The ideal thing to check lens focusing.
This calibrated lens focus chart is a boom for anyone that is truly critical about sharp focused images. It arrives as a flat card and is quickly assembled into what you see in the listings photographs, it can be taped together for a permanent chart or undone and made flat again for storage. This isn't a lot of money for the piece of mind it delivers, it allows you to reset the focus of your current lenses if needed and you can also use it to check any prospective lens you're considering buying. The calibration requires your camera to be mounted on a tripod for accuracy but if you're focus critical and who wouldn't be, a tripod will already be part of your kit, so this is not a problem for the serious photographer.
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