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The new iPhone 7 Plus has a dual camera system with one 28mm and one 56mm. Usually in smartphones where the camera is compact, you do not have real optical zoom. So how is the zoom feature, which gives a continuous zoom as could be seen in this video works.

Is this software interpolation between the two fixed lenses, or has the wide-angle 28mm lense some optical zooming capabilites? Also that we might have distinct distortion between the telelense and the zoomed wide-angle lense?

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First of all: those objectives are fixed-focal.

One objective being 56mm means that it does not see anything around the frame what 28mm objective sees. You cannot interpolate between something known and something unknown, it is not interpolation.

There is only one convenient way in which this can be implemented:

  • just one objective is used to record images at 28mm (or may be slightly past 28mm) or 56mm (those are 135-equivalents BTW)
  • each image in-between has interpolated 28mm frame data (one may say "digitally zoomed") with 56mm frame data mixed into the central part to improve resolution of central part of the image in such a way that a resolution increase is not apparent at transition from wide objective data to tele objective data
  • the resolution of output image being the same even though it does not mean anything at all: the outer part is interpolated to match one resolution throughout the range and the inner part is just partially mixed in to improve details

It is same interpolation which is used for digital zooming but it improves the central part of the image using "tele" objective.

Regarding the distortion: of course Apple would make it seamless too, it is a part of the trick. I hope that you do not mistake perspective as distortion, those are different concepts: perspective change results from shooting point change, geometric distortion is a property of optics. Perspective is exactly the same with any objective given that the shooting point is the same while distortion depends on the optics.

Usually the cropped central part of image made with some objective does not feature more distortions than image made with longer objective - the most distorted part of images is left outside of cropped area. I.e. second objective most probably does not improve distortion itself, it just improves detalisation.

Also geometry corrections are implemented in wide range of cameras for years already, so this means there are no real reasons for 28mm to exhibit more distortion at all. I do not know whether iPhone corrects distortion though.

  • Yes, something like that I meant by interpolation, thanks for making this more specific. But that exactly the point you are asking in your comment. A 28mm has another distortion as a 56mm (in the last the image is "flatter" an has less distortation, that also why its prefered for portrait), but when I digitally zoom in with the 28mm the distortion would be still the same, or said differently if I take the 56mm frame and put a zoomed 28mm frame over it, the closer I am to the center the more the images match [...] – StefanH Feb 21 '17 at 18:07
  • but the closer I get to the border the 28mm has some (more) distortion to than the 56mm picture. Is it understandable what I mean? – StefanH Feb 21 '17 at 18:07
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    Again, it doen't, it is the distance. Maybe this can make it clearer: photo.stackexchange.com/a/85528/32110 – ths Feb 21 '17 at 21:46
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    @StefanH In your link the camera was moved back each time the focal length was increased.That is what changes the perspective. If you take a photo with the 20mm lens from the same distance as was used for the 200mm lens and crop the wider image to match the same FoV the perspective will be identical. – Michael C Feb 21 '17 at 23:43

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