I Don't Get It

[Mark Stahlke]

In this thread Policar jokingly suggested adding a close-up filter to his 180mm lens instead of buying a 240mm lens. I was curious about how that would work so I dug out my Canon 500D close-up filter and gave it a try.

I can't make sense of the results. The Canon 500D seems to shorten the focal lenght of the lens. When I add the Canon 500D to a lens I have to rack the bellows in which gives me a wider angle of view. I was quite surprised by that, I expected the opposite.

I tried this experiment with 90mm, 125mm, and 210mm lenses. The effect is more pronounced with longer focal lengths. I also tried different subject distances - one foot to about twenty feet - with similar results. I didn't measure the subject size on the ground glass, maybe I should.

I don't get it. Can someone explain this?

[Jack Dahlgren]

In this thread Policar jokingly suggested adding a close-up filter to his 180mm lens instead of buying a 240mm lens. I was curious about how that would work so I dug out my Canon 500D close-up filter and gave it a try.

I can't make sense of the results. The Canon 500D seems to shorten the focal lenght of the lens. When I add the Canon 500D to a lens I have to rack the bellows in which gives me a wider angle of view. I was quite surprised by that, I expected the opposite.

I tried this experiment with 90mm, 125mm, and 210mm lenses. The effect is more pronounced with longer focal lengths. I also tried different subject distances - one foot to about twenty feet - with similar results. I didn't measure the subject size on the ground glass, maybe I should.

I don't get it. Can someone explain this?

Compound lens formula with lenses with focal lengths fa and fb

1/f = 1/fa + 1/fb - d/(fa x fb)

[Eric James]

From John Shaw's Close Ups in Nature:

Combined Focal Length = 1000/(1000/FL prime lens) + diopter power

The Canon 500D is a +2 diopter (see below).

So your observations are right on:

Your 90mm has a combined focal length of ~76 with the 500D in place:
1000/(1000/90mm)+2 = 76mm

And your 210mm lens has a combined focal length of ~148mm with the 500D:
1000/(1000/210mm)+2 = 148mm


He continues with magnification:

M=FL prime lens/FL of diopter

where the FL of the diopter is 1000mm/diopter power or in the case of the 500D: 1000mm/2 = 500mm, hence its name.

[Jim Jones]

Long ago accessory negative meniscus lenses were sometimes used to make the effective focal length longer. Kodak Telek and Zeiss Distar lenses were examples.

[Two23]

On both my Nikon 80-400mm and 70-200mm lens, the 500D makes focus distance about 18 inches. Your results don't surprise me.


Kent in SD

[Maris Rusis]

I use a variety of plus and minus dioptre supplementary lenses to extend or shorten the focal lengths of my view camera lenses.

The supplementary lenses are 75mm diameter spectacle lenses from the local optometrist at about $20 each. Surface polish quality is surprisingly good for CR39 plastic and at f64 or smaller on 8x10 (for contact work) image sharpness is ok even in the corners of the format.

75mm spectacle lenses also have the fortunate quality of fitting into a standard 77mm filter ring.

[Policar]

I feel obligated to chime in since I'm partially "at fault" for this thread. This really confused me for a while, too, until I thought about it in terms of a macro filter resulting in closer focus, meaning you'd have to move the front standard back to focus on infinity, resulting in a shorter focal length. Still seems counterintuitive to me, though...

I asked someone who knows about these things if this might be a viable method for adjusting focal length; he seemed to think it was a terrible idea w/r/t image quality... He did, however, say that reversing the front and rear cells (and threading a cable release through a light-sealed shutter, yikes) was a cheap easy way to make a decent macro lens. Some day I'll try that.