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Long macro tubes reversing direction of focus (1 Viewer)

opisska

Jan Ebr
Poland
Today, I have received my extension tubes for Canon DSLR and begun playing with them. After mounting all three (giving 65 mm of overall space) with a Canon 85/1.8 lens, I was completely flabbergasted as to what was happening with my focusing - and then after some experimenting, I got to the bottom of it: with this amount of extension, the focus acts backwards. The focusing range is incredibly small, on the order of a centimeter between "infinity" and closest focus position - and it's inverted! Focusing on "infinity" focuses a little closer to the camera than focusing on "0.8 m". This effect exists for three of the longest setups and switches back to normal (if still very shallow) focus range when using just the 31 mm ring - from there downwards, everything is normal and infinity is further from the camera than 0.8 m. Funnily, the autofocus seems as confused as myself and will not work in the inverted range. Moreover, the same thing happens also for the 50/1.8, but there only at the two longest setups, it switches to normal earlier.

Now I am not completely dumb when it comes to optics, but I just don't see any explanation for this behavior at all. The tubes are just empty space, they only change the distance and I don't see anything in "1/f=1/a+1/a'" that would have a local maximum ... Has anyone seen anything like this?

Btw., I would somewhat understand this in internal-focus lenses such as the 85/1.8, fine, weird things may happen there and it changes focal length with focusing. But the 50/1.8 focuses by moving the entire assembly up and down in the bloody barrel!!!
 

Troubador

Moderator
Staff member
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Canon only offer a 12mm extension tube and a 25mm tube and they don't recommend you use them together although they do concede some acceptable results can be obtained. This rather suggests that 65mm could be too much. If you imagine the lens (set to closest focus) without the ext tube and the cone of light coming to a focus at the sensor, then imagine where the focus point would be with the official ext tube in place it seems likely that the lens will no longer have the full range of focus available. And this is OK normally because you only want the close focus part of the focus range. However, mount more ext tubes on the lens and the focus point of the lens is moving further away from the sensor and the focus mechanism is having its operating range reduced further and further. Seems to me there could be a point where the extension tube is so long that the lens can no longer focus.

You might have done this already but with the full set of ext tubes fitted, try manually screwing the focus all the way to closest focus and then focus back and forth a little to see what happens, then screw the lens all the way to what would normally be infinity and then do the same. I am just wondering if there might be some useable focus at one end of the focus range or the other.

Lee
 

opisska

Jan Ebr
Poland
So first of all, I found out that the autofocus actually can take even the "inverted" behavior, I just need to give it enough light to work with. In general, the entire macro thing gets much easier when you shine a reasonably powerful light source on the target :) So it's not even a problem, it's just a big optical curiosity to me.

To me it's not surprising at all that the focus range gets very narrow with the tubes, that's really expected from the geometrical optics of the problem. The only real surprising thing is the inversion - that the direction of the focus reverses for very long tubes. But I think I have generally understood it, it's because the effective focal length changes with focus (that's something you can see pretty easily with most lenses if you take your time and measure the scales and distances apparently) - and at this extension it changes faster than the focus point. Still I am confused how this works for the 50/1.8 as that doesn't have internal focusing, but yeah, optics of lenses with many elements not easy.

Ultimately, I have developed a pretty handy way to work in the field even in "not optimal" conditions - the 70-200/4 works nicely with the entire 65 mm of tubes, given roughly 1:1 macro. It has the great advantage that changing the zoom a little changes the focusing distance a lot - so the zoom can basically be used as manual focus :) And then the autofocus fills in the fine details. Works even in the dark and for things that are under water!

 

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