Macro magnification...am I 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?


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

 

[Robin Usagani]

you only got it half right

137 cm at what resolution? 72 ppi? 300ppi? What resolution you want to print? Hell.. you can make it 100X magnification but your photo will be pixelated probably.

 

[Bend The Light]

you only got it half right

137 cm at what resolution? 72 ppi? 300ppi? What resolution you want to print? Hell.. you can make it 100X magnification but your photo will be pixelated probably.

So, it's meaningless, then?

I opened the document in PS, the document size was 137cm wide. That doesn't change with the DPI in Photoshop. So if I print that, full size, I get a print 137cm wide, do I? a 2.5cm ruler without resizing anything is now 137cm wide?

 

[Robin Usagani]

dont look at the document width.. look at the pixels. what is your pixel size?

 

[Overread16 4]

Here is the rough math:

Length of one side of your camera sensor in real life
Divided by
Length of a ruler captured in a frame along the same side

For example 1.6 crop camera sensor would have a length of 22.2mm
And you've captured 25mm of ruler length along the long side.

Which gives you: 22.2/25 = 0.888:1 magnification

With regular macro lenses at 1:1 it shows that you're just under what a regular macro lens would get (which makes sense as the very rough math for extension tubes is focal length of the lens/length of the tube in mm = x:1

 

[Robin Usagani]

He is also talking about digital magnification though.

 

[Overread16 4]

I don't think so - I think he was after the regular magnification and tried to apply some logic to the problem, but it failed because some key data wasn't quite what he thought it was.


Also - what is digital magnification in this context?

 

[Robin Usagani]

After reading it again, I have no idea overreag LOL

Bendthelight, just what overread was saying, if you had a 1:1 macro lens, you could put a subject that is a size of your sensor and you will be able to fill up your whole sensor with that subject.

 

[Bend The Light]

Cheers, guys. I have it now...I think.

What I have is 1:1 (nearly) with equipment costing me a total of £10 (possible less). This is acceptable.

I'm not even going to ask about printing the images and things like that.