8mm Forum: 16mm long throw lens enquiry

This is topic 16mm long throw lens enquiry in forum 16mm Forum at 8mm Forum.

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Posted by Den Brown (Member # 819) on January 22, 2009, 06:48 PM:

Anyone know much about these?

1) If I wish to project 16mm via an Elf NT2 placed 20 metres (65 feet) from the screen, which of these long throw lenses would be best: 63mm, 73mm or 100mm?

2) Which would give the largest image? (approx size?)

3) Which would give the sharpest/brightest image?

Many thanks.

Posted by John Hermes (Member # 1367) on January 23, 2009, 02:27 AM:

1. It depends on what size screen you have.
2. 63mm. The shorter the focal length, the larger the picture from the same distance to the screen.
3. Generally, focal lengths such as 50mm will have the fastest aperture and provide more light. Among the three you mentioned, the 63mm lens will probably be the fastest. Sharpness depends on overall optical quality, so any of these lenses may be the sharpest. Usually lenses on the extremes of the focal lengths have optical compromises and tend not to be the sharpest. There are a lot of determining factors, however.

Posted by Hugh McCullough (Member # 696) on January 23, 2009, 04:55 AM:

With a 65ft throw you will have approx the following screen widthes:-

63mm lens.....10ft
73mm lens.....8ft
100mm lens....5ft 10ins

Posted by Den Brown (Member # 819) on January 23, 2009, 07:11 AM:

Thanks.

No screen size limit as I will be screening onto a large white wall so I assume I should go for a 63mm as I need a large screened image.

Should I surmise that the only relevance of the barrel length is the screen size or do these lengths also affect brightness?

What is a good focal length rating for a long throw lens?

Regards.

Posted by Hugh McCullough (Member # 696) on January 23, 2009, 11:54 AM:

I think that you are getting confused between focal length, and the size of the lens aperture, expressed as an f number.

The focal length is the measurement from the rear lens element to the nodal point, and is an optical measurement that we need not worry about here.
This is expressed as, in your example, 63mm.

The aperture, or f number as it is often called, is a reciprocal of the focal length., and is an indication of the maximum amount of light the lens will let through.

If you measure the diameter of the lens, and then divide this into the focal length, you will then have the f number. IE if the diameter is 31.5mm, and you divide this into 63mm, it gives an f number of 2 (f2).
The smaller this f number the more light you will project onto the screen.

Posted by Den Brown (Member # 819) on January 23, 2009, 04:32 PM:

Hello,

Which of these lenses would be best (brightest) to project a large image with the Elf positioned 20 metres away from the screen?

50mm F1.2
63mm F2.2
76mm F1.5
89mm F2.8
100mm F2.2

Thanks

Posted by John Hermes (Member # 1367) on January 23, 2009, 08:15 PM:

The 50mm f1.2 will pass more light but the picture will be magnified more (larger) than the other lenses at the same distance. Therefore, the amount of light hitting the screen may or may not be more. Keep in mind, too, that an f-stop is a theoretical number, with the lens coatings and glass quality also being taken into consideration. This is usually more important with zoom lenses, but does affect all lenses.

Posted by John Whittle (Member # 22) on January 24, 2009, 10:37 AM:

It's also worth considering that the 50mm f/1.2 is the news formula with the best coatings and thus will most likely give you the best picture. Also the faster the lens (lower the f/ number) the less the depth of field which requires more critical alignment of the projector to the screen and lens to film gate. Any mis-alignment will show up as one side being out of focus or the inability to maintain razor sharp focus across the picture. The 50mm lens (made by Kowa for Eiki) is the last of a design that started at f/1.4 then f/1.3 and finally the f/1.2 design.

John

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