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Old July 2nd, 2006, 01:18 PM   #1
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focusing with 35mm adapter

Is it reliable to focuse with tape measure using lens barrel markings on 35mm still lenses when using an adaptor? Lets say a hdv cam + m2 + nikon35mm lens. Would a monitor be necessary?
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Old July 2nd, 2006, 02:14 PM   #2
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just look through the viewfinder or look at the lcd and turning the focus ring, till the image comes into focus. simple? an lcd might help.
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Old July 2nd, 2006, 03:57 PM   #3
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to answer the question, yes...but the distance is to the focal point of the lens, not the front of it...so where ever the ground glass is.
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Old July 2nd, 2006, 05:54 PM   #4
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It also depends on the gg-slr mount distance being absolutely correct.

If this isn't the case, whilst it is often perfectly possible to focus an image on the gg, the markings and actual focus distance might not correspond.
The act of focusing an slr lens just involves moving it towards and away from the film or gg (the lens elements don't move relative to each other) so IF the mount-gg distance is set exactly as it is on the camera that the lens was intended for then you should be alright using the lens markings.

I'm of the opinion that if using a 2.5" lcd, critical focus is quite difficult especially when the dof is so shallow. I'd rather rely on the barrel markings having checked their reliability on a bigger monitor.

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Old July 3rd, 2006, 09:02 AM   #5
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But if the slr/gg flange focal distance isn't exact, it's not difficult thing to make it exact by aiming at a distance and setting focus to infinity. Am I correct with that? Or else, the minimum distance on the lens barrel could be set and measured and a close object could be used.

I've personally been planning on measuring and having someone pull focus -- it will be difficult to even reach the lens once my setup is done. Although a larger LCD monitor would be really nice.
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Old July 3rd, 2006, 09:18 AM   #6
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One could also take the time to graph out the DoF/Distance from focal point of a particular setup if it's critical or seems wonky. Test the barrel marks and see if they work as expected...put something 5 feet from the focal point using a tap measure and set the lens to 5 feet and see if you have critical focus...test before you shoot. That's what they do in hollywood if they're not sure if something will work as expected. Nothing prevents you from doing the same.
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Old July 3rd, 2006, 02:30 PM   #7
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so, the distance is the from the focal point of the 35mm lens and not some virtual film plane in the camera body or the adapter? how do you know where the focal poin of the lens is?
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Old July 3rd, 2006, 03:06 PM   #8
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if you take the lens and hold it paralel to the surface of a white piece of paper, the distance from the back element to a crispy picture (when focus is set far and the subject is in that range) is the focal length. Since the back of the lens doesn't move in relation to the film plane, that distance is where they put the film plane. It is the magical distance to the film plane, but not mythical at all...just straight physics.
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