Focal Length w Extension Toobz

[TwoWheeler]

I know that extension tubes lengthen effective focal length, but when using a focus stacking calculator do I add the length of the tube(s) to the focal length of the lens?

 

[DWU2]

Seems like you would.

 

[TwoWheeler]

Seems like you would.

seems so to me too, but then, I’m not all that bright…

 

[Archibald]

I know that extension tubes lengthen effective focal length, but when using a focus stacking calculator do I add the length of the tube(s) to the focal length of the lens?

Extension tubes do not lengthen the focal length, effective or otherwise.

Ignore extension tubes for a moment. Changing the focus of a modern internal-focusing lens shortens the focal length the closer you focus. The focal length depends on the adjustment in the lens.

Now putting extension tubes between the camera and the lens without adjusting the lens leaves the focal length exactly where it was. It just changes the extension (and where it is focusing, obviously).

 

[Bryan Conner]

I know that extension tubes lengthen effective focal length, but when using a focus stacking calculator do I add the length of the tube(s) to the focal length of the lens?

I think the important information that would be needed in a focus stacking calculator when using extension tubes for macro photography would be the magnification ratio. Calculators which do not ask for this information are probably intended for general photography and not macro. The addition of extension tubes changes the magnification which is what is really important in focus stacking since you are wishing to calculate step size and that depends on depth of field which becomes more shallow as magnification increases.

If you need to calculate your new magnification ratio with the tubes, that is easy to do as long as you know the native magnification ration of your lens:

new magnification ratio=lens native magnification ratio+(extension tube length divided by lens focal length)

So, if a 100mm macro lens has a 1:1 magnification ratio, and I add a 25mm extension tube to the lens, the new magnification ratio would be 1.25:1.

 

[Archibald]

I haven't done stacking for a while, but here are the steps that typically one takes.

- Decide on the magnification to be used (agree with @Bryan Conner )
- For that magnification, work out the narrowest acceptable effective aperture to be used (narrower aperture results in excessive diffraction)
- Calculate the DOF for the magn and aperture. That is approximately the step size.

There are charts and equations to calculate these numbers.
Effective aperture = a * (1+m), where a is the marked aperture. Stay at f/22 or wider.
For DOF, I used the data that Canon published with the MP-E65mm lens.

There are equations to calculate DOF. They are all approximate only, because the lens exit pupil size matters but is never taken into account.

 

[TwoWheeler]

Extension tubes do not lengthen the focal length, effective or otherwise.

If you define focal length as “the distance between the nodal point of a lens to the film/sensor plane”, they do indeed change the focal length. Whether you’re moving the optical center of the lens via extension tubes or the throw of a zoom, you are still physically changing that distance.

 

[Archibald]

If you define focal length as “the distance between the nodal point of a lens to the film/sensor plane”, they do indeed change the focal length. Whether you’re moving the optical center of the lens via extension tubes or the throw of a zoom, you are still physically changing that distance.

Focal length is defined when the lens is focused on infinity.

What Is Focal Length in Photography? A Beginner's Guide

photographylife.com

https://www.nikonusa.com/learn-and-explore/c/tips-and-techniques/understanding-focal-length

The focal length is a property of the lens that does not change with where it is focused, as long as its optical properties are not changed.

Thus focal length does change if internal elements shift, as is common in modern internal-focusing lenses (or zoom lenses, obviously).

But otherwise, the FL stays constant. When focused on a subject at a magnification of 1:1, the distance between the nodal plane and the sensor is double the focal length. That distance does not become a new focal length.

 

[Archibald]

It's kind of cool that the German word for focal length is Brennweite, which literally means burn distance. Focus the sun on a piece of wood to burn it. The distance is the focal length. And the sun is effectively at infinite distance.

 

[Bryan Conner]

It's kind of cool that the German word for focal length is Brennweite, which literally means burn distance. Focus the sun on a piece of wood to burn it. The distance is the focal length. And the sun is effectively at infinite distance.

German has a lot of cool words. Two of my favorites are the word for a skunk: Stinktier....or stink animal. And a sloth is Faultier...or lazy animal.