There is so much misinformation written about the legendary TR capacity of various lenses I thought I'd present a few basic points to consider.
The Isco can only do its theoretical
TR of 1.3 when its back element, which is approximately 98mm in diameter, and its front element (about 134mm in diameter) does not vignette the beam from the projector. A 98mm diameter circular back element equates to an approximately 42mm x 28mm wide area of usable glass. The rest of the glass, above and to the sides of the light path, is wasted.
There are several factors in determining the size of the image when it reaches the back element of an anamorphic lens.1. Mechanical clearance
A projector with a protruding lens that allows the anamorphic lens to be mounted closer to it, clear of any mechanical obstructions, will deliver a smaller beam to the back element of the anamorphic lens than an optically identical projector with a recessed projection lens or an elaborate case the protrudes in front of the lens.
In some cases, like some Sim2 Domino models, the first point of mechanical clearance is about 60mm (2.4") out from the projection lens. The beam will be pretty big at this point, the closest you can mount your anamorphic lens.
On the other hand, all the Panasonics and the JVC RS series have lenses that are jut out well in front of the case. You can mount your anamorphic lens as close as 15mm (0.6") from these projectors.2. Focal length and f-number value of the lens
Larger imaging chips (like 0.95" DLP) require longer focal length lenses than smaller imaging chips (like 0.6" LCD chips).
The gauge of brightness of a lens is its f
-number equals focal length divided by the lens's effective physical diameter at its smallest point.
2/100mm focal length lens has an effective aperture twice the size of an f
-2/50mm lens. Hence the beams of light repesenting all the rays from individual points on the imaging chip, close to the projector
(where the anamorphic lens is mounted), will be approximately twice as wide with the 100mm focal length lens as with the equivalent 50mm focal length lens. This is so even if the throw, screen size and TR set-ups are identical
between the two projectors you're comparing.
As these beams of light (or "ray pencils") eminate from every point on the imaging chip, the size of the image close to the projector will always be bigger for DLP projectors than it is for LCD projectors for any given mounting offset, as the ray pencils at this short distance will add to the size of the image at that same point. They all end up as focused points on the screen, yes, but close to the projector there are significant differences in image size between projector types. These differences directly affect the ability of the anamorphic lens to accomodate beam width.
So far, it boils down to this: an anamorphic lens that easily accommodates a TR of 1.3 when it is mounted 15mm from an LCD projector, may only be able to accommodate a TR of 2.0 when it is mounted 60mm from a DLP projector. So, to say that the Isco will accommodate a TR of 1.3 is not anywhere near the full story. The mounting offset and the projector type have to be taken into consideration. They are absolutely crucial considerations.
An anamorphic lens that works with a given TR with one projector may not work with the same TR using a different projector.
Hence, Throw Ratio itself is not the sole determinant of whether an anamorphic lens will work in any given situation. At best it is a rough indication.
Which brings us to...3. Beam angle
Beam angle is related to TR as well.
Even though the beam may fit the back element of the anamorphic lens, its angle (between opposite sides of the beam at its widest) may be too wide to make it through all the elements inside the lens
It's quite possible for the end element (the one closest to the screen) to be too narrow, even though the back element (the one closest to the projector) is wide enough to accommodate the beam, if the TR is very small. This is especially true with projectors that force the user to mount his or her anamorphic lens way out in front of the projection lens, due to mechanical obstruction, or elaborate "designer" cases.Conclusion
In determining whether an anamorphic lens will accommodate a particular beam you have to consider, at least
, back element size, internal element sizes, mounting offset of a particular projector, type of projector (DLP v. LCD) and lastly Throw Ratio.
In addition, the anamorphic lens may not work as well when mounted, say, 3" from the projector, as opposed to, say, 3/4" from the projector. It may have, probably would have, a sweet spot closer to the projection lens where its optics work best.
The Isco and Schneiders are hardly likely to be exceptions to this basic rule of optics. You can only design-in so much flexibility to a lens. Originally, anamorphic optics were mounted almost flush with the film projector, literally screwed onto the end of the projector's prime lens, in very close proximity to its optics.
With digital projectors, and users' set-ups having an almost infinite range of mechanical constraints, zoom ratios, varying f
-numbers through those zoom ratios, Throw Ratios, throw distances, beam angles and so on, it's impossible to design an anamorphic lens which will accommodate all of the many possibilities with equal performance quality.
And anyway, if you can afford an Isco, you should be able to afford a bigger HT than TR=1.3