Another medium tele test — LaCA

This is a continuation of a test of the following lenses on the Sony a7RII:

• Zeiss 85mm f/1.8 Batis.
•  Zeiss 85mm f/1.4 Otus.
• Leica 90mm f/2 Apo Summicron-M ASPH.
• AF-S Nikkor 85mm f/1.4 G.
• Sony 90mm f/2.8 FE Macro.

The test starts here.

Enough pictures, right? I know, you’ve all been champing at the bit waiting forthe charts and graphs. Today I did a test of our four lenses designed to measure lateral chromatic aberration (LaCA), and the results are those beautiful graphs you’ve all been waiting for.

Just kidding about the waiting. But not about the graphs.

I set the camera up and aimed it at a LaCA target that I created. I lit the target with two Wescott LED panels set to full on and 5000K. I placed the target in the lower right corner of the image. ISO was 100. Here’s what the camera saw, with the Sony at f/8.

Here’s a crop of the target:

I did an aperture series for each lens. Then I brought all the raw files into Lightroom, developed them with default settings save white balance, exported them as TIFFs, and analyzed the CA in Imatest, using the upper right light to dark transition (BTW, I spot checked a few of the lower left dark to light transitions and got similar results.

Here’s what the measured CA area  looks like:

Well, that’s kinda weird.  The Otus LaCA gets worse when you stop down? And the Summicron, the lens that looks so polished and smooth, actually has the worst LaCA wide open? And what about the Batis looking so good? Maybe we should drill down a bit.

[Begin additional text] Those clever coders who make Lightroom one of the most opaque image editor ever written have a few tricks up their sleeves. As a reader points out in a comment to this post, Lr applies LaCA correction for the Batis 85, and you can’t turn it off. Here’s the message the Lr displays:

That’s why the Batis looks so good.

The same is true for the Sony 90mm f/2.8, except in this case the corrected LaCA is mid-pack compared to the other three lenses’ uncorrected LaCA.

Notice that, in both cases, the only thing the user can do is say “OK”. There is no “Don’t do that, you stupid program!” button. Pity.

So take the LaCA numbers for both native FE lenses with a large container of salt.  I will be redoing this with a different raw converter.

[End additional text]

Here’s the Batis wide open (that measurement isn’t on the graph, which just has whole f-stops.

The measured region is highlighted in red in the lower left corner. The curves are nice and tight, especially near the halfway point in the transition.

How about the Sony? Let’s take a look at how it did at f/2.8.

It’s sharper, but the LoCA is a bit worse. Remember, this is at f/2.8, and we looked at the Batis at f/1.8, so the Sony has a leg up here.

Now the Nikon, also wide open:

It interesting that the Nikon wide open doesn’t look much worse in the middle of the transition  than the Sony two stops down from f/1.4. The numbers are worse, though, because of the spread in the light part.

Now the Otus, also at f/1.4:

Excellent curves. The worst spread is in the dark tones.

The Otus didn’t do as well at f/8 as it did at f/1.4, which is a surprise. Let’s take a closer look.

It suffers the same way that the Sony suffered at f/2.8. There’s overshoot in both cases, the Otus in the red channel and the Sony in the blue. I’m thinking that the sharpening in Lr that you can’t turn off is adversely affecting the Imatest CA area readings. It would be instructive to redo this testing with a raw developer that allows sharpening to be completely turned off.

I’ve saved the head-scratchier for the last. The Summicron:

It’s just not very sharp, is it? And lots of LaCA compared to the others, although not huge in any absolute sense — remember, these are small pixels. The red and green are pretty tight, but the blue is not. Apo? Hmm…

One thing this test proves is the LaCA is not the most important spec, since the Leica measures the worst and looks beautiful. The LaCA of all these lenses isn’t bad. The Batis is amazingly good, after the Lr corrections.

Keep in mind that LaCA is one of the easiest lens aberrations to fix in post.