As discussed in this post, the design of lens attachments for cell phone cameras presents particular challenges to optical and optomechanical engineers. One common market goal is to decrease the minimum focus distance of a smartphone. In this post, we will discuss key considerations.
Most lens attachments are afocal which means that parallel light rays entering an afocal lens are also parallel when they exit it. However, the diameter of the exiting parallel light rays may be different from that of the incident ones. An afocal lens does not have focal length; its focal point is at infinity. The main purpose of an afocal lens is to modify the system focal lengths and 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.
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The magnification expected from a native camera phone lens and lens attachment can be estimated with the simple equations:
1/fcombo=1/fnative+1/fattach
The magnification can be estimated as
m=fnative/fattach
Using smartphone autofocus, we can make small changes in the 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 smartphone cameras:
- Focal length – 3.5-4.5 mm
- F-number – F1.8-2.5
- Minimum focus distance 70-80 mm
- Pupil location is inside the 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 aberrations found in native camera phone.
Design considerations:
- Lens must be afocal. This allows the lens attachment to be used with many types of smartphone cameras. The mounting to the smartphone body should correctly position the lens attachment relative to the camera lens pupil position.
- Lens usually has 2-3 elements and can be as small as possible to simplify the optomechanical design and attachment challenges.
- Different types of glass should be used for chromatic correction and to reduce tooling cost
Design result:
In the picture below, you can see what a well-designed macro lens attachment can do. The right side photo is not in focus because the camera focusing range is insufficient. The left photo was taken with a 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 including magnification, distortion, chromatic, and spherical aberrations.