Weird Image Shift issue with Nikon Zoom

[Frederic Sturm]

Hi All,

 

I think this is my first post here, usually I'm much more active over on the Steadicam forum.

 

Recently, I have encountered a weird issue with my Nikon 24-120 f4 zoom, and am hoping someone
can shed some light on this: pulling focus causes the image to shift left and right, up and down,

abruptly and -I think- randomly. This is definitely not what would usually qualify as breathing.

 

I have isolated all other causes like pushing the lens itself around a bit, it is definitely just due to turning
the focus ring. The faster it's turned, the worse the shifting, but even on very slow pulls it shifts at some
points.

 

Usually I'm not a big Fan of stills lenses, but for a recent documentary shot, it seemed like this
was a good allrounder on my BMCC, and it's a really nice and sharp lens overall. Now, of course I'm
used to this kind of shift happening when changing focal length of a stills zoom, which is why it's totally
unuseable in shot and I really use it as a "variable" prime, but while pulling focus?

 

Has anyone ever encountered something similar, Is this possibly due to a loose element in the lens,

i.e. might it need repair, or is that just the way it is with some stills lenses?

 

Thanks and all the best

 

Frederic

[Dom Jaeger]

Image shift is a problem I come across pretty regularly when servicing lenses. When a lens is focussed, the focus group (or sometimes the whole optical block) is mechanically moved along the optical axis. If there is play in the focussing mechanism that allows the elements to slightly cock or shift sideways, it can introduce some amount of image shift. Whether you notice it or not depends on a combination of the light-bending power of the elements and the severity of the play. It tends to occur when you change the direction of focus, and is also (obviously) more noticeable in a locked-off shot than a hand-held one.

 

I don't know that particular zoom, but stills lenses often have this issue in some form or other, since it's not a technical requirement for still photography that the image stays perfectly stable as you focus. However if the shift is excessive then it may be due to a loose focus cam follower screw, a loose element, damage or wear. If you can access another example of that lens to test you could determine if it's normal behaviour or not. Or someone more familiar with the lens might chime in.

[Frederic Sturm]

Hi Dom,

 

wow, thanks so much, I couldn't have hoped for a better answer!

 

It is noticeable, but I wouldn't say excessive. And you're absolutely right, it's most noticeable

when changing direction, but also when starting a focus pull.

 

I was planning to compare my zoom to another sample, and now know that it's worth the effort.

And another reminder for me to be extremely careful about testing all aspects of a stills lens

before using it for video.

 

Thanks!

[Dom Jaeger]

You're welcome Frederic.

 

You might find that another sample has slightly more or less shift, consumer grade lenses can exhibit quite some variation in build tolerances and quality.

Read for example:

http://www.lensrentals.com/blog/2011/10/notes-on-lens-and-camera-variation

[Frederic Sturm]

While -in still photography terms- this one really is a quite solidly built pro lens, I'm aware of variation in all kinds of gear, pro or consumer, of course. That link is really interesting, wouldn't have thought product-to-product variation to be that evident on lenses! Still a comparison should be good to tell if a certain degree of this shift it's normal or if I have an issue with my copy.

 

 

In the end however, this reminds me to be really careful about testing a stills lens before using it for video. While I'm not a fan of that practice at all, it sometimes does come in handy, especially on documentary-style shooting.

Edited July 23, 2014 by Frederic Sturm