Matti Haapoja compares the Sony A7sIII with the Canon EOS-R5 and clearly prefers the Sony. Dpreview says you can’t depend on the Canon cameras when shooting video

Sony A7sIII preorder at Adorama. BHphoto. Amazon. FocusCamera. Fotokoch. Calumet DE. WexUK. Park UK.

Matti Haapoja compares the Sony A7sIII with the Canon EOS-R5 and clearly prefers the Sony:

Dpreview also completed the Canon EOS-R5-R6 overheating test and concluded:

Our testing suggests that the cameras perform in exactly the way that Canon said they would. However, there is an important caveat that Canon’s figures don’t address: although the cameras can repeatedly deliver the amount of video promised, they may not always do so in real-world usage.

Even set to the mode designed to limit pre-recording temperature build-up, the clock is essentially running from the moment you turn the camera on. Video recording is the most processor-intensive (and hence most heat generating) thing you can do, but any use of the camera will start to warm it up, and start chipping away at your recording times. Consequently, any time spent setting up a shot, setting white balance, setting focus or waiting for your talent to get ready (or shooting still images) will all cut into your available recording time, and you won’t reliably get the full amount Canon advises.

Not only does this make R5 a poor fit for many professional video shoots, it also means that you can’t depend on the cameras when shooting video alongside stills at, say, a wedding, which is a situation that the EOS R5 clearly is intended for.

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Sony A7sIII Preorders:
Sony A7sIII in USA at Adorama (up to $400 off with lens). BHphoto. Amazon. FocusCamera.
Sony A7sIII in EU at Fotokoch. Calumet DE. WexUK. Park UK.

New Sony A7sIII accessories:
You will also have to preorder the world’s first CFexpress Type A cards at Adorama, BHphoto. Amazon.
And of course the new Sony MRW-G2 CFexpress Type A/SD Memory Card Reader at Adorama and BHphoto.