Macro Magnification...am I calculating correctly?

Bend The Light · February 10, 2011

Hi,

I like a bit of macro from time to time, but don't have the money for a proper lens. So I make do...

...so, I have a 135mm M42 lens which I use on my Canon 400d. I put macro tubes to it, attach an old flash on a bar, trigger with a cheap radio trigger.
I was interested in the magnification...so, I put 10.5cm of macro tube on with the 135mm, and took this photo, handheld. It's a JPEG straight from the camera.

Photoshop says the image is 137.16cm wide. Well, my ruler shows 2.5cm across the frame.
So, quick maths...137.16 / 2.5 = 54.864.

1cm in real life is 54cm printed at 100%?

So, I have 54x magnification? Is that how it works?

10 and a half cm macro on 135mm lens by Bend The Light, on Flickr

Cuong · February 10, 2011

Magnification is the ratio of the recorded image over the actual object. In your case, the width of the image sensor of the 400D is 2.22cm, so the magnification is 0.888 (2.22cm/2.5cm).

Cuong

Sand · February 10, 2011

Not as I remember it. Magnification is calculated by the ration of the image size on the film vs. the actual object. I assume in digital the sensor size can be substituted for the film size.
So if you have a full frame camera, the image size is 36 mm. The magnification is 36/25 =1.5 give or take a little.
If you have a dx camera, the image size is 24 mm. The magnification is 24/25 = 1.0 give or take a little.

puzzledpaul · February 10, 2011

<< Photoshop says the image is 137.16cm wide. >>

Photoshop can't know the size of something in the 'real world' by itself - all it's done - to provide the 137.16cm figure - is to divide the number of pixels across the image by whatever resolution happened to be present in the appropriate dialogue box (72ppi)

pp

Overfocused · February 11, 2011

In a microscope you could call it 54x I suppose.

In photography, magnification ratio refers to actual size of an object being focused onto the 35mm film plane. If there's a bug 5mm long 1:1 is saying that it's being projected/focused onto the 35mm film plane, and if you measured that projection on the film, it would also be 5mm long. 2:1 would be 10mm on the plane, 3:1 15mm, etc.
So with that, you'd need a 7:1 lens setup to fill your whole 35mm frame with said 5mm long bug, then you'll get a full resolution photo of whatever it is. Viola!

When purchasing a lens, all macro ratio ratings are relative to the 35mm film plane, just like focal lengths are. It's a standard, but with crop cameras that amplifies some and it needs to be recalculated. I think you could just multiply it by the ratio of the sensor since that's also relative to 35mm. A 1.6x crop camera would make a 100mm 1:1 35mm equivalent macro lens 160mm @ 1.6:1.

I think that math is right... multiplication should work for all the calculations since its all relative to 35mm. If not, someone correct me. It may not since the length of the lens isn't actually extending on a crop sensor... bah

Lord Vetinari · February 11, 2011

Cuong is correct re the lens magnification calculation which is the magnification normally quoted for macro shooting.
ie it's the sensor width cms/cms shown across the image. This just a ratio and the sensor size itself does not affect this magnification ratio. If you had been using a full frame camera there would have been about 4.0cms shown across the frame which would then give exactly the same ratio.
The print magnification is a combination of the enlargement magnification and the lens magnification.
Brian v.

Bend The Light · February 11, 2011

Thank you all, for that...I now realise my mistake. The image size in Photoshop is changeable depending on settings. The image on the sensor is not...it's fixed.

This is what should be used for the calculation. So, doing the above photo, I have at least confirmed that I am getting 1:1, give or take a fraction.

I am pleased with this, though.

Macro Magnification...am I calculating correctly?

Comments