" I only want to take two lenses with me on holiday. "
This is different than what I assumed you wanted. I thought you wanted a replacement lens for the heavy zoom 100-400mm. But you want a combo to compliment it? My suggestion (ef 400mm f5.6L) will certainly fill the bill for a better and lighter lens but it is in the same use class.
A lens to compliment your current zoom would make the ef-s 18-135mm a reasonable choice. The ef-s 17-55mm f/2.8 is usm would be a great addition. It is better in several ways. It has a fast, constant aperture and pretty good IQ.
Hello, thanks for all the suggestions and sorry I've not been able to post on here for the past week. I've decided to sell most of the lenses I own including the 100-400 L and upgrade to the 100-400 II. The new lens focuses from 3.2” which means I only need to take one lens on holiday with me. I’m trying to travel as light as possible and I think this is a good solution. Thanks again for your help.
You're equipping yourself pretty well, but for telephoto photography only. There's a vast array of subject matter out there which can be captured only with wider lenses, and you might not want to blind yourself to opportunities in that regime. I'd caution you to remember the old adage that "When the only tool you have is a hammer, everything looks like a nail."
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