How do extension tubes change a lens’s minimum and maximum focusing distances?
Asked 3/17/2012
3 views
2 answers
0
I know that adding extension moves the lens farther from the sensor/film and reduces the focusing distance range. Is there a way to estimate the new minimum and maximum subject distances for a given lens when using an extension tube or bellows? A formula would be ideal.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
14y ago
2 Answers
1
For minimum distance calculations, see here for details: http://photo.net/large-format-photography-forum/00Sswe
Extract:
"The core formula [for minimum focus distance] is 1 / f = 1 / u + 1 / v
Where f is the focal length of the lens, u is lens to subject distance, and v is lens to film distance.
More properly, v and u are measured from the focal point of the lens. For a symmetric lens, that's usually smack-dab in the middle of the lens. For a telephoto lens, it's way in front of the lens -- that's the definition of telephoto. For a retrofocus lens, it's way behind the lens.
Magnification is v / u."
[add your extension tube length to 'v' to see how the range changes.]
More calcs of all kinds here: http://photo.net/learn/optics/lensTutorial#part1
Originally by user4655. Source · Licensed CC BY-SA 4.0
user4655
14y ago
0
Generated from our catalog & community — verify before relying on it.
Yes. A good estimate comes from the thin-lens equation:
1/f = 1/u + 1/v
where:
- f = focal length
- u = lens-to-subject distance
- v = lens-to-sensor/film distance
To estimate the effect of an extension tube, add the extension amount to v, then solve for u. That gives the new subject distance for that focus setting.
In other words, extra extension increases v, which forces u to become smaller, so both the minimum and maximum focusing distances move closer. Magnification is:
m = v/u
A practical caution: u and v are measured from the lens’s principal/focal points, not from the front of the lens body. For a simple symmetric lens that’s roughly near the middle of the lens, but for telephoto and retrofocus designs it can be significantly offset. So calculations are approximate unless you know the lens’s optical principal planes.
This method is most useful for estimating the new close-focus range after adding extension.
Recommended products
UniqueBot
AI14y ago
Your Answer
Related Questions
Why do extension tubes reduce maximum focus distance and enable closer focusing?
How do extension tubes affect a lens’s maximum magnification?
How do you calculate subject size at minimum focus distance for macro lenses?
Why does a zoom lens seem to lose focal length at close focus, and how can I estimate its field of view?
How do extension tubes affect depth of field on a 105mm macro lens?