DaveJ wrote:
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#2 Your speed of focus will be at least as good as with your 5D MK II, but will probably be much faster and more accurate.
Considering we are talking about a 5D Mark II, that's not a very heavy lift.
The 5DII has a very primitive AF system. It's AF system is basically unchanged from the original 5D, which was intro'd in mid 2005. The APS-C Canon 20D (2004) used a quite similar AF system. It wasn't until the 5D Mark III (2012) that Canon would significantly upgrade the AF system.
5DII's AF system consists of 9 points visible in the viewfinder, within an oval that covers about 1/3 the image area. Only the center AF point is a higher performance and more sensitive dual axis type. There are also 6 hidden "assist" points that can be enabled, but only work in AI Servo mode. These points don't appear in the viewfinder, but essentially turn the circle etched in the center of the viewfinder into one big AF point (this circle indicates the spot metered area). When enabled the assist points add two more dual axis type. The optical viewfinder through a true pentaprism shows 95% of the image area. The 5DII's AF is "f/5.6 limited", meaning that it's unable to autofocus a lens/teleconverter combo with smaller than f/5.6 aperture. For example, it can autofocus a 500mm f/4 lens with a 1.4X teleconverter (a 700mm f/5.6 combo), but
not a 500mm f/4 lens with a 2X TC (1000mm f/8 combo). The 5DII's AF is rated to work down to -0.5EV light levels. From my own experience with it, I felt it actually did a little better than that. I was able to get my 5DII to focus, albeit slowly and with some hunting, at light levels that other cameras rated to -1EV could not focus. (Granted, it might have been a problem or incorrect rating with the other camera. I certainly didn't do any scientific testing of this.)
In contrast, the R5's AF is close to the "latest and greatest" from any manufacturer (Canon R3, Sony A1 and possibly Nikon Z9
might be slightly better in some ways). It has 5940 "Dual Pixel CMOS AF points" covering almost the entire image area. Only the selected and/or active AF points are displayed. The camera is able to face and eye detect (humans and animals) and has very sticky tracking ability across the frame. The R5's electronic viewfinder (which can be set so you can see in very low light conditions) covers 100% of the image area, all the AF "points" are equal in performance. It is able to autofocus as low as -6EV (a moonless night, starlight only, away from city lights) and can autofocus f/8, f/11 and even f/16 lens/teleconverter combinations.
Further, the R5's "dual pixel AF points" are embedded directly in the camera's image sensor. In other words, they are on exactly the same plane as the image sensor. In contrast, a DSLR like the 5D Mark II has AF sensors that are separate from the image sensor. In fact, they are located in the bottom of the mirror box and light is redirected to them via a secondary mirror, after passing through a semi-transparent primary mirror (the mirror that reflects the image into the viewfinder). The light path to the DSLR's AF sensors must be calibrated as closely as possible to match the more direct path of light to the image sensor when a photo is taken. This is rarely perfect, but is why many modern digital cameras have means of fine tuning the AF for different lenses. The mirrorless camera with the AF sensors right in the image sensor itself eliminates need for this type of calibration, making for highly reliable focus accuracy. In modern mirrorless there also is no light "lost" to the semi-transparent mirror and "folded" light path used in DSLRs, which is why the mirrorless are able to autofocus in much lower lighting conditions. The more than five stop difference between -0.5EV and -6EV means to autofocus the R5 only needs roughly 3% of the light versus what the 5DII needs!
These differences are huge and are why the R5's AF system will amaze someone accustomed to a 5DII's!
And this is just the proverbial tip of the iceberg. Beyond the AF system there are a whole lot of other reasons the R5 would be a very big upgrade from a 5DII.
Canon R5 versus Canon 5D Mark II:
https://cameradecision.com/compare/Canon-EOS-R5-vs-Canon-EOS-5D-Mark-IINOTE: Rereading my response, sounds like I'm really trashing the 5DII... I'm not. There's one in my "go bag" right now, in fact. It's a true classic that revolutionized how Hollywood made movies and television shows. It's just that there have been huge strides made improving AF systems and other stuff over the last 17 or 18 years! Plus mirrorless can do things DSLRs never could.Yes, there are a few... very, very few... ways that DSLRs might still be preferred. But the positives of mirrorless greatly outweigh the few negatives.