Andy,
What do you mean by this - my lenses aren't good enough or I need something longer for wildlife?.
Thanks for taking the time to reply
I've just talked about this in other threads. Basically, the lenses are designed for full frame and their lower pixel density. They have more than sufficient resolution to work well with full-frame cameras. Your APS-C 40D has the same pixel density as roughly a 25.6MP full-frame camera, so won't push the lenses and will work very well. However, the 18MP 7D will have a higher requirement on your lenses (it's like a 46.8MP full-frame) to get pixel-level sharpness. They aren't bad lenses - you just need to be aware of their limitations. Certainly what I've seen of a 300/4+1.4xTC on a 50D (15MP) doesn't stand pixel peeping - the image looks soft if cropped hard, but looks fine if you scale it back to the 40D 10MP level. The 100-400 will have similar but potentially worse issues as it isn't as sharp as the 300/4.
Discussed here:
http://www.talkphotography.co.uk/forums/showthread.php?t=206508
This is the reply I gave to TheNissanMan (apologies if this is pitched at too basic a level):
Originally Posted by TheNissanMan:
One thing that you do bring up in that post is APS-C & full frame... As a noob thats right over my head...
Your camera has an APS-C sensor - a so-called 'crop' sensor because it is smaller than the so-called 'full frame' 35mm sensor (which are about the size of an old 35mm film negative). The aspect ratio of the sensor (its width vs its height) 3:2 - this is the same for the full and APS-C sensors.
The crop factor is 1.6 for Canon APS-C cameras - so each side of the sensor rectangle is 1.6x smaller than the 35mm full-frame sensor. This means the area of the APS-C sensor is 1.6x1.6x smaller or 2.56x smaller.
...
So, a 10MP full-frame sensor has pixels that are 2.56x bigger than the pixels on a 10MP APS-C sensor. Or another way of thinking about it is, the pixels of the size they are on an APS-C 10MP sensor would fill a full-frame sensor with 25.6 million pixels.
The impact this has on lenses is... If your lens has just enough resolution (has been engineered in such a way) as to be able to show a detail that covers just one pixel on the full-frame, it implies that if you have a full-frame sensor with more pixels, your lens will be the limiting feature here and you won't be able to see any more detail in your pictures. The lens is designed with a target resolution in mind - to make it 'sharper' - i.e. so it can resolve smaller details, would make the lens more expensive, possibly bigger too and require the use of specialised glass.
If you then put the same lens on your APS-C sensor camera with the same number of pixels as your full-frame sensor, the smallest detail the lens can 'see' will be spread over 2.56 pixels of your APS-C sensor. To make use of your smaller sensor, you need a lens capable of 'seeing' these much smaller details. This is a bit simplified as there are other factors such as fill-factor, diffraction limits, AA filters etc., but it is broadly correct.
The reality is that most lenses that work on full-frame and APS-C body cameras have more detail-resolving capability than the current full-frame sensors and less than the current APS-C sensor cameras (You may notice that the number of pixels in a full-frame body is usually a lot less than 2.56x more than the APS-C bodies).
This is why in controlled tests (see DPReview as an example), they will do two tests, one with an APS-C body and one with a full-frame body. You will notice the same lens appears to be much 'sharper' on the full frame body than on the APS-C body for the same aperture and focal length value. This is no surprise given the lower demands on the 'smallest level of detail' that the lens must resolve on the full-frame body rather than the APS-C body. If you add in a teleconverter, the demands on the lens sharpness are even more acute.
Canon do have EF-S lenses, designed to be APS-C-only lenses. These are designed to only focus light over the smaller APS-C sensor (this circle of projected light is known as the image circle) and can be made smaller and use less glass. They can also be designed to cram more detail into the smaller image circle without making the costs go astronomic.
Unfortunately, Canon want you to have your APS-C camera then move onto the full-frame cameras in time (for very understandable commercial reasons). As such, they have not made the effort to produce many top-notch APS-C only lenses - they would prefer just to make the full-frame L-series lenses. They don't work as well on the APS-C cameras as the full-frame - they were designed for full frame. To design a full-frame lens with the resolving power for the highest resolution APS-C cameras would be 'wasted' if it was used on the current full frame cameras and would make the lenses prohibitively large, heavy and expensive.
I hope that goes some way to explain the full-frame vs APS-C and what relevance it has to your lens choice...
Andy