When we finally see the lens we have been looking for sale in the used section of a forum , it is easy to lose your head and buy it without looking at its overall condition. So here is a list of all the questions and points you should look for.

How old is the lens?

You ask this mostly to know if the lens is still under warranty or not.  While you are at it, ask for the date code to make sure the sellers info is correct.

Does it have any cosmetic damage?

While these dont change anything about the performance of the lens, it will decrease its value if you ever want to sell it back. So its value should be less.

If it is a weather sealed lens, look for damage on the weather seal ring

Unless you are planing to use the lens on a weather sealed body, it wont change anything about the lens performance, here again, it is a negotiation point!

Are there any dust spot inside the lens?

As time passes by, some dust will find its way inside the lens. While it is possible to clone the spot out in post production, the better solution is to send the lens to the service center. Another negotiation point!

Any scratches on the lens front element?

Lenses with damaged front element are very hard to sell because people usually dont want to buy them. So if you can get one with a huge rebate, go for it and send it for repair (call for for an estimate, prices fluctuate a lot depending on the lens!)

Does it come with original packages?

This is another thing that does not have any effect on the image, yet from experiences lenses that come with the whole packages sell faster and for more money than those without.

How good is the focus?

Mount the lens on your camera, go into AI Servo focus and while keeping the focus button pressed, focus on items at various distances. You want the focus to be smooth and gradual. Do the operation a few time. If the focus is not smooth, only get the lens if you are going to use it mainly for video. Focus issues are costly to repair (cost 300$ to fix a 400$ lens in my case).

Play with the focus and zoom ring

You want both rings to be smooth. A little bit of stiffness or slack is tolerable but keep in mind that it will only get worst as time passes by. If you plan to use the lens with a follow focus for video, you want the ring to be perfectly smooth.

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