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. So I wrote my own, and here it is. Be aware that the calculator uses simplified lens formulas, 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.
|Focal length (mm)|
|Near Focus Limit (mm)|
Image preview at near focus distance.
Image preview at far focus distance.
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.