old lenses - Canon EF 75-300mm f/4-5.6 III vs EF 100-300mm f/4.5-5.6 USM

I'm using a Canon 75-300 III, the non-USM version, that came in a "kit" along with my Rebel T6. I like its reach, but the optical quality isn't great, even if you step it down a bit, and it takes forever to focus.

Then I came across a used 100-300mm lens, a (very) old lens, but maybe optically better/faster to focus than my 75-300.

I have two specific and very related question in this context:

1. How would the two lenses compare?


2. What could be the issues of buying such an old lens, even considering that the store gave it an 8+ rating, meaning "Used very little, but obviously used. No major marring of the finish or brassing. Optics perfect. Mechanics perfect."

Answer

The 100-300mm f/4.5-5.6 USM is a better lens than the 75-300 III. It is sharper, has better build, and most importantly has very fast Ring Type USM Auto Focus. It also has a non-rotating front element which is great when using a polarizer.

A +8 rating from a reputable store means you should feel safe buying it and could always return it if there is a problem.

Still, I would NOT advise you to buy the 100-300 mainly because of it's lack of sharpness and lack of Image stabilization.

A MUCH better option is the EF-S 55-250mm STM. It will cost you $199 at that same seller, but it will be well worth it. It is much sharper, has great STM AF, and most importantly, it has Image Stabilization. It also has a non-rotating front element.

It is so sharp you can crop the 250mm image to match a 300mm field of view and you will still have a better image.