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

Question

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 1

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.

Answer 2

Everything in Mike Sowsun's answer is correct and I concur with his assessment.

There is at least one additional consideration that sometimes gives a current lens such as the EF-S 55-250mm f/4-5.6 IS STM an advantage over an older lens such as the EF 100-300mm f/4.5-5.6: Canon (and other manufacturers are similar) generally supports a lens for around seven years after it is discontinued. After that, replacement parts and service are no longer available via Canon's authorized service network.

This wouldn't be much of an issue when considering lenses in the price range of these two, as just about any repair would cost more than the price of a good used copy of either lens. The higher the price range of the two lenses in question, though, the more this becomes a consideration. At the other end of the scale, if one is considering a lens such as the long discontinued and no longer supported EF 200mm f/1.8 L (which uses focus-by-wire manual focus, so if the AF motor goes bad you can't even manually focus) versus the current EF 200mm f/2 L IS, then the availability of service or lack thereof is much more of a concern.