Comparing sharpness of 16-80 to other lenses

Started 9 months ago | Discussions thread
Birdie55 Regular Member • Posts: 247
Re: Questions

Alexis-C wrote:

"We see the 16-80 absolutely thrashing the 18-55 across the board"

I'm looking at the same graphs and that is not at all what I'm seeing (I have some experience reading those in a different context, when evaluating telescope eyepiece designs).

What I'm seeing is a generous amount of edge of field astigmatism on the 16-80, which is absent on the 18-55 (and also some field curvature). That doesn't only create ugly bokeh, it's also detrimental to sharpness in a very nasty way. Basically, astigmatism means that when you focus on edges oriented in a certain way (sagittal means pointing to the centre), lines in a direction 90° from it are unsharp.

If you have some softness but no astigmatism some amount of sharpening can recover some quality; if the image is astigmatic that is impossible without really ugly artifacts (since your features are not made fuzzy, but smeared in one particular direction).

In other words, in MTF graphs I always tend to use the _lowest_ of the two graphs (S and M). When you take that less charitable criterion, the 16-80's poor tangential/meridional performance at the edge makes it limp behind all the other lenses in edge of field performance (fortunately, it's not as dire in the middle of the focal range, but we have no graphs for that).

It's very striking when you compare it to the 16-55 f/2.8. That one has NO astigmatism at its extremes even though it's a fast f/2.8 constant aperture lens (it's essentially perfect except for a tad of field curvature at the tele end, but you could probably even focus decentrally to make the corners a tad better).

As far as the 18-55 is concerned, its main misfeature seems to be a relative lack of sharpness in the centre at 18mm when fully open (which means, in other words, that if you want to shoot at f/2.8 you're likely trading in some resolution). I'd bet there is some residual spherical aberration in the design or at least some spherochromaticism (i.e. green is better than other colours) when set both at 128mm and f/2.8. That usually drops sharply when stopping down the aperture.

Given that, nothing in the graph tells me whether the 16-80 at 18mm opened to f/4 would still beat the 18-55mm at 18mm opened to f/4, which is what the 16-80 would have to do to "trash" the 18-55mm. But we don't have the graphs for that...there's a hidden fallacy in overlaying graphs "wide open" from lenses that have different f/ratios.

Looking at the graphs, I consider the corner performance, even when both are wide open, to actually be better on the 18-55, with almost no field curvature and little astigmatism (i.e. the sharpness in the corners isn't much worse than in the centre, unlike on the 16-80).

What the MTF graph does _not_ tell you, though, is what distortion correction will do with the corners if you want a distortionless image. That too will be tricky for the 16-80, which has a large span of focal lengths which tends to introduce quite some distortion; astigmatism and some field curvature in the corners only makes it even more tricky to stretch without degrading the image.

In other words, I don't agree with the final assessment, especially not with the "weakest overall performance" bit. To me just looking at the graphs, it looks decidedly better than the 18-135, even when used wide open, probably more or less a wash with the 16-80 (if both used at f/4), and indeed not as sharp as the 16-55 f/2.8 (no prizes for guessing that, if you've seen what a Holy Handgrenade that one is in real life).

I could tell you about my evaluation of the two lenses but I'll give you the tl;dr version: when used as 18-55mm f/4 lenses the differences really are negligible. The 16-80 gives you an extended focal length range at the cost of corner sharpness if you stray outside of 18-55, the 18-55 a shorter f/ratio at the wide end (at the cost of a tad of image sharpness across the image if you use it).
The 18-55 is also a lot smaller -- my wife will _not_ take the 16-80 instead of a 35mm prime for a casual walk when she wants to be discreet, but she's OK with the 18-55.

I have today read almost all posts in this thread, and Alexis' is the first that is in my view credible, and helpful to me to understand the various lenses under discussion here. I have the 18-55 and the 55-200, and also the 16-55 and the 50-140. I have read most threads on the new 16-80, thinking it may replace the 18-55 one day. Now I am not so sure.

Interesting would be to have Alexis' analysis of the 16-80 versus the 55-200 for the overlapping range. As I also have the 14 mm, the wide end of the 16-80 is of less interest to me, for landscape anyway.

Many thanks, Birdie

 Birdie55's gear list:Birdie55's gear list
Fujifilm X-H1 Fujifilm X30 Fujifilm X-E2 Fujifilm XF 14mm F2.8 R Fujifilm XF 55-200mm F3.5-4.8 R LM OIS +3 more
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