Critical focus issue with Nikon adaptor

B

blando3

Well-known member
I have an issue that I hope someone can help me with. I have an AF100 with a 17-35 Nikon. I use the Novoflex Nikon adaptor with my Nikons.
I just noticed that with the 17-35, while opened up to F2.8, when I zoom in and set focus, if I then zoom out, the critical focus point changes. If I zoom back in, it returns to my original point of focus. Why is it shifting focus? Does that have something to do with the adaptor? I'm not sure it's always been this way, but I'm guessing it has. The camera has seen very little use. This isn't happening with my Lumix 14-140, fyi.
Any thoughts?
Thanks,
Andrew
 
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The lens is not parfocal because it is not a "true zoom" lens as in cine style zoom.

Some still lenses are parfocal though and will stay focused thru the focal length range.
 
It's got nothing to do with parfocality- that's a very common misconception.
Either the lens is not focused correctly or the adapter is not the right height. The easiest way to check the lens would be doing the same "zoom in, focus -zoom out- check focus" thingy on a Nikon camera to see if the lens behaves the same. Obviously if it holds the focus on the Nikon camera it must be the adapter.
 
It's got nothing to do with parfocality- that's a very common misconception.
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Do tell us more.

But I quote from Wiki :

Zoom lenses (sometimes referred to as "true" zoom) are ideally parfocal in that focus is maintained as the lens is zoomed (focal length and magnification changed), which not only is convenient, but also has the advantage of allowing more accurate focusing at maximum focal length and then zooming back to a shorter focal length to compose the image.[SUP][1][/SUP]
Many zoom lenses, particularly in the case of fixed lens cameras, are actually varifocal lenses, which gives lens designers more flexibility in optical design trade-offs (focal length range, maximum aperture, size, weight, cost) than parfocal zoom, and which is practical because of auto-focus, and because the camera processor can automatically adjust the lens to keep it in focus while changing focal length ("zooming") making operation practically indistinguishable from a parfocal zoom.
[h=2][edit] Cinematography[/h]Zoom lenses used for moviemaking applications must have the parfocal ability in order to be of practical use. It is almost impossible to stay in correct focus (as done manually by the focus puller) while zooming. This could be possible theoretically, but if the subject and/or camera also move, there are too many variables to correct consistently[SUP][citation needed][/SUP].


I also point out I have the same lens as the OP and it is not parfocal and behaves the same on any of my 3 adapters or SLRs.

Thats why I call them varifocal Lenses and not zooms ( as per Cine zooms)
 
OK Both ends of the zoom are corrected easily- long end with the front focus (lens scale) and wide end with the back focus adjustment(mount shimming)
the mid range is also adjustable, but to a limited degree and it's quite complicated (the air space of the variator and compensator section of the zoom adjustment)
parfocality refers to the mid range focus falloff since both ends are easily adjusted and should be correct from the factory.
The OP is talking about focus variation between the long end and wide end-hence my response.
Either the lens BF or the adapter height (or both) are incorrect . The lens may be non parfocal but we won't know that until the focus is adjusted correctly and both ends of the zoom match..
Hope this helps....
 
Cant see that it helps the OP much at all who must now think he has a faulty back focus or faulty focus range set up in the lens or a faulty adapter needing shimming.


Simply .

He needs to focus his shots at any of the points of his focal range with this particular lens and can not rely on the cine technique ( with cine style zooms lenses) of zoom in to focus...pulll back to focal length and trust it to hold focus.
 
Shooter said:
Cant see that it helps the OP much at all who must now think he has a faulty back focus or faulty focus range set up in the lens or a faulty adapter needing shimming.


Simply .

He needs to focus his shots at any of the points of his focal range with this particular lens and can not rely on the cine technique ( with cine style zooms lenses) of zoom in to focus...pulll back to focal length and trust it to hold focus.
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Or.... he can set up his camera and lens correctly and won't have to worry about focus on either end. This would be the technically correct way to deal with it. and the way it has always been dealt with in the motion picture industry (and photography)
 
optitek said:
Or.... he can set up his camera and lens correctly and won't have to worry about focus on either end.
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Nikon lens adapters for MFT cameras don't provide any user adjustments to "set up the camera and lens correctly". If the adapter's flange distance is machined correctly, the lens will focus at infinity when set to its farthest focus setting. If not, the lens will either fall short of infinity, or focus beyond infinity. This can only be corrected by physically modifying the adapter.
 
The reason they don't have "user adjustable" designs is that users are typically not competent to do that. Which your post seems to confirm:beer: I'm not saying that "user" can easily adjust them- I'm saying that I can easily adjust them as well as other professionals (craftsmen, technicians- whatever) that made it their business to do that.
Neither am I saying that "users" should be able to do that- they should not. If they are more power to them.
I hope it's clear now...
 
Despite what a lens technician (maybe) can do with it..

A lens designed for a still camera and by virtue of cost (usually)...does not maintain focus from one to other focal lengths without "re focussing".

The OP asked if it was an issue with his Adaptor.

You have side tracked the issue somewhat!

So provide the OP with a quote to "fix" his lens (guaranteed result) so that his issue is no longer an issue.
 
optitek said:
It's got nothing to do with parfocality- that's a very common misconception.
Either the lens is not focused correctly or the adapter is not the right height. The easiest way to check the lens would be doing the same "zoom in, focus -zoom out- check focus" thingy on a Nikon camera to see if the lens behaves the same. Obviously if it holds the focus on the Nikon camera it must be the adapter.
Click to expand...

What is the misconception of parfocality? This is not a parfocal lens, so why are you talking shimming the lens/adapter? You're really confusing the discussion.

This lens does not even have a hard stop at infinity. "As with most, if not all, AF zoom lenses, the exact position of the infinity mark when focused at infinity will vary with zoom position." http://www.kenrockwell.com/nikon/1735.htm
 
optitek said:
The reason they don't have "user adjustable" designs is that users are typically not competent to do that.
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No, the reason these adapters aren't adjustable is because they are rigid mechanical devices that were not designed to be adjusted at all. What you're talking about is grinding, shimming, and/or bending the mount surfaces to alter the flange distance, aka hacking the adapter. Not that there's anything wrong with that...
 
Paul F said:
What is the misconception of parfocality? This is not a parfocal lens, so why are you talking shimming the lens/adapter? You're really confusing the discussion.

This lens does not even have a hard stop at infinity. "As with most, if not all, AF zoom lenses, the exact position of the infinity mark when focused at infinity will vary with zoom position." http://www.kenrockwell.com/nikon/1735.htm
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Haha since when you believe everything that's on the net:evil:
Didn't your parents tell you?
But seriously- from the "user" perspective the lens is what it is and can't be changed.
From the expert perspective it's easily adjustable to address the OP concern.
Is that better?
 
I'm trying to understand your statement 'misconception of parfocality' and how it applies to the lens in question.

You say a professional can fix the lens so that it will hold focus through the focal length. I don't understand this point.

It's a vari-focal lens and therefore, it's inherent in the design that the focus will change with the focal length. Do I misunderstand?
 
Paul F said:
I'm trying to understand your statement 'misconception of parfocality' and how it applies to the lens in question.

You say a professional can fix the lens so that it will hold focus through the focal length. I don't understand this point.

It's a vari-focal lens and therefore, it's inherent in the design that the focus will change with the focal length. Do I misunderstand?
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Yes the focus may and probably change in the mid range of the zoom.
As I said earlier that's a complicated and somewhat unpredictable adjustment.
What I define as relatively easy operation is matching the focus on both the long and the wide end of the zoom.
That would, naturally bring the mid range closer and only then you can see if the lens is truly parfocal. Or test it on an optical bench which is even less practical.
now I've been involved in design , manufacturing and support of still lens adapters for high end cameras for about 4 years now. We have sold many and this issue comes up occasionally. Up until now, as I recall we have resolved every single issue that came up. But there have not been that many.
One thing we do that no one else does is adjust the height of our adapters very precisely. Of course it is mostly practical on an relatively expensive adapter used on a high end ($20,000 +) camera....I think the budget is the biggest obstacle in getting the adjustments done.
The adjustment itself is not that pricey but if it costs more than the adapter itself most users consider it price prohibitive.
And to do that right one would need both the adapter adjusted and the lens checked and possibly refocused to the standard benchmark.
That could potentially cost few hundred $$$. And that's the proverbial wall most users hit IMO...
 
..and one would end up with a lens that still can not be used as a true zoom because the mechanicals are not up to the task.

Most have plastic components and fairly sloppy engineering tolerences . Even the wear factor would be a show stopper.
 
Shooter said:
..and one would end up with a lens that still can not be used as a true zoom because the mechanicals are not up to the task.

Most have plastic components and fairly sloppy engineering tolerences . Even the wear factor would be a show stopper.
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I rest my case:grin:
The lenses hold up just fine BTW... the urban legend inspired perception prevails however...
 
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