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Cropping isn't the same as capturing at a different focal length.
by fredwu 2y ago
Cropping isn't the same as capturing at a different focal length.
- wvbdmp 2y agoIsn’t it optically? Ignoring lens imperfections and assuming infinite resolution, you should get the same image cropping vs. equivalent focal length, no?
- tobyhinloopen 2y agoI think it does, yes. Cropping 25% of the center of a 35mm F/2.0, you'd get the equivalent of a 70mm F/4.0, but with only 25% of the pixels obviously.
- GiovanniP 2y agoI expect depth of focus to be different.
- Kubuxu 2y agoYes, depth of focus will be larger, as signified by the larger f-number.
- tobyhinloopen 2y agoIt will not, I specifically included the F-stops for that reason. The depth of field is determined by the focus distance and the aperture of the lens. Both remain unchanged. Note that 35mm F/2.0 is the same aperture as 70mm F/4.0. Both lenses have an aperture of 17.5mm. (35/2.0 == 70/4.0) You can easily verify this with your favorite zoom lens. If you have an 24-70 F/2.8 available to you, you can verify by taking 2 pictures; one at 35mm F/2.8 and one at 70mm F/5.6. Crop the 35mm one to 25% area (half the width, half the height). Render both images to the same size (print, fill screen, whatever) and see for yourself.
- GiovanniP 2y ago> assuming infinite resolution this is an assumption that goes against the concept of "f-number" so if one does it, they should not expect to get to anything sensible.
- wvbdmp 2y agoI just meant sensor pixels, because you’re obviously losing those when cropping, but you get the same perspective as from larger focal length (since you’re not moving).
- GiovanniP 2y agoI agree that the images correspond to the same region in object space. Further assumptions on optical resolution don't work well, as the optical resolution depends on the f-number.
- Anotheroneagain 2y agoThe angular resolution depends purely on the aperture diameter, not the f-number. There should be no difference between capturing the image in high resolution, and blowing it up for a lower resolution sensor. All that should be needed is a 200mpx sensor that can output the entire frame in 12mpx, and 12mpx of the central area in full resolution. It's similar to how our eyes work.
- ratmeadow 2y agoI think it's not the same. Changing focal length changes the perspective warping, right? That's why fisheye lenses look crazy, and telephoto lenses "compress" depth. This might be a function of the sensor geometry too, though.
- john2x 2y agoCropping the centre of a fisheye photo will look the same as a normal or telephoto lens if they are taken at the same distance (the crop will have less resolution of course)
- ratmeadow 2y agoAfter looking it up, yes you are right, they are the same. I was thinking of changing the distance to subject instead.
- Anotheroneagain 2y agoFisheye lenses look crazy because they are deliberately made that way. Rectilinear lenses don't do it.