Tuesday, May 24, 2016

Cameras in - what do we rlly need?

An awful lot is being said about mobile phone cameras just lately; and we have to thank for opening the conversation with the introduction of their One which has half as many pixels at twice the size of anyone else's. Before we begin I'm going to get some ground rules. I won't use marketing terms; so I will talk about how many pixels a camera sensor has, and what size they are. You won't hr me talking about mega- or ultra- anything. Next is that I will show my mning with diagrams - I'm a visual person and it helps me to explain.Plse br with me on this post; there's a lot to rd but it's worth it at the end.
A digital camera uses a lens to focus an onto an oblong of special material which has a of sensors on it; ch sensor is called a pixel and usually contains three sub-pixels; a red, a blue (actually two blues but don't worry about that) and a green one. ch of these sends a signal to the camera depending how much light of that colour is falling onto it. Bigger pixels produce a bigger signal for the same amount of light than smaller ones. When signals are small, the amount of noise (erroneous signal levels produced simply by electrons moving around) is a bigger percentage of the whole signal because noise is a constant backdrop. When the signals are bigger therefore, it's sier to pick out the signal, from the noise. More noise mns that the signal produced by a smaller pixel is less accurate which mns the final photo may not have exactly the same colour from ch pixel receiving the same light.

Camera sensors line up their pixels in rows and columns - and the camera's rating is often quoted by counting them. For instance a 10MP camera contains ten million pixels (roughly). These are arranged in a rectangular grid with a side length ratio of 16:9 - so that's 4213 pixels across and 2370 pixels high. If you have a sensor which is 7mm across, ch pixel is 7/4213=1.662 microns across (microns = thousandths of a millimetre).

In the rl world, standard sensor sizes and pixel sizes exist; a 13 million pixel camera in a phonewill have pixels about 1.1 microns across; this gives a sensor width of about 4.55mm (there are 4128 pixels across the ). In the One, the best photos are 2688 pixels across, and the pixels are 2 microns across giving a sensor width of about 5.4mm. Figure 1 - 4 million large pixels
Figure 2 - 8 million (smaller) pixels on same size sensorNow we can start looking at construction. Take a look at Figure 1. This shows a camera with large pixels - the lens is focussing the onto an ar exactly the same size as the sensor. In Figure 2, we gave the camera more pixels (which rlly does give more detail in the but at the expense of noise and grain - and also less sensitivity at low light levels). Of course; straight away you want more, BIGGER pixels. Figure 3 shows that if you make that happen, the doesn't cover the whole sensor; the light falls on a little bit of it. So - in Figure 4 we move the sensor further away to ensure the covers all the pixels. Figure 3 - 8 million large pixels with no other changes
Figure 4 - Sensor moved away, but the lens size is unchanged.However; the same amount of light, entering the lens, is now sprd over a wider ar: this dims the light hitting ch individual pixel, which reduces the signal output and incrses the noise. The only way to change this is to make the lens bigger - the aperture wider - so that more light gets in. This is shown finally in Figure 5. Figure 5 - all required changes incorporated.So - how far back do we have to move the lens? Simple trig shows us the answer. Imagine the "field of view" of the lens is about 60° (not unrsonable): the light coming in from the left and right cross over in the centre of the lens and make a triangle with the sensor as the base... an equilateral triangle is formed with all angles at 60° and all sides the same length (let's call it 5.4mm to stick with the One's sensor size). So the lens must be 2.7mm away from the sensor. If we now make the sensor an 8MP one (3800x2140 pixels) that gives a width of 7.6mm - the lens still has a viewing angle of 60° so that mns the sensor has to be 3.8mm away from the lens. Obviously I have used 60° as the lens viewing angle because it makes the angles, sines and side lengths sy to calculate - the s are probably quite different in rl life but you get the id: to get double the megapixels you have to move the sensor further from the lens, which mns you need a bigger lens to gather the light needed to adequately illuminate it. Lenses very rapidly incrse in price with size - a lens 4mm across instd of 2mm might cost three or four times as much. (BTW - the large circular opening on the back of the phone is NOT the lens; the lens is the tiny dark speck in the centre of it).

In practical terms for a smartphone? This mns that, to get an 8 million pixel camera, with pixels 2 microns across, your phone would need to be about 16-18mm thick (at lst at the camera's loion); and the phone would cost upwards of US$1000 - possibly even more - JUST because of the lens.

This isn't something which can be corrected with better design, or better lenses; the only way to mitigate this incrse in size and cost would be to make smaller pixels which had the properties of larger ones - and that my friends is the nirvana of a pixel scientist! The very best technology available is going into these - and with today's tech we're stuck with these limitations. In another article I'll talk about why I believe 4MP at 2 microns across is enough.

This article is also to be found on its authorpersonal blog.
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