Does a 50mm lens become less 'fast' on APS-C if the aperture is larger than the sensor?
Asked 7/27/2010
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I’m comparing a 50mm f/1.7 and a 50mm f/1.4 used on an APS-C camera. Since a 50mm f/1.7 has an entrance pupil around 29.4mm wide, which is larger than the APS-C sensor’s diagonal, does that mean the sensor is already getting all the light the lens can deliver? Or is a 50mm f/1.4 still meaningfully faster than f/1.7 on APS-C?
More broadly, does sensor size change a lens’s maximum aperture, or is that only related to image circle and vignetting?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
16y ago
2 Answers
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f1.4 will always be 2/3rds stops faster than f1.8.
The diameter has nothing to do with whether or not part of the sensor is hidden. That is a separate measurement referred to as vignetting, and not the image circle's light level. The image circle's light level/brightness is directly affected by the aperture of the lens design.
FF lens simply means the image circle is designed to cover a full frame sensor (which can be film). Using it on a APS-C will be using the inner part of the image circlce.
An APS-C lens of the same focal length and speed could have been created at a smaller size the image circle does not need to be as large, but the lens would need to be redesigned.
Also, note that the Pentax 50mm f1.7 (if this is what you have) is generally regarded as sharper and/or more contrasty than the Pentax 50mm f1.4 at common apertures up until f2.8 or so.
Pentax 50mm f1.4 advantages include one third f-stop faster, rounder aperture blades for rounder highlight bokeh only when stopped down. It may or may not have "better" and smoother bokeh as that is not simply a function of aperture blades and I have not seen any other comparisons.
Originally by user389. Source · Licensed CC BY-SA 4.0
user389
16y ago
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A 50mm f/1.4 is still faster on APS-C. Sensor size does not change the lens’s f-number or light intensity at the sensor plane.
The f-number describes how bright the image formed by the lens is, independent of whether you record the center with APS-C or the full image circle with full frame. APS-C is just using a smaller central portion of the lens’s image circle. That affects field of view, not the lens speed.
Whether the aperture diameter is larger than the sensor diagonal doesn’t mean the sensor is “fully exposed” in a way that cancels the difference between f/1.7 and f/1.4. That idea is separate from image circle coverage and vignetting. If the lens covers the sensor, the f/1.4 lens still delivers a brighter image than the f/1.7 lens.
In practice, f/1.4 is about 2/3 stop faster than f/1.7/1.8, regardless of APS-C or full frame. Sensor size mainly changes how much of the image circle you use. Depth of field changes only when you change framing or distance; simply using a smaller sensor is like cropping the image.
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