Settle an Argument

[DogHeadGod]

My friend reckons you can zoom in (i'll use my kit lens as an example) to 70mm focus the camera perfectly on an eye then zoom out to 50mm and take the shot and the focus will remain perfect.

I think when you zoom the internal lenses are moving relative to each other and therefore you would need to refine the focus after zooming back to 50mm. I reckon i proved this to her by zooming in to 70mm, autofocusing, zooming back out and pressing the shutter to here it autofocus again before taking the shot.

What say you?

DHG

[RexK_Cozumel]

depends on the lens. Some of them will hold the focus some wont

[Mr Guy]

There's more variance in your answer than you think, and your methodology is highly flawed for proving your point. It is not NECESSARILY true that the act of zooming the lens will alter the focus. The better quality the lens, the better job they do of maintaining focus when being zoomed. On top of that, there's other forces at play if you're relying on autofocus to be your demonstration of what's taking place, the two primary ones being:

A) Your autofocus point isn't necessarily where you think it is. The dots in the viewfinder are to give you a rough idea of where autofocus is looking, however, if you zoom in and out it's perfectly possible that your autofocus points are going to pick different points of contrast to autofocus on. You could EASILY autofocus beautifully on the eye at 70mm, and then as the eye lashes enter the field of view, your camera decides, Nope, THOSE are better for focus and botches your composed shot.

B) Your depth of field changes with different focal lengths. Fortunately, in THIS case, that works in your favor, but it can still screw with your autofocus. Calculate it and see. At 70mm you have a lot less room for error in your focus range. It's possible for your camera to refocus based on that depth of field changed what the camera's AF sensors see, particularly in tricky lighting conditions, but it's also possible for those limits changing to skew what you perceive as focusing "right" and "wrong" at different zooms. You'll notice this more with Canon or Nikon than with Pentax, as Pentax uses a slightly slower focus confirmation method that double checks the focus by bracketing the autofocus.


Focus and recompose should go without saying as being particularly flawed in with this method.

In real world practice, most lenses DO change focus slightly when being zoomed, but that's not intentional as far as I know. You'd need test shots, manually focused while zoomed in, on a tripod, with a stationary subject, being extreeeeeeemely careful to zoom without touching the focus ring, with a well built zoom lens, to prove you're wrong. In practice, you'll appear to be right most of the time, which may be good enough for your purposes.

[Doug Pardee]

What RexK said. A lens that will do that is called parfocal.

The motion picture industry relies heavily on parfocal lenses, because they need to be able to adjust focal length in the middle of a take. Most still camera lenses, however, are not parfocal because you can always refocus before you take the photo.

By the way, the term zoom lens technically refers to a lens that is parfocal. The lenses that we call "zoom" lenses are more correctly called varifocal. Too late, though, we'll probably always call them zoom lenses.

[daft_biker]

Originally Posted by Doug Pardee

By the way, the term zoom lens technically refers to a lens that is parfocal. The lenses that we call "zoom" lenses are more correctly called varifocal. Too late, though, we'll probably always call them zoom lenses.

And just to annoy pedants even more they stick a macro label on some

[m.c.adams]

Not to hijack the thread, but a big THANKS to Mr Guy for the link to the DOF caulculator. Too cool.
Mark

[NetG]

On the other hand - this works pretty well with the level of focus and larger depth of field you get with a point and shoot. That was the trick how I always got focused where I wanted to be when shooting concerts with my point and shoot. I'm having to relearn focus with my dSLR, because it most definitely doesn't work for me now!

[Mr Guy]

Originally Posted by m.c.adams

Not to hijack the thread, but a big THANKS to Mr Guy for the link to the DOF caulculator. Too cool.
Mark

If you liked that one, I find this set invaluable when I'm trying to understand the maths involved (particularly field of view related questions): http://www.tawbaware.com/maxlyons/calc.htm