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07-21-2017, 08:38 AM   #1
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Help with magnification calculation

Greetings all,
Yesterday I was playing around on the back porch with lens stacking and curious on the magnification calculation.
M200/4 lens with reversed Rodagon 105/5.6. My calculations would bring this to just under 2:1.
However, I didn't realize I had a Sigma Achromatic Macro Lens (from the Sigma 50 Macro) on the M200.
Would there be any additional calculation added?
Not that it makes any difference for the image, just curious on the math.

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07-21-2017, 09:05 AM   #2
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Because the additional diopters change the focal distance, I think it would, but I have not the foggiest idea how to calculate it. I think if I really wanted to know I would take a picture of a ruler and work it out from there.
07-21-2017, 09:10 AM   #3
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I hadn't thought of that, thanks.
07-21-2017, 09:42 AM - 1 Like   #4
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Just take metric ruler and photograph it.

Divide the sensor size dimension by what you measure on the image.

Example:
Full Frame sensor 36 × 24mm
Measured 18mm
36 / 18 = 2× magnification

07-22-2017, 07:21 PM   #5
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QuoteOriginally posted by Not a Number Quote
Just take metric ruler and photograph it.

Divide the sensor size dimension by what you measure on the image.

Example:
Full Frame sensor 36 × 24mm
Measured 18mm
36 / 18 = 2× magnification
Thanks, I'll dig up a metric ruler. As I'm always experimenting, could come in handy for other calculations in the future
09-14-2017, 06:29 AM - 1 Like   #6
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Magnification is defined as the ratio of the linear size of the object on the sensor plane and its size on the image plane, which is none other than its real size. So if you have a ruler or caliper or something and just measure the size of whatever feature you want from the image above, for example the ball point, you have one of the needed quantities. If it is anything like the bic pen on my desk now the diameter should be approximately 1mm. Then you just measure the pixel size (horizontally for instance) of the ball point on your image and multiply by (sensor_size/horizontal resolution). If it's an APS-C sensor, for example, horizonal sensor size is 23.6mm. This gives you the size of your imaged object on the sensor plane. Then taking the ratio yields the magnification. Can't do the calculation right now as exif data are missing, besides not knowing if the attached picutre has been resized.

However, you don't need anything special to calculate the magnification, just to be able to measure the real size of the object you are photographing.
09-14-2017, 09:29 PM   #7
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Thanks simon_199, I figured there wouldn't be much difference in the magnification by the added diopter. Calculating the stacked lenses I used is a set formula but was curious if there was a formula for the added diopter. Never thought about going back to the basic of simply measuring.

09-23-2017, 02:02 AM - 1 Like   #8
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It would indeed make a difference to the calculation but not one that I have a formula for.

Maybe it's something it's something to add during the winter month downtime to http://extreme-macro.co.uk/calculators/

But I completely agree, a shot of a ruler is by far the best way to measure (the equations we use are approximations because of thin lens assumptions)
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