Close Up Lenses and Focal Length

[Fragomeni]

I'm working on some macro and micro experiments and a question came up that I've never really had to think about before and it has me stumped. I'm using a 120mm Rodenstock Macro lens and in my experiments I've played with putting some Hoya close up lenses (+1, +2, +4) on for additional magnification. My question is does the use of a close up lens change the actual focal length of the lens being used? This came up because when I go to compensate for bellows length I need to know the focal length of the lens to figure out my compensation. Does the close up lens change my focal length or is it factored in some other way? Thank you.

[vinny]

Just measure your bellows as you would for any other macro shot. At least that's what i do.

[Fragomeni]

Thats what I started to do but to appropriately calculate the compensation necessary for the extended length of the bellows you need to know the focal length of the lens. If I were to compensate using the focal length of the lens (not taking into account the close up lens if it does effectively change the focal length) then the compensation would be off.

I've found some equations that supposedly demonstrate how to calculate the change in focal length when using close up lenses but I'm having a hard time understanding them. Primarily I just need to know if a close up lens does actually change the functional focal length of a lens so I can figure out if its really necessary to even worry about this.

[ic-racer]

Use the magnification (image height comparison) method to get your bellows factor.

Yes, diopter lenses change the focal length. You can see for yourself, just focus at infinity with the diopter in place and with it off.

[Fragomeni]

Can someone simply provide an equation to find the new focal length with the diopter in place and an explanation for using the equation? That will be the most useful thing I can think of right now.

[ic-racer]

Off the top of my head:

+1 diopter is one meter focal length (1000mm)

A 120mm lens is +8.3 diopters

Add them together and you get +9.3 diopters or 107.5 mm focal length (this is a 'thin lens' approximation, best bet is to actually measure it on your standards).

But, you don't need to know this for the bellows factor.

[Mark Sawyer]

First you need to understand what "diopter" means. It's a reciprocal of the focal length of the lens in meters. A +1 diopter lens has a focal length of 1 meter (1000mm), a +2 has a focal length of 1/2 meter (500mm), a +4 has a focal length of 1/4 meter (250mm), a +10 is 1/10 meter (100mm), and so on...

To determine the focal length of the diopter plus your lens, add the two focal lengths together and divide by 4. Example: a 300mm lens and a 500mm (+2) diopter is 300mm + 500mm = 800mm, divided by 4 gives you a 200mm lens.

Don't use the stops indicated on your aperture scale in any calculations. Besides the error from the change in focal length, the aperture must be measured through the front element, and the addition of a diopter will magnify it and change the measurement significantly. Simply calculate the f/stop after you focus: measure the bellows extension and the aperture diameter (through the front element), and divide the former by the latter. No need for calculating the bellows extension factor with this method, as it is already figured in when you measure the bellows extension.

Hope this helps.

[Fragomeni]

K, this is the equation I was looking for. I knew I saw it somewhere before.

To determine what I was looking for i.e. new focal length with diopter lens added:

1) Calculate the dioptric power of a lens -- d=100/f (f=focal length in cm)
2) Add the diopter of your lens to the diopters of the close up lens. Ex. I want to add a +4 diopter to my 120mm lens so 120mm= 8.3 diopters. 8.3+4=12.3 diopters
3) Calculate the new focal length of the lens plus the diopter lens -- f=100/d = 100/12.3= 8.1 = 81mm

Don't try to use the stops as indicated on your aperture scale; besides the error from the change in focal length, the aperture is to be measured through the front element, and the addition of a diopter will magnify it and change the measurement significantly. Simply calculate the f/stop after you focus: measure the bellows extension and the aperture diameter (through the front element), and divide the former by the latter. No need for calculating bellows extension with this method.

This is very interesting, especially because I hadn't yet considered the effect of the diopter lens on the aperture settings. So what you're saying is I can attach my diopter lens, focus getting everything where I want it and then calculate my actual aperture by measuring the bellows extension and the aperture diameter (through the front element), and divide the bellows extension by the aperture diameter? This will give me the true aperture as opposed to what it written on the lens? Just want to make sure I understand that right. And using this method alleviates the need to compensate for bellows extension?

[Mark Sawyer]

...So what you're saying is I can attach my diopter lens, focus getting everything where I want it and then calculate my actual aperture by measuring the bellows extension and the aperture diameter (through the front element), and divide the bellows extension by the aperture diameter? This will give me the true aperture as opposed to what it written on the lens? Just want to make sure I understand that right. And using this method alleviates the need to compensate for bellows extension?

Yes, this is correct. The f/stop is simply the focal length (bellows extension) divided by the aperture (measured through the front element). That's all it ever is, one very simple ratio.

And just to make it clear, when I say "through the front element" that means through the diopter lens too, as it is now your front element, (unless you're putting it on the back of the lens, which would work too).

BTW, if you end up doing longer than one second exposures, as is common with macro photography, don't forget to figure in the reciprocity failure!

[Fragomeni]

Yes, this is correct. The f/stop is simply the focal length (bellows extension) divided by the aperture (measured through the front element). That's all it ever is, one very simple ratio.

And just to make it clear, when I say "through the front element" that means through the diopter lens too, as it is now your front element, (unless you're putting it on the back of the lens, which would work too).

BTW, if you end up doing longer than one second exposures, as is common with macro photography, don't forget to figure in the reciprocity failure!

Thanks for the help! This should be an interesting experiment. Much appreciated!