Film-Tech Forum ARCHIVE: 16mm lenses

This is topic 16mm lenses in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Ron Toole (Member # 4860) on 02-19-2010, 03:37 PM:

Anyone ever hear of this - a standard Eiki portable 16mm projector (like SSL version), standard with a 50mm lens - removing the front element and turning that 50mm into something like a 28mm. Anyone ever done this? What's the effect of removing the front element of a lens?

As far as I can tell, on the standard, non-modified lens, there's just a single front element, and single rear element.

Thanks for any input you might have.

Posted by Monte L Fullmer (Member # 2797) on 02-19-2010, 06:28 PM:

Prob get a bigger picture at a shorter throw when that front element is taken off ....

Posted by Ron Toole (Member # 4860) on 02-22-2010, 10:03 AM:

I compared them against a 25mm lens.

For a same-sized image, the throw distance was 18 for the 25, and 20+ for the 50 without the front element.

I figure the modified 50 has a focal length of somewhere near/ around 28mm.

What do you think the effect on brightness and sharpness would be without a front element?

Posted by Ian Parfrey (Member # 5122) on 02-22-2010, 12:07 PM:

It's highly probable that you would experience optical distortions of one sort or another, most likely barrel or pincushion distortion. In barrel distortion, if a grid was projected through the lens, the lines would bulge out from the centre of the image- much like the slabs of timber used in making barrels, hence the name. Pincushion distortions would have the projected lines converging inwards to the centre.

The front element may not only correct for these two distortions- they can also correct for aberrations in color and sharpness ( or fringing) of the image. Chromatic aberration is where differing colours are focused in separate focal planes- one colour slightly in front of the screen and another colour slightly behind.

Spherical aberrations are caused by the lens focusing the inner or central rays at a different plane than the outer rays. This manifests itself in soft centres (hmmm chocolate!) and sharp images extremes or vice versa.

So, even if a lens has only two elements, by removing one or the other will in no way guarantee a usable end product, but as a bit of fun and experimentation its an interesting thing to try and see what will happen.

By the way, the lens may be effectively useless after disassembly so if that's of no concern, hook in and see what happens.

...and take a few photographs and post them up. I'm interested to see what you come up with.

Cheers
Ian

Posted by Ron Toole (Member # 4860) on 03-05-2010, 09:13 AM:

RE: the pictures of the lens (25mm vs. the customized 50mm) - I haven't got any smpte standard film, so I've been trying to make make some. Whoa. I do hope to have comparison shots up soon.

The show for which these custom lenses have been created is a one which consists entirely of color. So a sharp edge isn't necessarily a prime concern, though, of course, this makes color rendition and luminosity very important.

It's a curious way to go and I'm eager to see the results, once I can get some frames that provide good comparisons. Thanks for both your patience and suggestions.