What 400mm do you use? Also what camera?
A crop camera gives you more "reach" with any given focal length. In part, this is a myth (the image area is actually just cropped).... but fact is that using an APS-C camera "puts more pixels on target" with distant subjects, due to a sensor that's much more densely packed with pixel sites.
The angle of view of a 300mm lens on a crop camera is roughly equal to the angle of view of a 420mm lens on a full frame camera. However, a crop camera has more than double the number of pixel sites per square millimeter than a 21MP full frame camera. The result is a bit of "free teleconverter" effect... "extra reach" without the penalty of lost light (as with an actual teleconverter).
Full frame sensor is approx. 24x36mm image area, or about 864 square millimeters. With a 21 or 22MP camera, that makes for roughly 25,000 pixel sites per square millimeter. An APS-C crop sensor is approx. 15x22mm, or about 330mm. WIth an 18 or 20MP camera, that makes for around 54,000 or more pixel sites per square millimeter.
So, the first thing I'd recommend is to try using one of the recent 18 to 20MP crop sensor cameras, if you aren't already doing so. A full frame camera gathers more fine detail by it's very nature, but if you have to crop the heck out of the image to frame a distant subject the way you want, you are going to lose that and more. A crop camera can work better in these situations. (Note: you mention making really large prints.... And for enlargment purposes a full frame camera would be better, but only if you can get closer and "fill the viewfinder" with the subject.)
Next is the resolving power of the lens itself. The Canon 500/4 and 600/4 IS Mark II lenses are incredibly sharp. They also work very well with teleconverters. One of the advantages of using a more modest focal length with a teleconverter (Canon calls them Extenders), is that you get two focal lengths. This can be handy when you simply don't know what distance you'll be working.
For example, say you want to photography large wildlife like deer or elk and set off with an 800mm. At 100 yards, the field of view of that lens on an APS-C camera is about 5.5 x 8 feet. On a full frame camera, FOV is roughly 9 x 13 feet. So with deer, you might be able to get a "tight" shot of a smaller mule deer at 100 yards with the APS-C camera, but would need to "back up" to fully frame a larger subject like an elk or moose. With a full frame camera, you would be able to photograph the larger subjects at 100 yards, but not much closer.
By using a shorter lens with a teleconverter, you are better equipped to handle both situations.... subjects that are closer as well as those more distant. But, adding a teleconverter "costs" both light and some image quality. You have to weight these factors against each other and make a decision, which will work best for you.
Some examples...
700mm (EF 500/4 IS "Mk I" with EF 1.4X II, on 8 MP APS-C Canon 30D camera)...
Now, the 500/4 lenses are pretty big and heavy.... even the "Mark II" which Canon did a great job reducing in size and weight a bit. They are largely "tripod only" or at least "monopod" lenses. A 500/4 pretty much fills a large backpack. The tripods I use with mine are sturdy carbon fiber, lighter than most, but still about 9 or 10 lbs including head and other accessories beefy enough to safely accomodate my 500/4 and 300/2.8 lenses. The 800/5.6 IS is slightly larger than a 500/4, but not as large/heavy 400/2.8 or 600/4. These last two lenses can require more specialized support. There are backpacks designed to accomodate just the 600/4 lenses, with a camera body, a couple much smaller lenses and a tripod.
For more portability, I do a lot of handheld shooting with a 300/4 IS lens, sometimes pairing it with the same 1.4X teleconverter. This lens isn't much bigger or heavier than a 70-200/2.8 zoom.
Here are a couple shots done with the 300mm alone (no teleconverter), on a crop sensor camera (18 MP 7D)...
300/4 IS with 1.4X II Extender (on 7D crop camera)....
Of course it works well on a full frame camera, too. 300/4 IS alone (on 5D Mark II, some cropping done)....
300/4 IS plus 1.4X II Extender (on full frame 5D II)...
Shooting really long distances with very long focal lengths complicates getting a steady shot (more likely to need a sturdy tripod), plus you are shooting through a lot more atmosphere and will see effects from that. Purely experimentally, I stacked both 2X II and 1.4X II teleconverters behind my 500/4 IS and used that rig with a crop camera (6MP 10D).... all on a sturdy tripod, and roughly 500 yards from the subject...
Would have been better with a more current, higher megapixel camera. But, even so, it's equivalent to a 2240mm lens on a full frame camera, there's considerable loss of IQ to using two teleconverters, and even more loss to all the atmosphere between me and the subject. Not really practical, for a number of reasons.
I've been pretty much a "prime only" shooter when it comes to lenses longer than 200mm. However, today there are some excellent zooms that give a lot of flexibility and are worth consideration.
On a crop sensor camera, the Canon EF 100-400L IS is a lot of lens. Very handhholdable and portable, too. (When zoomed and focused so the barrel is retracted, it's not much bigger than a 70-200/2.8 zoom). Sigma has the 120-400 OS, and Tamron has just introduced an affordable 150-600mm. Neither of these are "Canon L-series", by any means.
However, since you are considering an EF 800/5.6 IS, I'm guessing you have a pretty sizeable budget for a lens. That being the case, you might want to consider the EF 200-400/4 IS 1.4X.... This is a pretty amazing zoom that Canon just recently introduced. It's not small or compact by any means, but with a built-in/matched 1.4X teleconverter, it becomes a 280-560/5.6 IS lens at the flip of a lever. That would be a very versatile zoom on a full frame camera, and quite a beast on a crop sensor camera. I'd want at least a monopod, for anything more than a quick handheld shot or two. A sturdy tripod with a gimbal mount would be even better, though it's not as big as a 500/4 IS.
I haven't used the 200-400 1.4X personally, but have seen some people making great images with it. I found an interesting comparison and fairly thorough review of the 200-400 here (I got an obnoxious pop-up banner ad... hope you don't). This is bound to be one of the most versatile lenses Canon has ever offered.
In the end, there is no substitute for simply getting closer to your subjects. Stalking skills, blinds, attractants... and a lot of patience... are necessary for wildlife photography. I worked with the coyote for four months, to be able to shoot portraits of her with a 135mm lens. The black tail mule deer, safe in a county park and somewhat acclimated to people, also allowed me to approach closely (I kept my monopod handy just in case I needed to defend myself!). The ground squirrels were shot at a busy park where they're accustomed to people and tend to ignore them. It's also a great location to photograph birds. .
No matter how long a lens you have, there will always be subjects just out of practical reach. Those are the times I just sit back and enjoy the show.
***********
Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
FLICKR & PRINTROOM
I really appreciate all your information and lens recommendations. I am going to research some of the ones you talked about. I agree that nothing is better that actually being up close to the subject. I will look into the sigma. I purchased a Tamron lens and it was my first non- ef canon lens and while it takes great photos the quality of the body I have not been real impressed with so I was a bit skeptical of getting away from Canon again. I have allied a couple of local camera shops to check in to rentals to see what I was most comfortable using. Thank you again for the time you put into sharing your response with me. This is a picture of a mule deer I shot in Colorado in the wild but I got pretty close, I assumed the local neighbors may have been feeding them.
