Telephoto lenses for smartphones – Tech stated
In my former short article, I talked over the influence of creating glass components into cell phone camera lenses. Here I’ll look at an additional cellular phone lens conundrum, that of smartphone ‘telephoto’ lenses.
Yet again, we start off with the complete limit on a lens’s resolution capability, which is the diffraction restrict presented by the 5mm entrance pupil (aperture) diameter, which is dictated by the slimness prerequisites of a cellular phone. The angular resolution restrict of .008° is calculated from that entrance pupil and the wavelength of the visible light.
In observe, the smallest complete angle of watch from this kind of a lens is about 10° if we want to produce 1K pixel impression, regardless of sensor sizing. This corresponds to a ‘full-frame’ focal size of 200mm, which is usefully ‘long’, but not at any time going to pull in distant topics. This limitation is given fully by the aperture size of the lens.
So, let’s now appear at two various design solutions to delivering ‘200mm equivalent’ functionality in a mobile phone. The initially is just to use a quite tiny picture sensor guiding the lens. Pixel dimensions of around 1.3 microns can create quite superior results with contemporary technological innovation, and a 1K sensor so equipped would end up at about 1.3mm throughout, which in sensor sizing terminology is ‘type 1/10’. The focal length needed to make a 10° angle-of-check out on this sort of a sensor is 7.4mm, which could be equipped within the area boundaries offered. By the way, this lens would have an f-selection of 1.5, which is really possible for a effectively-corrected lens.
Answer two is to use a ‘periscope’ lens, in which a mirror or prism at the rear of the front factor(s) of the lens turns the rest of the lens by 90° so that it lies sideways in just the cell phone. Discover that it does not make it possible for for a larger entrance pupil, due to the fact its dimension is constrained by the prism/mirror, which in change is dictated by the slimness prerequisite. But the periscope style and design does permit a for a longer period lens construction, so a much larger sensor might be specified. Suppose a ‘type 1/2.4’ sensor, 6mm throughout, is to be utilised. To give 10° angle of look at a focal length of 34mm is demanded, which can be packed into the periscope tube. The lens will have an f-number of 6.8.
A periscope structure will allow a for a longer period lens within a slimness constraint. But there are other ways of providing a slender angle of watch, which might prove to be a much better general alternative.
In optical terms, neither resolution is much better than the other. Both equally acquire the exact sum of light from the identical angle of look at, and use the exact same angular sampling charge. Which is far better arrives down to useful considerations. The small sensor could have a dilemma with the restricted saturation capacities of its modest pixels limiting the use of extended publicity occasions. But that may possibly be mitigated by compositing many exposures. Without having likely via a structure cycle and evaluating the success, any dedication as to which would give superior results would be purely speculative.
It is having said that very likely that the compact digicam would be more cost-effective to manufacture and less complicated to combine into the cell phone, which is possibly why periscope-fashion cellphone lenses do not feel to have caught on.
In particular contemplating that there is a answer that outperforms them both equally.
The available entrance pupil of the lens, and the ensuing angular resolution, decided the pixel dimension of the tiny sensor. The sensor dimensions was confined by the 10° angle of watch. Putting the little pixels on a sensor substantial sufficient to give a ‘normal’ angle of check out with the lens would deliver a extra flexible option allowing both ‘telephoto’ and ‘normal’ options. Of system, these types of a sensor would pack in a lot of pixels, possibly 48 million of them – but which is not unusual these times and the promoting folks might see it as an gain.
There is yet another lesson from this entire discussion. If you want to seize distant subjects in significant detail, then the lens desires a huge entrance pupil, the much larger the greater. This is why significant athletics arenas are inclined to be encircled by photographers making use of lenses with 150mm entrance pupils.
Bob Newman is at present Professor of Laptop Science at the College of Wolverhampton. He has been operating with the layout and progress of superior-technological innovation equipment for 35 a long time and two of his products and solutions have won innovation awards. Bob is also a digital camera nut and a keen beginner photographer.
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