Canon EF, EF-S and RF lenses represent Canon's take on top grade photography interchangeable lenses. What differentiates them from the technical standpoint?
For professional photography the EF series still predominates due to sheer choice available and numbers in the field. Three new EF telephoto lenses were introduced in 2018. The RF for mirrorless series is gaining traction though, and has dominated new releases in 2019-2020.
The EF lens format dates back to the end of the film era. The lenses are designed to illuminate an image circle which contains a 35mm film frame. Many of the EF lenses still available new started production in the film era and have retained relevance into the digital sensor age. Actual development and production of digital sensors covering the entire 35mm frame took some time. The initial releases of the EOS 1Ds and 5D had to wait until 2002 and 2005. Earlier digital SLRs had smaller 'cropped' sensors, with the still-used APS-C format being 1.6 times narrower than FF at about 22mm. These smaller sensors didn't (and still do not) use the full light-gathering capability of the EF format. An APS-C camera starts at disadvantage when it comes to grain- ('noise'-) free images in lower total illumination situations such as fast exposures.
There are a huge variety of EF lenses available. Some, like the EF50mm f/1.4, date back over a couple of decades. Optics just itself hasn't changed that much. This is still a very nice lens in my opinion, unless you want sharp results wide open. The colour and contrast are great. This is one example from about ninety lenses built to suit the mount, about half of which are current. There are several EF-fit lenses for 'macro' use, i.e. ultra close-up, with the MP-E 65mm giving an image up to 5 times the real size of the subject on the 35mm sensor. It is then blown up much further when it appears on your display or print. There are massive telephoto lenses in the series with large apertures and long focal lengths above 600mm. There are tilt-shift lenses for specialised perspectives. There's an amazing ultra-wide zoom covering an 11-24mm focal length range. There's a lens for just about everything, with a lot of overlap. Even older lenses like the 1998-2014 produced EF 100-400 f4.5-5.6 IS USM MK1 can work pretty well on a digital camera if used carefully. The heron shot on the homepage was done with this lens and is substantially cropped, so this lens can give fairly sharp results. Again the colour and contrast of the lens are nice and more pleasing than some competing newer designs. It has a large fluorite glass element and 16 other internal lens elements. I've found some of these older lenses can be mechanically quirky and trouble-prone though, so I'd recommend purchasing with care and a good warranty. They still seem to fetch substantial prices.
This is not to say that lens design is not improving. The 2016 EF 16-35 F4, for example is a spectacular design in terms of sharpness to the edge of frame at wide aperture, and also has an extremely effective image stabiliser. My sample is not quite so good at the 35mm end, and lenses in general are subject to some sample variation. Lenses are probably becoming more correct, technically, although Canon still release portrait-oriented primes which are very expensive but could be criticised objectively. They take nice pictures, and that is the slightly mystical 'art' element. They seem to have been optimised subjectively rather than just with test gear.
It was decided to continue the APS-C format for several reasons after full-frame digital was introduced. Predominantly, lower cost of production of the sensor itself. A camera sensor is made from a small part of a dinner-plate-sized wafer of processed silicon. The possibility of getting a defect in the part of the wafer a particular sensor is taken from increase with the sensor area. An APS-C sensor also normally gives a higher density of pixels, so if your subject is far away, it will often give you better resolution in the image (see the article on sensor size for more information). APS-C sensors used with full-frame EF lenses also neatly avoid the most commonly degraded parts of the image circle, those nearer the edges. They can actually give you a cleaner result with a given lens, if everything else is working for you.
Canon's EF-S lenses were made with only APS-C cameras in mind. They don't work on full frames. The image circle coming out of the lens is too small and only correctly illuminates the approximately 22x15mm dimensions of the smaller format. But this allows the designs to be optimised for lower weight and cost. Some of them are just really nice lenses in their own right. I'd include the EF-S 10-22mm ultra-wide zoom and the EF-S 15-85mm general purpose zoom in that category. Those lenses, used with a 100D, 550D, 60D or 7D, or later models of those body series, will provide some excellent results.
Canon's latest series is the RF line for the mirrorless R, RP and R5 full-frame cameras. These are reported to be superb lenses with a potential small performance edge over the corresponding EF models. This is possible because the lens rear element can be positioned closer to the sensor, there being no mirror assembly in the R-series cameras. This can reduce the amount of dispersion and spherical and similar aberrations but it seems to me Canon already had a good handle on these things with the newer EF lenses. Additionally there are more electrical contacts at the lens and body. EOS D and R cameras use a serial data bus format here. The extra contacts on the RF system potentially allows a more parallel electronic data transfer process to be performed, increasing communication speed during the short time available around when a shot is taken. Again, there isn't really a huge problems there with the EF lens system here, as you can get very fast data speeds through a single bus connection. The bigger issue for many is EVF delay and quality and those factors work for the EF system cameras. I don't presently see the RF system as the definitive way forward for all photographers.
The RF lens range is still smaller at this time, although growing rapidly. And while an EF, or indeed EF-S, lens can be attached and used with an R camera, using a low cost or included adaptor, the reverse is not true. RF lenses are for R cameras only. However, Canon is focusing (sorry) on releasing more and more of these lenses and paying less attention to the EF and EF-S formats. There are several due during 2020.