Testing: Longitudinal Chromatic Aberration and focus shift

With lenses offering an aperture of f2.8 or larger I test for longitudinal CA (loCA, a.k.a. “axial color” or “bokeh CA”). The Zeiss shows some magenta coloration in the foreground (left) and greenish hues in the background (right). This also shows up in real-life shots but is well controlled and not as pronounced as from the Nikon 58/1.4G. Rendering of white out-of-focus targets that are well behind the plane of sharpest focus do not exhibit the nasty green halos that some large-aperture primes produce – see the flower-shot in my sample-images section.

Stopping the lens down does not produce a noticeable focus-shift.

Zeiss 55mm f/1.4 Longitudinal Chromatic Aberration (loCA)

100% crop, 55mm, f1.4, left = foreground, right = background

This also shows up in real-life shots especially under high-contrast conditions (specular highlights like sun on water or other reflecting surfaces or trees against the sky) and gets more pronounced the closer you focus. Unfortunately it is not easily corrected in post-processing, unlike lateral CAs. See the following worst-case example shot from around 1.3m.

Longitudinal CA: shot with Zeiss 55mm f/1.4 at f1.4 on a D800
Main image and all 50% crops: 55mm, f1.4, 1/1250 sec, 100 ISO, shot from 1.3m
foreground, f1.4, 100 ISO		center, f1.4, 100 ISO		background, f1.4, 100 ISO

The coloration is much milder than from the Nikon AF-S 58/1.4G and at f2.8 the effect is greatly reduced but still visible (longitudinal CA at f2.8).

For an example of loCA on specular highlights have a look a the sample image of the creek on the next page.

Sharpness and contrast

Let’s have a look at the theoretical performance (MTF-charts) compared to the Nikkor 58/1.4G first:

Zeiss Otus 55mm f/1.4 versus Nikon 58mm f/1.4G MTF
Zeiss 55mm at f1.4		Nikon 58mm at f1.4

These charts show the lens-performance at the largest aperture f1.4. Higher values are better and the closer the dotted and the continuous lines of each color are together the less astigmatism (= resolution depends on the orientation of the test-pattern) the lens has. The x-axis displays the distance from the optical axis (=center of the sensor) in mm. I’ll show you the real-life performance at 4 mm (center), 13 mm (APS-C-corner), and 20 mm (FF-corner) on a D800.

The MTF-chart of the Zeiss Otus displays the contrast-curve at 10, 20 and 40 lp/mm (from top to bottom), which is a bit unusual. Other manufacturers like Nikon on the right display the contrast-curves at 10 and 30 lp/mm.

From the charts the new Zeiss should perform on a higher level than the new Nikon 58/1.4G regarding overall contrast and sharpness (contrast with finer details) across the sensor. Even the 40 lp/mm curve of the Zeiss (showing the contrast at very fine details is better /higher than the 30 lp/mm curve from the Nikon. But it looks like the Zeiss exhibits more astigmatism at very fine details than the Nikon. Let’s see how this theoretical performance translates into real life results in the sharpness test based on Siemens-stars – and how the performance increases when you stop this lens down.

What follows are near-center results (first column) followed by APS-C-corner results and FF-corner results on a Nikon D800. The D800 results from the APS-C-corner should be a very good approximation for performance on a 16MP APS-C sensor (like the Nikon D7000), because the pixel-pitch of both sensors are the same. But differences in the AA-filter and micro-lens-design of a D800 and a D7000 might yield different end-results.

Processing was done in Lightroom 4.4 from RAW at camera standard settings. Noise-reduction is set to 0, sharpening to 70/0.5/36/10, with no extra tone, color, or saturation-adjustment. White-balance was adjusted to a neutral white and I did some exposure compensation to make the brightness match. CA-removal is ON. The new Zeiss is one of the very few lenses that exhibit almost no field-curvature at the shooting-distance for this test. So the following crops from the center, APS-C-corner and FF-corner of each aperture were taken from the same image.

The following are all 100% crops!

Zeiss Otus 55mm f/1.4 with Nikon D800 100% crop from center		Zeiss Otus 55mm f/1.4 with Nikon D800 100% crop from APS-C-corner		Zeiss Otus 55mm f/1.4 with Nikon D800 100% crop from FF-corner
55mm, f1.4, 100 ISO		55mm, f1.4, 100 ISO		55mm, f1.4, 100 ISO
55mm, f2.0, 100 ISO		55mm, f2.0, 100 ISO		55mm, f2.0, 100 ISO
55mm, f2.8, 100 ISO		55mm, f2.8, 100 ISO		55mm, f2.8, 100 ISO
55mm, f4.0, 100 ISO		55mm, f4.0, 100 ISO		55mm, f4.0, 100 ISO
55mm, f5.6, 100 ISO		55mm, f5.6, 100 ISO		55mm, f5.6, 100 ISO
55mm, f8.0, 100 ISO		55mm, f8.0, 100 ISO		55mm, f8.0, 100 ISO
55mm, f11, 100 ISO		55mm, f11, 100 ISO		55mm, f11, 100 ISO

These 100% crops directly from a 36MP D800 sensor show the awesome performance of this lens. Even at f1.4 only the full-frame corner suffers from a little haloing. Diffraction is setting in at f11. So for your landscape shots f8 would be your best bet. Distortions are minimal.

Following is a direct comparison to the Nikon AF-S 58mm f1.4 at f1.4:

Zeiss Otus 55mm f/1.4 with Nikon D800 100% crop from center		Zeiss Otus 55mm f/1.4 with Nikon D800 100% crop from APS-C-corner		Zeiss Otus 55mm f/1.4 with Nikon D800 100% crop from FF-corner
55mm, f1.4, 100 ISO		55mm, f1.4, 100 ISO		55mm, f1.4, 100 ISO
Nikon 58mm f/1.4 with Nikon D800 100% crop from center		Nikon 58mm f/1.4 with Nikon D800 100% crop from APS-C-corner		Nikon 58mm f/1.4 with Nikon D800 100% crop from FF-corner
58mm, f1.4, 100 ISO		58mm, f1.4, 100 ISO		58mm, f1.4, 100 ISO

As you can see, the Zeiss clearly beats the new Nikon lens in this test.

Performance at large distances

The Siemens-star test-targets are shot at a distance of 40x focal length (i.e. at 2.2m). But performance of lenses also depends on the shooting distance. Therefore I do another series of test-shots of a landscape dubbed the “Unremarkables” where you can measure distances in km, not meter. I use this scene to show you how the lenses perform when almost everything is at infinity. I set White Balance to a standard daylight value to make them comparable across lenses shot at the same day and also try to make exposure comparable. There’s no tinkering with vignette-control so you see it here as it is produced by the lens. Focus was manually acquired at the largest aperture in live-view and not changed for other apertures.

You can click on the image to access the large original. Please respect our copyright and only use those images for personal use.

The main image shows the complete scene at maximum aperture to give you an impression of the angle of view and to judge vignetting. This is followed by one row of 100% crops at f1.4, f2.8, and f5.6 each from the middle, the APS-C-corner and the right FF-corner.

Unremarkables at 55mm: Infinity shots with Zeiss 55mm f/1.4 Otus on a D800
55mm, f1.4, 100 ISO; Below: 100% crops from the main image at different apertures
f1.4, 100 ISO, center		f1.4, 100 ISO, APS-C-corner		f1.4, 100 ISO, FF-corner

f2.8, 100 ISO, center		f2.8, 100 ISO, APS-C-corner		f2.8, 100 ISO, FF-corner

f5.6, 100 ISO, center		f5.6, 100 ISO, APS-C-corner		f5.6, 100 ISO, FF-corner

Performance is very impressive even wide open just right into the FF-corner. So also at distant targets the lens does not show field-curvature. But it has some hefty vignetting at f1.4. Stopping down to f2.8 brings center sharpness to an excellent level and the vignetting is greatly reduced. Sharpness across the frame is very even already. At f5.6 you profit from another reduction in vignetting. Compared to the Nikon 58/1.4G the Zeiss clearly leeds in sharpness but it does show stronger vignetting than the Nikon.

All-in-all the lens is an excellent performer near and far even on the very critical 36MP sensor of a Nikon D800. So you can really use this lens wide open and get very good results. Just be prepared to deal with vignetting which might or might not be a problem depending on your subject.

Rendering of out-of-focus subjects

This test is for the rendering of point-light sources in the background. The circle of confusion that is produced by this test is pretty indicative of Bokeh performance (in the background) and light fall-off. Ideally the out-of-focus image of the point-light is evenly lit and perfectly circular, with no “onion-rings”, and without coloration. Large aperture lenses normally produce an effect known as “cat’s eye” the further away from the optical axis the point-light is projected. This is due to optical vignetting in the lens barrel when light enters the lens from an angle. For an explanation of this effect have a look over at toothwalker.org.

Zeiss 55mm f/1.4 Otus with Nikon D800 50% crop from center		Zeiss 55mm f/1.4 Otus with Nikon D800 50% crop from DX-corner		Zeiss 55mm f/1.4 Otus with Nikon D800 50% crop from FX-corner
55mm, f1.4, 100 ISO		55mm, f1.4, 100 ISO		55mm, f1.4, 100 ISO

The Zeiss 55/1.4 produces a large circle of confusion albeit 10% smaller in the center than from Nikon’s 58/1.4G. It exhibits a very even light-distribution across the circle with almost no onion-rings and only a faint smattering of green outlining. This is a good prerequisite for high Bokeh quality. But it shows some obvious cat’s-eye effect towards the borders/corners of the sensor and the diameter of the circle shrinks faster than from the Nikon 58/1.4G – which is in line with the stronger light fall-off of the Zeiss. The clipping on the top of the right-most cat’s-eye seems to come from the mirror-box of the D800 body and effects all circles at the top of the FX-sensor (showing up at the bottom of the image). The clipping is gone by f2.8 and by f4 the cat’s-eye effect is almost gone in the FX-corner. For real-live Bokeh performance head over to the next page.

Flare/ghosting

Shooting normal or wide-angle lenses always runs the risk of catching a strong light-source like the sun shining directly into the lens. This could produce strange colorful ghosts-images or reduce contrast considerably through flare and glare.

The appearance of flare and ghosting depends on factors like the aperture and the angle of the light hitting the lens. So to judge the proclivity of a lens for these artifacts I went through a series of well calculated shots against a strong light source to provoke glare and ghosting.

Flare/Glare: shot with Zeiss 55mm f/1.4 Otus at f8 on a D800

55mm, f8, 100 ISO

The image shows one of the worst flares that I could produce with this lens in the upper right corner – and that is really minimal. Of 22 shots under similar conditions 4 others showed some flare and 3 others shot almost straight into the light source produced some weak ghosts. There’s little veiling glare in these images and the blacks are black.

In three shots where the light-source was just outside the frame I got some brownish streaking effect over the door jamb (the lens-shade was mounted).

Streaking: shot with Zeiss 55mm f/1.4 Otus at f1.4 on a D800

55mm, f1.4, 100 ISO

But you can avoid this by shielding the light from shining into the lens. So all-in-all the lens handles contra-light very good.

Now let’s check out my Zeiss Otus 55mm f1.4 sample images, or if you’ve seen enough, head over to my verdict.