Jump to content


Photo

Formula for distance focus per degree rotation focus ring


  • Please log in to reply
5 replies to this topic

#1 vvanwillegen

vvanwillegen

    New

  • Basic Members
  • Pip
  • 2 posts
  • Other

Posted 22 August 2012 - 02:47 PM

Dear users,

I'm wondering: is there a formula for calculating the distance of the focus after rotating the focus ring x degrees?

Greetings,
Vincent
  • 0

#2 Leo Anthony Vale

Leo Anthony Vale
  • Basic Members
  • PipPipPipPip
  • 2010 posts
  • Other
  • Pittsburgh PA

Posted 22 August 2012 - 11:53 PM



I'm wondering: is there a formula for calculating the distance of the focus after rotating the focus ring x degrees?



There is no formula. Rotating the focus ring is a mechanical function, not an optical function.
The amount of rotation varies from one lens design to another. There is no standard.
  • 0

#3 Dom Jaeger

Dom Jaeger
  • Basic Members
  • PipPipPipPip
  • 1604 posts
  • Other
  • Melbourne, Australia

Posted 23 August 2012 - 02:15 AM

Yes, it depends on the lens, they all have different focus barrel throws.

Even within a highly standardised series like say Zeiss Ultra Primes, manufactured to extremely tight tolerances, there may be slight variations in the total length of the focus scale throw (ie from infinity to minimum) between lenses of the same focal length. So each Zeiss focus ring is engraved with a focal length and a code - GG or HH etc - that identifies the length of throw and has been matched to the lens. Other high-end manufacturers hand-engrave each focus scale individually. One of the problems with cheap cine lenses is that they use generic focus scales that sometimes don't match up.

In terms of the spacing between distance marks within a focus scale, it's a type of logarithmic progression. To be perfectly accurate you'd need to include nodal points and back-focus distance in the formula, but an approximate version is

x = f² ÷ D

where f is the focal length, D is the distance to the object, and x is the distance the lens needs to be racked out from its infinity position to focus on the object. Depending on how much rotation of the focus ring causes how much focus travel (which is a function of the mechanics, be it helical thread or cam), you could work out some sort of relationship between degrees of rotation and focus distance, but it won't be linear.
  • 0

#4 vvanwillegen

vvanwillegen

    New

  • Basic Members
  • Pip
  • 2 posts
  • Other

Posted 29 August 2012 - 08:22 AM

Thank you for your detailed reply.

Today I will compare the formula against some prime lenses.
  • 0

#5 Tom Jensen

Tom Jensen
  • Basic Members
  • PipPipPipPip
  • 1234 posts
  • Camera Operator
  • Dallas, TX

Posted 29 August 2012 - 08:47 AM

Curious as to what you will do with this data
  • 0

#6 Chris Millar

Chris Millar
  • Basic Members
  • PipPipPipPip
  • 1642 posts
  • Other

Posted 29 August 2012 - 08:56 PM

Another way about it would be to tabulate it yourself with actual readings. Put those into a simple look up table into your 3D package/motion control whatever you're making - interpolate points inbetween (linear/cubic whatever complexity you choose) or maybe do a nice curve fit in excel/matlab etc... to get a function and avoid the look up table altogether Posted Image
  • 0


FJS International, LLC

Ritter Battery

Wooden Camera

Willys Widgets

Abel Cine

CineLab

Rig Wheels Passport

Glidecam

Paralinx LLC

The Slider

Metropolis Post

Tai Audio

Broadcast Solutions Inc

Opal

CineTape

Aerial Filmworks

rebotnix Technologies

Media Blackout - Custom Cables and AKS

Technodolly

Gamma Ray Digital Inc

Visual Products

Rig Wheels Passport

Media Blackout - Custom Cables and AKS

Wooden Camera

rebotnix Technologies

Broadcast Solutions Inc

Technodolly

Ritter Battery

FJS International, LLC

Opal

Willys Widgets

CineLab

Tai Audio

Abel Cine

The Slider

Gamma Ray Digital Inc

CineTape

Metropolis Post

Aerial Filmworks

Visual Products

Paralinx LLC

Glidecam