The zoom eyepiece allows astronomers to reach a multitude of focal lengths (magnifications) using only a single unit. In principal this can be a very desirable application as it negates the need to swap eyepieces around, it can be a real pain with an undriven mount, when you've spent ages locating an object only to lose it again in the time it's taken to swap lenses and refocus.
However, there is usually a payoff to be made for this convenience, the typical zoom eyepiece can suffer from dim inferior images due to the large amount of glass used in their design (the S-Waler uses an epic 9). Each focal length can never be 100% as good as a fixed eyepiece for any given focal length, although recent advances have started to close that gap. Also zoom eyepieces can be quite heavy (the Waler weighs a whole 1lb) so there can be issues with balancing your telescope.
It has to be said that traditionally I am not a fan of the zoom eyepiece, I find the dip in optical quality negates any advantages brought about by having myriad focal lengths in just one eyepiece. When you consider that I have a natural dislike for Antares you may be forgiven for thinking this Antares made Speers-Waler is not going to fare awfully well. However, there are always exceptions to the rule.
As well as being a zoomer the Waler is a wide field eyepiece, boasting a field of view at 89 degrees at 8mm and 84 at 5mm. I think in reality the field of view is more like 87 down to 80, that's still a cracking amount of field though.
Optically this eyepiece is very sharp, surprisingly so. Views remain tack sharp and crisp at all focal lengths, right to the edge of the field. The views are ever so slightly dimmer than some similarly focal length-ed eyepieces in my collection but the difference is small and almost imperceptible. Even when objects were allowed to drift right to the edge of field there was barely any degradation or distortion of image.
The eyepiece can be infinately adjusted to achieve any focal length desired, it does have handy little click stops every 0.5mm so you know when your exactly on a whole or half number, but other than that your free to make minute adjustments to obtain any focal length you like.
So far the S-Waler performs much better than expected visually and offers you a massive amount of freedom in choosing focal length but can it get the hat-trick and offer comfort to the eye weary observer?
The answer is an emphatic yes! There is a very comfortable 12mm of eye relief coupled with a varying field stop of 11mm at the eyepiece's 8 mm focal length and 7mm at the 5 mm focal length. Together this makes for a very relaxed viewing experience, no need to keep your eye pushed too close to the lens and no need to peer through a pin hole either.
If there was ever a single product to change my opinion of a company then this is it. £250 may seem expensive but what you're buying is multiple high quality eyepieces in one, put in that context it doesn't seem so bad.
This one receives a high recommendation from me as it offers a seemingly infinite amount of focal lengths between 5 and 8mm, with wide field high quality views that can be enjoyed in comfort..
The Waler retains all the plus points that a good zoom eyepiece should have, convenience being the biggest and offers none of the drawbacks such as significantly degraded images or ridiculously short eye relief, truly a triumph of optical engineering from the little Canadian company.