I’d been debating about lenses to take with me on a wildlife trip to parts foreign, Namibia, in this case. If my subjects were to be wildlife only, the choice would be very straightforward; all I would need would be my super-duper  Canon 100-400 mkII L series Lens. With its close-focus distance of 0.9m, it does for close-ups of insects, too. The debate comes in with any landscape work, which we also have thrown into the mix.

Going from wide angle to the 100mmm starting point of my long zoom lens is less than straightforward with an APS-C Cropped Sensor Camera. Canon, makes some lovely L-series lenses for a full frame camera but they really only have consumer lenses covering that range on a cropped sensor camera. I started out thinking I’d make do with our Canon 17-55mm F2.8 lens and ignore the gap up to 100mm. Then I remembered my Sigma 18-300mm F3.5-6.3 “travel” lens. I was expecting the image quality to be lower but I thought I’d do a test. I set up a shot and took it on both lenses, each set at their 55mm Focal Length.

To my surprise, the Sigma 18-300 seemed to be both sharper and clearer (maybe it has more contrast) than the Canon. Right, decision made: not only is my travel lens sharper but it’s also a little lighter and covers a much greater range overlapping with my primary lens. The weight could prove critical ‘cos we’re limited to an 8kg carry-on limit.

Then I started wondering about a comparison between my long Canon professional lenses, the 100-400 mentioned above and my Canon L-series 300mm F4 prime lens. Now I was in for a real shock.

I set up a couple of tests using a tape measure to compare image size. My limiting factor was the Minimum Focus Distance of the 300 mm prime: 1.5m. Both other lenses the 100-400 L series and the could be set to 300mm focal length and could focus at that same distance. Click, click, click.  Here’s a composite of my results.

Three different images but all of a ruler at 1.5m distance, taken on lenses all apparently set to 300mm focal length. All three images are clearly different sizes but they should all be the same, right? So what’s going on?

What’s going on is known as focus breathing, a bizarre name for a symptom whereby a lens’s actual focal length varies at different focus points. In essence, it gets weaker at closer focus distances.

The top image is, if you will, the default – it’s the Canon 300mm prime lens and shows a little more than 9cms of the ruler.

The middle image is taken with the C anon 100-400mm lens set at the same 300mm focal length (I checked using the EXIF data). It shows almost 12cms of ruler, 33% more. The 100-400 is known to “suffer” from noticeable focus breathing so I was expecting some difference.

What blew me away, though, was the third image, the result from the Sigma 18-300mm lens set at 300mm. It’s showing a whopping 19cms of ruler, fully double the amount of the 300mm prime benchmark.

In other words, the image from the 300mm prime lens is a thumping great four times the size of the image from the 18-300 lens. Yikes! Now THAT’s what I call focus breathing.

The travel lens is still good; I’m glad I know what’s going on, though.