Chromatic aberration in binoculars - what it is, and how high contrast only makes things worse

Chromatic aberration can have a significant effect on your binocular's performance, especially when looking at views with high contrast. it results in poor focus and colour fringing. This article explains what chromatic aberration is, and why high contrast makes things worse.

When I'm talking about binoculars to customers, most often bird watchers, but also hunters, we often discuss how they might perform in high contrast environments.

What, you may ask, are these? And why is it important?

The answer is associated with chromatic aberration.

A while back, I was out at Lake Acraman in the South Australian outback. We were out for geology, but of course, we were also looking for birds. Because we were in the desert, the sun was shining strongly, and the shadows cast by the trees are deep and sharp.

My mate Dean had a pair of Vortex Razor binoculars - a magnificent tool. Being newer to birding, on the other hand, I had a far less salubrious pair - a stopgap while I saved up for something better. And I had a problem. The brighter the sun got, the worse my binoculars seemed to be performing. I really thought there was something wrong with them. I couldn't get the focus right - just a few hours ago, in the early morning, they were functioning fine. Dean's pair were fine. What on earth was going on? Was it the heat?

It wasn't until after the weather changed that I figured it out. It was chromatic aberration.

What is chromatic aberration?

Chromatic aberration, as the name suggests, is an optical problem related to colour. In the visible range, different wavelengths of light appear to our eyes as different colours. Light we see as blues has a wavelength of 400-500 nanometres (nm), what we see as greens is 500-580 nm, and light of more than 580 nm we see as red hues.

But lights of different wavelengths have other differences. Different colours refract (bend) to different extents in a lens (and yes, I'm aware that I've used the word "different" five times in two sentences - get over it). In the diagram I've scribbled here, the light that hits the lens, particularly at the edges, splits into its component colours, just like on the cover of my all-time favourite album.

Light with the shortest wavelengths (blue and violet) is refracted more than light of longer wavelengths (reds). Because of this, near the focal point of the lens, the blue light comes to a focus closer to the objective lens than reds. Greens focus somewhere in between.

Two photos with different levels of chromatic aberration

To show you what chromatic aberration does, I've simulated the effect here. It's a magpie on the oval at my local park. The first photo is the original, and I've separated the red, green and blue images that the camera records in the top left. All the channels are pretty sharp, and so the combined colour photo is pretty good. 

Now have a close look at a second photo. It's not quite the same. I've used Photoshop to blur the red and blue channels a little bit. You might be able to tell the difference in the top and bottom insets - they're slightly out of focus. Once the colours are recombined into the colour image, the blue and red light are both a teensy bit out of focus. 

Check out the interfaces between black and white in the two images. In the good image, the black and white come right up against each other - there's a sharp difference between them. In the second image, there are unwanted colours at the interfaces. The thin black line between the Magpie's mandibles looks a bit purple (the adjacent blue and the red have smeared over into where they shouldn't be) and that tiny hair in the end of the Magpie's bill looks green (again, the blue and the red light have dispersed away, Pink Floyd style).

Different contrast in lighting conditions

Here are two more photos showing different lighting conditions. I didn't simulate this, these are two genuinely different photos. I took one photo when the sun was shining, and a second one when it had gone behind a cloud. I had my camera on a tripod, and I used the same aperture (so the depth of focus would be the same in each photo).

In the sunny photo, the lit areas are very bright, and the nearby shadows are very deep. You can hardly see my binoculars in the thicket there. For those of you who use histograms, I've attached one, which shows dim pixels, medium ones, and a good number spread all the way up to a nearly-overexposed level. 

In the cloudy photo, where the light is scattered and much more even, the difference between bright and dark areas is much less. Because of this, you can easily make out my binoculars in the thicket. In terms of the histogram, there are more pixels in the middle, neither under- nor over-exposed, and there are fewer dark or bright pixels. 

The difference between the photos is contrast - the sunny photo has much higher contrast than the cloudy photo.

The effects of chromatic aberration are worsened in high contrast light

Getting back to my original situation, out in the desert, why was the effect not so bad in the morning, but worse as the day went on?

The difference is in contrast, and how it changes. In the morning, the light wasn't so bright and direct, so adjacent areas of bright and dark weren't so pronounced. As the sun climbed higher, though, they became very harsh. These interfaces are where chromatic aberration becomes critical, as we saw in the Magpie photos. This is why the chromatic aberration became more and more intrusive.

Nearly all binoculars produce some level of chromatic aberration. If yours produce a lot, they might not be too bad to look through in low contrast conditions - such as a cloudy day or in twilight. However, when the sun becomes strong and direct, that chromatic aberration is going to become intrusive. Bird and whale watchers will miss catching those important field marks, hunters will miss critical outlines, and everyone will get frustrated trying to get a sharp focus.

Of course, the false colours you might notice will become more obvious as well. I was looking at field marks on a Dusky Woodswallow a while back and found that single white primary wing feather was either red or blue, depending on where I pointed my binoculars. It was awful.

What's the solution to chromatic aberration?

There are a couple of different ways to manage chromatic aberration.

The most common method is to switch to low-dispersion glass, or extra-low dispersion glass. Using this type of glass, the distance between the blue focal point and the red focal point is smaller. This doesn't eliminate the effects of chromatic aberration, but it's less (and sometimes much less) intrusive.

Very often binocular makers will market an ED (extra-low dispersion) range alongside the standard glass range. Examples are Vortex Viper and Vortex Diamondback or Bushnell Engage and Bushnell Prime. Equally, spotting scopes and telescopes come in standard and ED varieties. Obviously, for cost and marketing reasons, low-dispersion binoculars are more expensive than their standard glass siblings.

Parenthetically, another way (a beautiful and expensive way) of nearly eliminating chromatic aberration is to have a triplet of lenses out front. By juggling lens profiles, refractive indices and dispersion levels, lens designers can get red, green and blue light to focus on the one point. I think the finest pair of binoculars I've ever looked through is the Leica Ultravid HD Plus 7x42. And my refractor is a triplet. However, I won't go into this as it's beyond the scope of the article. Besides, Newton's third law of bird watching applies here: for every opinion there is an equal and opposite opinion.

If you have a pair of binoculars with chromatic aberration, and use them in dull or some other low-contrast environment, you'll probably be quite happy. However, when the sun comes out or gets particularly bright, you'll have all sorts of problems with poor focus and false colours.

Using binoculars with low chromatic aberration means that when you view adjacent bright and dark areas, like in the Magpie photo, the interface between those light and dark objects remains sharply-defined: your views won't be blurred or have fringes of false colour.

A binocular with low chromatic aberration, such as low-dispersion glass, will outperform a binocular with standard glass. In a high-contrast environment, the difference in performance will become dramatic.