AGeoJO Offline Upload & Sell: On
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What will you be using the lens for? I hope not only for test charts in future . Seriously though, the so called decentering issues are more for 35mm lenses or wider. Longer focal lengths lenses generally are not designed to take flat images in real life, unless you are in to reproduction of art pieces and for that macro lenses will be better anyway.
I have had and enjoyed my GM 70-200mm lens but mostly I used them for portraits and landscape (for compressed perspective/view) and any other than flat objects. However, if you really want to find out whether your lens is centered, it is fairly easy to do.
You mount the camera/lens using the lens foot on a sturdy tripod in a horizontal format. You aim at something flat, like a block wall (some people frown on this) and set it as perpendicular as possible (although it is not that crucial as you will see later). You focus carefully on the center point, and take 3-4 images using an aperture f/2.8, f/4 and f/5.6 or even at f/8. Without changing the position and focus distance or anything else, flip the camera upside down via the lens collar. Take the same set of aperture range. And compare the images in your monitor. So, the area what is taken with the left side of the camera in the normal position is now taken with the right side of the camera in the flip position. A comparison between the two sets should give you a good idea of centering aspect of the lens. Technically, it can be done with the upper and lower but due to the rectangular format of the sensor, it is less pronounce hence we pay more attention to the left or right side only.
Hope this helps but seriously, go out there and use the lens as you envision of using the lens and enjoy photography and don't dwell too much or too long on testing lenses.
Joshua
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