Focus breathing

If you watch Tony Northrup's you tube channel you will know he often mentions the issue of focus breathing, particularly with 70-200 lenses at closer distances.

Having just bought a DA 200mm prime I thought I would compare it with the Sigma 70-200 at 200mm. Both shots at f2.8 from the same shooting position about 9 feet from the subject.

This from the DA 200mm



This from the Sigma at 200mm



A significant difference, as you can see.
edit: I don't know why these are sideways but they still illustrate the point

I'm not completely sure what you're getting at but some zooms seem to appear as if they have shorter focal length at closer focussing distances than they do at infinity. Notably the Pentax 18-250 gets close to 200mm at close distances! Is this what you mean? The DA 200mm should be pretty accurately 200mm but the Sigma may well not be at close focus.

Mmm looking at the photos it looks like either the sigma is around 205 and the pentax at 200 or pentax at around 195 mm and sigma at 200mm. I wouldn't know how you'd go about testing it though.

The Pentax will be 200mm (or should be as it's a prime). The Sigma is noticeably shorter than 200mm at close focus, though it will be a true 200mm at infinity.

All IF lenses focus by changing the focal length. The DA 200mm is probably about 167mm at it's closest focus distance of 1200mm.

Should be around 200mm at infinity, but they are rarely that exact.

The Sigma looks a lot smaller about 83% of the size of the 200mm so it could be about 140mm at 1200mm...... a bit more at 2740mm (9 feet).

That's focus breathing.

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Other way around

Thanks for the clear explanation, makes perfect sense now. The term "focus breathing" is about as obvious as an extremely non-obvious thing, however.

It's odd how these terms spring up, a bit like bokeh. We lived without them for decades and then suddenly they're in everyday use. The amazing power of the internet I guess.

Focus breathing is an issue in video terms, where the 'width' of the shot changes as the camera focusses on items near and far in the scene.

In photography terms, where you're talking one still frame in isolation to others, it's not so much of an issue.