Extension Tube Calculator
Ever since I tried the Phottix Close-Up Filter, I've been looking for something better. A dedicated macro lens was just out of my budget - and I wouldn't want to carry it on my travels either - so I started thinking about extension tubes.
But while there were some extension tube calculators out there, none of them showed me what I wanted to know: what would the final image look like? They also never mentioned how the extension tube would affect the focusing distance and thus the working distance[1]. So I wrote my own, and here it is. Be aware that the calculator uses simplified lens formulas[2], which will not give precise results for actual real-world lenses but should give you at least a feel for what the end result will be. The focus distance is measured from the entrance pupil of the lens.
| Camera | |
|---|---|
| Sensor | |
| Camera Lens | |
| Focal length (mm) | |
| Near Focus Limit (mm) | |
| Extension Tube | |
| Length (mm) | |
| Focus | |
| Near | |
| Far | |
| Span | |
| Reproduction Ratio | |
| Near | |
| Far | |
Image preview at near focus distance.
Image preview at far focus distance.
Footnotes
[1] | The distance from the lens to the subject. This should be "comfortable" - so you can get close enough to isolate the subject from any distracting foreground, but also not so close that you end up mashing the lens into it. |
[2] | Taken from the excellent The Mathematics of Extension Tubes in Photography[a] by Douglas A. Kerr, P.E. |
Links
http://dougkerr.net/Pumpkin/articles/Extension_tubes.pdf |