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The Bottom Line.
This is a very affordable lens that provides extreme magnification or close-ups. It has sturdy construction but poor ergonomics. The manual focus and fixed aperture will be severe limitations to all but the most advanced photographers. It is priced to appeal to the average photographer, but its lack of auto focus and auto aperture makes it less user friendly. Besides, you will rarely if ever require this much magnification. I get more enjoyment from my Sigma 28-200mm, and my Canon 75-300mm lenses with auto focus and fast variable apertures. Overall I cannot recommend this lens without reservations.
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Fixed Aperture Only.
The more advanced lenses sold by Canon, Nikon, Pentax and Sigma, have built-in electronics that along with the camera, allow for fine control over the aperture and focus. The Samyang lens does not have such electronics. This means that you will need to have a good working knowledge of controlling the exposure with ISO and shutter settings on the camera. The fixed aperture will appear smaller with increased focal length. As you slide out the focal length adjustment tube (zoom), the aperture or the F-number is marked next to the focal lengths. They are calibrated as follows:
650mm-----f8
800mm-----f9.5
900mm-----f11
1000mm---f12.3
1100mm---f13.5
1200mm---f14.7
1300mm---f16
These are only marked on the lens, to help the photographer decide the appropriate exposure and shutter speed on the camera.
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