K20D AF and Zoom

My present non-DSLR camera (Fuji s100FS) will maintain AF when I zoom, even when in AF-S mode.

As I zoom I hear the AF working, and the image re-focuses.

Does the K20D do this, either with body-driven lenses or in-lens AF drive?

Thanks for any advice

Nick

Welcome to the forum.

With body-driven lenses at least (the only type I have), AF is unaffected by zooming. However, AF-S, after it has finished focusing, won't re-focus if you zoom. You need to activate it again if you want to check or alter the focus - by pressing the button again.

You don't always need to re-focus after zooming, by the way. It depends on the lens.

Thanks Pete.

I am not quite sure what you are saying. Sorry.

When you say AF is not affected by zooming, do you mean it stays in focus as you zoom?

Most SLR lenses are true zooms, so the focus doesn't change with zooming. For example, my 16-45 is tricky to focus at 16 because of the massive depth of field, so I frequently focus at 45 then zoom out to compose.

Bridge and compact camera lenses tend to be vari-focal rather than true zoom to save space, hence the camera needs to re-focus as you zoom.

Ahhhh....OK. I did a lot of SLR work in film, but all my stuff was totally manual. However I just tried my old Tokina 50-250 on my old film camera, and it definitely needs re-focusing. Is it later (D?)SLR stuff that is as you say?

Sorry. Not just arguing. Technology moves so fast....I went from SLR to non-DSLR for several years, and it's all got away from me.I am great at photographic theory, but fings 'as changed!

EDIT: I did read in a review of my camera that it was a problem because you had to re-focus. What I did not realise was that it may have been compliment to the camera (badly written) in that it took care of this.

A word of caution here. Just like some lenses are more apochromatic than others, even if labelled Apo, then some zooms are more true zooms than others.

In other words, many "zooms" do in fact wander slightly in their focus when they are zoomed. It's not a perfect world.

John is right of course, and indeed some lenses are intentionally vari-focal to allow the lens designer to concentrate on pure optical quality at the expense of ease of use. The Pentax FA 24-90 springs to mind, along with some of the early Vivitar Series 1 lenses.

There are two separate issues here. One is whether refocusing is required after zooming. It is in some cases, not in others in practice.

The other question is whether the AF works while you are zooming. The answer to that is yes, but zooming doesn't make AF-S refocus once it has already focused.

An obscure exception to this is when using old Power Zoom lenses. I just got one of these today (FA 28-80)! Zooming with this does automatically make the camera re-focus, even when set to AF-S mode.

Anyway Nick, both zooming and focusing with the K20D is so quick and easy that you don't need continuous AF whilst zooming. Normally you'd zoom in a fraction of a second by turning the manual zoom ring on the lens, then half-press the shutter button (or AF button if you prefer).

What are your thoughts/plans about the K20D then?

[There are two separate issues here. One is whether refocusing is required after zooming. It is in some cases, not in others in practice.]

If it's arbitrary, to me that means it will be needed.If I don't know, I assume the worst case, I guess.

[The other question is whether the AF works while you are zooming. The answer to that is yes, but zooming doesn't make AF-S refocus once it has already focused.]

Sorry, I am completely confused. What do you mean by "the AF works while zooming", if it's not re-focusing when I zoom? Are you referring to AF-C?

As far as my plans go, I am not sure. I am still tossing up whether I can _justify_ the jump to DSLR. I know all the reasons. It's just justifying it. I am also trying to work out whether a DSLR will have enough extra photographic features to make it worth it. The S100FS has problems, but it's a pretty powerful camera.

I mean the AF works whilst zooming if you happen to manually activate AF at the same as turning the zoom ring. This is true for AF-S as well as AF-C.

Note. Power Zoom lenses (which you are unlikely to use because there aren't many of them, and none are particularly good) are excepted from some of the following:-

Zooming: This is done manually by turning/pushing a ring on the lens. Doing this does not activate AF or make AF-S re-focus.

Auto-focusing: This is done by half-pressing the shutter button or pressing the AF button. It only operates while you hold your finger on the button.

Once AF-S has finished focusing, it will not attempt to re-focus unless you lift you finger off the button and press it again.

AF-C will re-focus if the subject goes out of focus, including when that is caused by zooming, but only while you are pressing the button.

In practice with this camera, you will rarely (if ever) need to focus and zoom at the same time. You simply zoom, then re-focus if necessary afterwards. This is a quick and easy process, and usually the image is not so blurred while zooming that you can't easily see what you're zooming in/out on. In fact often no, or only very slight, focus adjustment is needed after zooming if you focused before.

Have a play with a Pentax dSLR in a camera shop and seeing how quick and enjoyable it is to use.

The manual can be downloaded from here: http://www.pentax.co.uk/en/product/19385/body/downloads/Photo_Digital_SLR.html

OK. Thanks Pete. I took most of the above as read (same as my camera). The explanation of the first question is clear now. As I said, I assume that my camera at present "presses" the shutter to maintain focus.

Interestingly, the reason I do not use AF-C on the one I have now is that it is genuinely continuous. All the time. If you listen carefully, you can here it zip zip zip all the time. That's why I avoid it, to save battery and prevent useless wear. I wish it was as you describe.

I really should get to a shop and try one, I know. Trouble is I am a fair way away from such a shop and when I go into town there are a dozen "real" errands to do.

Nick