As discussed in this post the design of lens attachments for cell phone cameras present particular challenges to optical and opto mechanical engineers. One common market goal is to decrease the minimum focus distance of a smartphone and in this post we will discuss key considerations.
Most lens attachments are afocal, this means parallel light rays entering an afocal lens are also parallel when they exit; however, the diameter of the exiting parallel light rays may be different from the diameter of the entering light rays. Thus, an afocal lens does not have focal length, or, its focal point is at infinity. The main purpose of an afocal lens is to modify system focal lengths and so is generally used with other prime lenses.
The operation of a macro lens attachment when the native lens is focused at infinity is presented in the next picture.
The magnification expected from a native camera phone lens and lens attachment can be estimated with the simple equations:
and the magnification can be estimated as
Using of smartphone autofocus we can some make small changes working distance (+- 3-5 mm)
Since the cameras inside the Iphone, Samsung, HTC and others have similar properties a single lens attachment can usually be designed for multiple phones.
Key optical parameters of smart phone camera:
· Focal length - 3.5-4.5 mm
· F-number - F1.8-2.5
· Minimum focus distance 70-80 mm
· Pupil location is inside of camera lens
· Distortion - less than 2 %
· Resolution - 300-350 lp/mm (usually limited by imaging sensor)
· Number of elements - 4-5 plastic aspherical lenses
Typical design goal:
· Focus 2-5mm (microscopic applications)
· Visual magnification (250/f) -20-25X (microscopic applications)
· Focus 10- 20 mm (macro applications)
· Visual magnification (250/f) - 10-15X (macro applications)
· Maintain resolution, chromatic aberrations, spherical aberrationsfound in native camera phone.
· Lens must be an afocal. This allows the lens attachment to be used with many type of smartphone cameras. The mounting to smartphone body should provide correct position of lens attachment relative to pupil position of camera lens.
· Lens usually has 2-3 elements and be as small as possible to simplify optical mechanical design and attachment challenges.
· Different types of glass should be used to maintain chromatic correction and reduce tooling cost
In the picture below you can see effect of using a well-designed macro lens attachment. The right side photo is not in the focus because focusing range of camera is not enough. Left photo was made with macro lens attachment.
You can find these design principals in products from iPro(Shneider optics), Photojojo, Moment lens and Olloclip. All offer well designed lenses with high image quality. The lens attachments have comparable optical parameters as magnification, distortion, chromatic and spherical aberrations.