Film-Tech Forum ARCHIVE: Magnacom effect on F number of long focal length lenses?

This is topic Magnacom effect on F number of long focal length lenses? in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by George May (Member # 1060) on 12-18-2006, 06:54 AM:

I'm rationalising the set of (long focal length) lenses I carry for a mobile drive-in. One of my options is to use a Magnacom with one main lens to give me a range of focal lengths. In particular, I have an f1.6 105 mm Isco I'd like to use. Using this with a Magnacom I think I can get an effective focal length of over 170 mm. But the diameter of the Magnacom suggests that my f number will rise to well above 2.5.

In the past on these forums people have expressed diametrically opposed views on the effects of Magnacoms on light loss. Can anyone supply me with any useful pragamatic quantitative information to help me here?

Thanks.

A Happy Christmas to you all.

Posted by Louis Bornwasser (Member # 3063) on 12-18-2006, 03:11 PM:

There is no question that there is light loss with Mangecoms; this is transmission loss.

Whether the gain provided by the lower f stop of the prime lens will offset the transmission loss in the magnecom is the question.

My guess is that it is a bit of a wash: light will be about the same. Your advantage will be variable size of picture and the possibility of the center focussing slightly further away, lending some better focus to a curved screen. I used this to great advantage in the days of 70mm. Louis

Posted by Mark Gulbrandsen (Member # 72) on 12-18-2006, 05:30 PM:

I believe that Dwight Lindsay at Schneider once quoted me 92% transmission for the Schneider Magnacom. There is definitely not much glass in there to create a loss.....! Older "coke bottle" Magnacoms will be worse though, er should we consoder em all coke bottles [uhoh]

. The biggest objection that I have with them is loss of contrast and image distortion both of which are more than just negligible.

Mark

Posted by Louis Bornwasser (Member # 3063) on 12-19-2006, 10:56 AM:

Glenn Berggren once told me that the new multi coated Magnecom had 20% more light than the old ones. He had previously said that there was no light loss on the old ones.

Therefore we have a lens with light GAIN?????

Visions of a drive in with a 500 watt bulb and 45 magnecoms are what I still think of when the discussion leads to Margecoms. Louis