# Thread: How to extend the focal length on a DIY lens?

1. ## How to extend the focal length on a DIY lens?

Good morning!

I have a problem with focal lengthening in DIY fixed focal length lenses.

For example, I have two plano-convex lenses with a focal length of 500mm. Using them simultaneously obviously shortens the focal length. Wanting to make a duplet lens from these lenses, with a distance between them of 100mm, the focal length decreases, all the way to 278mm.

I would like to know from the respected forum members if there is any rule/method of adding additional lenses to the system, so that in the end the focal length is larger and not smaller.

Going back to the example. What lens or how should the lens system be constructed to get a focal length of 1000mm.

Thank you very much for your time.

2. ## Re: How to extend the focal length on a DIY lens?

To achieve 1000mm, start with two meniscus of 2000mm, or +0.5 diopter. Place them facing outwards.

Or one +1 close up, facing inwards.

See how that works out.

What format do you need to cover, and what aperture do you need?

Sent fra min SM-S901B via Tapatalk

4. ## Re: How to extend the focal length on a DIY lens?

For simple lenses, the thin lens approximation is usually sufficient. If you have two lenses of focal lengths f1 and f2, and they are separated by a distance d that is less than the focal length, the focal length of the combination ftot is given by:

1/ftot = 1/f1 + 1/f2 - d/(f1*f2)

See for example https://jackwestin.com/resources/mca...tion-of-lenses or https://www.cis.rit.edu/class/simg23...inlenses-4.pdf

If the lenses are separated by a very small distance d, then the third term is small and you have 1/ftot ~= 1/f1 + 1/f2. If the two lenses are equal, then you get that the focal length of the combination is one-half the focal length of the individual lenses.

If the lenses are convex, they have positive (converging) power, and when you combine two of them, you get a lens with more power (more convergence), so a shorter focal length. You can't easily add two positive lenses to make a less powerful lens. The ways to get a long focal length such as 1000mm are:

1. to start with a pair of individual convex lenses that are 2000mm or longer.
2. start with a pair each of which is longer, like say 1500-1800mm, and increase the inter-lens separation, although this comes with other disadvantages.
3. use a combination of a positive and a negative.

5. ## Re: How to extend the focal length on a DIY lens?

Here's a table & instructions:

http://www.subclub.org/fujinon/softfocus.htm

6. ## Re: How to extend the focal length on a DIY lens?

Fellow constructors. Thank you very much for the direction. I am getting down to studying and experimenting.

7. ## Re: How to extend the focal length on a DIY lens?

I have a 1890's self casing Cycle 5X7

I changed lenses to a working 1950's shutter with series filters

I can use glass plate and standard film

The OE lens came with Tele, closeup, dark yellow filters

Really heavy wood box, but perfect for group shots

5X7 Cycle Camera by TIN CAN COLLEGE, on Flickr

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