# Why does a fisheye add-on lens for smartphones need refocusing when a wide-angle adapter doesn't?

I'm playing with some low-cost clip-on fisheye lens for phones:

When I clip the fisheye over the phone, the camera needs refocusing. But the 0.67x wide-angle + macro combo doesn't. What's the theory that fisheye needs refocusing? Should refocusing be needed usually?

The camera lens gathers light rays from outside world and projects these rays so an image forms on the surface of a digital sensor chip at the rear of the camera. If the lens is too close or too far away from the chip, the image will be blurred. The distance lens to chip is a variable based on the closeness or farness of the object being photographed. If critical focus is to be achieved, the lens must be repositioned base on subject distance.

The cellphones we carry sport super tiny cameras. We fit lenses to cameras based on the size of the imaging chip. A super tiny imaging chip such as the type used in cellphones calls for camera lens that is approximately 3 millimeters in focal length. That’s short, compare to a film camera that typically spots a 50mm lens. The interesting thing about short lenses is, a short focus lens is less critical as to focus distance as compared to its larger kinsfolks.

If you mount an additional lens atop a “normal” camera lens, the power of the optical system is altered. This results in a change is the angle-of-view. A negative powered supplemental lens delivers a wider angle whereas a positive power supplemental allows closer focusing than usual.

Your wide-angle supplemental lens is labeled 0.67. This reveals its multiplying factor. We multiply the exiting focal length by 0.67 to discover the power of the combination. The math is 3 X 0.67 = 2mm. Thus revised focal length is 1mm shorter and the result is a wider angle of view. Add a “macro” to this and you again alter the focal length. How much does the “macro” alter this array? Not much! The typical “supplemental” close-up lens is a +3 as to power. That will change the power of the array by only a fraction of a millimeter.

What about a “fish-eye” supplemental? This is a special design. It works like looking through binoculars backward. Add this backwards telephoto to your cellphone and the focal length is impressively changed to give about a 180° field of view. Such a modification requires a radical repositioning of the lens to chip distance.

• Thanks for the explanation. With fish-eye, is it that the multiplying factor is much lower, like 0.4x, so the resulting focal length of 1.2mm (3 * 0.4) is much more significantly shorter? Does that mean phone sensors can cope with a 1mm difference (0.67X) but not 2.8mm (0.4X)? – John M. Mar 16 '17 at 4:40
• What I know about cellphone sensors is a thimble full compared to the Pacific Ocean. -- Sorry about that! – Alan Marcus Mar 16 '17 at 4:46

Because of how wide the fish eye lens is, the alignment with respect to the camera's existing lens will make a big difference to the focussing required.

My guess is that positioning the lens consistently enough to use the same focussing every time is more difficult than fine tuning the focussing after placement. The same effect will occur with other lenses but to a much lesser extent (so they don't need adjustment)

• But what's special about the fisheye that the "default" focus cannot cope? The wide-angle + micro is fine though. – John M. Mar 14 '17 at 12:03
• @JohnMunroe the fish eye lens is simply more extreme, on the wide angle lens, the effect is not strong enough to matter. – Harry Harrison Mar 15 '17 at 9:50