Jump to content


Photo

How does a focal lenght doubler work?

doubler focal lenght

  • Please log in to reply
2 replies to this topic

#1 davide sorasio

davide sorasio
  • Basic Members
  • PipPip
  • 93 posts
  • Student
  • New York

Posted 24 April 2018 - 03:07 PM

Hi guys,

I was wondering if anybody could give me a scientific exolanation of how focal lenght doublers work and the reason why there is a 2 stops loss.

Thanks in advance


  • 0

#2 David Hessel

David Hessel
  • Basic Members
  • PipPipPip
  • 166 posts
  • Other

Posted 24 April 2018 - 04:12 PM

It enlarges the image circle and as a result dims the brightness of the image in the process. 

 

As an example to make this concept more clear imagine a movie projection say 10 feet away from a wall and it is projecting an image on a 50" screen. This is similar to the way a lens works, it projects and focuses an image at a certain distance onto a certain size sensor just like a projector.

 

Now take that projector and move it back to 20", now the image is much larger, 100" across, so the screen has much less image on it and the image that falls on the screen is enlarged. Also as you move the projector back the image gets dimmer because it is spreading the same amount of light over a much larger area.

 

This is essentially what a doubler does, it increases the size of the projection on the sensor which magnifies it. What once filled the entire sensor is now spread well beyond it and since a large amount of its light is no longer falling on the sensor the brightness of the image also decreases.


  • 3

#3 James Malamatinas

James Malamatinas
  • Basic Members
  • PipPipPip
  • 151 posts
  • 2nd Assistant Camera
  • London

Posted 27 April 2018 - 04:31 AM

A super clear explaination - thanks!


  • 0



Also tagged with one or more of these keywords: doubler, focal lenght

The Slider

Technodolly

Abel Cine

Willys Widgets

Tai Audio

Rig Wheels Passport

Aerial Filmworks

Wooden Camera

Visual Products

Broadcast Solutions Inc

Gamma Ray Digital Inc

CineLab

FJS International, LLC

rebotnix Technologies

Paralinx LLC

Media Blackout - Custom Cables and AKS

New Pro Video - New and Used Equipment

Glidecam

Ritter Battery

CineTape

Metropolis Post

Aerial Filmworks

Technodolly

CineTape

The Slider

Visual Products

Media Blackout - Custom Cables and AKS

CineLab

Abel Cine

Gamma Ray Digital Inc

Wooden Camera

Tai Audio

Broadcast Solutions Inc

FJS International, LLC

Metropolis Post

Ritter Battery

Rig Wheels Passport

Paralinx LLC

Willys Widgets

rebotnix Technologies

New Pro Video - New and Used Equipment

Glidecam