Getting off Auto - should you buy the RF lens?

OK, so it's been a while since I did a GoA thread - lots more via the index. But I did say on the Moving to Mirrorless thread I'd try and find time to compare the Canon EF100-400L Mk2 with its RF replacement, the 100-500L. A bit of spare time and some better weather mean I've finally got around to it.

So, some basics. This shows the two lenses side by side. There's nothing in it really, the 100-500 is slightly longer but when you add the necessary EF/RF adapter to the EF lens, it's nit-picking to spot a difference

The 100-500 is the lighter of the two, 1.7Kg vs the 1.9Kg of the 100-400, though I didn't bother removing the (different) lens feet on the lenses. In the hand there's not much in it. I don't have an RF 1.4x extender to play with as yet, but it's a "feature" of the 100-500 that you cannot close the lens right down when you have one attached - ie you have to extend the lens to 300mm before you can attach it. This I really don't like - I use the 100-400 as a walkabout lens and lock it closed until use to keep it safe. Having the barrel of the lens stuck out all the time if you have the extender attached feels like a poor design decision, but (being realistic) I wouldn't use the extenders regularly on these lenses anyway so it's probably academic.

What IS very noticeable however, is the longer minimum focus of the 100-500. Canon have been a bit misleading with the specs on this one. According to the numbers, the RF lens is better (0.9m vs the EF's 0.98). However, this is a lie. Sure, the RF lens will focus at 0.9m, but only at 100mm. At 500mm (which is what we're interested in for creepy-crawly pics), it's nearer 1.2m. Conversely, the WORST the EF will do is that 0.98m (actually, I measure it a bit less) at 400mm. At 100mm it's around 0.5m. So not really comparing like for like here - I suspect they didn't want anything about the RF lens to look worse on paper.....

Is it an issue? Well, you work around it obviously, but it's an annoyance for certain, especially if you're used to using the older lens.

What else to say - stabiliser is better on the RF lens and AF is a bit faster too (as you'd expect), but what we're all interested in is the image quality rather than small incremental improvements in handling, so here we go. All on the Canon R7 body at 1/1000 sec using a helpful Puffin as a subject

First off, some distance shots (a few metres). The 100-500

and the 100-400

These are cropped to the same size, so they show the extra reach of the 500mm. Useful gain in size here, so I tried the 1.4x on the EF lens

Definitely softer as you'd expect, though I haven't sharpened any of these images. They've all been processed with PS Elements & Topaz De-Noise with the same settings to try and keep things as equal as possible

Now some close-ups. These are at minimum focus for each lens with no crop in post-production. The 100-500

and the 100-400

This shows us two things - firstly, the sun went behind a cloud a bit so a direct comparison of the images is not ideal, but more importantly, I think the magnification is bigger with the EF lens. [EDIT - looking at them one above the other on the thread, I think maybe my initial impression was wrong & that the 100-500 is slightly bigger. What's your opinion?} Remember, both these are uncropped. This is not what I expected, the Canon spec says magnification is improved on the RF, 0.33x vs 0.31x. Except, hmm, let me guess, what's the betting this is the same issue as the minimum focus? ie the EF magnification is quoted at 400mm, the RF's at 100mm. I'm not sure, but one thing's for certain, the difference is tiny, so if you're hoping for bigger bug pics with the newer lens, you'll be disappointed.

To get a better comparison in the same light, I re-shot the last two pics then cropped to the same field of view. First the 100-500

then the 100-400

Flicking between the two images, I do think the 100-500 is the better lens from an image quality perspective. It has the edge on the older EF model - perhaps not surprising, the EF lens has been around for a few years now and technology moves on. However, the new lens will set you back near on £3k and only you can decide if the small improvements are worth the extra cash should you already own the 100-400. It also depends what you use it for - if you use it for buggy pics close in, the 100-400 might actually be the better lens. 

No clear cut answer for this one I'm afraid! Perhaps it's simply a reflection on how good lenses have got in recent times, we're hitting the law of diminishing returns with new models....