Originally posted by FozzFoster I might get a public lashing or two for suggesting this:
Maybe they are making a 70-200 that doesn't focus breath...
It's funny thing this focus breathing. Extending the lens in and out while focusing inhales and exhales air, which is actually real breathing and will lead to dust getting sucked into and collected in the lens. Internal focus largely neutralizes this behaviour.
Hence my F 70-210 breaths every time I focus. My DA 60-250 with internal focus doesn't breath the same way, but it's a "focus breather." Looking at the same two lenses, you will also notice that on my 70-210, the IQ falls of at the long end as the front element extends. The 60-250 on the other hand maintains constant test chart resolution all through it's range.
Focus breathing is a poorly conceived term not understood by most who use it and coined by as far a I'm concerned someone ignorant of the factors it addresses. Start with the basic physics, as the lens moves forward to focus, the resolution in the centre of the lens is spread over a wider area, meaning at the longest point in it's range it has a fraction the resolution it had when closer to the sensor.
So your choice in focus breathing is, do you want to maintain resolution through the whole range of the lens and limit dust being sucked into the camera and lens, or do you desire the best possible resolution only at the farthest focusing distance and more dust in your lens and camera? Focus breathing also means, the lens has been optimized for optical performance, not strictly magnification without regard to test charts performance. There's no free lunch.
Focus breathing lenses are the best possible technical solution.