100mm Macro focus distance w/tubes

Just wondering - the 100mm macro has a minimum focus distance of roughly 6 inches from the front of the lens. I got a set of Kenko extension tubes and with all tubes attached, the focus distance only drops to about 4.75". I was hoping they would drop the MFD more than that for something I am working on (too much to go into right now).

Does this sound right - only about a 1.25" drop in MFD with all tubes?

looking at the link below, and with my own experience i thought it'd be more than a 20% gain...i've never measured though...you could add one of your TC's to get even more magnification...

https://photography-on-the.net/forum/showthre​ad.php?t=578274

Why are you worried about the minimum focus distance? I just worry about MFD when it's too little.

Are you trying to achieve a certain degree of magnification? Canon states that their 25mm extension tube on the 100mm macro gives 1.36:1 mag (about one and one third life size, according to the reference chart in Canon EF Lens Work III). A little rough math suggests that adding a full set of Kenko tubes (12+20+36=68mm) should give around 1:97:1 or very close twice life size magnification.

To estimate this, I just divided the increase in mag (.36) by 25mm, then multiplied that by 68mm. It's not exact... I'm sure there is a better and more accurate way of calculating the new degree of magnification... but it would be close enough for most things I'd ever shoot with a macro lens.

I'm not sure it's practical to try to get much more than 2:1 with a standard macro lens.... though it might be possible with more extension. You might want to look into a 65mm MP-E that starts out at 1:1 and goes as high as 5:1 mag... Should be able to rent one if you only need it for a particular job.

Andre is right, too... You could combine extension tubes with one or the other of the teleconverters for higher magnification. You can't attach Canon TCs directly to the 100mm macro... but can if you put at least a 12mm tube in between them. The TC will increase magnification 1.4X or 2X (not taking into account the extension tube), while MFD remains the same.

Am I not correct that lenses that focus internally change the focal length of the lens to achieve that focus but formulas assume a fixed focal length lens that is focused by extension? I have no idea how to compensate for this but it would seem that adding 68mm tubes might give more than 2x because the prime focused to the near limit is no longer a full 100mm. No?

The way to calculate the magnification is
Original magnification + (tubes length/ Lens FL)
The fl of the 100 macro is in reality about 70mm at max mag.
The original mag is 1.
The tubes add around 68mm
In this case:
1 + (68mm/70mm) ~= 2

At 1:1, the 100 Macro has a focusign distance of 31 cm. Since the focusingin distance according to optical laws at 1:1 equals 4X the FL, at 1:1 its FL is 31cm (=310 mm) / 4 = 77.5 mm.

When going for even larger magnifications, i.e., by adding extension tubes, the WD doesn't decrease a lot, it actually decreases less that the increase in extension, but the magnification will still go up considerably. IOW, the camera has to move backwards more than the object will get closer. And this is non-limear, because it involves inverse mathematical relationships.

With an aditional 68 mm of extension tubes, the magnification now becomes 1.877 X, at a focusing distance of 341.8 cm.

All of this can be calculated with the aid of standard optical formulas .

Kind regards, Wim