Many people believe that 35mm Full Frame is the ultimate in sensor quality. Other’s argue that smaller sensors are just as good while being cheaper.
It’s tempting to believe that smaller sensors equate to cheaper cameras. No doubt about it, smaller sensors can equate to smaller cameras, and smaller anything is often cheaper to manufacture.
While the cameras in a phone may be tiny things, part of the reason they’re cheaper is how they’re constructed. Most phone camera lenses are simply glued together then glued to a piece of plastic that holds the lens array in place over the sensor. Full size cameras and lenses can’t be glued together like this as they would just fall apart with use. The structural parts of a modern camera lens, even lower priced lenses like a Canon RF 50mm f/1.8, are usually made of metal and screwed together.
It seems to me that the build quality in a modern camera lens is worth something over the cheaper glued together construction techniques found in cellphones.
As a general rule it assumed that larger sensors require larger lenses and smaller sensors require smaller lenses. While it is true in one sense, it is false in another. In actual fact there is a situation where there would be no difference between camera lenses due to sensor size.
3 commonly available sensors sizes are MFT, APS-C, and 35mm Full Frame. In addition to this there are smaller sensors on cellphones and larger sensors in Medium Format.
You would think that if a lens covers 35mm full frame, that lens can be proportionally shrunken down and produce the same quality of image on a smaller sensor. It’s tempting to believe they could just take the Canon RF 50mm f/1.2 L, shrink it down by half, and have a 50mm f/1.2 lens with a smaller sensor. Wouldn’t that be easy? Unfortunately, physics doesn’t work that way, and if you think about it, it makes sense.
If we assume that each sensor in the comparison is 24 megapixels, then in order to produce the same brightness per pixel the lens will have to be the same size on each camera type.
However, if each sensor isn’t the same resolution but instead is the same pixel density. Then the relationship is different. In this case, once the image circle has been shrunken down to the smaller sensor, it will be relatively brighter because a smaller image circle has more concentrated light in it. Since each sensor has the same density (same size pixels), the smaller sensor will have a brighter image with the same size lens. (This has nothing to do with adapted lenses unless you’re using a speedbooster.)
The problem here is that MFT sensors would have to have the same pixel density as much larger sensors which means they’ll need to be much lower resolution to match the noise performance of a larger sensor. If you’re willing to accept lower resolution then the noise levels in the image will be about the same for smaller sized sensors like MFT. However, larger sensors with more pixels will usually still have an advantage because the extra pixels can be oversampled. For instance, the Canon EOS R5 can oversample its 8k sensor. This produces a higher quality 4k image from 8k worth of data. While this might seem like a waste, the point is that more pixels provides more ways to achieve a higher quality image.
It doesn’t change the fact that if a lens is designed for a smaller sensor with a smaller image circle then it’s probably going to perform very similarly to a larger sensor camera. The gotcha is the fact that the lens really has to be the same size or of very similar size to the full frame equivalent. The lens is the light gathering unit, if the lens is small, the the amount of light gathered is small. But if the lens has to be the same size or very nearly the same size then there isn’t much benefit in having the smaller camera.
It seems that the amount of light gathered by each type is rather similar. But one difference that can’t be easily accounted for is the effect of bokeh. Generating out of focus backgrounds is the specialty of larger sensors. This is a simple relationship of the size of the sensor to the size of the world. Larger sensors can frame the subject at closer distances. Being closer generates more background blur, something that many photographers love to see in their images. In this case there is an inverse relationship between lens size and sensor size. With smaller sensors a larger lens (with a larger aperture or longer focal length) must be used to match the bokeh creating capability of larger sensors.
Ultimately there are advantages and disadvantages to both small sensors and large sensors. This is very different from film. In the film world larger film stocks are universally superior to smaller film stocks. This just shows how incredibly good modern digital sensors are that this is even a topic of conversation. In the days of film cameras APS-C sized film was considered low quality. 35mm film also struggled amongst professionals for a long time until the film stocks were improved. But in the digital world this is all changed. MFT is good enough for many uses, particularly video shot at 4k resolution or less. If you’re shooting 4k video there isn’t a whole lot of reason to consider moving up to full frame these days.
It also seems like the smaller sensor cameras get improved faster than the larger sensor cameras do. While both have definitely gotten better in the past 5 – 10 years its really the smaller sensor cameras that have improved the most. It seems that large 35mm full frame sensors are still higher quality overall but they definitely need an improvement of some sort if they’re going to stay that way for the foreseeable future.
