When is 500mm not 500mm?

When it's a zoom.

As we'll be selling on our Sigma 150-500mm soon I thought it would be good to finally compare our telephoto lenses for focal length and sharpness. We had upgraded our 150-500mm to a Canon 100-400mm on the assumption that it would give better results but we have never performed real side by side comparisons.

Shots were taken with our 500mm f/4 (with and without 1.4x extender), 100-400mm and 150-500mm all at maximum aperture and maximum focal length, at around 4.5m distance. All shots were taken on a tripod mounted 1Dmark4 in manual mode. Here is a combined image of the results:

Combined images after cropping to same physical size

The images were cropped to the same physical image size (something bird photographers will often want to do), in this case 9cm, and exposures pushed/pulled to be roughly equalised.

It can be easily seen that the 150-500mm is nowhere near 500mm at close distances. Using the pixels/cm rating of each shot the Sigma works out to be around 415mm and Canon 100-400 works out to be 351mm. This is quite an extreme example, most birds won't be this close!

At the same physical size at f/5.6 or f/8.0 the 100-400mm gave sharper and more detailed results, even though it was with fewer pixels. This gives some weight to the claim that although the Canon has less focal length you can more than make up for it with superior image quality.

The exposure changes are interesting as well. The base exposure was fixed with the 500mm+1.4x at f/5.6. At the same f/5.6 the bare Canon 500mm was 2/3stop brighter, Canon 100-400mm 0.5stop brighter and Sigma 150-500mm 0.1stop darker. These results have a wide margin of error but the Sigma clearly has a lower T-stop rating (ie lets less light through the glass).

The main conclusion is that optically the Canon 100-400mm is a better lens than the Sigma 150-500mm even when distance limited (the Sigma does have a price and IS advantage, though). The marginal extra focal length doesn't make up for the poor image quality and slow optics, particularly wide open. We're glad we made the change!