Why digital camera sensors don't need to be big
Posted by Graham Stratton Tue, 17 Oct 2006 08:55:00 GMT
For a long while I thought that Nikon had made a mistake by not using full frame sensors. Surely Canon’s large sensors were going to collect more light and therefore always be more sensitive? But then I realised I was wrong. The sensor size in a system does not affect it’s sensitivity – only its potential resolution, and there seems to be no problem with cramming enough pixels on a small sensor.
So, why did Canon start making full-frame cameras? Simple, because they have lots of nice 35mm lenses around. And maybe because they couldn’t fit as many pixels as they wanted on a small sensor. But mainly because of the lenses.
Lenses
The job of a lens is to take all the light which comes from a certain set of directions and enters the front of the lens, and focus it onto a sensor. Whatever size our sensor is, there will be the same amount of light entering the front of the lens. With a smaller sensor the lens just needs to focus it onto a smaller area. Since there is always the same amount of light coming in, a sensor of the same sensitivity will yield the same quality of image when the light is focused onto it, regardless of its size.
Using a 1.6 times crop factor sensor with a 35mm lens is exactly equivalent to using the same lens with a 1.6 times teleconverter on a full-frame camera. In both cases, only a small amount (about 1/1.6^2 ~= 0.4x) of the light collected by the lens is projected onto the sensor. Using the teleconverter increases the focal length by 1.6x, and at the same time reduces the f-number by 1.6 times.
f-numbers are most usefully thought of as a ratio of subject brightness (per unit of solid angle) to sensor brightness (per unit area). Hence when one uses a teleconverter to spread the light out over a greater area, the effective f-number decreases. Since a small sensor needs to get more light on a smaller sensor, a higher f-number is needed. But since the light can be focused onto a smaller area, a higher f-number can be attained.
Effective f-numbers
The concept of focal length multipliers seems to be generally accepted. A 50mm lens on a 1.6x crop camera acts like an 80mm lens on a full-frame camera. But what is not yet accepted is the concept of an aperture number multiplier. A 100/2.0 on a 1.6x crop camera acts not as a 160/2.0, but rather as a 160/3.2.
So, am I saying that if you wanted an effective 80/1.2 for a cropped camera, you’d need a 50/0.75? Yes, I am. Isn’t that impossible to make? No.
Suppose you had a 0.625x teleconverter. Connect this to a full-frame 80/1.2. What do you get? A 50/0.75 (but which only covers the cropped sensor). (Admittedly it is not actually possible to make such a teleconverter, as you would need to move the lens closer to the sensor, just as a 2x teleconverter moves it further away). A 0.625x teleconverter would make any full-frame lens into the equivalent cropped-sensor lens, just as the a 1.6x teleconverter makes a full-frame lens behave as if it were on a cropped-sensor camera.
The future
So, we can all move to smaller sensors with lighter lenses, then? Er, no. Lenses will still need to be just as big. The fact that current cropped-sensor lenses are light is because they have an effective f-number that no one would buy in a full-frame lens.
So far, the move to small sensors has yielded lighter, cheaper lenses. Manufacturers have used the transition to sell lenses with effective f-numbers that would never have sold before. To get effective f-numbers where we need them, we will need to see some striking specifications. Expect to see f/2.0 zooms and f/1.0 primes.
I heard a ruumour that Canon are considering doubling the sensor size since they’ve reached the limit of the sensitivity of 35mm sensors. That’s not going to happen. The interesting question is as to whether Canon will abandon full-frame sensors at some point. As they’ve so far only produced a couple of L-series quality EF-S lens, I don’t expect it to happen soon. In fact, Canon seem to be trying to pull people towards full-frame sensors. That’s fine, provided they can make them nearly as cheaply (actually I believe that full-frame sensors are currently very expesive to make, though they may get cheaper – although chips normally get cheaper because they get smaller). Full frame is also a great idea from a marketing perspective, as most people still believe full-frame is fundamentally better.
I expect to see more EF-S (cropped sensor) lenses from Canon soon. If Canon don’t produce such lenses, I think we will see more people buying from manufacturers who are making cropped-sensor only lenses, such as Sigma. By designing for cropped sensors only, manufacturers can make lenses which are over a stop faster but roughly the same size, weight and price as full-frame lenses of the same focal length.
Notation
Isn’t all this ‘effective’ focal length and aperture stuff getting a bit confusing? Yes, but I don’t think it’s going to change. Lenses for cropped sensor cameras will continue to be specified as if they produced an image which covered a full 35mm sensor.
Conclusion
So, is sensor size really arbitrary? Well, not entirely. Ignoring by far the most important aspect, which is what lenses already exist, there are a couple of other considerations:
a) Whether the manufacturer can fit enough pixels on the sensor to get the resolution they want
b) Whether getting enough logic or light on a small sensor makes it warmer, and therefore more noisy
Does this mean you shouldn’t buy a full-frame camera? Well, if you’re not sure you could give it to me instead. But seriously, there are a lot of very nice 35mm lenses out there which you can only take full advantage of with a full-frame sensor. But what it does mean is that you shouldn’t feel you are missing out on potential sensitivity by buying into a system which only supports cropped sensors, providing you feel that the lenses you want exist or will exist.
Very clearly put. It would probably be useful to propagate this explanation rather more widely!
Hmm, not so sure about the crop factor/f-stop thing. Using a smaller sensor doesn’t change the f-stop of the lens, I think. The amount of light per unit shouldn’t change because you use 1/1.6th of the cone of light with 1/1.6th of the surface. Am I missing something?
One more reason why full-frame sensor is less practical: the angle of the light rays. The pixels on the sensor work by collecting photons that fall almost perpendicularly on them. The bigger the sensor, the bigger the angle away from the vertical that the light rays follow. I believe the Canon sensors use microlenses above each pixel to bend the light in the appropriate angle. I suspect that the future belongs to 1.6x sensors, but FF will be manufactured for at least 10 more years.
Deian,
What I am saying is that a 100/2.0 on a cropped camera will produce exactly the same image as a 160/3.2 on a full frame camera, assuming they both have the same pixel count and sensitivity. Here I define sensitivity to be output power per photon per pixel, and assuming the pixel lenses cover the same percentage of sensor area on both sensors. This seems to be a good assumption between Canon’s 5D (FF) and their 20D (1.6x crop), allowing for the different pixel counts.
I agree that the amount of light per unit area doesn’t change between FF and cropped, but the amount of light per pixel does, hence the lens acts as if it were slower.
On the subject of microlenses, Leica’s M8 has special offset lenses to address the problem of angle of incidence with wide-angle lenses.
A field of view crop, found on all sub-fullframe DSLRs, does NOT result in a loss of light, per pixel or in any other way. Assuming two identical sensors, with the same distance between photosites, but one having the same size as a full 35mm film frame, and the other a typical ‘APS-C’ sized frame, there is no light fall off onto identical photo sensors. Obviously, there are no identical sensors, as the actual density of the photosites continues to rise on the APS-C sized sensors. Loss of f/stop is obvious, on a teleconverter… doubling the focal length, with the same effective f/stop on the lens, results in a loss of aperture, even though the actual amount of light is not lessened by that same amount. Look at the various teleconverters made by the big camera makers… some produce a 1 1/3rd stop loss, others a full 2 stops. Some, like the 1.4x converters made by Nikon are even less… and yet when I meter a scene with a converter between my camera and lens, I do not lose that much light (as recommended by the maker of the converter). The actual loss of light has to do with extra optics and so on… if one were to use the actual theoretical definition of f/stop, focal length divided by the largest diameter of opening, a 2X teleconverter would always produce a doubling of the aperture. Yet, many 2x converters are specified to lose only a bit more than 1.5 stops.
Sure, two sensors, one being full frame, the other being smaller than full frame, would have different ‘pixel densities’ which could result in less light per photosite. But, several medium format sensors (Leaf, PhaseOne) were the same size as a 35mm film frame, a couple years ago… with 22-30 million photosites. So, would that result in the same ‘loss of light’? No! Density of the photosites still does not affect the amount of light striking the sensor. The ONLY thing affecting light, before it hits the sensor, is the lens, dust, and the source of said light. Granted, the QUALITY of the photosites could affect the final image, but one needs to remember, using ISO numbers for sensitivity between film and digital is dangerous. 100 ISO on film is rarely the same sensitivity on a very recent digital camera sensor… moreover, it is even dangerous to assume that ISO is relevant between makers of sensors. I’ve got a Canon that reliably meters 1/3 to 1/2 stop off, compared to my Nikon and Fujifilm… using the same exact lenses in manual focus mode. Either the Canon is less sensitive, but has the exact same sized sensor as both my Nikon and Fuji, or the lenses are suddenly working differently because I changed the camera?
A smaller sensor collects less light. But that does not mean that f stops of the camera change. You just have to user smaller photo sensors which measure light less accurately and therefore introduce more noise into the recorded photo.
What a load of rubbish. A small sensor is simply a cropped version of a full frame. The narrow field of view therefore acts like a magnified focal length. However, this makes nigh on any difference to the f stop of the lens being used. The reason why the concept of F Stop multiplier is not accepted is because it isn’t true. What a load of rubbish. Trying to make your theory hold with the teleconverter impacts is not correct because a smaller sensor is simply a cropped effect and not really a focal multiplier. Back to school for you me thinks.
after all this puzzling, the simple logic- the amount of light depends on your lens diameter. with the FF sensor,all gathered light falls on the sensor which covers the spacial andle, divided by your f number
With the APC sensor, only the cropped amount of light falls on the cropped sensor, divided by your f number, resulting in the same energy on the sensor per steradian of angular coverage in both cases
FF lenses are not slower assuming identical sensor sensitivites.
Well Said Ariel. All makes perfect sense to me and the other message boarders. The same can not be said of the author of this column. Where I come from ‘Rambling’ does mean a BSer so he has aptly named himself as right Rambler here :D.
I see that the comments on this article have got to the ‘let’s all jump on board and laugh at this idiot even though we have nothing useful to add’ stage. Please keep the personal comments to yourselves.
Yes, of course the same lens on a smaller sensor doesn’t give the same result. But the point is that if camera manufacturers were prepared to modify their lenses then there should be no loss though using smaller sensors.
There’s no reason for the Four-Thirds guys to not be able to compete with the FF guys. But the equvalent Four-Thirds lens to Canon’s 70-200/2.8 on a FF camera would be a 35-100/1.4, and as yet no such spec exists. There are now f/2 zooms in the Four-Thirds range, but no faster.