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As we all know, we will get a cropped image when we use the same lens on FF and APS-C. The image is kind of zoomed-in. THe image quality of the two images would depends on the sensor quality.

If I now managed to attach a DSLR lens to get a focused image on an iPhone sensor (which is very small with high pixel density), will I get an astonishing zoomed image with high resolution?

  • Or said more simply; if an iPhone sensor could be mounted in a DSLR what would be the crop ratio for X lens? (50mm?) – Alaska Man Oct 20 '20 at 2:11
  • Crop factor is (diagonal of the mentioned sensor) / (diagonal of 35 mm film). It does not otherwise matter which physical body or which lens. – WayneF Oct 20 '20 at 2:47
  • yea, can put it like that. And I wonder can we 3D print some kind of an adapter for mobile phones, so that we can get a super tele lens with a relatively cheap kit DSLR lens. – Ricky Oct 20 '20 at 2:48
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    Are you first going to remove the iPhone's built-in lens? – Michael C Oct 20 '20 at 4:49
  • @MichaelC, for convenience, I guess we shouldn't remove the built-in lens, but is it possible to get a better image in this setting? – Ricky Oct 20 '20 at 6:07
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The iPhones have sensors with about a 6 mm diagonal(depending on the model). A typical full frame camera has a diagonal of about 43 mm, so the crop factor is about 7. That means that the lens acts like a lens of 7 times the focal length in terms of field of view. A standard 50mm DSLR lens becomes a 350mm lens in terms of field of view. It is like you take a picture with a full frame DSLR and crop it to 1/7 the size in each direction. You have about as many pixels as the DSLR, but there is blurring due to diffraction because the pixels are so small.

I have a Nikon P900 with a slightly larger sensor, so the crop factor is 6. The lens goes out to an actual 357.5 mm focal length, which is equivalent to 2000mm on a full frame 35mm. I also have a Canon 7D with the 100-400mm zoom, which has an effective focal length of 640mm. I find (YMMV) that I prefer the Canon shots when the right focal length is about 1300mm or less because the larger pixels and less zoom range make a sharper image, even cropped by a factor 2 or so. When the target gets smaller than that, I prefer the Nikon with its higher pixel count. The Nikon will not stop down smaller than f/9.5 because of diffraction, so don't ask it to give you much depth of field.

The arithmetic in this answer is approximate, but accurate enough for the subject at hand.

  • Thank you for your answer. You mentioned there is blurring. I wonder how much blur will we get? The DSLR lens should be much better than the one one iphone (i guess) and I just wonder will we get a very good photo in such setting. Thanks again for your explanation! – Ricky Oct 20 '20 at 2:55
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    It is not about the DSLR lens vs the iPhone lens. When light passes through an aperture there is diffraction. The smaller the aperture, the more the diffraction. When the diffraction moves too much light from one pixel to another the image blurs. The larger pixels of a larger sensor are less sensitive to this. – Ross Millikan Oct 20 '20 at 3:00
  • How does leaving the iPhone lens between the external lens and the iPhone sensor affect this answer? – Michael C Oct 20 '20 at 17:51
  • If you leave the iPhone lens in place, you need to consider the effective focal length of the lens system, not just the focal length of the DSLR lens. Without the iPhone lens you have to mount the DSLR lens with the mounting flange some 43mm from the sensor so the image is focused. I don't know how to figure out what the focal length or positioning of the DSLR lens needs to be if the iPhone lens is left in. – Ross Millikan Oct 20 '20 at 19:54
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The new imagined lens would appear zoomed (with say a 28 mm lens instead of 4 mm, which would be around 7x zoom, which is a real actual zoom by the longer lens). And it will be a lot of pixels, but it will still be a very small "cropped" image of tiny area, with crop factor of about 6x. So it would have to be greatly enlarged about 6x more just to view it at the same regular viewing size as full frame would need.

And enlarging 6x more reduces the viewing dpi to 1/6 dpi. The full frame image would have to be enlarged about 9x to view 8x10 inch size, so this tiny sensor has to be enlarged about 9x6 = 54x to view 8x10 inch size at 1/6 the dpi.

If using the same lens, the APS-C "appears" zoomed compared to the full frame image, but it is just an illusion. The lens image (from the same lens) is of course exactly the same image (just cropped smaller). It only appears zoomed after it has to be enlarged 1.5 or 1.6x more to view it at the same viewing size as the full frame. This extra enlargement fools us and the smaller camera costs less, and we like it, but the full frame image outperforms it, which is why you see the pros at the football sidelines using very long lenses on full frame bodies.

You can also simply zoom any existing image in the photo editor to see the same enlargement illusion, but enlargement costs dpi.

  • Thank you for your answer, WayneF. – Ricky Oct 20 '20 at 6:28
  • How does leaving the iPhone lens between the external lens and the iPhone sensor affect this answer? – Michael C Oct 20 '20 at 17:47
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Theoretically, you would get a 7x more zoomed image,

But lenses have a limited resolution and the resolution of a given generation of DSLR lenses more or less matches the resolution of the same generation of DSLR sensors. And being 7x denser your iPhone sensor would require lenses with 7x more resolution. So in practice, for the same final quality, and with ideal lighting conditions, the iPhone would not produce much bigger images than the DLSR the lens is intend for.

  • How does leaving the iPhone lens between the external lens and the iPhone sensor affect this answer? – Michael C Oct 20 '20 at 17:47
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Don't do this. As already stated, you get about a crop factor of 7 on the lens. That means that the lens provides about 50 times the image area than your sensor actually utilised. The typical smartphone camera these days has more than 20MP so your DSLR lens has to produce an image with a resolution corresponding to about a 1000MP full-frame camera even though only a tiny fraction of the image circle it projects on will actually get used. All the rest of the image circle's coverage for which the camera has corrective elements and glass of non-trivial thickness will be wasted.

So the results will be comparatively lack-lustre if you have an outstanding DSLR lens, and bordering on awful if you have an average one.

  • How does leaving the iPhone lens between the external lens and the iPhone sensor affect this answer? – Michael C Oct 20 '20 at 17:47
  • @MichaelC Well, the premise was that you get the contraption to focus. "Straightforwardly" you'd need to get closer than the flange distance for that (or you get an upside-down image, and making the focusing distances combine reasonably would be close to impossible). Practically that would mainly be relevant for closeup/macro shots where you'd likely use the DSLR lens in reverse, focused to infinity, and put the object in question at flange distance. For "real" shots, you'd rather use a tele converter. Either way, the image circle size mismatch means that a lot of glass is pointless. – user95069 Oct 20 '20 at 18:13

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