Why does a subject look larger on a smaller sensor with the same lens?
Asked 11/17/2022
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2 answers
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I’m trying to understand sensor size and crop factor. If I use the same lens at the same focal length on full frame and on a smaller sensor, my understanding is that the lens projects the same image either way, and the smaller sensor just captures a smaller crop from the center.
If that’s true, why do comparison diagrams often show the subject looking bigger on the smaller sensor? Shouldn’t the subject be the same size on the sensor image, with only the frame cropped tighter? Is the apparent increase in size caused by higher pixel density, or is it simply because the smaller-sensor image is enlarged more for display or print?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
3y ago
2 Answers
4
It all about the various levels/forms of magnification/resolution...
There is the lens' magnification/resolution... that determines how large the subject is projected onto the sensor, and how much detail/resolution the sensor might record.
There is the sensor magnification/resolution (pixel density)... that determines the size the subject is displayed at when enlarged to a certain degree (e.g. 100%), and the maximum amount of the available (lens) resolution recorded.
And then there is the print/display magnification; which when combined with the viewing distance determines the relative magnification, visible details/resolution, and the depth of field (relative sharpness of details).
The reason the smaller sensor resulted in a larger subject in that example is because they were output at the same print/display size/physical dimensions; which in-itself is independent of the pixel density... but the pixel density will/can still have a significant impact on the visible detail.
Originally by user70370. Source · Licensed CC BY-SA 4.0
user70370
3y ago
0
Generated from our catalog & community — verify before relying on it.
With the same lens, same focal length, and same camera position, the lens projects the same image regardless of sensor size. A smaller sensor does not increase optical magnification—it just records a smaller central crop of that image.
So on the sensor, the deer is not inherently “bigger” because of crop factor. It only looks bigger in many examples because the smaller-sensor image is enlarged more to be shown or printed at the same final size as the full-frame image. Since the frame is tighter, the subject occupies more of the displayed image.
Pixel density is a separate issue. Higher pixel density can let the camera record more detail from that cropped area, but it is not what creates the crop-factor effect itself. The main point is:
- same lens + same position = same projected image
- smaller sensor = narrower field of view (center crop)
- same output size = smaller-sensor image must be enlarged more, making the subject appear larger in the final display
So your understanding is basically correct: crop factor changes framing, not the lens’s magnification.
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