I've been playing around with photography for close to a year now, almost all of that has been wildlife so the couple of lenses I have bought have been longer focal length. Ocassionally though, I see a flower or an insect I love. I haven't had bad results just using whatever lens I have on at the time. Below is a crocus taken with the kit lens (Canon 18-55 mm f5.6 ) and a bumblebee (Canon 100-400 mm also a f5.6 the only good piece of glass I have, my third lens is the Canon 55-250mm my camera is an old Canon Rebel T4i). I don't think I am ready to splurge on a macro lens yet. So, if I want to take a close up should I be using the the widest lens I have, does it matter?
Has anyone, yet, mentioned the Canon 500D Close-Up adapter? If the OP's 100-400mm lens is the II version, it should play well with the 500D Close-Up adapter, which threads onto the lens, in the manner of a filter.
The EF 100-400L II IS has a magnification ratio that is already quite good, and a rather short Miminum Focusing Distance spec, so it is, already, more suited than most telephoto lenses, to close-range photography.
Notably, the 500D can be used at the same time as extension tubes.
Cheaper, single-element screw-on close-up adapters do not usually perform as well as the diopter-type Canon 500D. My wife learned this, when she bought a set of single-element close-up "filters." (She is a Nkon shooter, but the 500D will work on Nikon lenses, too.)
I have not (yet) personally used the 500D, nor extension tubes, because I was fortunate to start DSLR photography with a bag of pre-owned Canon gear, that included a quite nice Tokina 100mm f/2.8 AT-X Pro Macro lens. Indeed, it was an interest in close-range and macro shooting that prompted me to start DSLR shooting. I soon added an EF 100/2.8L Macro IS, in order to be able to shoot in all weather conditions. (I was a police officer, at the time, and there was a new emphasis on photographing evidence, at more types of crime scenes.)
I have no personal experience with close-up filters, but all that I have heard or read from those who have used them is that they are a definite compromise in terms of quality of result. I chose not to comment on them as I have not used them, so I did not include them in my posts.
If you care read the thread through (always a good idea), Waddizzle had this to say about close-up filters:
"The macro filters and adapters that screw on to the filter threads of a lens are not very good, because they are habitually made form low quality glass"
""The macro filters and adapters that screw on to the filter threads of a lens are not very good, because they are habitually made form low quality glass"
I doubt seriously he has ever used them. It is his opinion not a fact he knows. My personal experience because I do have a full set of close-up filters, you know their are several powers, is they do work. However, at today's current level of camera and lenses and post editors anything with few exceptions that screws on to the front of a lens is not good.