TechRadar Verdict
The Canon RF 10-20mm F4L IS STM is a spectacular lens.
There are some compromises.
First, this is an f/4 lens, not an f/2.8.
If you were expecting Canon’s breathtaking 10-20mm zoom to be HUGE, then you’re in for a surprise.
Another is that it relies on digital corrections although this may have no practical impact.
And then, of course, theres the price.
The widest zoom there is!
As usual with Canon RF lenses you get a focus ring, custom control ring and a zoom ring – and on this lens, a function button, too.
The STM AF actuators deliver fast and near-silent autofocus, too.
But when you put the camera to your eye, your world changes.
Your first test is likely to be to check sharpness at corners.
At regular print and display sizes, this lens is sharp from edge to edge.
However, chances are, you wont ever see it.
Shooting handheld, its extremely difficult to avoid any convergence in vertical or horizontal lines.
These two shots, taken from the same position, give you an idea of the RF 10-20mm’s zoom range. The top image was shot at 20mm; the lower image at 10mm.
Otherwise, the slightest shift in your position or the cameras angle can radically alter the perspective.
The fact is that it’s a triumph in specifications, performance and handling.
All thats left is to start saving the cash to buy one.
This lens relies heavily on digital corrections. This JPEG has in-camera corrections and is dead straight – any irregularities are in the wall, not the lens’s rendering.
Its on sale for around $2,299 in the US and 2,579 in the UK.
Should I buy the Canon RF 10-20mm F4L IS STM?
If you hate duplicating focal lengths, the RF 10-20mm F4L IS STM is for you!
But this is the same image uncorrected, showing extremely strong barrel distortion and corner cropping. In almost all practical circumstances, though, you won’t be seeing uncorrected images.
I checked out performance both wide open and stopped down.
My aim was to replicate as far as possible the kind of uses this lens would be put to.
Real-world tests seldom match official CIPA figures, which are typically a “best case” measurement.
Interior shots such as this quickly reveal the difficulty of controlling convergence in lenses this wide. The camera was kept dead level for this to keep the verticals straight.
But tilting the camera upwards to show more of the ceiling produces extreme convergence. It’s fine as a creative effect, but hard work to control if you don’t.
You might imagine that focal lengths this short would produce practically unlimited depth of field, but that isn’t the case. At 10mm, the angle of view is so wide that you can get really close to foreground subjects, way beyond the limits of depth of field. So that here, the background is blurred, even at f/7.1.
Predictably, the RF 10-20mm isn’t cheap; but it isn’t ruinously expensive for an L-series lens, either.
You can get striking compositions just by pointing the camera upwards, because objects from both sides of you converge.
