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What Aperture is best for best focus?

Emin Henri Mahrt

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The design of your spherical lens, it's focal length and arrangement of element plus the anamorphic adapter will determine the widest aperture you can reasonably work at.


You said "century optix like" so I'll assume it's one of several "generic" anamorphic adapters that pop up every now and again.  These could be based on the Century Optics or Optex design (the coating looks the same) while internally being different enough in their calibration that what I say may be totally off but at 50mm I wouldn't expect you to get useful focus at anything wider than ~f/5.8 unless the Canon FD design is just more compatible with a front mount adapter than my 50mm F.Zuiko. 


Focal lengths wider than 50mm will allow larger apertures while focal lengths longer than 50mm will need to be further and further stopped down.  What I've been able to determine with mine is that it simply will not focus with a physical pupil size of much larger than 8.57mm (what you have with a 24mm lens @ f/2.8).  So at 50mm you achieve this pupil size @ f/5.8 and this is consistent with my 105mm f/2.5 Nikkor which forces me to nudge it just above ~f/12 for useful focus.


Diopters change these factors.  Something like a Tokina +0.4 achromat gives you about a stop wider performance while sharpening up the image at the expense of infinity focus.  SLR Magic has a new diopter set that will do the same thing (for a lot less, since the Tokina achromat is OOP and being sold as either used or very rare NOS).  


You didn't say you were using a diopter but if it came with the lens when you bought it you might have just been tempted to always use it.  This will limit your ability to focus beyond a certain point.  Also, some folks experimented with flipping the rear element on these adapters which, in some cases, sharpened them up at wider apertures but not without introducing limitations elsewhere.  Buying a used adapter has always been risky but with so many DIY channels on Youtube and more enthusiasts than ever before using these adapters you do have to be mindful of what modifications might have been made by the previous owner.


Bottom line though, you'll have to determine the limits of your anamorphic adapter foreach and every lens you intend to use it on.  The big, expensive anamorphic cine lenses are expensive because the manufacturer goes through and calibrates a dedicated anamorphic optic for each focal length.  That's how they're able to offer a line of lenses that are all rated the same.  When you're using dedicated cine lenses you can work the way a film crew does and have a stop that you use for all or most of a film, lighting to that stop and a fixed ISO.  


Folks using anamorphic add-ons must adapt their shooting and lighting to work within the limitations of every adapter + lens combination.  Each time you swap taking lens you are dealing with new minimums and maximums to be mindful of.  That's just one of many compromises we have to make since we're not spending tens of thousands of dollars per lens.

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