There is no rational answer to your question. Unless you can afford to buy
"every" focal length, the usual approach to space the focal lengths more or
less geometrically. For example, you might pair 35 and 24, or 28 and 21. You'd
rarely combine 35 and 21, and definitely not 24 and 21. The difference in
coverage is either "too large" or "too small."
I'd suggest that, as you already have an 18-35 zoom (which can provide 21mm
when you need it), a 24mm lens would be a better "match" with the 35-80 zoom.
Going from 35 to 21 is simply too much of a "jump." And as wide as 24mm is, 21
is an extreme wide angle you're less-likely to use.
Now let's _really_ confuse the issue... There are two reasons for selecting any
particular focal length. The first is "practical" -- it either gives a bigger
image or wider coverage when you can't get closer or step farther back.
The second is that a particular focal length has a specific "look" you like.
For example, 300mm provides a significantly "flatter" perspective than 100mm
(assuming you can step back far enough), while 21mm exaggerates depth much more
than 28mm (all questions of coverage aside).
So ask yourself... Do I feel lucky? I mean, do you often pull the zoom all the
way back to 18mm when shooting wide shots (for whatever reason)? If so, the 21
would probably be the better choice. If not, go with the 24.
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