Well I've been playing with the 400mm almost exclusively for 2 weeks, at least 1000 frames, most on the 20D but a few hundred were on the 10D. I figured - prompted by others - to share some thoughts and observations on the comparison of the lenses.
This is not a professional lens test and I have not measurebated either lens. For the most part these are my opinions, pure and simple, just opinions. There are some facts, and I'll try to be clear about them. And this is a comparison between the two lens, not to any others.
This thread will initially praise the good and great points of the 400 Prime, and may initially seem to be putting the 100-400 in a bad light. This is not the case at all. They each have their place IMHO. So I will make 2 statements to start:
If I had only one lens it would be the Canon 100-400mm L.
It will be a very cold day in hell before you will be able to pry the 100-400mm from my hands.
Sharpness and Contrast and Stuff
As to sharpness and contrast and chromatic aberration and such image quality stuff... Well, my thoughts are not very conclusive. I can say that I think the Prime is a little crisper and has better contrast. But these are things that really need to be measured properly, or at least compared in a measurbating way. Michael Reichman has done some simple sharpness & contrast tests between the two. They favor the 400 Prime, and it's quite visible in his tests.
Let's just say for now that I like the Prime a little more, but I've yet to take a shot that really proves it.
The most impressive thing I've found about the 400mm prime is it's AF speed. Compared to the 100-400 it's a rocket. There is no comparison. You aim, you press, it locks on. I have been able to lock onto blue jays so far away that they're almost the size of the sensor square in the viewfinder. Whether low light or poor contrast it's obvious that the 400 Prime AF performs much better than the Zoom.
That paragraph is a little short, and I'm not sure that's it carries the necessary punch. So I'll add more:
The AF on the Prime rocks! It simply blows away the Zoom. The Prime on a 10D blows away the Zoom on a MkII. I'm not kidding. I'll take the Prime for birds in flight any day of the week, no questions asked. It rocks.
I think the Prime feels a *tiny* bit better than the Zoom when manual focusing. But it's certainly faster to focus manually. I'm sure that any prime's internal focusing elements have to move a lot less than a zoom's. Of course it would seem faster, and make for faster AF.
Now this may not be completely fair... With the 10D and Zoom it was pretty darn difficult to manual focus - almost impossible really, for me at least. But with the 20D and the Prime it's not only possible, but in fact it's quite easy! I've gotten some very sharp shots with the 20D + Prime + 1.4 TC, and it was easy to focus. With the 10D + Zoom + 1.4 TC it took a long time to focus on something and I never got a decent shot.
I highly doubt that this is completely due to the 20D.
I definitely feel that the 20D is better than the 10D for manual focus, and I think the Prime is a *lot* better than the Zoom for manual focus. This is one of the reasons why I think the Prime is better for clarity and contrast. Again, I have no proof shots, and I'm also comparing the 10D to the 20D in these statements so it's not really fair. But I truly feel that the clarity, contrast, and manual focus abilities of the Prime are better than the Zoom.
Size & Weight
The 400 Prime is much thinner than the Zoom. It's several inches longer when stowed, but several inches shorter with the Zoom at 400mm and the hoods attached.
The Prime is listed as being 3 ounces lighter than the Zoom, but it feels a bit lighter still. It's very easy to make the Prime a few more ounces lighter since it's extremely easy to remove the tripod ring...
To remove the tripod ring from the Zoom you have to remove the lens from the camera, loosen the ring tightening screw, turn the ring until the detents match the lugs, and remove. On the Prime you loosen the tightening screw, then pull the screw out to release the lock. The ring then opens on a hinge and comes off. No need to remove the lens from the camera. I like this little feature a lot - simple, but it quickly & easily reduces weight - especially since I often have a large quick-release plate attached. I can leave the plate on the tripod and just pop the lens on and off.
The hood on the Zoom is a big plastic affair that attaches rather clumsily in my opinion. I have never liked this hood - it's difficult to get on since it's easy to tilt the hood and then it doesn't screw in right. It's mildly easier to get off. I just don't like this hood. Though, admittedly, it's not *that* bad, and it does it's job. I just don't like it.
The hood on the Prime is always attached and is metal. A turn releases it from it's friction hold and then you extend it. If you wish - and I recommend it - you can then turn it a couple times to screw it in and lock it. I like this hood a lot better. One thing that I *really* like is that I can now mount a polarizer on the zoom, since I can easily pop the hood down and turn the polarizer to the desired position. Even without this I like the Prime's hood a lot more.
Macro and Close-Up Shooting
Minimum focus distance on the Prime is 11.5 feet - almost twice that of the Zoom's 5.9 feet. For small bird shooters (like me) and butterfly/dragonfly shooters (like me) this is simply a huge difference. When I tried the Prime on a bird on SuperFeeder the Prime could not focus on the bird (even with the switch set correctly - before anyone asks. LOL!). I went back and got my 12mm tube. Still no go, and I had to use the 20mm tube. This causes some pretty dramatic light loss (1/2 stop I'm guessing) on a lens that is already f/5.6.
This has a huge impact on my macro shooting. I like to use tubes rather than close-up lenses like the 500D. The 500D confines you to a pretty small zone, between 18-22" or working distance, so you're also constricted to a limited magnification range. Tubes allow you much more leeway in distance and thus magnification. Also remember the fact that the Zoom has the inherent ability to adjust magnification with either tubes or close-up lenses.
Checking the specifications I found that the Zoom has a Magnification of 1:5 while the Prime gets 1:8.3. I'm going to assume that this is due to the close-focusing abilities of the Zoom. Magnification should be equal with both lenses at 400mm and shooting from the same distance. But the Zoom's close-focusing benefits means that you can get closer thus get greater magnification without tubes, or with less tubes at the same distance. This means less light loss, so higher shutter speeds or more DoF.
There is no comparison on close-up or macro shooting - the Zoom kicks the daylights out of the Prime. If you have *any* desire to do such shooting then the Zoom is your lens. Period.
IS - Image Stabilization
The Zoom has IS, the Prime does not. Man what a huge difference this makes! I know that people love their IS, and rightly so, but when you compare two 400mm lenses with and with out IS it's extremely apparent. You 100-400 owners - go out and spend a day with IS off. You'll really respect it.
So for low-light situations or shaky people, the Zoom wins again, hands down. The Zoom with IS gets you a LOT of pictures that the Prime would miss.
However, IS doesn't fix everything. Even though you can shoot at 1/160 at 400mm, it doesn't means you should. Just about every bird on the planet moves faster than 1/160 even when they're sitting still. Yeah, that sounds strange, but I've got lots of "almost" shots where the bird is in the process of opening it's beak, or even blinking. Big birds are generally fine as long as you grab a standard posing shot. Small birds are a different story. Larger animals or just generally slow ones are fine.
I'm not knocking IS here - I love it, and I want it on every lens. But just realize that it doesn't fix everything.
They're both L. They look it and feel it. They're quite comparable.
Except for the hood.
So how do these lenses handle, how do they work in real-life situations, what's my feeling on using the two... Tough question. I find that the Prime feels much lighter than the Zoom. With the tripod ring removed it's probably 6 or 7 ounces lighter than the Zoom. That's a lot, really.
The skinnier Prime feels funny to hold because I'm so used to the fat Zoom. But I'll get used to it I guess. Though something in between might be perfect. I'd say that I think I prefer the fatter Zoom because it's a bit easier to hold steady. For me at least.
The longer Prime can be tougher to lug around. The Zoom's push-pull feature means that it can get shorter quicker, and this can be nice in the field, depending on the type of terrain. One *could* compact the hood of the Prime, but then you'd want to put the lens cap on, and then you might lose a shot.
Each of these lenses has their place. Each has benefits, and each has some weak points. I own both lenses and will continue to own both lenses. I go out in the very early morning with the Zoom and switch to the Prime when the light comes out. I'm ready to switch at any time.
I like them both. A lot.
Which one should you buy?
Well, I'd say that it depends on you. If you do close-up or low-light photography then I think that the Zoom is the definite answer. If you like versatility then I think that the Zoom is the answer. If all you want is normal (not close-up) shots in bright light, then the Prime will do for you but you'll be missing some benefits of the zoom. If you never do low-light and you want birds in flight or fast AF or demand sharper shots then the Prime wins hands down.
The Final Word
If I had only one lens, it would be the Canon 100-400 f/4.5-5.6 L IS.
But I like the Prime, too.
I'm pretty sure that's it. Then again I'm writing this between tests at work so it's not like I have undivided attention. Please let me know - and I'm sure you will - if I forgot anything or if anything was unclear.