Macro Confusion

Mystery57

Mystery57

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Ok I understand that when I use my Canon 100mm macro lens on my 5d2 that gives a focal length of 100mm.

If I add extension tubes totalling 68mm does that give an effective length of 168mm, or do the tubes calculate out differently.

When experimenting with some earlier I was surprised that my previous near focal length was dramatically cut down from lens only, and I had to get much closer to the subject with the tubes.

Any tips etc would be great
 
The tubes actually reduce the focal length. Not sure how you could calculate it mm for mm but I am at about 6" with a few tubes on. Some people use a teleconverter as well to get back to a longer focal length but I haven't tried that yet.
 
My understanding is that tubes doesn't change focal length, it changes focus distance. It brings the range of focus distances towards your sensor, thus gaining more magnification.

A change in focal length would mean change in angle of view. I don't think macro tubes change that?


Another thing I'd like to know that is sort of related to this topic is how is aperture calculated? f8 with 25mm tube becomes?
 
Interesting thought about that teleconverter, I tried a kenko on when using lens in normal mode and it worked quite well doubling it up, didn't try it macro though - something to try I guess either works or it doesn't.

With all the tubes on this morning I was about 2.5 inches from a spider when photographing it
 
Mystery57 said:
Ok I understand that when I use my Canon 100mm macro lens on my 5d2 that gives a focal length of 100mm.

If I add extension tubes totalling 68mm does that give an effective length of 168mm, or do the tubes calculate out differently.

When experimenting with some earlier I was surprised that my previous near focal length was dramatically cut down from lens only, and I had to get much closer to the subject with the tubes.

Any tips etc would be great
Click to expand...

Nothing changes the focal length of a prime lens - as wuyanxu pointed out it is the focus distance which is changed.

If you added tubes of 100mm to the 100mm macro lens then, because at maximum macro you are already getting a 1:1 magnification on the sensor you will get approx 2:1.

If you used 100mm tubes on the 100mm f2 lens (NOT the macro lens) then you would get a magnification of 1:1.

Using the usual array of tubes which gives approx 60+mm extension you will get approx 1.6 magnification at maximum macro on the 100mm macro lens - on the 100mm f2 lens you would get 1:0.6.

In the same way you will get approx 1:1 magnification using 50mm extension tubes on the 50mm f1.8 lens, but because the lens has a focal length of 50mm compared to 100mm of the 100mm Macro lens you will be much closer to your subject for the same magnification.

Which is why I often use kenko extension tubes on my 70-300mm L IS lens - to allow me to take close up shots of bees etc without getting too close.

And if you use 100mm tubes on the 100mm f2 lens you would lose 1 stop of exposure i.e you would need to double the exposure.

Which is why the MP-E65 super macro is very dark at 5x magnification - because you lose 5 stops exposure - which is 32x less light reaching the sensor.

.
 
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