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Vario-Switar 18-85 2.5 - is it parfocal?


Jay Bawar
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Hey guys,

Just got the 18-85mm vario switar for my rex4 bolex and the focusing is throwing me off a bit. 

First, is this supposed to be a parfocal lens? 

Secondly, I think there might something wrong with the lens/my camera as when i focus it to infinity through the bolex and zoom in to 85, the picture is not in focus (obviously aimed at something really far away). I have to pull focus to about 20 ft or so for it to be sharp zoomed in. Is this normal? 

Pretty new to this so any help is appreciated 🙂

Cheers!

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Any zoom made for cine use has to be parfocal. Some are better than others at keeping perfect focus, and of course they need to be correctly set and not too worn. But your Kern zoom should hold focus through the range.

An important rule with cine zooms is to always focus at the long end (zoomed in) first, and then pull out to whatever wider view you want. You can’t focus zoomed out and then expect focus to hold as you zoom in.

If you find the distance marks are out when you focus at the 85mm end (which sounds like this is the case if infinity is sharp when the scale reads 20ft) then the lens focus scale has possibly slipped and needs re-setting. The other possibility is that someone has removed the front focussing section and then screwed it back in starting on the wrong thread start. 

If you find the focus goes soft as you zoom back out to the wide end, then the lens needs its back focus adjusted (or possibly the camera flange depth itself is out).

You do need to make sure you have set the camera eyepiece diopter for your eye, so that the ground glass grain is visible.

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57 minutes ago, Dom Jaeger said:

Any zoom made for cine use has to be parfocal. Some are better than others at keeping perfect focus, and of course they need to be correctly set and not too worn. But your Kern zoom should hold focus through the range.

An important rule with cine zooms is to always focus at the long end (zoomed in) first, and then pull out to whatever wider view you want. You can’t focus zoomed out and then expect focus to hold as you zoom in.

If you find the distance marks are out when you focus at the 85mm end (which sounds like this is the case if infinity is sharp when the scale reads 20ft) then the lens focus scale has possibly slipped and needs re-setting. The other possibility is that someone has removed the front focussing section and then screwed it back in starting on the wrong thread start. 

If you find the focus goes soft as you zoom back out to the wide end, then the lens needs its back focus adjusted (or possibly the camera flange depth itself is out).

You do need to make sure you have set the camera eyepiece diopter for your eye, so that the ground glass grain is visible.

Thanks Dom! The ground glass is focused to my eye and i have a berthiot 10mm that focuses perfectly through the viewfinder at all distances. 

Actually I forgot to mention that the focusing ring when i first got it would skip frequently when pulling focus too fast. I think i helped it by applying pressure and turning the focus wheel. Im not sure if this helped spread grease throughout the lens, im no expert!! It still skips from time to time. Probably the lens needs a CLA. Do you know anyone in North America that can do this? Im in canada and i dont think Jean Louis Seguin is working anymore (no response to my emails 😞

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