8-24mm long eye relief zoom

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This Meade zoom eyepiece is fully multicoated, with an 8mm to 24mm focal length range and a soft rolldown rubber eyecup. Focal lengths are marked in 1mm increments for repeatable power settings (for example when two eyepieces are used in a binocular viewer). It uses a lanthanum rare earth glass field lens for a long eye relief that’s suitable for virtually unvignetted use with eyeglasses.

As with almost all zooms, the field of view of this eyepiece decreases as the focal length increases. This results in a narrower apparent field of view at low powers and a wider apparent field at high powers than you can achieve with similar focal length single magnification eyepieces. This is just the reverse of what most people expect from eyepieces – which is a wider field at lower powers, where you’re usually observing larger objects that don’t need as much magnification to fit in the eyepiece field.

Of course, when you factor in the increasing magnification as the focal length decreases, the actual field of view still gets narrower as the power increases. However, a single magnification eyepiece in the 24mm focal length range will typically have an actual field of view that’s 25% wider than the zoom set at the same focal length. At an 8mm focal length, the zoom will typically have a field 10% wider than a single magnification eyepiece of the same focal length.

Likewise, the usable eye relief changes as the focal length is changed, in this case decreasing somewhat in the middle focal lengths and increasing to a maximum at the focal length extremes. Contrast is also generally a little lower than you’ll find in single magnification eyepieces of similar focal lengths. If these minor anomalies are not a problem for you, a zoom offers the convenient ability to precisely match the observing magnification to the seeing conditions, as well as offering a virtually infinite number of magnifications in a single reasonably-priced eyepiece.

The specifications given to the left are for the zoom set at its longest focal length of 24mm. With a 2000mm focal length telescope, such as an 8” f/10 Schmidt-Cassegrain, the 40° apparent field at 24mm will result in an actual field of view that’s 0.48° across.
At the 16mm focal length, this zoom has a 42° apparent field (giving a 0.34° actual field with the same 8” f/10 Schmidt-Cassegrain) and 12mm of eye relief.
At a 12mm focal length, the apparent field is 45° (a 0.27° actual field with the same 8” f/10 Schmidt-Cassegrain) and the eye relief is again 12mm.
At an 8mm focal length, the zoom’s apparent field increases to 55° (a 0.22° actual field with the same 8” f/10 Schmidt-Cassegrain), with 15mm of eye relief.

Eye Relief:
Eye relief is the distance from the last surface of the eye lens of an eyepiece to the plane behind the eyepiece where all the light rays of the exit pupil come to a focus and the circular image is formed, sometimes called the “Ramsden Disk.” This is where your eye should be positioned to see the full field of view of the eyepiece. If you must wear glasses because of astigmatism, you’ll usually need at least 15mm of eye relief or longer if you want to see the full field of view with your glasses on.

A note on our eye relief figures: Quite often, our eye relief figures will differ from those of the manufacturer. This is because we measure the “usable” eye relief, while the manufacturers specify their usually-longer (but technically correct) “designed” eye relief.

The eye lens of the eyepiece is normally recessed below the rubber eyeguard or rubber rim of the eyepiece to keep the lens from being scratched during use. An eyepiece might have a “designed” eye relief of 15mm (and the eye relief will truly measure 15mm from the eye lens to where the image forms). However, if the eye lens is recessed 3mm below the eye guard, the Ramsden Disk forms only 12mm above the eyepiece body (the 15mm “designed” eye relief, less the 3mm of eye relief made unusable by having the eye lens recessed into the body of the eyepiece). This “usable” eye relief of 12mm (measured from the rolled-down eyeguard – the closest point you can get your eye to the eye lens – to where the image forms) is the eye relief figure we would measure and list in this website.

Why is it important to list the “usable” eye relief? For those people who don’t wear eyeglasses while observing, a few mm difference between the eye relief they expect from the manufacturer’s literature and the shorter eye relief they actually get in real life doesn’t mean a lot. They can simply move a little closer to the eyepiece to see the full field, and never realize that the eye relief is a little shorter than they expected. However, some people must wear eyeglasses while observing, because of severe astigmatism. These observers can’t move closer to the eyepiece if the eye relief is shorter than expected because their glasses get in the way. For these people, the real life “usable” eye relief is more important than the technically correct but sometimes not fully usable “designed” eye relief. We measure and list the actual usable eye relief so that people in the real world can pick the eyepieces that will work best for them.

16mm at low power
Focal Length:
This is the length of the effective optical path of a telescopeor eyepiece (the distance from the main mirror or lens where the lightis gathered to the point where the prime focus image is formed). Focallength is typically expressed in millimeters.

The longer the focallength, the higher the magnification and the narrower the field of viewwith any given eyepiece. The shorter the focal length, the lower themagnification and the wider the field of view with the same eyepiece.

8mm-24mm
Number of optical elements:
7
Barrel Size:
The industry standard diameter, as of 2011, is 1.25" and is the most common.  Other sizes available are .965" and 2".
1.25"
Weight:
The weight of this product.
7 oz
Warranty:
1 year
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Meade - 8-24mm long eye relief zoom

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Meade - 8-24mm long eye relief zoom
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This Meade 8- 24mm zoom eyepiece has its focal lengths marked in 1mm increments for repeatable power settings (for example when two eyepieces are used in a binocular viewer) . . .





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