Anyone able to work out extension tube magnification?

[Kell]

I recently picked up some Minolta extension tubes from the Bay and happened to watch a video yesterday of someone stacking loads. As I already had some Canon tubes (and an MD>EOS connector) I thought I'd stack them all to see how close I could get.

Please bear in mind this was not meant to be a serious attempt at Macro Photography, just one of those "I wonder" moments.

But I'm lost at how to work out the magnification.

So, in the shot attached I have.

A Minolta 35-70 Macro lens. It's MACRO at 70mm and maximum is 1:4.

Attached is then 14, 21 and 28 extension tubes. Then the MD>EOS connector, then a set of 13, 21 and 31 tubes. Then the EOS>RF connector.

I think, reading online, you take the original focal length and then divide that by the length of the tubes so 70 / (14+21+28+13+21+31) 128 = 0.54.

but then it was already at 1:4/0:25. I think you add those figures together to get 0.79

Is that correct? It doesn't seem right.

As it happens, the shots weren't great. I'm not sure if it's user error, or the resulting magnification is too much for the lenses, but none of my attempts were really in focus.

Confused.

IMG_7905 by Kell, on Flickr

LR7A5917 by Kell, on Flickr

I also used one of the Minolta tubes (can't remember which now) on my Sony yesterday to get this, which I was happy with.

Macro 58mm by Kell, on Flickr

 

[Mostly Vintage Cameras]

If the extension equals the focal length you have 1:1, some people would call this "true macro" as opposed to "close focus" your zoom offers. Twice the extension than focal length I.e. 100mm of tube on a 50mm lens is 2:1 or twice life-size. 25mm tube on a 50mm would be half life-size or 1:2.
Hope this helps
MVC

 

[zx9]

If the extension equals the focal length you have 1:1, some people would call this "true macro" as opposed to "close focus" your zoom offers. Twice the extension than focal length I.e. 100mm of tube on a 50mm lens is 2:1 or twice life-size. 25mm tube on a 50mm would be half life-size or 1:2.
Hope this helps
MVC

The above is correct assuming that a normal lens is in use, it can go a little pear shaped with a retro focus wide angle a telephoto design or a zoom lens, for simplicity a 50mm standard lens preferably reversed, or a fixed focal length macro lens will make magnification and exposure calculations much easier.

 

[RyanB]

This site is a great resource, here is a link to the ext tube calculator.
link

Assuming you are using f5.6 ( per the image) the above say you're getting around 5.8:1 magnification. The dof is going to be next to nothing which is why almost all macro at those levels is done with stacking to get anything usable. You will also be contending with diffraction big time.

Personally I would use your widest prime focal length/ sharpest at its widest aperture lens you have and minimum amount of ext tubes for the magnification you want fot the best IQ.

 

[myotis]

I recently picked up some Minolta extension tubes from the Bay and happened to watch a video yesterday of someone stacking loads. As I already had some Canon tubes (and an MD>EOS connector) I thought I'd stack them all to see how close I could get.

Please bear in mind this was not meant to be a serious attempt at Macro Photography, just one of those "I wonder" moments.

But I'm lost at how to work out the magnification.

So, in the shot attached I have.

A Minolta 35-70 Macro lens. It's MACRO at 70mm and maximum is 1:4.

Attached is then 14, 21 and 28 extension tubes. Then the MD>EOS connector, then a set of 13, 21 and 31 tubes. Then the EOS>RF connector.

I think, reading online, you take the original focal length and then divide that by the length of the tubes so 70 / (14+21+28+13+21+31) 128 = 0.54.

but then it was already at 1:4/0:25. I think you add those figures together to get 0.79

Is that correct? It doesn't seem right.

As it happens, the shots weren't great. I'm not sure if it's user error, or the resulting magnification is too much for the lenses, but none of my attempts were really in focus.

Confused.

IMG_7905 by Kell, on Flickr

LR7A5917 by Kell, on Flickr

I also used one of the Minolta tubes (can't remember which now) on my Sony yesterday to get this, which I was happy with.

Macro 58mm by Kell, on Flickr

All the zooms I have used with a "macro" setting have achieved their close focusing by adjusting the focal length as well as the focus (as do many primes). And I have never found calculations to work with zoom lenses, even though I have successfully calculated magnifications (at least roughly) when using primes. But the calculations ar designed around using "simple" lenses not the complex lens designs found on DSLR, and probably mirrorless cameras as well.

I would ignore the calculations and just photograph a ruler and physically measure the differences between the ruler and the image on the rear screen or on the computer, where you can adjust for the difference between sensor size and the mage on the screen i.e. if the sensor spec says the image length is 35.8mm just compare that to the units showing on the photographed ruler.

 

[Kell]

Thanks all.

I may be tempted to try it with a prime just to see if it's better, but probably (definitely) won't be something I'm going to invest much time in. I like using one or two tubes to give me that option, but combining them like this maybe takes more skill and/or patience than I have.