Doug Kerr
Well-known member
We often hear it said, in the case of a lens intended to support a certain frame size but used on a camera with a smaller frame size (such as a Canon EF lens on a Canon EOS 7D body), that we may enjoy improved performance of the lens "since only the center of the lens is used". That statement is wholly incorrect.
I will use this figure to discuss this matter.
It shows the famous fictional "thin lens", and since it is fictional anyway, we have shown the aperture stop being right inside it. (Thus we need not deal separately with the entrance pupil and the exit pupil.)
Each row of panes of the figure shows a different camera and lens "setup".
We assume that our cameras regard three "point" objects, "X", "Y", and "Z". Object X is on the lens axis, Y off the axis by a bit, and Z farther off the axis. We will look at the rays from each of these objects in separate panes in the row.
In the top row, A, we have our larger sensor and the lens at its maximum aperture. We follow five rays from the object through the lens until they converge to form a point image on the sensor. This works fine for all three objects.
Note that in this situation, rays from any of the objects pass all portions entire lens.
In the next row, B, we have gone to a camera with a smaller frame size (smaller sensor), but still have the same lens, still operating at its maximum aperture. For the objects X and Y, everything works fine, but the rays from object Z do not form a point image on the sensor but rather alongside it (on the sensor mask). In other words, object Z is out of the field of view of this lens with this frame size - it will not be seen in the image. Its rays are shown dotted to remind us that they are wholly ineffectual.
Note that there is no notion here of "only using the center of the lens". Rays from all the objects still pass through the entire lens.
In row C, we have gone back to our original camera, with the larger sensor, but have now stopped the camera aperture down quite a bit. We now see that the rays from the object that would have passed through the outer reaches of the lens no longer do so, being blocked by the aperture stop. We see that this effect is the same for all three objects.
The blocked rays, before they reach the aperture, are shown dashed as a reminder that they are doomed.
Now here we could rightly talk about "only using the center of the lens".
In row D, we have the camera with the smaller sensor and as well have stopped down the lens aperture. We see that, as before, rays that would have passed through the outer reaches of the aperture no longer do so, being blocked by the aperture stop. Then, in the case of the object Z only, the rays that do pass through the lens land off the sensor (since of course that object is out of the field of view of this lens with this sensor).
Best regards,
Doug
I will use this figure to discuss this matter.
It shows the famous fictional "thin lens", and since it is fictional anyway, we have shown the aperture stop being right inside it. (Thus we need not deal separately with the entrance pupil and the exit pupil.)
Each row of panes of the figure shows a different camera and lens "setup".
We assume that our cameras regard three "point" objects, "X", "Y", and "Z". Object X is on the lens axis, Y off the axis by a bit, and Z farther off the axis. We will look at the rays from each of these objects in separate panes in the row.
In the top row, A, we have our larger sensor and the lens at its maximum aperture. We follow five rays from the object through the lens until they converge to form a point image on the sensor. This works fine for all three objects.
Note that in this situation, rays from any of the objects pass all portions entire lens.
In the next row, B, we have gone to a camera with a smaller frame size (smaller sensor), but still have the same lens, still operating at its maximum aperture. For the objects X and Y, everything works fine, but the rays from object Z do not form a point image on the sensor but rather alongside it (on the sensor mask). In other words, object Z is out of the field of view of this lens with this frame size - it will not be seen in the image. Its rays are shown dotted to remind us that they are wholly ineffectual.
Note that there is no notion here of "only using the center of the lens". Rays from all the objects still pass through the entire lens.
In row C, we have gone back to our original camera, with the larger sensor, but have now stopped the camera aperture down quite a bit. We now see that the rays from the object that would have passed through the outer reaches of the lens no longer do so, being blocked by the aperture stop. We see that this effect is the same for all three objects.
The blocked rays, before they reach the aperture, are shown dashed as a reminder that they are doomed.
Now here we could rightly talk about "only using the center of the lens".
In row D, we have the camera with the smaller sensor and as well have stopped down the lens aperture. We see that, as before, rays that would have passed through the outer reaches of the aperture no longer do so, being blocked by the aperture stop. Then, in the case of the object Z only, the rays that do pass through the lens land off the sensor (since of course that object is out of the field of view of this lens with this sensor).
Best regards,
Doug
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