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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?
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.
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.
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.
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.
