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Better than parfocal?


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I've been playing with my Canon 700D, Magic Lantern 3x crop mode, and the 55-250 zoom lens (which is 264-1200mm equivalent in crop mode) and I've noticed a funny thing about it.

If my kid is on the far side of the football field and I zoom all the way in to 1200mm and then manually focus (which is horrible at this distance on this lens BTW) then when my kid runs towards me and I naturally zoom out, the lens shifts its focus closer to me, effectively keeping my kid in focus, and actually helping me!

I know about parfocal lenses that maintain the same focus distance throughout the zoom range, but do lens manufacturers deliberately design lenses so that zooming out helps you to maintain focus like this?  Or is it just a random happy coincidence it zooms closer and not further away?

Considering that the difference between focusing on players on the other side of the football field and focusing on the cars and houses on the other side of the field is a smaller tweak of the focus ring than there is free play in the focus ring, it's pretty difficult to adjust focus, and combined with the Canon screen and lack of good focus assists (that are fast and clear enough to keep up with sports) it would be nearly impossible to film at larger apertures without this lens behaviour!

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3 hours ago, mercer said:

I know that was a trick we used to do with camcorders, zoom in really tight on your subject, get focus and then zoom out and your subject is still in focus. 

that's because it's a parfocal lens. with old cameras, you didn't have punch in focus. you zoomed with your lens, focus and the frame the shot.

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