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Accuracy of online macro magnification calculators

Started 3 weeks ago | Questions
Sir Jake Regular Member • Posts: 108
Accuracy of online macro magnification calculators
1

I tried to calculate the magnification ratio of several lenses with different Raynox add-ons with this site:

http://extreme-macro.co.uk/calculators/

The calculations give me astronomical results, such as Tamron 50-400mm on the 400mm end + the Raynox 150+250 yields a magnification ratio of 6.65x.

Has somebody tested and verified whether the calculator is right or not?

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Joseph S Wisniewski Forum Pro • Posts: 35,461
Appears to be satisfactory…
1

Sir Jake wrote:

I tried to calculate the magnification ratio of several lenses with different Raynox add-ons with this site:

http://extreme-macro.co.uk/calculators/

The calculations give me astronomical results, such as Tamron 50-400mm on the 400mm end + the Raynox 150+250 yields a magnification ratio of 6.65x.

Has somebody tested and verified whether the calculator is right or not?

A Raynox 150 is 4.8 diopters. A Raynox 250 is 8 diopters. Diopter values add when you stack them close, so both Raynoxes together is 4.8+8=12.8 diopters.

The 400mm is 1000/400mm=2.5 diopters.

With the 400mm focused at infinity the magnification will be 12.8/2.5=5.12x

I don’t know how close the 400mm focuses or what the change in focal length at its closest distance is, but 6.65x is a very reasonable value. I tend to favor using the rear (aka "tube") lens at infinity so I'd call it a 5.12x combination.

I punched in a combination I do know (Nikon 85mm f/1.4 + Canon 500D) and their answer matched mine at close focus, so I'd say it's OK.

Personally, I'd never stack two Raynox closeup lenses in front of a 50-400mm zoom. Aside from the optical quality of that combo not being all that, there's a problem with speed. The Tamron is rated f/5.6 at 400mm, that's a 71mm aperture. The Raynox will choke that down to 32mm or f/12.5. That's approaching diffraction limiting.

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