focus in relationship to f-stop

[Robert86]

Hello peeps,

I've been having a difficult time finding info on this subject, even in my books at home. Now, say you were to measure the length from camera to subject and you focus on that person. And say you were outside in the sun, so you're stopped down to f-16 for a 50speed film. You would have a deep depth of field. Now say you were to open up(adding ND filters to compensate for opening of iris of course) to f-5.6 or one, two, three or four stops from f-16, whatever it is. You're depth of field becomes shallower right, but how much more shallow does it become with every stop? This is something I really need help with. Does every stop wider become a certain amount of feet/meters shallower? I need to learn about this area before I invest in follow focus tools. Help me please! Ahhhhhh!

-I posted this in the Cafe ala DVX forum just because the cinematography section has been kinda dry lately and a lot of information has been pumped out of this section lately.

[Karl151k]

If you are using cinema lenses, there's a pretty good chance there is a depth-of-field scale on it. This is instrumental in figuring out exactly what range will be out of focus at a given F-stop. I think (and this is speculative) that the amount the BG goes out of focus with each stop is 1/4th, so between f16 and f5.6 (4 stops), the "blurryness" will double in intensity. However, I may be way off. I'm sure someone will come in with a forumla for you soon enough. =)

[Robert86]

Thanks for helping with what you can Karl. I am using cinema lenses and shooting film. So, this is important because I won't truly know what I shot until it gets back from the lab but I want to know what I will be getting. Again, thanks for helping.

Anyone else want to pitch in a bit. :)

[Isaac_Brody]

Depth of field is also dependent on what lenses you're using. Are you using 16mm prime lenses? Which lenses are you using?

[Robert86]

Depth of field is also dependent on what lenses you're using. Are you using 16mm prime lenses? Which lenses are you using?

Yes. I mainly be use 16mm primes. I have an Angenieux 18.5mm and an Angenieux 25mm.

[Karl151k]

Are you shooting with that ARRI-S that I see in your user icon? If so, shouldn't you be seeing exactly the DoP you'll be getting since it has a mirror shutter? I havent used one in a while, but I though that when you changed the F-stops on an ARRI-S that the eyepiece got darker and you could preview the DoP (albeit darkley). I could be imagining things though...

[Robert86]

Are you shooting with that ARRI-S that I see in your user icon? If so, shouldn't you be seeing exactly the DoP you'll be getting since it has a mirror shutter? I havent used one in a while, but I though that when you changed the F-stops on an ARRI-S that the eyepiece got darker and you could preview the DoP (albeit darkley). I could be imagining things though...

That's my baby alright. It does have a mirror shutter but even when wide open, T1.1 I can still see everything clearly, when it should be a very shallow depth of field. So I don't think it's a preview of the DOF.

[Karl151k]

The DoP diffrences on 16mm arent going to be as apparent as on a 35mm camera or even an SLR. Since the film gauge is smaller, the lens is focusing its image into a smaller area of film which results in a generaly wider DoP (with more things in focus). At longer focal lengths like 70 or 85mm (in 16mm), you should start to really see the DoP narrow down, but with 18 and 25mm lenses even at t1.1, it might not be as obvious through the viewfinder although it will likley show up upon projection/telecine.

[Robert86]

Ok. But is there a determined way to figure out how much narrower the DOF gets with every opening of a stop? In feet or meters?

[Karl151k]

There probably is. I can't think of a way to do it presently. But, it's gotta be do-able somehow. Anybody else know?

[GageFX]

The actual depth of focus is determined by the circle of confusion of the exact lens you are using.

http://www.luminous-landscape.com/tutorials/understanding-series/dof.shtml

This doesnt give you a handy pocket chart to quick reference, but will explain it all.

-GageFX

[Barry_Green]

Spend some time messing around with this, it should answer all your questions:
http://www.panavision.co.nz/main/kbase/reference/calcFOVform.asp

[Robert86]

This doesnt give you a handy pocket chart to quick reference, but will explain it all.

Even the amount of depth of field gained or loss(in feet/meters) from every opening or closing of a stop? Thanks.

[Isaac_Brody]

Spend some time messing around with this, it should answer all your questions:
http://www.panavision.co.nz/main/kbase/reference/calcFOVform.asp

Barry that's awesome. :dankk2: :thumbsup:

[Robert86]

Sweet. Thanks, I hope both links answer my question.

[GageFX]

Yes. very cool Barry. i so much prefer qucik references to complex math.

-GageFX