Swarovision and the globe effect: Frequently asked questions

During the last couple of weeks I have received plenty of emails concerning the new Swarovski binoculars and their globe effect. Since many questions raised therein are repeating themselves, I want to discuss them here - along with the hope to find somewhat more time for myself after opening my mailbox every morning ;-) Actually, the answers to most questions could be found when carefully reading my text on globe effect and pincushion distortion, but I understand that not everybody is keen on studying lengthy technical documents that are not easy to understand at times.

I have now received Swarovski's official information which I am allowed to disclose here. According to their specifications, the new Swarovision does in fact have a certain, limited amount of pincushion distortion. The distortion parameter, as defined on the aforementioned webpage, amounts to 0.74, a value in between the circle- and tangent condition. This means that the globe effect is already significantly reduced when compared to the tangent condition (k=1), and at the same time the amount of pincushion distortion is much smaller than found with the conventional angle condition (k=0). Nevertheless, the approach chosen by Swarovski may imply that the globe effect appears undercorrected to the average binocular user. The following FAQ are addressing the consequences of this fact.

Here we go:

Question 1: Is the globe effect a psychological phenomenon?

The globe effect is a phenomenon related to our visual perception. Already 150 years ago, Helmholtz has demonstrated with his checkerboards that our vision is generating a small amount of barrel distortion. Once we use a binocular with a pincushion distortion smaller than our visual distortion, the resulting image, a combination of the instrumental and the visual imaging, contains a residual barrel distortion that is causing the globe effect of the panning binocular.

Question 2: If the final image contains a barrel distortion, how comes it shows straight lines?

In the case of a visual barrel distortion, it can be shown that straight contours appear straight whenever we look at them. The overall curvature generated by the barrel distortion remains well hidden because it shows up on contours only that are far away from our direction of view. But once the image is moving as a whole in front of the eye, the barrel distortion becomes obvious in form of the rolling globe.

Question 3: Why do some people perceive the globe effect while others do not?

The amount of visual distortion mentioned in Q1 appears to be individually different. Why this is so, and details about its statistical distribution among the population are currently subject to scientific studies. It also appears that eye-glasses significantly alter the perception of distortion and hence the intensity of the perceived globe effect. For example, glasses for short sighted people are often adding some barrel distortion to the perceived image.

Question 4: Is it possible to get used to the globe effect?

Most likely, yes. At least if the person is exclusively using binoculars of low distortion. But once again, individual differences may exist.

Question 5: Why does pincushion distortion eliminate the globe effect?

Because to the resulting image it makes little difference at which point a distortion enters the equation: Our visual system adds some barrel distortion, all right, so we compensate that with a similar amount of pincushion distortion and obtain a final image with none of them. Well, this is not exactly true: As long as the direction of the binoculars optical axis and our direction of view coincide, we are home, otherwise this compensation is less than perfect. That is why, when using a binocular with pincushion distortion, we see those annoying curved lines whenever we point our eyes toward the edges of the field.

Question 6: Why has Swarovski introduced that globe effect?

According to Swarovski, the optical design philosophy of the Swarovision is to offer an image of equally high quality over the entire field of view. This is achieved with a high resolution, reaching out almost to the edge of the image. The observer is then encouraged to make use of the entire field of view, which naturally implies conditions under which the eye is pointing far off-center. As was mentioned in Q5, residual pincushion distortion would become an issue then, and the perception of curved lines would be incompatible with an image that was intended to be perfect to the edge. For that reason, Swarovski has chosen a compromise that eliminates the major part of pincushion distortion at the cost of a certain residual globe effect while panning the binocular.

Question 7: Is there any optimum solution?

Since the quantity of perceived barrel distortion is most likely individually different, a perfect solution for everybody may not exist. But with their approach to add a minor amount of pincushion distortion, somewhere in between the circle- and tangent condition, Swarovski is probably on the right track. Based on the numbers regarding the distribution of visual distortion, that are currently available to me, a quantity of pincushion distortion between k=0.6 and k=0.8 should be offering a reasonable balance between both residual distortion and globe effect. The lower limit, k=0.6, would eliminate the globe effect to most users while the pincushion distortion is already beginning to degrade the image quality during off-center observations. The upper limit, k=0.8, would practically eliminate the pincushion distortion but at the same time cause some trouble to those who are sensitive to the globe effect. Thus, absolute perfection is impossible. Maybe, after a couple of years we will have the "Swarotune", allowing the user to set the particular amount of distortion that suits him best?

Your question is not in here? Feel free to ask, and I shall complete the list.

Update: Simulation - is it worth removing the globe effect?

k = 0.74

k = 0.6

For k=0.6, the globe effect has disappeared (lower left). But when observing the edge (lower right), the lines now display a somewhat stronger curvature than before (upper right). It would be a matter of taste whether the residual globe effect (upper left) or the residual pincushion distortion (lower right) appear more annoying to the user.

Remark: For those observers with visual distortion much different from l = 0.6 the situation would appear different. Those with lower barrel distortion, l > 0.6, would find the globe effect less annoying than those with higher barrel distortion, l < 0.6.