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(More customer reviews)I own this in the Canon EOS mount.. they are in fact all the same, just ships with a different T-mount adapter, and the 420-1600 just ships with a 2x teleconverter.
This is a very light weight lens, especially for its focal length, it has no aperture blades, it's completely empty on the inside, its like an old style hand telescope. When sources of strong constrast/light are coming toward the camera, expect heavy chromatic aberration, free of chromatic aberration in other situations.
However, I've found a particularly excellent use for this lens, which is taking great photos of the moon on a tripod with a shutter release cable on my Canon 30D.
Although be warned, the lens is very physically long, although light, it makes focussing extremely difficult, especially on a tripod, as there is no tripod collar, you attached the camera to the tripod, then when you zoom the lens all the way (linear pull zoom) the lens is over 30cm long (12 inches!), although light because its empty, its supported on the tripod from the camera base, so soon as you touch the lens or camera, or the focussing ring, the image in the view finder shakes like crazy while trying to focus, making it hard to focus on the most useful application with this lens... on a tripod.
You can only use this lens hand held in full day light, and if you're doing so you'll need a shutter speed of 1/500 sec to eliminate blur from your own movement (due to the massive focal length), 1/250 sec is the minimum for sharp shots I could personally get.
And shooting in daylight, whether hand held or tripod, you're going to want to use a 1A sky filter AND a polarising filter (or at least one of them) to increase colour and contrast, luckily this thing comes with a lens hood.
The top of this lens screws off easily, so you can keep the lens pieces clean, if they ever develop fungus, good thing about this lens is water wont damage it, as it doesnt even have aperture blades, just glass and coated metal.
Though since the tip screws off you could cut a circle out of some black card then a hole in the middle and insert it into the middle of the lens to create your own aperture to try and improve sharpness and increase depth of field, and reduce aberration and flare etc.
I hope this helps.
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Kenko Vari 8000S - Telephoto zoom lens - 420 mm - 800 mm - f/8.3-16.0 - Canon EFKit includes:♦ 1) Kenko 420-800mm Super Telephoto Zoom Lens♦ 2) Precision Design T-mount Adapter♦ 3) Rear Lens CapCapture images magnified up to 16x that of a standard 50mm lens with the Kenko 420-800mm Super Telephoto Zoom Lens mounted to your favorite camera.This zoom is manufactured in Japan with multi-coated all glass elements and a built-in lens hood for superior contrast and resolution with an aluminum alloy body that is surprisingly lightweight and compact. Macro focusing even at 800mm down to a remarkable 5.25 ft, it can also be used reveal all the details and textures of small subjects. This powerful zoom offers all the versatility you need for wildlife, sports, nature and surveillance photography at an affordable price.This lens kit includes a T-mount lens adapter which allows this lens to be used with Canon EOS cameras. Also includes a compatible rear lens cap.

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