Need help deciding on ≈100-400mm lense, wildlife photography.

Not considering a extender @RacePhoto. Dont want the quality loss. The "natural" crop of the APS-C sensor is enough.
But a 70-200 2.8 with extender does have a lower f/s. So maybe that would be a possibility.

Thinking of the push pull MK1 canon. Not the new one. Bit to expensive.
Hogna, by now you probably have a lens. If not this may help or at least may help someone else in your position.
I used the 100-400 mkl for ten years. I know that lens inside and out. It's extremelty sharp in the middle and the less sharp edges generally fall into the OOF, DoF areas anyway.
First, on the quality lose using an extender. Quality lose is more myth than truth. Don't get me wrong, there is a lose but you'd be hard pressed to see it even at the pixel level. I've done LOTS of comparisons and pixel peeped them all and it's VERY hard to see the differences. You can spend a lot of time looking for them, that's how unapparent they are. They are there but it's not worth avoiding an extender. You COULD crop but I assure you, none of the cameras you're talking about have enough pixels to deliberately crop to mimic the view of an extender. Deliberately throwing away 1/2 of the mp will lower your IQ way more than adding an extender.
400mm is pretty short for anything smaller than a bear! Birds, reptiles, squirrels etc are really small. It was very rare I ever took the 1.4x of my lens but a crop camera does help a lot.
Anyway you need all the help you can get with small animals. And this is probably just me but I don't use 3rd party lenses. If I'm having a problem and I need help from Canon they can't help me with a 3rd party lens, only the camera portion. Many of them are more difficult to Micro Adjust as well and dead -on focus is critical with small animals.
Good luck with your choices!

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