This isn't a question, just the passing-along of something I stumbled across years ago. While playing around setting up some "table-top" photography, I taped a common magnifying glass to the front of the lens. I was surprised to find that it shortened the distance I was able to focus from around 16 inches down to about 2 inches in front of the lens. I know that the magnifying glass was far from "fine optics", but the images were surprisingly clear. Of course there would be many variables involved depending on the hardware used, but it doesn't cost much to experiment. If anyone else has tried this, or does try it, I would be interested in hearing about the results. Best regards to all...-Dave

-- Dave Richhart (pritprat@erinet.com), April 02, 2000

Answers

Dan Smith, a regular contributor to this forum uses the two elements achromat close-up lenses from Nikon (around $30?) with excellent results.

-- Paul Schilliger (pschilliger@vtx.ch), April 02, 2000.

This is just a version of a close up lens (a cheap single element one, at that!) The ones designed for cameras are probably much better. If you want the best in image quality using these devices, get the two element versions, which are better corrected for distortions. Nikon and Canon both make them, and I believe the better third party ones are two element types, as well. They do give good close up performance for many applications, without the need to compensate for light losses from extreme bellows extensions.

-- Ron Shaw (shaw9@llnl.gov), April 03, 2000.

What does the magnifying lens or filter do to the DOF issues? Very interesting concept...

-- Bill Glickman (bglick@pclv.com), April 04, 2000.

The depth of field I wound up with was very shallow...only several inches. I was using a 14-inch commercial ektar with a 3-inch magnifier taped to the front. It was just a surprise to see the effects it provided in 8x10 format. I know it's not fine optics, and you guys don't have to remind me of all the lenses that can be bought that I know will produce superior results. I just posted the idea to promote a little experimentation... something to try on those days when it's freezing cold and windy outside, and the sky looks like pea- soup.

-- Dave Richhart (pritprat@erinet.com), April 04, 2000.

I discovered the magnifying glass trick 28 years ago (in jr high school) using a Canon FT-ql and a 50/1.8 lens. As far as quality goes, it demonstrates nicely all the abberations of a single element lens.

-- Tim Brown (brownt@ase.com), April 04, 2000.

I don4t know if it works with LF, but with 35 mm if I want large magnification I mount a short lens (24 or 50 mm) reverse and wide open on a longer lens (105 or 200 mm). Great results! You can first fix the lenses with tape or buy a cheap adapter ring. However, I haven4t try that with LF. Maybe it is possible to mount a "small format" lense in front of a LF lens with good results. Does anyone try that? Best regards

-- Ivo Chao (ichao@neuro.ukat.gwdg.de), April 05, 2000.

The "power" of supplementary close-up lenses is usually given in dioptres. e.g. a #1 close up lens is 1 dioptre; a #2 is two dioptres etc. A dioptre is a rather archaic indirect measurement of focal length, being 1 metre divided by the actual focal length in metres. It's commonly used by opthalmic opticians for spectacle prescriptions.

How this is useful in practice is that a #1 (1 dioptre) lens attached to any focal length of taking lens will allow focussing at 1 metre when the prime lens is focussed at infinity. A #2 lens will focus at half a metre (500mm), and #3 at one-third of a metre (333mm), and so on. I'm sure you get the idea by now.

To work out the combined focal length of prime lens and supplementary lens together, take the prime lens and convert it to dioptres, add the supplementary lens power in dioptres, and convert the sum back to millimetres by dividing it into 1000.

Let's take a 150mm lens, with a #3 close-up lens for example: 1000/150 = 6.6666 dioptre + 3 dioptre = 9.6666 dioptre. The combined focal length is 1000/9.6666 = ~103.45mm. So you would need an additional 103.45mm of bellows extension to achieve a life-size image with this combination.

In conventional focal length measurement, the formula is a bit more awkward: 1/f_total = 1/f_prime + 1/f_supplementary

or: f_total = 1/(1/f_prime + 1/f_supplementary)

The ease of the dioptre to give combined "power" by simple addition was why it was conceived in the first place, and why it's still in current use.

-- Pete Andrews (p.l.andrews@bham.ac.uk), April 07, 2000.

Thanks Pete. Most helpful comment!

-- Paul Schilliger (pschilliger@vtx.ch), April 07, 2000.