Anamorphic lenses: FOCUS THROW INDEX

With my method of combining a taking lens with an anamorphic adapter in order to create a single focus anamorphic lens, there needs to be a way of defining whether a particular adapter is suited to a particular taking lens.


In order to do this I take the measurement of how much the anamorphic adapter increases in length when focusing it from 1.5m to infinity.  This range is chosen as it represents the typical range of an adapter designed for 16mm film projection (these are the most suited and commonly available adapters).

I call this the focus throw index, because no-one appears to have used this measurement before, so I could call it anything I wanted.  Unfortunately this also means that you cannot find these values online.  I will change that shortly in case anyone ever wants to know this stuff.

In order to check compatibility, we measure the same value on the taking lens.  It is a reverse value, only because the taking lens will get shorter as it gets focused from 1.5m to infinity, whereas the anamorphic adapter gets longer.  This is of course exactly what we want, because the entire success of building the single focus rig is that based on the combination of the two lenses remaining the same length during focusing.  


If the indexes match, then that combination might indeed work.  If they don't match then the taking lens cannot fully operate the anamorphic's focusing and the system won't work as a single focus solution.


Just how accurate the index needs to be matched is an interesting question.  In practical terms I have found it sufficient to measure the focus throw index to the nearest millimetre, so a ruler is usually of sufficient accuracy to determine the index.  Slight discrepancies will mean that your rig will go slightly out of focus towards one or other ends of the focus range, and this is arguably still quite a useful rig in practical terms.  And stopping down will always compensate and increase that range.  And fortunately the majority of lenses (both taking and anamorphic) appear to behave in a linear fashion during focus changes, which is lucky otherwise we would encounter zones of sharpness and non-sharpness as we racked focus with a combined rig.



I will measure the index of an adapter.  I will then find a lens that matches the value within about 1mm.  Then I attach an adapter to the lens and focus them on a close subject and measure the entire length of both lenses (or both lenses and camera, it doesn't matter for this part).  I then focus them on infinity and measure the entire length again.  If the measurements are the same (ie there is no change in overall length of the combined lenses), then the combination of that adapter and that lens is a great candidate for building into a single focus rig.


Most people don't have access to lots of different adapters and/or taking lenses.  The usual situation would be that you have one adapter, and by definition it would have one value for its focus throw index.  You could then look up the index for taking lenses and find one or more that match, and then set about finding one of those to purchase in order to build your single focus rig.

A list of values for different adapters will be included here over the next while.

© 2020 by mort'mer/mortmer/Bruce Mortimer