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Topic: Looks like I still need a scope lens
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Martin Jones
Phenomenal Film Handler
Posts: 1269
From: Thetford , Norfolk,England
Registered: May 2008
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posted March 25, 2013 12:00 PM
Not strictly true, Joe... the distance between the front and rear elements changes slightly.
The purpose of the adjustment is to compensate for the two DIFFERENT Focal Lengths of the Camera Lens/Anamorphic combination when SHOOTING film...
When an Anamorphic is added to a camera lens (taking a 50mm lens used with a 2X Scope lens as an example) the combination has a Focal Length of 50mm in the vertical plane, but only 25mm in the horizontal plane. This is not a problem at small apertures as the "Depth of Field" of BOTH Focal Lengths is sufficient to give good focus at most distances. But as the aperture of the camera lens is increased, objects at some distances may go "out of focus" in one axis only, depending on where the Camera Lens is focused. This give a strange "fuzzy" effect. The solution is to vary the distance between the elements, which actually varies the "squeeze ratio" slightly (and hence the "horizontal Focal Length"), thus tending to compensate for the loss of depth of field.
That's a crude description, but then it is a crude method of dealing with the problem.
An anamorphic is actually a Galilean Telescope (Opera Glass) in one axis only, and because it uses negative and positive lenses in the same ratio as the "Squeeze", separated by the sum of their focal lengths, it is AFOCAL (i.e. it does not affect Focus in any way). As soon as you vary the distance between the elements, this relationship no longer holds, and focus is affected DIFFERENTLY in the two different axes! So such adjustment is(strictly)a bodge. But it does "work" to an adequate degree.
There is a different requirement for Projection. Because there is only one Object Plane (the Film) and one Image Plane (the Screen), there is NO "depth of Field" problem and (contrary to popular belief) there should be no need to re-focus the Anamorphic for different screen distances: the Infinity setting should give the correct distance between elements, and the correct "Squeeze".In fact, Anamorphics such as the Iscomorphot, which was sold for "projection use only" do not have an adjustment.My own home made anamorphic (now about 58 years old!), which consists of two simple elements, gives good focus at all screen distances WITHOUT adjustment. Likewise the Iscomorphot that I used to own.When I started to use my home made lens for filming I had to build an adjustment facility in to obtain good results at low light levels. Also, I found that in high brightness conditions my commercial Magnarama (1.75:1)gave acceptable focus on the same subject over a wide range of distance settings, and I use the same ("infinity")setting at all screen distances when projecting.
[ March 25, 2013, 02:53 PM: Message edited by: Martin Jones ]
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Retired TV Service Engineer
Ongoing interest in Telecine....
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