Focusing with Zoom Lenses (1 Viewer)

I have been to a local retailer to test their scopes. They only had Bushnell & Leupold brands, so that's what I tried. I wasn't particularly pleased with the image either of them provided & I am leaning towards a Zeiss 65 Diascope after reading many reviews and thoroughly searching this site.

My question is: On the scopes I tested, I noticed that after zooming to maximum power, I needed to refocus to get a clear image. - Is this normal for all scopes, regardless of their quality?

I have never seen this covered in reviews so I'd like your comments.

Thanks. |:$|

Yep, I would say so unless you happen to hit "true" focus initially. I have a 20-60 zoom and the focus is more forgiving at 20x ( there is greater depth of field/focus ),but if you zoom up to 60 invariably a slight adjustment is necessary. However you can zoom back to 20x without any adjustments.

Thanks, I guessed that might be the case but I just wanted to check so I knew what to expect. I didn't realize that you could zoom back down though without refocusing. Now that I think about it in terms of the depth of field though, it makes sense.

Somewhere along threads I've seen, I thought I read that you didn't need to refocus when zooming up in power with the Swarovoski scopes. I see now though, that can't be true.

Even with the Swarovski you need to refocus when increasing zoom.

What is said in posts above is true with the better zooms, but there are zooms where best focus at high power is also not even near to best focus at low powers. The first-generation Leica zoom was like this, and the Opticron zooms I have seen have also required considerable re-focusing whenever you adjust the zoom setting, irrespective of which way you do it. So, the best case scenario is that if you have a tack-sharp image at maximum mag, you can back off to any setting without needing to re-focus. Personally, I do not put much weight on this feature or the lack thereoff.

Kimmo

That was probably something of mine you read, Andy. Both of my Swarovski ATS80HDs are the same in this regard: focus at one point in the zoom range and you will be in focus at other points within the range, subject only to the following:

1: the depth of field is much less at high zoom levels, so to zoom in without refocusing you need to be exactly in the centre of the focus band at low zoom. I.e., focus approximately at 20X and you will almost certainly need to adjust a fraction at 60X. But if you get it exactly right, then you are fine at the higher zoom too.
2: That was an over-simplification I just wrote. At full zoom, the point of focus is very slightly closer than it is at 20X. Focus exactly at 20X, zoom in, then you will need to adjust a tiny fraction for precise focus. With practice, you can do this without looking: it's always the same amount. Going in the other direction, you don't need to bother, as you are still within the depth of field.

I actually rather like this last aspect. Because I generally focus at 60X but always shoot at 20X, it means that I am usually focussed very slightly in front of the bird - not enough to throw the bird out of focus at 20X, but maybe just enough to throw the background a little further out - and that generally makes for a better shot.

Hi there

Yes, I have this problem but LOVE my Zeiss 65 + zoom - it's fab, pin sharp, bright, neutral and lightweight.

Cheers

B