Just a little background. I shoot landscapes but have been managing to get by with telephotos and using a Rokinon 24 f/1.4 (fully manual) when I need a wide. My widest Full Frame AF lens is a Canon 28 f/1.8 which I have had since I started photography. As most people, I started out in the crop sensor body world, when I had a Tokina 11-16 f/2.8 which I loved, however when I moved up to FF, this lens wasn’t as useful to me anymore.
That being said, this initial impression is targeted at a quick study of performance at 28mm to determine where my 28 prime would fall in my lineup, if it even needs to be there anymore.
I did comparisons at 28mm on the 16-35 and compared them at f/4 with the same ISO. I used A mode because I did not feel the shutter speeds would have varied much at matching apertures anyway. IS was left on for the 16-35 and my 28 prime does not have IS.
For these tests, I have used Lightroom’s compare feature and with the exception of the first image, all will be at 1:1 or 100% zoom
This first one is 28mm f/4 on both lenses and at first glance looks indistinguishable.
Lenses are usually sharpest at the center so the next set does not show much. Equally sharp on both in my eyes.
The next two are right middle edge and left top corner
To me these are both appropriately sharp and there is no distinguishable difference.
The next set, I decided to test the 16-35 f/4 vs 28 at f/1.8 wide open. Most lenses are always a little soft wide open so I will then follow up with a f/2.2 test to mitigate the inherent softness wide open. The 16-35 will remain wide open at f/4 with IS on however.
Here are the f/1.8 images. Compared at similar zoom points. The Left is the 16-35 zoom and the Right is the 28 f/1.8
As expected, wide open, the f/1.8 is significantly more soft on edges and corners. The center remains usable however which is the only reason I have held on to this lens as an AF alternative on my FF camera. The added stops of light make this still a fairly usable night photography camera especially once you stop it down to f/2.2 as seen in the following set. 
As you can see, f/2.2 is still softer however the added stop of light allows for faster shutter speeds and a shallower depth of field.
In conclusion, I will continue testing the 16-35 f/4 L IS as my new wide angle FF AF lens. I believe the IS will allow me to take images at slower shutter speeds, potentially 1-2 stops more than I was with my 28 f/1.8. This in turn should compensate for the loss from f/1.8 to f/4. I have yet to test this in real world situations though.
I will have to determine whether or not the f/4 is able to perform in low light settings. If the lens struggles to focus in darker settings or the IS is unable to compensate the slower shutter speeds needed at a maximum aperture of f/4, I will have to continue holding on to the 28mm f/1.8 as a primary AF wide angle night lens or continue to use the Rokinon 24 f/1.8 MF lens. The Rokinon is sufficiently sharp at f/1.8 however the limitation of it being a manual focus lens makes it highly impractical for day to day usage.
Canon 16-35 f/4 L IS
- Pros
- Wider
- Covers a wider range as well, including 28mm
- Sharp wide open
- IS
- Cons
- f/4
- Difficult to focus in low light due to f/4
- Limited bokeh at f/4
Canon 28 f/1.8 II
- Pros
- f/1.8
- Smaller and cheaper
- Equally sharp to the 16-35 f/4 at matching settings
- Decent bokeh at f/2.2
- $ half the price of the Canon 16-35 f/4
- Cons
- Softer wide open f/1.8 and still soft at f/2.2
- Prime lens, no zoom range
- Flares significantly
Recommendations: I would still consider the 28mm as a cheaper sufficient wide angle lens. I have countlessly shot 3 photo panoramas on the 28mm to create a higher resolution wide image and have been happy with the results. With this method you also reduce distortion on the edges. The 16-35mm f/4 may be sharper however I foresee difficulty creating any panormas if needed due to distortion on the edges. Also depending on the photography you are doing, I do not see significant differences between the two lenses even with the 28mm f/1.8 wide open at a web/online resolution. The artifacts and soft corners are only noticeable at full scale and only until about 10% from the margins. The centers remain sufficiently sharp in comparison to the 16-35mm f/4.
I did not purchase the 16-35 f/2.8 II due to the sharpness issues that have been well documented online and did not purchase the f/2.8 III for cost. As an occasional portrait and predominantely landscape photographer I did not feel a wide f/2.8 lens would be used as much to make it worth purchasing (will rent if needed only).
Addendum: This lens is perfectly capable in low light. The IS compensates sufficiently and allows for sharp photos without pushing the ISO to compensate for faster shutter speeds. You lose quite a bit of bokeh at f/4 but in my experience this is more noticeable because of the wider focal length, not so much the aperture limitation.
Canon 6D 16mm, ISO 1600, f/4, 1/15sec
Purchase links for lenses discussed. Disable adblocker if they don’t appear.
Canon 28mm: http://amzn.to/2gSKCG0
Canon 16-35 f/4 L IS: http://amzn.to/2gN7xzW