Macro Extension Tubes

[EchoingWhisper]

How does Macro Extension Tubes work? I've Googled but still no luck.

How do they reduce focus distance of a lens and how do you calculate? If it is not possible, what is needed for a Nikkor 18-105mm lens for 1:1 magnification?

 

[Overread]

The rough maths for extension tubes is as follows:

Length of the extension tubes in mm (as stated on the tubes) - divided by - the focal length of the lens attached (for a zoom its point on its zoom range) = magnification : 1

for example a 100mm lens on 50mm of tubes would be
50/100 = 0.5:1

or a 50mm lens on 50mm of tubes
50/50 = 1:1

so as you can see your 18-105mm would give you quite a wide range of possible magnifications with a set of tubes. However note that the distances will decrease as the magnification increases and there is a chance for the point of focus to even move inside the lens itself (thus preventing it from ever being focused). The distances are less certain and I've no idea of the math to calculate them, however you would certainly get a usable range at the 1:1 mark.

Also note make sure you get good tubes, suck as Kenko brand ones - with the contacts to ensure that you retain your control over your lens, primarily aperture control.

 

[joealcantar]

Here is a good read to get you started on using Kenko (Mfr's Name) tubes.
Kenko extension tubes - Andrew Trevor-Jones - Nature Photography
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Shoot well, Joe

 

[mjhoward]

Overread... that is a good rough estimate. More precisely though, you need to know the total extension to calculate this. Lenses already have some magnification built in and this needs taken into account. So for instance, your 18-105mm lens already has 0.2x magnification built in (according to B&H specs). If we were to calculate the required extension for 1:1 at 105mm, you'll note that there is already 0.2*105=21mm of extension built in. So you will need another 105mm-21mm=84mm to achieve 1:1 at 105mm focal length. If you set the lens to 60mm, then you will only need 48mm extension but you will have less working distance.

 

[EchoingWhisper]

Overread... that is a good rough estimate. More precisely though, you need to know the total extension to calculate this. Lenses already have some magnification built in and this needs taken into account. So for instance, your 18-105mm lens already has 0.2x magnification built in (according to B&H specs). If we were to calculate the required extension for 1:1 at 105mm, you'll note that there is already 0.2*105=21mm of extension built in. So you will need another 105mm-21mm=84mm to achieve 1:1 at 105mm focal length. If you set the lens to 60mm, then you will only need 48mm extension but you will have less working distance.

Thanks. If the closest focus distance of the lens is 0.5m, what will it be after adding a 50mm extension tube?

 

[EchoingWhisper]

for example a 100mm lens on 50mm of tubes would be
50/100 = 0.5:1

This is weird. Isn't it that the longer the focal length the higher the magnification?
If so, 50mm tubes on 50mm lens give higher magnification. Is that true?

 

[mjhoward]

for example a 100mm lens on 50mm of tubes would be
50/100 = 0.5:1

This is weird. Isn't it that the longer the focal length the higher the magnification?
If so, 50mm tubes on 50mm lens give higher magnification. Is that true?

You're thinking of Field of View. A longer focal length has a narrower field of view.

 

[EchoingWhisper]

So to get a larger flower, should I go for a higher focal length or a shorter one?

 

[EchoingWhisper]

The lens' original magnification is 1:4 to 1:5 at 105mm. How do I get it to 1:1?

 

[mjhoward]

The lens' original magnification is 1:4 to 1:5 at 105mm. How do I get it to 1:1?

Your 18-105mm lens has a magnification of 0.2x. You need 84mm of extension to get 1:1. Whether or not you want a longer or shorter focal length depends really on how much working distance you want. A shorter focal length lens doesn't require is much extension as a longer focal length to achieve the same magnification so there is also that tradeoff. In some cases of too much extension, as Overread has mentioned, your working distance can be negative (inside the lens) so it would be impossible to focus.