Can a full-frame lens be made effectively faster on a smaller sensor by shrinking its image circle?
Asked 8/25/2017
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If I use a lens designed for a larger format on a much smaller sensor, I know that at the same f-number the sensor receives the same light per unit area. But what if I add optics to compress the larger image circle down to the smaller sensor area? Would that make the image brighter on the small sensor, and if so, how does that affect focal length, f-number, field of view, and depth of field?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
8y ago
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Assuming I understand what you're saying, the answer is no. Or, depending on how you look at it, yes, but. The optics you use to squeeze that full-frame image into the smaller size are effectively reducing the focal length of the lens. Let's say that your crop sensor is ⅔ the (linear) size of your larger sensor, like the difference between APC-S and full-frame.
And, let's say you have a 50mm f/2 lens. Wide open, that means the apparent aperture is 25mm — because 50mm/25mm = 2.
Your projection effectively creates a 33mm lens with a smaller image circle (think of it from a field of view perspective: by shrinking the image circle, you're recording that much more of the scene, so, wider angle). But now, your f-number is 33mm/25mm, or about f/1.3.
So, yeah, you get a lot more light in. But you do it by converting the lens to effectively have a smaller f-number.
This is exactly what the Metabones Speedbooster does. Read more about it here: How can a speedbooster improve the light performance of a lens?
Originally by user1943. Source · Licensed CC BY-SA 4.0
user1943
8y ago
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Generated from our catalog & community — verify before relying on it.
Yes — but only because the added optics effectively change the lens. A plain larger-format lens used on a smaller sensor does not give more light per unit area at the same f-number. The small sensor just crops the image.
If you add optics that compress the large image circle onto the smaller sensor, you are using a focal reducer (the opposite of a teleconverter). That concentrates the same light into a smaller area, so the image becomes brighter. But this also reduces the effective focal length by the same factor, which lowers the effective f-number by that factor as well.
So a 50mm f/2 lens with a strong reducer might behave more like a shorter, faster lens — for example something closer to 33mm f/1.3 if reduced by 0.67×. You gain brightness and a wider field of view, but it is no longer meaningfully the “same lens at f/2.”
Depth of field and perspective follow the new effective focal length and framing. Image quality also depends on the reducer’s quality. This is a common idea in astrophotography.
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