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Home > Reviews > Eyepieces > 12mm and up > Smart Astronomy EF27 Eyepiece

Smart Astronomy EF27 Eyepiece
By david elosser - 3/16/2007

The Smart Astronomy EF27 Eyepiece

Have you been looking for a quality wide field eyepiece for under $100? Would you like to find a pair of really good low power eyepieces that don’t cost more than your Denkmeiers? Smart Astronomy has two new eyepieces, the EF series. The “EF” stands for “extra flat” field. They come in 27mm and 19mm focal lengths, and I thought I would give these eyepieces a try. The 27EF is the target of this review.

As I pulled the 27mm eyepieces out of my mailbox, the clouds opened up, the sun came out, and a huge low voice boomed out "We are giving you clear skies tonight to test your new eyepiece!" Wow! It's nice to have contacts in high places! :-) First look is impressive. The appearance, fit and finish is nothing short of excellent. The lettering is large and easy to see. The eyepiece housing is ergonomically designed and easy to grasp. You get top and bottom eyepiece caps, and even a small cleaning cloth. Now that’s a nice touch! There is a twist-up eye cup and I discovered later that with the eye cup retracted, there is plenty of eye relief for eye glass wearers. For me, that’s another nice touch! I can easily see from the design that the EF27 has potential as a binoviewer eyepiece. It is small and lightweight for this focal length (see stats below) and the eye cup can be adjusted so that you can properly align your eyes with the eyepieces.

Here are the physical stats of the EF27mm eyepiece as I measured them:
Weight: 185gr (5-1/2 oz)
Field stop diameter ~26.5mm (roughly same true field of view as a 32mm plossl)
Maximum housing diameter 45mm (47mm at the eyecup)
Height above the chrome barrel: 50mm with eyecup retracted
Twist-up eye cup and ergonomic grip
Very generous eye relief (great for eye glass wearers)
Cost: currently under $100 USD

First test was done in daylight. I compared the EF27 with a GSO 32mm plossl, a 28mm XL Pentax (discontinued 1.25" version), and the Stellarvue 23mm FMC. I targeted the door of my shed to look for field distortions. The 32 plossl and the 23FMC show some pincushion, more so with the plossl. The EF27 though, showed no pincushion or field distortion that I could detect. Looks like this eyepiece is going to live up to its name-sake! Sharpness, contrast, and detail of the 27EF were excellent on center. However, the eyepiece looses definition just a bit at the edge. Considering the cost of this eyepiece I would consider this acceptable. It has about the same image performance as the 23FMC, and the 32mm plossl shows the best sharpness at the extreme edge of the field. As I expected, the image is not quite as good as the 28mm Pentax XL, and the Pentax has better resolution at the edge. I have not tested the EF27 in a telescope faster than F6, but just judging from what I see in two of my F6 scopes I would think that performance might not be as good in faster telescopes due to the loss of edge detail, so the Pentax might be a better choice for fast scopes.

As promised, I had clears skies the first night for testing. :-) In all of the telescopes I tested the EF27 performed extremely well. Now, in my 80mm/F6 achromat, I actually like my 32 plossl on large open clusters or the Sword of Orion. But the EF27 gives more magnification with a darker sky and is tack-sharp over most of the field. To some, this fact might make it more desirable than the 32mm. In my 8"/F6 Dobsonian, for example, I liked the extra magnification and contrast that the EF provided, and I would prefer this eyepiece over the plossl. As I mentioned before, the Pentax edged out the EF in sharpness and edge definition, but not really by much. In fact, given the cost of the EF, I would say that the performance was remarkably close. The EF does have the advantage over the Pentax of being much smaller and lighter weight, so balance is not as much of a problem on alt-az arrangements. (Also, the Pentax XL is discontinued.) The EF also shares the generous eye relief and twist-up eye cup features of the much more expensive Pentax. On-axis sharpness in my Dob was excellent. At 44x, not only could I see the rings of Saturn well, on the evening of March 7th I picked out the moons Tethys, Dione, and Rhea lined up on the western edge of the rings, spanning only 22 arc seconds across at a maximum distance of only 1 arc minute from the center of Saturn. Using a Barlow, Powermate, or Meade TeleXtender turns the EF into a makeshift but nice lunar/planetary eyepiece. With or without any Barlow assistance, I detected no flares or ghosts. I would describe the contrast of this eyepiece as excellent. Lateral color is minimal, and I am impressed at just how “clean” the image is, with the background sky being black right up to the planet. Bright stars show up as lovely jewels. Orange, white, or bluish white, all bright stars had nice saturated colors. Airy discs were perfectly round with no comet tails. This eyepiece also performs very well in my 102mm/F7.75 doublet apochromatic. I would definitely recommend considering this eyepiece over a plossl or Erfle for long focal length telescopes, as the longer the focal ratio of your telescope, the better it performs on-edge.

32mm GSO Plossl, 28mm Pentax XL, and EF27 (L-R)

Note the purple tint to the EF27's coatings versus the greenish tint of the other two. I compared the EF19 (19mm) to another national brand of similar construction and price and judged the EF coatings to have noticeably better contrast and image brightness.

Lunar performance is equally impressive. Looking at a waning gibbous moon through my 80mm/F6.9 doublet apo, the moon was tack sharp from top to bottom. Again, there are no ghosts or flares, even when used without any filters. The Moon even flared very little when I placed it out of the field of view. No doubt this eyepiece is extremely well protected against unwanted light. Libration was not good for the western limb at that time, but at 22x I could still see one mountain peak poking out from the limb at Mare Orientale. Off-axis lateral color was so slight I would call that a non-issue.

Binoviewing: If you have one of the ubiquitous Norin-made binoviewers (WO, Burgess, Orion, Stellarvue, etc) these eyepieces have more field of view than the binoviewers are capable of giving, so there is considerable vignetting. Other than that, the pair performed very well in my Stellarvue BV1's, giving a low power option for lunar/solar viewing when the seeing is lousy. In premium brands, like Denkmeier and Televue, there is no vignetting so you get the full potential of field of view. The physical size and weight of the EF27 make it an excellent choice for binoviewers.

Eyecup retracted.
Eyecup fully extended.


Bottom line: It is unrealistic to expect a $99 low power wide field eyepiece to match the performance of a premium eyepiece like a Pentax or a Televue. The Smart Astronomy EF27 however, met or exceeded my expectations for a $99 eyepiece. With the possible exception of performance issues with telescopes faster than F6, in my opinion anyone buying the 27EF would be happy with the views they get for their money. In fact, I’m glad I bought mine!

David Elosser
Kernersville NC

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