8mm Forum: Eiki/Elf 25mm lens image size

This is topic Eiki/Elf 25mm lens image size in forum 16mm Forum at 8mm Forum.

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Posted by Mark Todd (Member # 96) on March 13, 2009, 07:35 PM:

Hi can someone please rell me what the picture would be across the top at 10 feet.
Many thanks Mark.

ps looking for one by the way.

Posted by John Hermes (Member # 1367) on March 13, 2009, 09:23 PM:

In the back of the Elmo CL instruction manual there is a projection image and image size chart. It is in meters, but at three meters (pretty close to 10 feet),with a 50mm lens, the image width is 0.6 meters. Accordingly, with a 25mm lens, it would be 1.2 meters, or just under four feet wide. It would probably be almost four feet on the nose at 10 feet, since three meters is about 9.75 feet.

Posted by Hugh McCullough (Member # 696) on March 14, 2009, 05:13 PM:

Mark

The picture size at a 10ft throw using a 25mm lens would be
3ft 9ins X 2ft 10ins.
Information gleaned from www.cinephoto.co.uk/amazing_film_facts.htm

Posted by John Whittle (Member # 22) on March 14, 2009, 06:04 PM:

FWIW there is a simple way to compute this:

width of aperture x 1/focal length x distance = width
(use same units in all measurements inchs or millimeters)
for 16mm:

.380 (width of 16mm aperture x 1/1 (1 inch lens) x 10 feet = 3.8 feet or 3 feet 9.6 inches.

You can revise the formula to figure distance for width, etc.

A word of warning on lenses shorter than 2 inches (50mm).
1. Usually the fastest forumla is the 50mm lines (i.e. f/1.2)
2. The best field sharpness and contast is usually with the 50mm lens
3. Shorter lenses have less depth of field such that focusing is much more critical and projector to lens alignment must be perfect in all directions (centering, left to right and top to bottom).
4. Shorter lenses are generally a "slower" design such as f/2.5
5. Shorter lense usually don't have the snap or contrast of the 50mm lens.

Why? Because they spent all the time and money on the development of the 50mm lenses which progressed rapidly in the 60s and 70s. The designs of the shorter lenses for 16mm have basically been unchanged since the 50s.

Also be aware that there are people that mount 8mm lenses in sleeves to fit 16mm projectors and those lenses don't have the coverage and will often vingette the edges of the picture.

And finally ... if you use a scope adapter lens, using a prime lens of less the 38mm will be problematic in getting a picture without the image being vingetted by the inside of the scope lens and again all those warning about focus are just 2x more important with a scope adapter.

John

Posted by Raj Patel (Member # 1257) on March 26, 2009, 03:38 AM:

I've got an original EIKI one which i found at foster films last year, i think they still have one. As stated good for size but definitely be aware of the loss of contrast and sharpness, i mainly use the 50mm still.

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