Calculator for Macro

[tartar]

Hello,

I'm glad to discovered this website. (Sorry for my English, I am French…)
While browsing the “photography tools” section, I wonder if there are any errors in the calculations. Or more precisely, I wonder if they are still valid for Hybrid camera:
For example, on the “macro camera lenses” page, if I use the “Magnification Calculator” tool, it tells me that the focusing distance must be at least 4 times the focal length of the lens. However, if we take the Nikkor MC 105mm for example, this is not valid, because the focusing distance is 290mm (1x magnification).

Another example:
On the "Macro Extension Tubes & Close-Up Lenses" page, I don't get the same results as in reality with "Extension Tube Calculator":
I have the Nikkor MC 105mm macro lens and 29mm extension tube. Using the calculator it tells me that I get a magnification of 1.28X. While in reality, I get a magnification of 1.64 (22mm filling the width of my 36mm sensor, mounted on a Nikon Z6).
Is this difference explained by the difference in draw/Tirage (space between sensor and lens) a reflex (46.5 mm) and a hybrid (16mm)?

Could you tell me if I'm making a mistake?

Thank you!

[odds]

...it tells me that the focusing distance must be at least 4 times the focal length of the lens. However, if we take the Nikkor MC 105mm for example, this is not valid, because the focusing distance is 290mm (1x magnification).

I guess the nominal 105mm is valid when focused at infinity. Lenses tend to shorten the focal length when focusing closer. The 4x rule implies a reduction from f=105mm to f = 290/4 = 72.5mm at 1x. I believe the shortening, or "breathing" depends on lens design and I do not know if 105 to 72.5 is valid for your lens.

A couple of links:
https://photographylife.com/focus-breathing
https://www.dpreview.com/forums/post/65840489
--
Odd S.

[tartar]

Hello,

Thank you very much for your answer!

Indeed, the lens breathing could explain this difference.
On the other hand, for the "Extension Tube Calculator" to be valid (I talked about it in the second part of my message), the focal length would have had to be 30mm at the minimum focusing distance. The difference seems to be huge?!
Thanks again for this answer, I hadn't thought of that.

[Alayahfsho]

Maybe try adjusting the focal length manually in the calculator to better match what you're experiencing with the lens. Sometimes breathing affects close-up shots more than expected.

[Dave Humphries]

Hello tartar,

I agree with odds and Alayahfsho, while it is probably 10 years since I last tried macro with a Nikon 105mm Macro lens, on a D7100 - I do recall the focus-breathing being much more than expected.

Dave

[Chataignier]

A wise man once said that any activity involving more than 7 variables is an art and not a science. Macro is an art.

[DanK]

One of the links is about ghosting, which is a very common issue with focus stacking, particularly with depth map methods, which are the most commonly used. It really has nothing to do with the core question in the OP, which is why the numbers don't match what sources say they should.

I'm not an expert in the mathematics of optics, but I do a lot of macro and have puzzled over this numerous times. There are several reasons why the results of the classic formulas, e.g., the thin lens formula, don't work in macro work. Focus breathing is a big one, but the complexity and assymetry of the lenses matter as well.

I have consistently had magnification ratios with extension tubes that are quite different from predicted by the simple equations.

The bottom line for me is to test the lens, with and without extension tubes. A ruler and a tripod are all you need to do this.