As discussed in previous post on smartphone attachment lenses, users of iPhone, Galaxy, HTC One, Nexus 5, and others smartphones cannot interchange lenses to alter focal length like with a SLR camera.
However, it is possible to vary the focal length with afocal attachments fitted in front of the native lens. This is a bit like using a fixed lens camera to take photos through a telescope eyepiece. Afocal attachments of the telescopic type are created by the separation of a pair of lens elements by a distance numerically equal to the sum of their focal lengths. This modifies the principal planes to infinity, i.e. the device has no focal length. Incident parallel light emerges parallel but the diameter of the beam is changed (see picture).
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The magnification in this arrangement is given by
m = f1 / f2 = feff / fcamera
when the camera is focused at infinity. For closer distances, the actual magnification from the image size on the sensor depends on the focal length of the camera lens. The main problem of front lens attachments is that these lenses greatly increase the minimum focusing distance.
Using this method, it is possible to create wide angle lens attachments for unique image capture. Typical requirements include:
- Magnification – 0.6-0.7X (0.4-0.5X for super wide angle)
- Resolution (center) – 250-300 lp/mm (close to resolution of native lens)
- Resolution (corner) – 200-250 lp/mm
- Distortion – < 1%
Below you can see how a lens with these parameters changes the captured image.
No lens vs wide angle lens
It is also possible to use this design method to create a telephoto attachment. Typical requirements include:
- Magnification – 2-2.5X
- Resolution (center) – 250-300 lp/mm (close to resolution of native lens)
- Resolution (corner) – 100-150 lp/mm
- Distortion – < 1%
Design result:
As opposed to a wide angle lens, the telephoto lens attachment decreases the smartphone camera field of view (FOV) presenting a magnified image. This lens also differs from a macro lens attachment because of its long focusing distance and is used in a wide range of working distances.
No lens vs telephoto lens