35mm lenses on SR3

Mike Budde · March 11, 2004

This weekend I'm shooting with an SR3, and i'm using 35mm Prime Lenses.

I'd assume that the depth of field will be equal to 35mm, because of the amount of glass in the lens, right? Or is depth of field determined more by the format's size (the area where the light is exposed)? Please help my confusion.

Thanks,

Mike

David Mullen ASC · March 11, 2004

A 50mm is a 50mm and has the depth of field of a 50mm -- but when used on a smaller negative area like 16mm, has a narrower field of view (looks more telephoto.)

Therefore if you want the same field of view of a 50mm on a 35mm camera, you have to use a 25mm (approx.) -- and a 25mm has more depth of field than a 50mm. Understand? This is why 16mm generally has more depth of field than 35mm - the focal lengths are generally shorter.

So the main problem with using 35mm lenses on a 16mm camera is that you need shorter focal lengths to get wider-angle shots. Most 35mm prime lens sets start at 18mm or 25mm, while you need lenses that start more at 9mm or 12mm, for example.

Mitch Gross · March 11, 2004

It doesn't matter what format the lenses were designed to cover, it only matters what format you are shooting in. Different lenses hae different depth of field characteristics based on the quality of their construction, but that doesn't particularly have anything to do with the shooting format. Use 16mm depth of field charts or a Sam-Cine calculator or the pCam program for the Palm Pilot (free download).

Mike Budde · March 12, 2004

Thanks guys!

In conclusion to the advice you both gave me; a 50mm is a 50mm is a 50mm with the depth of field of a 50mm. However, after the light has passed through the lens the depth of field is determined by the size of the negative area.

When shooting 16mm i should use 16mm charts, and when shooting 35mm i should use 35mm charts - because each negative area is a different size. The negative area is the starting point when determining depth of field. Depth of field charts can then show me the difference between a 50mm and a 25mm.

Please correct me if i'm wrong.

Thanks,

Mike

Mitch Gross · March 12, 2004

You are effectively correct, but not precisely considering the way you got there. Essentially, depth of field is depth of field as well, it really depends on what you use as your circle of confusion (c.o.c.) which is the measurement of when a point of light is considered in focus. So here's where we get to what you were saying--people generally use different c.o.c. for different formats, not because the image is captured onto a smaller negative but because that smaller negative will have to be magnified more in projection. So the acceptable c.o.c. for something finishing to video can be far larger than that of something finishing to a 35mm film print.

Are you completely confused now? :D

Mike Budde · March 12, 2004

Nope, that was the missing link.

Thanks,

Mike :D

Michael Nash · March 13, 2004

This topic seems to come up time and time and time again. The answer is always the same. I wonder if we could make up a little laminated chart like the one for HMI safe speeds that says "A 50MM LENS IS A 50MM LENS. THE FOCAL LENGTH REMAINS THE SAME REGARDLESS OF FILM FORMAT -- ONLY THE HORIZONTAL ANGLE OF VIEW CHANGES WITH DIFFERENT FILM FORMATS. DEPTH OF FIELD REMAINS THE SAME, BUT SHOULD BE CALCULATED BY THE C.O.C. OF THE FILM FORMAT."

I don't mean to sound insulting to the students who ask this question as it is a little confusing at first, and you have to start somewhere. But sometimes it gets out of hand, and students will INSIST that it's somehow the other way around. A very intelligent friend of mine who's a cinematography student at USC went through this very argument with me. He even called Clairmont to get a "technical" answer, only to end up more confused (and adamant) than ever. Frustrating.

David, are you still editing the "Cinematography" book? This seems like a topic deserving of a once-and-for-all explanation for cinematography students.

Mitch Gross · March 14, 2004

This is the only concept that I've had to explain more often than how the DVX100 lays down 24p onto 60i video. And I'm talking about people who should know better.