Kern POE 100 Parfocal?

Christopher Trice · October 18, 2012

Hello,

I have recently picked up a Kern POE 16-100mm 100 lens and am wondering if the lens is parfocal? Do be clear, does the lens maintain focus if I zoom in and out? I'm curious because I find it often much easier to zoom all the way in, focus and then zoom out.

Thank you.

Robert Ditto · October 18, 2012

That is a good question. I do not believe so, but I am not at home at the moment. When I get there here in a bit, I will check and get back to you. I have one of these lenses that came with my first EBM, but have never used it. I use Tamron SP SLR zoom lenses when filming, and Hexanon Primes.

Christopher Trice · October 19, 2012

Thank you Robert,

The reason I started down this line of thought is the following quote from the manual.

"Manual variation of the focal length or power-zooming from 16 to 100mm, without affecting either relative aperture or picture sharpness." (p.1)

Now, this could be translated as meaning parfocal or just another example of oddly worded manual-speak from Bolex.

Chris Millar · October 19, 2012

I own one and with a correctly adjusted back focus don't have any reason to believe it isn't...

Robert Ditto · October 19, 2012

And the answer would be a no, you loose focus as you zoom in either direction.

Sincerely, Respectfully,

Robert

Dom Jaeger · October 19, 2012

Sure it's parfocal, it's really only still photography zooms that you need to worry about. One of the main design parameters of zooms made for cine cameras is that they hold focus through a zoom.

How well a particular cine zoom holds focus depends on a few factors - the quality of the lens, the zoom range, but most importantly whether the back-focus is exactly correct, and whether there is any wear to the zoom barrel cam tracks or followers. If the back-focus is even slightly out the zoom will progressively lose focus towards the wide end.

When assessing a zoom, a technician can make a graph of the zoom curve, mapping how far the focus drifts in both directions of the zoom travel. There are always certain tolerances allowed, often the focus is good at both ends but may form an S-shape drift through the range. On higher end zooms an adjustment can be made to the spacing between variator and compensator to correct the curve.

From my experience the Kern zooms are usually pretty good at holding focus, but wear or dried lubricant in the zoom mechanism can cause a focus jump when the zoom direction is changed.

Robert Ditto · October 19, 2012

At least let me clarify,

I mounted the lens on my Bolex EBM and zoomed all the way in on the house next door to mine which is approximately two acres across the span. I focused the lens and zoomed out and lost focus as I did so.

Then I tried the reverse with the same results.

After that, I zoomed mid way, focused the lens, and zoomed in both directions, loosing focus either way.

I do not know if this is a fault with my lens, I have never had it collimated or serviced as I dont use it. It is kept with my Bolex collection items.

Sincerely, Respectfully

Robert

Christopher Trice · October 19, 2012

Thank you everyone for your responses. I'm not sure if the back focus is correctly adjusted but I should find out soon as I shot three rolls today - one of which tested the technique I described above. To be clear, I wasn't attempting to zoom while shooting. I just wanted to set the focus while zoomed in, zoom out and then shoot.

Thanks again!

Chris Millar · October 19, 2012

Your lens back focus needs adjustment

Since it's off already you may as well give it a go yourself - but beforehand just to be sure ... your aspheron knob isn't turned is it ? (if you have one on your model)

Chris Millar · October 19, 2012

Thank you everyone for your responses. I'm not sure if the back focus is correctly adjusted

Quick way to find out if it is definitely wrong is to look through the lens and check - however, a failed result here (i.e. a perceived pass) doesn't indicate it is technically correct as it's really up to your perception or the size of your grains on film, whichever bounces off your priorities first.

In other words, there are three places your back focus can be - too far forward, too far back or in the range that you define as acceptable - which, for all intents and purposes is as 'perfect' as it'll ever get .

Maybe there is some agreed upon metric amongst lens technicians? Within reason, I'd feel free to break it.

Christopher Trice · October 19, 2012

Chris,

Thank you for your input. My lens does not have the aspheron knob (or I could not find it) but in all honesty lens adjustment is not something I would feel comfortable doing.

I think my plan of action will be to get the film developed and confirm the issue and then find someone to service it. Part of the reason for this rationale is that I like to know something is broken before I go about fixing it.

Thank you again. I'll let you know how it all turns out.

CGT

Robert Lewis · October 19, 2012

Chris,

Thank you for your input. My lens does not have the aspheron knob (or I could not find it) but in all honesty lens adjustment is not something I would feel comfortable doing.

I think my plan of action will be to get the film developed and confirm the issue and then find someone to service it. Part of the reason for this rationale is that I like to know something is broken before I go about fixing it.

Thank you again. I'll let you know how it all turns out.

CGT

Christopher,

I have two Bolex cameras fitted with the second series POE lenses, and have very recently had them serviced by Bolex, so I think I can say that they are set up correctly. In relation to both lenses, having set the diaphram to its widest setting (using the pre-selection system to the first stage position on the button) I zoom in to whatever it is in the distance that is to feature in my picture and then focus on it. If, say it is to be a building, I zoom in fully on the building and focus. Then if I wish to have a wider angle shot of what it is I focussed on, I zoom out until I have the composition as want it. As I zoom out every thing remains in focus until I reach wide open (subject to depth of field considerations). After processing, I have a projection print made and on projecting I find that the focus is just right. I think I can therefore say that the manual for the lens is correct in what it says. Of course, if you are using the lens on auto setting, as the light value changes as you use the zoom, so the depth of field might well change because the auto diaphram changes. Because, however, focussing was done with the diaphram wide open, any change in the diaphram setting done automatically will only lengthen the depth of field and not reduce it. Apologies for going into such detail, but it just occurred to me that you might not be focussing correctly in the first place. I think that if you focussed initially with the diaphram open at other than its widest, you could be suffering from the depth of field changes as you zoom which, in turn could impact on focussing. If I am going into detail which you already know, please excuse me.