Canon 300 f/4 IS vs Canon 300 f/2.8 IS

Canon 300 f/4 L IS USM and its big brother Canon 300 f/2.8 L IS USM. Both lenses without lens hood. Click here to enlarge.
 
Since early 2008, I own the Canon 300 2.8 L IS, that is considered one of the finest Canon lenses ever made. The Canon 300 f/4 is $2800 cheaper, much smaller and lighter: the 2.8 is really worth the price and weight difference? I have tested the two lenses side by side to answer to this question.  
 
In terms of built quality, the two lenses are similar - sturdy and well built - except for weather sealing: the f/2.8 is sealed, while the f/4 has no weather sealing. Without the large hood, the f/2.8 is not much longer, but it is clearly wider, and it is about 1 kg heavier. Both lenses have "2 stops" image stabilization, even though the IS of the 2.8 is slightly more advanced (it can be kept active when the lens is mounted on a tripod, as far as the support is not too stable).  
 
Both have a fast ultrasonic AF motor, with full time manual focus. Without teleconverters, both the f/4 and the f/2.8 are very fast, even though the f/2.8 is a little better. With the 1.4x TC, the f/2.8 has a more marked advantage, and with the 2x TC the difference is huge: on prosumer SLRs the f/4 does not even work in AF with the 2x, and on 1 series SLRs it is extremely slower than the f/2.8 + 2x. Other than that, the f/4 has a two steps focus limiter, while the f/2.8 has a three steps limiter, that gives more choices (and in some situations it can make a big difference).
 
 

Specifications

 
 

Image quality comparison

I have tested the lenses on my Canon 1DsIII (21 megapixel, FF). The lenses was mounted on tripod; I have used mirror lock up and self timer. The following images are 100% crop from the center of the unprocessed RAW file.  
 
 
 
300mm:

Without teleconverters, both lenses offers excellent image quality, even though the 300 f/2.8 has an extraordinary sharpness that the 300 f/4 never reaches.  
 
 
 
420mm (300 + Canon 1.4x TC):

At f/4, the 300 2.8 + 1.4x gives about the same image quality of the 300 f/4 + 1.4 at f/5.6; at comparable apertures, instead, the 300 shows better sharpness and contrast.  
 
 
 
600mm (300 + Canon 2.0x TC):

At f/5.6, the 300 f/2.8 image quality drops, even though it is still usable; with the 300 f/4 + 2x this aperture is not available. At f/8 and f/11, the 300 f/2.8 shows more detail and contrast, even though the difference is not huge. Other than that, the 2.8 has less chromatic aberration.
 
 

Macro capabilities

This is a test photo of my small Phalaenopsis orchid, with a big print of the tree frog into background. The difference between the 1:4 macro ratio of the 300 f/4 and the 1:8 macro ratio of the 300 f/2.8 is huge - the 300 f/4 focusses much closer, so you can get much more magnification.  
 
If you want a telephoto lens with "near macro" capabilities, the 300 f/4 is a better choice than its big brother.
 
 

Conclusions

If you are looking for the very best image quality, there are no doubts that the 300 f/2.8 is the winner; other than that, being 1 stop brighter may be a bit advantage in low light, or with prosumer SLRs that focus only up to f/5.6. That said, the 300 f/4 performs admirably for its price - it cost a fraction of the 2.8, but it gets quite close in terms of image quality and it has even a much shorter minimum focussing distance, that gives better macro capability.  
 
In conclusion, even though personally I have preferred the 2.8 for its image quality and aperture, if your budget is limited you won't regret the 300 f/4: it is a truly excellet lens for its price, with an image quality close to the IQ of professional superteles.