Here are a few more ideas from yours truly. Feel free to shoot them down...
: Everything else being equal, the larger the crop factor of the camera body you buy the faster the glass you need for each situation where shallow DoF, fast shutter speeds and/or low light ability is key.
I've offered a few posts over the years exploring the implications of sensor size when choosing a camera and recently came to the conclusion that the only parameter that truly determines a camera's performance, for the typical sensors in use in the larger cameras today, is the maximum diameter of the entrance pupil a lens can accept. For simplicity I'll refer to this as the "lens diameter" in what follows. In summary:
- For a given field of view the ability to generate a shallow DoF is dependent on lens diameter and not sensor size.
Have large sensor cameras reached their limit? refers.
- For a given detector efficiency and field of view the ability to "stop the action" is dependent on lens diameter.
Stopping the action requires a particular shutter speed and one needs to be able to record the arrival of enough photons in that time to ensure a particular image quality (lack of image noise).
- For a given detector efficiency and field of view the ability to work in low light is dependent on lens diameter.
It's the same deal as above: one needs a certain number of photons per pixel to get the noise level down and for the number of photons to be the same the lens diameter can't change.
- For a given pixel count and field of view the ability to avoid diffraction softening while offering maximum DoF is dependent on lens diameter used (actual).
Full-frame or Cropped Sensors: Which is best for Landscapes? refers.
From the above it would seem that the sensor size has nothing to do with the prime concerns of a photographer and that a 50mm f/2 lens in front of a full-frame sensor performs identically to a 25mm f/1 lens in front of a crop factor 2 sensor or even a 10mm f/0.4 lens in front of a crop factor 5 sensor as found in some of the larger compacts. Each combination provides the same ability to give shallow DoF and the same photon collecting power. The output is indistinguishable in terms of the images we extract from the memory card, right? Well no, not quite!
There are two stumbling blocks, the last of which is probably insurmountable. The first is that as you shrink the sensor the area of silicon that can be devoted to each pixel also shrinks. While that may not affect the ability to convert photons into free electrons it does, with current fabrication techniques, limit the number of electrons that can be stored before the storage is full and that limits the maximum dynamic range. Technology may overcome this but the second limitation, below, is a deal breaker.
In the last case above, with a crop factor of 5, to maintain the capabilities I mentioned at the start one needs a lens of 10mm focal length and an f-number of 0.4. Lenses even as fast as f/1 are rare beasts and incredibly pricey. Even though the sweet spot needed for a crop factor 5 sensor is a lot smaller I doubt we'll ever see an f/0.4 lens as a mainstream product. There are limits to what can be made!
As an aside, the lens diameter argument is why compact/superzoom cameras, which are great little tools in their own right, will always be at a photographic disadvantage when compared to their larger cousins in every way except for size.
The same logic, driven by the lens diameter, also applies to medium format cameras so one could argue that for pixel counts typically found in today's DSLRs there's no advantage optically in such a camera as one can achieve all the advantages one needs with the sort of glass already available for those DSLRs. But if higher pixel counts are needed then the sensor fabrication argument above would indicate that medium format is the way to go unless and until there's a breakthrough in storing those electrons. Of course there's much more to any lens than its diameter (entrance pupil) so one has to read the reviews to determine optical and mechanical quality but however good a lens is in these respects the lens diameter still limits what can be achieved.
With this in mind and looking at the current crop factor 2 to 1.5 cameras, field of view considerations mean that crop factor 2 lenses can have focal lengths that are shorter than their 1.5 to 1.6 equivalents by very roughly
about 25%. But the same "lens diameter" argument implies that they should also be about 25% faster (minimum f-number about 25% smaller) to enable them to achieve similar DoF or light-gathering power for a particular field of view. A very cursory glance through the lens catalogues tells me that this isn't typically the case so kudos to a third party manufacturer like Voigtländer for spotting the gap and offering their Nokton F0,95/25mm
Before the micro four-thirds community rise up in arms I'll offer that not only are there some great micro four-thirds cameras around but Canon, to take just one example, do exactly the same with their EF-S lenses. They are designed to work solely with their 1.6 crop factor cameras so one might hope that they would be 1.6 times faster than their full-frame equivalents (lenses with focal lengths 1.6 times longer). Not so, though Canon might defend themselves by pointing to the various fast primes they have on offer. No doubt the micro four-thirds community can point to their legacy lenses as well.
So, right at the start, I offered that:
Everything else being equal, the larger the crop factor of the camera body you buy the faster the glass you need for each situation where shallow DoF, fast shutter speeds and/or low light ability is key.
That statement doesn't mention the lens diameter (maximum entrance pupil) directly but, of course, it follows directly from it. The "each situation" part implies a particular field of view and that implies shorter focal lengths as the crop factor increases. Add a requirement for constant lens diameter to the mix and the f-number has to go down in proportion to the crop factor.
So far as purchasing decisions are concerned it does seem bizarre that it's often true that the full-frame cameras are typically less limited when it comes to selecting fast glass.