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doug briggs

parfocal vs. varifocal lens

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I would be grateful on any advice on the zoom lens on my nizo 561 macro. I've noticed that, on shots where I've zoomed in on an object and focused, start shooting and slowly zoom out, then on viewing the processed film, the image I'd originally zoomed in on and focussed on becomes out of focus as I zoom out.

 

I researched type of lenses and obviously it comes down to whether the camera's lens is of a parfocal or varifocal type. I understand that any cine camera would have to have a parfocal lens, to preserve the focussing on an image during a shot where the zoom might be changing.

 

Does anyone know if super 8 cameras (and my Nizo 561 in particular) do indeed only have varifocal lenses? I would be very grateful if anyone can suggest a super 8 camera which they know for certain has a proper parfocal lens.

 

I also thought that the correct procedure for focussing on objects is to zoom in to maximum zoom, focus and then zoom out to take the shot. If the camera doesn't have a parfocal lens, does this not therefore make this technique redundant, guaranteed to actually lead to an out-of-focus shot?

 

Any advice gratefully appreciated!

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The lens is out of collimation. It should certainly hold focus, as should any zoom. In fact if it's not designed to, it is, as you say, not properly a zoom but a varifocal. They haven't existed for decades.

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Interesting side note 1: some projection lenses are variable primes in that way.

 

Interesting side note 2: Most - if not all - servo-focus DSLR zooms are really variable primes which rely on the automated focus tracking in the camera to simulate true zoom behaviour.

 

P

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All Super 8 zooms should hold focus, at least within the tolerances of what were often low budget lenses. But if the back focus is off, which is to say the distance from the rear lens element to the film plane is not exactly right, the focus will drift as you zoom out to the wide end of the zoom range. The wider the zoom goes, the more critical the back focus becomes.

 

With macro zooms the rear group of optics can be moved in order to focus at very close distances - if this group does not return to the correct position for normal filming the back focus will be off and the lens will not hold focus. I'm not sure that's what's happening with your lens but it's a possibility. Alternatively the camera may just be out of tolerance.

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All Super 8 zooms should hold focus, at least within the tolerances of what were often low budget lenses. But if the back focus is off, which is to say the distance from the rear lens element to the film plane is not exactly right, the focus will drift as you zoom out to the wide end of the zoom range. The wider the zoom goes, the more critical the back focus becomes.

 

With macro zooms the rear group of optics can be moved in order to focus at very close distances - if this group does not return to the correct position for normal filming the back focus will be off and the lens will not hold focus. I'm not sure that's what's happening with your lens but it's a possibility. Alternatively the camera may just be out of tolerance.

Dom is right on as usual!

 

G

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Dom is right on as usual!

G

Dom deserves a small statue for all the technical answers and good advice he has given.

Edited by Gregg MacPherson

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Many thanks to you for taking the time to reply. I must say I'm a bit confused - reading round a bit today I found conflicting opinions! I should say that the object I've zoomed in on becoming out of focus when I zoom out is hardly noticeable in the viewfinder - only when the processed film is being projected and viewed is it noticeable. But then the viewfinder is a pretty small picture!

 

i also had a look for a nizo manual today, and found this on-line:

 

http://imperfectcinema.com/wp-content/uploads/2011/02/Nizo-801-Macro-Manual.pdf

 

The 801 shares many features with the 561, although I think it's regarded as one of the top of the range of the Nizo Super 8 cameras. In the manual under the section 'Setting the focal length' it staes that the 801 zoom lens 'is actually a variable focal length lens'. I'm not sure whether they actually mean, in saying this, that it's a varifocal lens.

 

Anyway, one last request - I'd be very grateful if anyone could point me in the direction of an experienced camera technician in the UK who might be able to adjust my lens, if this is indeed the problem.

 

thanks again

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Dom deserves a small statue for all the technical answers and good advice he has given.

That's very kind Gregg but I'd prefer beer actually. :D

There are of course many people with a great variety of experience who make up this valuable community, who generously share their knowledge and some that have done so consistently for quite a few years, with no expectation of statues or beer. Just helpful people. Hooray for them I say! It's just nice to be able to feel part of this community.

 

In the manual under the section 'Setting the focal length' it staes that the 801 zoom lens 'is actually a variable focal length lens'. I'm not sure whether they actually mean, in saying this, that it's a varifocal lens.

Believe me Doug, your Nizo zoom was designed to be parfocal, like every cinematography zoom except the very earliest ones. In the manual you link to, under "Faults and Remedies" at the end it mentions focus problems, with the remedy being first to make sure the eyepiece diopter adjustment is correctly set, and second to always set focus at the telephoto end of the zoom. No point saying that if it's not parfocal. It's really only some still photography zooms (and as Phil noted, some projection lenses) that aren't designed to hold focus adequately through their focal range.

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