Macro Magnification...calculating correctly?

[bend the light]

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?

http://farm6.static.flickr.com/5293/...85ba42ff_z.jpg
10 and a half cm macro on 135mm lens by Bend The Light, on Flickr

[Fixer]

Can’t wait to see who comes up with the answer lol

[petrochemist]

Macro magnifications are usually calculated using the sensor size not a print out/screen display. Both the second two allow you to enlarge without increasing the information...
A full frame 35mm is 35mm diagonally, with the APS (cropped) sensors being smaller. Not even a larger format camera will get you anything like 137cm (A sensor over 1.3m across the long side)

[bend the light]

Yes, I have it now. It's sensor size. I have 25mm showing on a sensor that is (apparently 22.2mm wide) which gives me 22.2/25 = 0.9 (nearly) to 1.

Nearly 1:1 with kid worth less than £10. I like that.

[john crossley]

magnification = ext tubes / focal length + 1 x 1.6 (crop factor) = 2.8444444

There again I could be wrong.

[petrochemist]

magnification = ext tubes / focal length + 1 x 1.6 (crop factor) = 2.8444444

I'm not sure about the +1 part (I remember the guide that when extension + focal length you get 1:1)
In addition I think it applies to having the lens focused at infinity. If the lens is focus closer the ext tubes part becomes Total extension as focusing the lens adds extension internally.

[robski]

I would go along with the 0.88 figure.

Crop factor has nothing to do with magnification. It is related to the field of view (FOV) a lens will give you for a given sensor size. In a nutshell it is saying the FOV is equivalent to using a 1.6 x longer focal length lens on a 35mm frame camera.

If the magnification was 3 then a 7mm object would fill the frame width on a 400D.

As a rule lens magnification calculation formulae are prone to error if they purely rely on the lens focal length and do not take into account a focus distance or native lens magnification. The main problem being (especially for telephoto lens) the stated focal length is not constant though out the focus distance range. In this case it maybe something like 135mm at infinity and reduce to something like 100mm at the closest focus distance.

For anybody who is interested I have a M42 screw thread 90mm Macro lens which I not longer have any use for. I bought it new for my Practika film camera some 35 years ago (only bit of kit I have left from that era). Missing front cap but otherwise OK. PM me if interested.