ.

I second what you said, but when you quote David saying he's wrong, you are going a bit quick your self, if I may say...

What David ment is that whatever the lens was designed for (16 or 35) it will give the same field of view, on a 16 camera. He of course didn't mean a lens would give the same field of view whatever format it would be used with.

I'm afraid that quote was cut off its context and then confusing...

(I'm some time confused, David is rarely so, if never...)

Sounds like there has been the same kind of confusion beetween Thomas and Stephen BTW...