'Bigma' focus question

[Paul Marshall]

Hi All,

I've read much on various forums that lead to me buy this lens recently. But when I stuck it on my camera (D200) I found that the focus point drifts as you change focal length, or zoom.

An example: With the camera on a tripod and focused on a subject approx 5 metres away, at 500mm the lens indicates a distance of, say 4.5 metres. Zoom back to 400mm and the lens goes out of focus. Re-aquire focus and the lens distance indicator has moved to say 5.5 metres. And so this goes on throughout the entire zoom range. The variation in the above example is from 4 metres to 7 metres, static subject and static camera.

I've never come across this on a zoom lens before (and I've used many) and I'm sure this can't be standard for this particular lens, but maybe it is?

For shooting wildlife this obviously is a nightmare. Zoom in to get your focus on a birds eye, for example, then zoom out to get your framing right. And whilst you've done this the lens has gone out of focus! If you're lucky you might be able to re-focus quickly on the correct part of the subject but I think the reality is that you'll get a shot that's not correctly focused. Even worse, with a lens such as this it's quite probable that it'll go off on a focus hunt just as you're wanting to press the shutter.

Someone please tell me that my lens (brand new) is a duffer and this is not a standard feature of this otherwise great lens.

Many thanks,


Paul

[glsammy]

Seeing your post, made me rush down stairs to check my Bigma!

So far as I can see mine doesn't lose focus when changing the zoom level. If it does, it finds the correct focus just about instantly. In fact the focus speed of the lens is one of its strong points.

I suggest you send it back. When I first received my Bigma, it had a focusing fault. I wondered why I couldn't get an in focus shot. Sigma repaired it promptly.

[HowlinWolf]

Paul,

I don't have, or have used, the Sigma lens you refer to, but it appears that this lens is not a true zoom lens. Instead, it is most likely a varifocal design, which is why you have to re-acquire focus when you change focal lengths. It is easier, and therefore much cheaper, for a lens manufacturer to make a high quality lens to this design than a good true zoom lens which will keep focus as you zoom in or out.

HW

[Ollie]

Well I have just checked mine out and it does lose focus when zooming in or out, but as Graham said it's very fast just about instantaneous to re-focus. I never thought it was a fault with the lens and still don't. Just assumed it was common of all zoom lenses as I've never really used any other zoom. That's if I have understood your question correctly
Roger

[HowlinWolf]

Ollie,

It's not a fault in the lens; it's just the way these optics are designed. A true zoom lens is an expensive thing to make, and are relatively rare.

HW

[glsammy]

Originally Posted by Ollie

Well I have just checked mine out and it does lose focus when zooming in or out, but as Graham said it's very fast just about instantaneous to re-focus. I never thought it was a fault with the lens and still don't. Just assumed it was common of all zoom lenses as I've never really used any other zoom. That's if I have understood your question correctly
Roger

My test was a very quick one, looking through my windows this morning so I take your point that it does lose focus, but as Roger said, it re-focuses so fast I've also never regarded it as a problem, it certainly doesn't stop of hinder me in use at all.

[Paul Marshall]

Thanks everyone, at least now I know where I stand. I guess the way to think of it is as a series of primes all in one lens and not as a "true zoom".

I am very surprised that this aspect of the lens design hasn't been mentioned in anything I've read about it (a lot!). I appreciate the point that it is very quick to focus, it certainly is, but it's where it focuses that worries me. If we were talking about shooting landscapes it wouldn't be a problem but potentially fast moving wildlife...I dunno.

Getting to 500mm at a reasonable price is always going to be a problem so I guess it's a case of beggars can't be choosers.

Does anyone know if Sigma's 170-500mm is the same design? Also is the 170 any good at the long end?

Trouble is I'm a TV cameramen by trade and the standard way to focus is by zooming in, focusing and then zooming out. Having done that on a daily basis for twenty years might make using this lens tricky for me. Guess I'd better start saving for a 500mm prime!!

Thanks again everyone,

Paul

[matt_xyz]

presumably the use of AF servo mode on your camera would allow the camera to automatically adjust its focus as you zoom in or out? I've never used the Bigma, but its performance in this regards sounds the same as all other zooms that I've used. I'm not sure many are going to perform as well as a TV camera!

Matt

[glsammy]

Originally Posted by Paul Marshall

Thanks everyone, at least now I know where I stand. I guess the way to think of it is as a series of primes all in one lens and not as a "true zoom".

I am very surprised that this aspect of the lens design hasn't been mentioned in anything I've read about it (a lot!). I appreciate the point that it is very quick to focus, it certainly is, but it's where it focuses that worries me. If we were talking about shooting landscapes it wouldn't be a problem but potentially fast moving wildlife...I dunno.

Getting to 500mm at a reasonable price is always going to be a problem so I guess it's a case of beggars can't be choosers.

Does anyone know if Sigma's 170-500mm is the same design? Also is the 170 any good at the long end?

Trouble is I'm a TV cameramen by trade and the standard way to focus is by zooming in, focusing and then zooming out. Having done that on a daily basis for twenty years might make using this lens tricky for me. Guess I'd better start saving for a 500mm prime!!

Thanks again everyone,

Paul

The Bigma has a better focusing system. The 170-500mm lacks the Hyper Sonic Motor. The Bigma is generally regarded as the better of the two lenses, mainly because of the HSM.
You wont find it lacking at all when tracking moving objects. For flight shots you'll rarely have to zoom out, even if you do the focusing is more than capable of tracking anything in flight.
Here's a couple of examples, taken recently:





I've plenty more in my Gallery.

[NickCantle]

Superb shots Graham