Using extension tubes with macro lenses...

[GeneX]

Can anyone offer any insight?

I’ve started using the Extension Tube Calculator on this website with both 105mm and 40mm 1:1 macro lenses. The new closest focusing distances regarding these lenses are slightly off and I’m wondering whether the new magnification factors indicated are inaccurate as well.

Any help greatly appreciated…

[DanK]

The magnification factor will likely be understated, if I understand this correctly.

My understanding is that at least for internally focusing macro lenses, the actual focal length at minimum working distance--at 1:1--is usually quite a bit shorter than the nominal focal length. For example, while this is a distant memory and may not be accurate, my recollection is that the effective focal length of my 100mm macro at 1:1 is close to 70 mm. For that reason, the impact of adding EXTERNAL extension is larger than the normal formula predicts, as that formula depends on the relative magnitude of the focal length and the external extension. When I add 68mm of extension (a full set of Kenko tubes) to my 100mm macro, I get roughly 2:1 (I haven't measured it precisely), which is consistent with entering a focal length of about 70 mm in the calculator.

[rpcrowe]

Perhaps this might help...

http://seeinginmacro.com/macro-photo...o-calculation/

[GeneX]

Thanks for your illuminating replies.

Looks like a lot more trial and error is the way forward!

[William W]

Dan’s answer is correct.

Please see comments #12 and #19 where Urban provides a detailed explanation of Actual and Nominal Focal Length.

REF: LINK

***

Note also the CiC Tutorial alludes to the differentiation between Actual and Nominal in the note, which is under the Magnification Calculator (my bold and underline now for emphasis):

“Notes: the "focusing distance" is measured as the distance between camera sensor and subject, and the "lens focal length" is the actual lens focal length (without multipliers).”

REF: LINK

WW

[William W]

Looks like a lot more trial and error is the way forward!

Not really. Once you (reverse) calculate the ACTUAL Focal Length of your two lenses when they are at Macro Focus, you just use that number as the FL.

WW

[DanK]

once you know the actual focal length at 1:1, you can approximate the magnification with (FL+EL)/FL, where FL is the actual not nominal focal length and EL is the extension length. In the example I gave (70+68)/70=1.97:1, which is pretty close. Brian Valentine, one of the best macro photographers I know of, tested this empirically and got 2.14:1.

Re minimum focusing distance: small differences in this aren't important, in my work at least. I just move the camera until it is in focus. After years of macro work, I can't even tell you what the minimum focusing distance or minimum working distance of my macro lenses are. All I need to know is that this number is very small and is smaller with shorter focal lengths.

[GeneX]

Thanks everyone, I think I'm on track.

So at 1:1, my 40mm macro lens has an actual focal length of 40.75mm, therefore:

(40.75mm + 68mm extension)/40.75 = magnification ratio of 2.66X at a minimum focusing distance of about 205.5mm from image sensor.

With a distance of just over 200mm from the camera image sensor to the outside edge of the lens filter [with the lens itself at full extension], I have a working distance of 5mm and a few atoms!!

I worked out the actual focal length of my 105mm macro at 1:1 as 78mm, so all is good.

At reproduction ratios smaller than 1:1, does the focal length of the lens revert to its nominal length?

[DanK]

At reproduction ratios smaller than 1:1, does the focal length of the lens revert to its nominal length?

Gradually, as magnification is decreased, that is, as distance to the subject increases.

[george013]

Thanks everyone, I think I'm on track.

So at 1:1, my 40mm macro lens has an actual focal length of 40.75mm, therefore:

(40.75mm + 68mm extension)/40.75 = magnification ratio of 2.66X at a minimum focusing distance of about 205.5mm from image sensor.

With a distance of just over 200mm from the camera image sensor to the outside edge of the lens filter [with the lens itself at full extension], I have a working distance of 5mm and a few atoms!!

I worked out the actual focal length of my 105mm macro at 1:1 as 78mm, so all is good.

At reproduction ratios smaller than 1:1, does the focal length of the lens revert to its nominal length?

Where did you get that number 40.75 from?

George

[GeneX]

Where did you get that number 40.75 from?

George

I used the Extension Tube Calculator tool on this site.

I set the 'Native Magnification' to 1, the 'Extension Amount' to zero and experimented with the 'Lens Focal Length' number until the 'New Closest Focusing Distance' matched the lenses' native closest focusing at 1:1.

EDIT: The 40mm macro lenses' focal length at 1:1 is also the minimum focus distance (163mm) of the lens divided by 4.

ie: 163 ÷ 4 = 40.75

[rpcrowe]

As an aside... The only time I used extension tubes with a macro lens is when I wanted to also use my 1.4x TC. I needed to add an extension tube between the rear of my Tamron 90mm f/2.8 Macro Lens and the TC because there was not enough space between the rear element of the lens and the front element of the TC.

I never calculated the magnification with that combination...

[george013]

I used the Extension Tube Calculator tool on this site.

I set the 'Native Magnification' to 1, the 'Extension Amount' to zero and experimented with the 'Lens Focal Length' number until the 'New Closest Focusing Distance' matched the lenses' native closest focusing at 1:1.

EDIT: The 40mm macro lenses' focal length at 1:1 is also the minimum focus distance (163mm) of the lens divided by 4.

ie: 163 ÷ 4 = 40.75

Where do you get the "native closest distance"from?

George

[DanK]

If it's the nikon lens, it's externally focusing. Does that mean that the actual and nominal FL at 1:1 are the same?

[GeneX]

Where do you get the "native closest distance"from?

George

I got the native closet distance from the 40mm Macro lens itself.

[GeneX]

If it's the nikon lens, it's externally focusing. Does that mean that the actual and nominal FL at 1:1 are the same?

Yup, according to my calculations, actual & nominal FL at 1:1 remain the same at 163mm.