Why does this lens lose focus when you zoom in but not when you zoom out?

Question

This is regarding the Sony 18-105 E-mount constant-aperture zoom.

I took at photo at 18mm, focusing manually:

I then zoomed to 105mm, and took another photo, keeping the same focus:

Notice that the shot is badly focused.

This problem doesn't happen if you start with a 108mm photo, focusing manually:

And then zoom out all the way:

Why is focus lost when you zoom in but not when you zoom out? Is this lens varifocal only when zooming in, and parfocal when zooming out?

I read the theory that you can see better if you focus when you are zoomed in, but this camera has a 9.6x focus assist function, which lets you see what you're focusing.

Besides, cropping the original photo (the first in this post) gives a better result:

than zooming in (the second in this post). Why is this happening? If my focus was indeed off in the first photo, cropping it and examining the photo at greater magnification should reveal the mis-focus. But the original photo seems to be perfectly focused.

The conclusion seems to be that this lens is varifocal only when zooming in. Is this conclusion correct?

All these shots are with manual focus, F8, ISO 200, aperture priority mode, with a 2-second timer to avoid shake. This is on the Sony NEX-5R.

Answer 1

The lens is not parfocal in either direction.

What you have discovered is the difference between narrow Depth of Field (DoF) at longer focal lengths and deeper Depth of Field at wider focal lengths. The focusing error you introduce when you focus at 18mm and then zoom to 105mm is greater than the shallow DoF at 105mm even at f/8, so you notice how out of focus the photo is. The focusing error you introduce when you focus at 105mm and then zoom to 18mm is well within the DoF at 18mm and f/8 because the focusing distance is much greater than the hyperfocal distance for 18mm @ f/8.

Answer 2

That is simple. Your lens does lose focus on zooming because it is not parfocal.

Depth of field: While zooming out you will shorten the focal length which results in a much deeper depth of field. The gain in depth of field sort of compensates for the inaccuracy of focal pane itself.

The other way around the focal pane moves in the other direction and the loss of depth of field seems to strengthen this effect.

The direction in which the focal pane moves while zooming and the current distance to the sujet, that is supposed to be in focus, plays a role too.

Direction: The focal pane comes closer when zooming in (longer focal length). This results in some stronger visual effect of unsharpness than when the formerly correct focal pane moves away on zooming out while it was rather far away anyway.

Distance: If you focus on something far away with a short focal length and then zoom in, then it may well be that your sujet still stays within the (sort of hyperfocal) depth of field after zooming in.

Try the same on some tabletop set and you will see the effect much stronger.

Answer 3

I just noticed the same effect on my Sony camcorder. Except that for just an instant after zooming in, it is in focus. Then it changes the focus to further away. Then it slowly re-focuses correctly. I think what's going on is that it automatically tweaks the focus further zooming in, and closer when zooming out. The idea being that if you zoom in you're more likely looking at something further away, and if you zoom out you're more likely looking at something closer. It's a way of guessing what the focus should be without waiting for the autofocus to catch up. Now that I know about it, I might want to swap between manual and auto during a shot just for this reason.