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"What is the lock switch on the back of the Canon EOS 90D used for, and when would it be useful to me?"

Chris Waymouth, Beaconsfield

BRIAN SAYS… Virtually all EOS cameras with a rear control dial have a lock switch, that’s designed to prevent an inadvertent change of exposure. The quick control dial on the rear sets exposure comp or the aperture value in Manual mode. If you are carrying the camera on the shoulder strap, or putting it in your bag while its switched on, you may unknowingly turn the dial. The lock prevents this, and you’ll see LOCK warning on the top LCD and viewfinder if you turn it while locked. However, if you are reviewing images or press the MENU button, the lock is ignored and you can use the locked controls without having to unlock. Recent cameras like your EOS 90D can have several controls of your choice locked; main dial, quick control dial, multi-controller, touch screen etc.

"Which lens would you recommend, the RF 24-240mm or RF 70-200mm F4L for my EOS R? I already have a Tamron SP 24-70mm f/2.8 lens"

Steve Harrison, Bristol

BRIAN SAYS… At the time of writing the RF 70-200mm F4L IS USM is not yet available, so it’s hard to judge it’s qualities. However expectation from the performance of the RF 24-105mm F4L IS USM is excellent, so I expect at least the same if not better.

The Canon RF 24-240mm lens is a great lens for every day photography, the wide to telephoto range suits many subjects

One of the limitations of the new RF 70-200mm lenses is that they are designed to be super compact with rear elements positioned close to the camera, meaning that unlike the EF 70-200mm f/4L IS USM models you cannot use them with an extender. The RF 24-240mm F4-6.3 IS STM is a compact wideranging zoom lens, that combined with the lens correction processing in the EOS R cameras delivers great results, and it makes a great all-purpose lens. Whilst it is similar sized to the RF 70-200mm F4L IS USM model, it is a little heavier at 750g. However it does use less expensive 72mm filters.