Found a Canon front mounted 1.4x converter in a thrift store. It is huge with a 67 mounting thread and about 100mm front lens element. Is it possible for a front mount design to offer similar quality to a rear converter? I'm waiting on adapter rings to try this out. Always thought rear converters were superior, but wonder why Canon would build such a beast.
Thanks,
barondla
Jan. 24, 2024
6
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There may possibly be exceptions, but unless the front-mounted unit is dedicated to a particular lens, a rear-mounted teleconverter will almost always be superior.
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They do not do exactly the same job. A rear teleconverter cannot change the size of the entrance pupil. A front teleconverter may increase the size of the entrance pupil substantially (depending on the lens) making a much faster lens.
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There is more difficulty designing a universal front-TC, in my experience, because I have tried a combo that was absolutely horrible; I bought a Canon one years ago for my Sony F707, because the threads matched, and it pretty much worked like a prism away from the center, the CA was so bad. I later bought a Sony one recommended by Sony for the camera, and it worked fine.
I'm not an optics expert, but my intuition tells me that rear TCs are dealing with light bundles that are already behaving like a theoretical thin lens, having already gone through all of the corrective elements and all the folding of light needed to effect focal lengths that are impossible with a theoretical thin lens, given the depth of the sensor chamber, and lenses that are physically shorter or longer than their focal lengths. So, in my intuitive "guestimation", there are far more main-lens optical wild cards for front-mount TCs, and they therefore need to be designed more specifically for specific lenses, and are not very interchangeable.
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Not exactly the same thing, but I bought a front-end wide-angle converter that was recommended by Sony for the F707, and it worked well except for one thing; dust on the front element of the adapter was in focus enough to be visible, even with the lens wide open!
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Good point. The Canon front mount converter has 1.4x magnification and the front element is about 1.4x larger than the rear glass. Unlike a rear mount converter there seems to be no light loss. A definite plus.
Thanks,
barondla -
If someone else points the main lens at you, and you can see the entrance pupil wide open, and then if they attach the 1.4x, if the pupil gets larger from your perspective, then the adapter will capture more total subject light, but exposure could still be lower with the same shutter speed (the f-number is larger) if the entrance pupil isn't at least 1.4x as wide.
The type of TC that goes between the body and the lens definitely increases the minimum f-ratio, but it does not shrink the entrance pupil, and therefore still gets the same total subject light (except the tiny losses from the extra glass).