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Canon 16-35mm f2.8L Mk2: Appendix: focus and depth of field.
#1

First, my apologies for not putting "mark2" in the titles of the parts. I've no experience of the mark 1.

One another review, as slejhammer pointed out to me, the reviewer found that his copy of this lens repeatedly focused in front of his focus point. In my trials over the past few days I was convinced that, on occasion, mine was doing the same: the daffodils in the arlier part of this series seemed to have a dodgy focus-point.
The plot thickens just a little here. Having looked at the originals, I found that I had in fact focused on a petal that was not in the visible crop.Doh!(Zig slaps forehead).
Well, I took a series of about another 100 shots today to investigate this before I send it back. After much messing about with distance, depth of field, focal length and aperture, I did notice that 3 of those shots did indeed seem to focus on a point in front of where they should have. Two of these were by a centimetre or so over a distance of less than a metre, at least one other was by about 20 cms over a distance of 5 metres.
I then got the washing-line out in the garden, and with the help of clothes-pegs, proceeded to take many a shot along the line to see if there was any focusing upon the pegs that I hadn't focused upon!
As I then found no discrepancy, I surmised that this was all user error (in my case only, I hasten to add), so decided to find out why.
I found that the "definites" had the common features of being f2.8 on a focal length of 16mm.
Now, it's my feeling that this lens does (very) occasionally decide its focal point to be ahead of where it should be...but at other times I think that maybe I've not quite covered the desired focus point with the camera's central sensor(I only use the one). I've of course found that 16mm covers a wide area...and that the lens resolves such an amount of detail that even minor errors can be revealed to be howlers. It's a bit like the unforgiving quality of making a musical error at lo-fi...only to find that the mistake is rendered in startling clarity upon hearing it at 24-bit!

Mercifully, though f2.8 is soft, particularly with 16mm(necessarily so, given the nature of what one is asking the whole lens to be able to do), it still "does the biz" if one considers the type of shots it would be used for. And of course, who needs to worry about corner and edge definition when shooting a central subject at a large aperture.

Here is shot, uncropped, showing one of the many that worked. As it is f2.8 at the 16mm end, bear in mind that getting the focus point just right with some subjects can be quite an effort: we are talking extreme wide angle, after all.
You'll also see the unavoidable darkening of the corners, not actually vignetting, as I did not have the polariser on. But, hey, it's only about a third of a stop and easily removable with software...

[Image: IMG_0003full.jpg]

It's not meant to be a great shot, more of an illustration to show what we're up against in terms of attempts at precision.
I focused on the base of the 3rd tuner in. Though we are at f2.8, notice that at this distance away, we're minimising "bokeh"(sorry, I hate that word: I tend to see it on fanboy sites where people have more money than photograhic discernment(sounds like me at present, to be honest..Rolleyes )
That was quite a squint, getting the focus point right.
Here's a 100% crop:

[Image: IMG_0003crop.jpg]

Considering it's at the softest, then, it's not bad.

To stress the point when all is well, here is the similar thing at the 35mm end. Notice how, with the closer distance, the pleasing blur. f2.8 here is quite dramatic...this is what I was referring to...it would be quite noticeable if I got this "out" by even a jot.

[Image: IMG_0016full.jpg]

Again, notice that with careful avoidance, one can give the appearance that edge and corner sharpness are not a concern.
You'll also notice at the 35mm end that corner darkening(on this 1.6 crop camera) is not as evident as at the 16mm end.

I've again done a 100% crop to show how this end resolves at the centre compared to 16mm. I think you'll agree that it's worth squeezing as many challenges as one can out of this lens.

[Image: IMG_0016crop.jpg]

Finally, for those who have been following this.....OK, OK, I haven't quite sent it back yet...
Wink

All my stuff is here: www.doverow.com
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