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kirkmacatangay
30-Jul-2013, 20:10
Hi,

If I have a 10" lens and wanted to put a -315 mm dipole at the back, how would you calculate the new focal length? Or is it a straight subtraction or some kind of product formula? I assume the entrance pupil will stay the same but with the increased focal length, the f-stop will get larger.

Kirk

ic-racer
30-Jul-2013, 20:24
Is that a 3.17 DIOPTER lens (315mm) on the back of a 10" (254mm or 3.9 DIOPTER) lens? That would give about 7 (3.17 + 3.9) diopter lens or around 143mm. But I may not be understanding the question.

polyglot
30-Jul-2013, 20:45
Assuming you mean diopter, they add. So 10" = 254mm = 3.94 diopter. -315mm = -3.17 diopter, so the combine result will be about 0.76 diopter = 1300mm focal length.

The physical aperture size will probably remain the same and since the new focal length is about 5x longer, the f-number will be about 5x larger.

Tin Can
30-Jul-2013, 20:48
I thought I was on the Ham Radio forum!

E. von Hoegh
31-Jul-2013, 07:03
I thought I was on the Ham Radio forum!

Me too, I thought maybe they'd opened a 63cm band and he wanted a halfwave antenna for it.:eek:

Bob Salomon
31-Jul-2013, 07:41
Are you looking to put a minus supplementary lens on your camera lens to make an longer focal length lens?

kirkmacatangay
31-Jul-2013, 10:08
Hi Bob,

Yes I am considering it. I understand the penalty of reduced detail and color fringing among other things. I plan to shoot bw so less of an issue and contact printing so detail is not as high a priority.

Kirk

ic-racer
1-Aug-2013, 12:28
Hi Bob,

Yes I am considering it. I understand the penalty of reduced detail and color fringing among other things. I plan to shoot bw so less of an issue and contact printing so detail is not as high a priority.

Kirk

Of course the title the thread is has the word "Negative" in it (my mistake above, corrected by Polyglot). So, in that case, yes, I have had experience with a negative 3.3 diopter accessory lens. However, the one I use has six elements and is pretty well corrected. It turns a 150mm into a 300mm.

Bob Salomon
1-Aug-2013, 12:45
Hi Bob,

Yes I am considering it. I understand the penalty of reduced detail and color fringing among other things. I plan to shoot bw so less of an issue and contact printing so detail is not as high a priority.

Kirk

The formulas are:

Effective F stop = indicated F stop x the combined focal length of the camera lens and the negative lens / camera lens focal length.

new focal length in inches = original focal length in inches x 40/40 + diopter power of the negative lens
This is approximate. It assumes that the nodal points of the main and the attachment lenses actually coincide. Which they won't.

Using a -2 diopter on an 8" lens wll increase the bellows extension from 8" to 13.3".
You will need a bunch of bellows. Many people might recognize these types of supplementary lenses by their other name; Tele-lenses.

ic-racer
1-Aug-2013, 15:41
You will need a bunch of bellows. Many people might recognize these types of supplementary lenses by their other name; Tele-lenses.

Exactly! Mine says "Teleconverter" right on the barrel. But of course it is a misnomer, because it just makes a longer lens, and not a telephoto.

polyglot
1-Aug-2013, 15:57
Exactly! Mine says "Teleconverter" right on the barrel. But of course it is a misnomer, because it just makes a longer lens, and not a telephoto.

You sure about that? The teleconverter I have (for 35mm though) really does make a telephoto. The effective focal length becomes much longer than the physical lens size.

kirkmacatangay
1-Aug-2013, 15:59
Thank you everyone for your replies. I got the name from Surplus Shed. The objectives are called dipoles and not likely as well correxted. Kirk

Dan Fromm
1-Aug-2013, 17:01
Are you sure? I ask because a a google search of www.surplusshed.com dipole got one hit. http://www.surplusshed.com/pages/item/r3077.html

ic-racer
1-Aug-2013, 17:45
You sure about that? The teleconverter I have (for 35mm though) really does make a telephoto. The effective focal length becomes much longer than the physical lens size.

Yes, the Horseman converter lens is just a well-corrected diopter, not a teleconverter. Needs accessory bellows to focus at its new 300mm focal length, see here: http://www.largeformatphotography.info/forum/showthread.php?86618-Hands-on-Topcor-2x-Converter-amp-Horseman-FA-Extension-Back&p=851815&viewfull=1#post851815

polyglot
1-Aug-2013, 18:58
Neat.

kirkmacatangay
1-Aug-2013, 20:31
Hi Dan, I would love to provide a link but the -315 mm lens is no longer on the site. It is very likely that I am mistaken but I will keep an eye out for a similar page. Kirk

E. von Hoegh
2-Aug-2013, 07:05
Thank you everyone for your replies. I got the name from Surplus Shed. The objectives are called dipoles and not likely as well correxted. Kirk

Thay may call them dipoles, that doesn't mean they are dipoles.

http://en.wikipedia.org/wiki/Dipole

https://en.wikipedia.org/wiki/Dipole_antenna

Asd for using a converter on your lens, I wouldn't bother.