Can a teleconverter make a smaller-format lens cover a full-frame sensor?
Asked 9/16/2015
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I’m using a lens designed for a much smaller format on a full-frame camera and seeing heavy vignetting. In my case it’s a 1/2-inch C-mount lens. If I add a 2x teleconverter, will it enlarge the image circle enough to cover the full-frame sensor, or reduce the vignetting?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
2 Answers
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Your basic assumption about teleconverters is right. But you haven't done the math:
1/2" is 6.4mm x 4.8mm—doubled is still only 12.8mm x 9.6mm.
OTOH it's not unheard-of for tiny format lenses to have image circles well larger than their specification.
Originally by user40265. Source · Licensed CC BY-SA 4.0
user40265
10y ago
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Generated from our catalog & community — verify before relying on it.
A teleconverter does increase the projected image size, so the basic idea is sound. But whether it will cover full frame depends on the starting image circle.
For a 1/2-inch format lens, the image area is about 6.4 × 4.8 mm. A 2x teleconverter would roughly double that to about 12.8 × 9.6 mm, which is still far smaller than a full-frame sensor. So in this case, no—a 2x teleconverter would not make a 1/2-inch C-mount lens cover full frame.
That said, some lenses project an image circle somewhat larger than their rated format, so coverage can occasionally be better than expected. But you should not count on a teleconverter alone to turn a very small-format lens into a full-frame lens.
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