Lenses and Optics
Wide-Angle Zoom MTF and Variations
We recently published the MTF and variation curves of standard range lenses and now are going to tackle wide-angle zooms.
As we’ve seen, zooms have more variation than primes and wide-angle primes have more variation than standard range primes. Also, the MTF curves of wide-angle primes are weaker off-axis than standard range primes. So my expectations for wide-angle zooms were pretty low. I expected to see a lot of copy-to-copy variation and some pretty weak MTF curves away from the center. As with most zooms, I expected the wide-angle zooms would be distinctly better at one end than the other.
Before we get started, let me address the inevitable emails that are coming, asking if I’ll run out and test your favorite zoom lens tomorrow. The answer is no. This takes an incredible amount of time. Each wide zoom report you see below took a week of testing full time; the four lenses we’ve got here are nearly a month’s work. These were chosen on a pretty simple basis; they are the 4 most popular rental lenses in this category. Someday I’ll add the Canon 17-40mm (trust me, 17-40 fanboy, you’re not going to like the variance), the Nikon 16-35mm f/4 VR, and the Tokina 16-28mm f/2.8. But it won’t be until next month, as I have ADD and I’m really bored with wide zooms right now. Not to mention a large backup of lenses needing tested and adjusted for Lensrentals.
The optical bench confirms a couple of things that most of us already knew. For example, the 16-35mm f/2.8 is weakest in the middle of its zoom range. At the 16mm end, it’s at it’s best in the center, but gets pretty weak in the corners. At 35mm it’s not nearly as good in the center as it was at 16mm, but it holds up better in the corners. There’s no question, though, that if you have a 24-70 f/2.8 lens in your bag you’ll want to take it out for all your 24mm and longer shots.
The newer 16-35 f/4 IS L lens is clearly better than the f/2.8 (although remember it’s being tested at f/4). It is, like the f/2.8 lens, at it’s best at the wide end, but it holds up very well at 24mm and 35mm. Overall the 16-35mm f/4 IS gives very impressive performance. But I know what you’re thinking. Would the 16-35 f/2.8 lens be just as good if we were testing it at f/4? Well, I’ll show you.
Stopped down to f/4, the 16-35 f/2.8 L dramatically improves in the center at both ends of the zoom range. It’s now about as good as the 16-35 f/4 IS at both 16mm and 35mm, although it is a bit different. At 24mm, though, it’s still not nearly as good as the IS lens. (BTW – those of you who notice the astigmatism jump at 16mm off axis when stopped down — this is a real change and not an artifact. This lens has some higher order aberrations that don’t improve much with the aperture reduced. Basically the sagittal plane gets much better and the tangential plane doesn’t.)
OK, let’s compare a couple of the even wider zooms: the Nikon 14-24mm f/2.8 and the Canon 11-24mm f/4. The Nikon is the gold standard for a wide-angle, wide-aperture f/2.8 zoom. The Canon has broken new ground for just how wide you can go with a zoom. I do want to point out that due to limitations of our optical bench we can’t completely measure the Canon lens at 11mm from edge to edge. Instead of 20mm off axis (nearly to the corner) we can only measure to 17.5mm off axis. So the ‘corners’ on the graph below at 11mm look better than they really are because it’s not measuring as far to the corners as the other lenses.
These lenses are both just awesome. They are both at their best at the wide end, but both stay really quite good to the long end of the zoom range. Even at f/2.8 the Nikon is as good as the 16-35 Canon is at f/4, except right in the center. And the Canon 11-24, in the center, is the best of all the wide-angle zooms. In summary, though, I’d have to say that all of the wide-angle zooms look better than I would have expected. Sure, the Canon 16-35 f/2.8 L isn’t as good as the other lenses, but at the wide end it’s still very good, and that’s where many of us shoot these lenses most of the time.
But the MTF curves are the averages for 10 copies of each lens. Let’s take a look at how much copy-to-copy variation these lenses have; at what the chance is that the lens you buy performs similarly to these numbers.
Last week we posted the Consistency graphs for the 24-70 f/2.8 zooms and saw they ranged from 4.3 to 6.6 at various focal lengths. As zooms tend to do, the 24-70s all had more variability at one end of the zoom range. All of those zooms had Consistency Scores of less than 5 at the 70mm end, meaning there was a fair amount of copy-to-copy variation. My expectation was that the wide zooms would be about the same as the 24-70 zooms, or perhaps even worse.
So, as is so often the case, the results surprised me a bit. The wide-angle zooms had higher Consistency Scores than the standard range zooms did. The Canon 16-35 f/2.8 L clearly had more variation than the other wide zooms, which wasn’t shocking; it’s an older design and we’ve been commenting on the copy-to-copy variation with that lens for quite a while. The 16-35 f/4 IS was clearly more consistent, and the Nikon 14-24 f/2.8 was excellent.
The Canon 11-24 f/4 though was just shockingly good. We did not expect anything like those numbers. It’s by far the most consistent zoom we’ve tested; far better than many prime lenses. I have no idea what Canon is doing or how they are doing it, but the copy-to-copy variation in most of their new lenses is minimal. I know people love to think that better inspection or QA procedures would accomplish this, but that’s not really the case. A lot of it has to do with designing the lens so that tolerances are not so critical. Put another way, that means that a tiny movement of an element doesn’t cause a huge change in the optics.
The Different Look
I know some of you prefer the 2-D interpolated graphs rather than the MTF curves we used above so I’ll add those here. Remember, while this gives a nice, simplified, means of comparison, like any simplification you lose some data. Remember, too, that for these graphs we average out any astigmatism so I strongly suggest looking at the MTF charts for astigmatism (how far apart the dotted and solid lines of each color are) if nothing else.
Leaving out some data to make graphs like theses does reduce accuracy, but it gives a nice, easy comparison that shows which lenses are strongest where. As always, I suggest you take a trip over to The Digital Picture where our MTF data is posted in their lens comparison tool, letting you compare any two lenses side-by-side. They also post the 3-D graphs that I don’t have the space to post here.
Roger Cicala and Brandon Dube
Author: Roger Cicala
I’m Roger and I am the founder of Lensrentals.com. Hailed as one of the optic nerds here, I enjoy shooting collimated light through 30X microscope objectives in my spare time. When I do take real pictures I like using something different: a Medium format, or Pentax K1, or a Sony RX1R.