Measure and Compensate for Lens Spherical Aberration

The majority of large-aperture camera lenses suffer from an optical defect known as ‘spherical aberration’. This phenomenon causes the lens to shift its focus merely by changing the aperture.

If you’re unaware that your lens has this problem, it can be very mysterious and frustrating. If you have a mirrorless camera, you may be able to easily compensate for it. If you are using a DSLR camera, you’re basically out of luck.

A large-aperture lens with spherical aberration

Spherical aberration causing focus shift

As shown above, lenses with spherical aberration cause focus shift by cutting off light rays when you stop down the aperture. The majority of light rays focus nearer to the lens front at a wide aperture (the ‘circle of least confusion’). When you stop down the aperture and block the outer-perimeter light rays, the majority of the remaining light rays are focused farther from the lens rear.

Going from f/1.4 to f/2.0, the area of light rays decreases by 50 percent. If you stop down to f/5.6, the area of light remaining is just 6.25 percent. The circle of least confusion is therefore much smaller at f/5.6, and also much sharper.

To know how much spherical aberration a lens has, I use the free MTFMapper program , written by Frans van den Bergh. He provides a “Focus distance” chart file that can then be printed and mounted. This chart can be used for both focus distance tests and also for evaluating longitudinal chromatic aberration.

The test chart is photographed while rotated by 45 degrees about the vertical. The left side of the chart is farther away from the camera. The chart needs to fill the frame for best results.

If you’re using a DSLR camera, it is going to auto-focus with the lens aperture wide open. The aperture closes down just before taking the shot, but focus doesn’t get adjusted at this point. If your lens has spherical aberration and you stop down the lens, the best focus gets ‘automatically’ missed.

Fully-manual lenses aren’t a problem, because they remain at whatever aperture you set while you manually focus. Any spherical aberration is always being compensated for.

Nikkor 85mm f/1.4 (left) and f/2.0 (right)

In the shots above, I show a close-up of the chart at f/1.4 on the left, and f/2.0 on the right. I focused the camera using f/1.4, and then stopped down the lens for the shot at f/2.0 without re-focusing the f/2.0 shot. The f/2.0 photo shows a distance shift of 5.1 mm away from the chart center where it is in the sharpest focus. That’s the effect of spherical aberration.

I chose my Nikkor 85mm f/1.4 AF-S G lens for this demonstration, but all of my fast lenses exhibit this optical defect. Some fast lenses handle the problem better than others.

The chart center is indicated in orange above. The green curve shows the area of best focus, with its peak at optimal focus (also shown via a vertical blue line). Peak focus is also indicated with a “cycles per pixel”, or “c/p”  MTF50 value. This number can be converted into other units, such as lines per millimeter, if you know the camera sensor pixel dimensions.

By the way, I chose to have the MTFMapper program use the green pixels on the sensor for these measurements. I could have chosen red or blue pixels instead. The differences between measurements of red, green, and blue are the essence of measuring “longitudinal chromatic aberration”, also known as LoCA.

If the camera is always focusing while using the f/1.4 aperture, then a shot at f/2.0 would miss the sharpest focus by 5.1 millimeters at this particular distance (1.3 meters away).

Nikkor 85mm f/2.8 (left) and f/4.0 (right)

In the shots above, I show the chart at f/2.8 on the left, and f/4.0 on the right. I focused the camera using f/1.4, and then stopped down the lens for the shots at f/2.8 and f/4.0 without re-focusing the f/2.8 or f/4.0 shots. The f/2.8 photo shows a distance shift of 12.2 mm away from the chart center, where it is in the sharpest focus. The f/4.0 shot shows a distance shift of 16.1 mm from the chart center at this apertures’ best focus.

If the camera is always focusing while using the f/1.4 aperture, then a shot at f/2.8 would miss the sharpest focus by 12.2 millimeters at this particular distance (1.3 meters away), and the f/4.0 shot would have missed by 16.1 millimeters.

Nikkor 85mm f/5.6

Similarly, the f/5.6 shot has its best focus at 18.8 millimeters away from the chart center. If the camera focused using the f/1.4 aperture, it just missed best focus by 18.8 millimeters.

By f/5.6, the focus shift is largely masked by the depth of focus. The MTFMapper software is very sensitive, though, and can still note that the center of focus is shifted.

Different lens, same focus-shift problem

Shown above, I marked where the focus plane moved for a Rokinon AF f/1.4 lens. I tried to focus on the chart center, but this lens focused a bit far at f/1.4. I took more shots (without re-focusing) at different apertures, and the arrows indicate where the sharpest focus landed. This lens has less severe spherical aberration than my Nikkor 85mm f/1.4 lens.

How to avoid this ‘missed focus’ problem

If you’re using a Nikon or a Sony mirrorless camera, then you have a remedy for this focus-shift problem. These cameras allow you to auto-focus at the shooting aperture, instead of having to focus at the widest aperture. This slows down the focusing speed a tiny fraction, but you gain sharper photos by doing so when you have a lens that has spherical aberration. On Nikons, the camera won’t stop down any further than f/5.6 while auto-focusing; it stops the aperture down the rest of the way just before taking the shot. Camera focus starts to get much more sluggish at apertures narrower than f/5.6.

If you’re using a Canon mirrorless camera, then you’re presently out of luck. They don’t allow you the option of auto-focusing at the selected aperture, but instead always use the maximum aperture to focus. Maybe a future firmware update will allow this option.

As previously mentioned for all DSLR cameras, they focus at the maximum aperture. They are pretty much forced to do this because they need as much light as they can get in order to focus quickly. The only way to avoid the focus-shift problem would be to focus-calibrate the camera at the selected aperture. When you change apertures, you’d need to re-calibrate focus.

If you never use high-speed lenses, then this discussion is basically a “don’t care”. Spherical aberration focus-shift was always a source of major frustration when I only had DSLRs for my fast lenses. Thank goodness for mirrorless cameras.