Who Chose the 300 f4 over the 100-400 for Birding and Why?

dv312 wrote:

I've considered both before settling down for the 100-400mm

I have the 75-300mm also and it may be for sale if I stop using it (not touched since I had the 100-400mm)

1) If money is no issue , I'd have gotten the 300mm too

2) The 100-400mm is already a hefty lens, the 300mm even more, so that's something to consider if you have to lug it around on long treks; I do so weight saving is a must for me

I cover 6-12 miles on a day out photographing gators and birds. Compared to using a Canon or Nikon of similar reach, the 300mm is very light. It really is not that heavy and can easily be handheld all day long.

3) Love to have the f4 on the 300mm but for birding reach at 300mm may (will) not be enough based on my experience ; adding a tele converter would reduce light to almost the same as the 100-400mm; not to mention adding weight and size; the extra 100mm is sorely needed if you 're serious about birding

I have to disagree. Some skill at getting closer is worth more then extra reach, especially for birds. Now dangerous animals can be a different story but they tend to be bigger and don't need as much reach anyways. I seldom need more then 600mm of reach, those times I do it is for special situations that come up rarely. As Danny says in his very good post about using manual focus lenses "Contrary to what a lot of people think we do with long tele lenses, we use them at close range. The closer to the subject the better and that applies to any tele lenses really. That's where the details are, in being close to the subject. The idea is to let the subject get as close as possible before you push the button. You are better off not taking the shot and wasting time if its too far out to start with. You need to learn what you can get away with using PP and cropping after the shot. Learning that saves a lot of wasted shots."

Example of getting close.  Was so close he filled the frame, had to add canvas and use content aware to improve composition.

4) I use the shorter end of the 100-400m to shoot landscapes; much less so with the 300mm

Most have other lenses for that type of work and landscape is not why people would buy a 600mm effective prime for.

5) I'd think both have good enough IQ for everyday's use; should not be the deciding factor for selecting one over the other; the 4 points above are more compelling IMHO

The IQ in ideal situations is not the problem, it is when the situations are less then ideal.  Getting photos accepted to stock agencies with µ4/3 can be challenging when pushing the ISO.  The 300mm is going to help keep ISO lower which improves the number of photographs you can get accepted.

Cheers,