Canon’s announcement of a new RF 100-500mm f/4.5-7.1L has sparked vigerous online discussions, pro and con -- especially with regard to the rather slow f/7.1 max aperture at 500mm.  Inevitably, comparisons are being made with the highly regarded EF 100-400mm f/4.5-5.6L II.  Assuming one finds the EF lens to be useful, how does the RF 100-500mm compare as a potential replacement, albeit only for use on R-series cameras? Since there has only been a development announcement as of May, let’s stick with what we know at this point, and what may reasonably be inferred based on optics, engineering and precedent, leaving aside wishes, flights of fancy and fervent hopes that have little basis in fact and a low probability of being realized. All considerations are, of course, conditional on obtaining and testing a real lens. Points to consider:

• If one is interested in using the EF 100-400 with a 1.4X extender in order to gain additional focal length, then the RF 100-500 may be more attractive than otherwise. In this case, the fair comparison is the EF with and without 1.4X extender vs the RF bare lens. The extender adds cost and weight to the EF 100-400 lens and probably degrades optical quality throughout its range while it is in use. In addition, there is the inconvenience of putting on and taking off as well as managing another accessory. Adding the 1.4X extender to the EF lens makes it a 140-560mm lens, losing some at the wide end in comparison with the RF lens while gaining at the long end.  The important of these differences is, like beauty, in the eye of the beholder.

• The RF will have a built in control ring which the EF lacks. To gain similar functionality if the EF lens is to be used on an EOS R-series MILC camera, purchase of a separate control ring adapter would be required.  At a minimum, the basic adapter would be needed, if not the control ring version.

• Optical and performance considerations include: a) What is the f-stop of the RF at 400mm? The answer is unknown at this point, but it is possible that it will not be f/5.6, but rather somewhat slower. b) What is the optical quality of the RF between 400mm and 500mm? If it is better than the EF with 1.4X extender, then the RF has an advantage. c) Also, will the RF lens autofocus with an RF 1.4X extender and what will be its optical quality? If the answers are positive compared to the EF with 2X extender, this confers an advantage to the RF lens, albeit with a slight loss of maximum focal length.

• How well will AF acquisition and tracking of the RF compare at 500mm relative to the EF with 1.4X extender?  The fair comparison will be the bare RF lens to the EF with 1.4X extender, not to the bare EF at 400mm.  At f/7.1 the RF may AF slower than at 400mm on either lens -- but will it be faster than the EF 100-400 with extender, at f/8?

• Size and weight are currently unknown. From the one photo available showing the EF 100-400 side-by-side with a mockup of the RF 100-500 (DPReview, from Canon display at WPPI), the RF lens looks a bit longer and appears to have a slightly thicker barrel, implying a modestly larger, longer, heavier lens.  The 500mm focal length makes this almost inevitable -- although one also has to consider the weight and length of an adapter and an extender when using the EF lens on an R-series camera. But, appearances in photos can be misleading, and newer designs may be lighter than old ones, even if overall size is larger. Specs are TBD at this point. 

• Price is an unknown. As a newly introduced lens, the RF may well initially cost more than the current street price of the EF, a differential that may persist for quite some time, although one has to consider the added cost of purchasing a 1.4X extender for the EF, if such isn’t already owned.

What's the Answer?

How one judges these factors is a matter of performance, operability, use cases, personal preference and cost. For some, and assuming the specs and tests of the new RF lens are favorable, this may be a worthwhile upgrade from the EF lens, particularly if one is inclined to use a 1.4X extender on the EF lens.  For others, the current EF lens may be more than satisfactory for the foreseeable future.

Some have expressed a wish for a different lens, perhaps with a faster max aperture at the long end or a different focal length range -- or any of a plethora of other possibilities. Any such lens would be a different lens and would likely be different in weight, size, optical characteristics, potential use cases and cost. Therefore, as desirable as such an alternative might be, this would nevertheless not be an evenhanded comparison to the EF 100-400mm lens.