Calculating magnification using objectives on cameras

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leander
Posts: 68
Joined: Sun Jun 19, 2011 8:39 pm
Location: USA

Calculating magnification using objectives on cameras

Post by leander »

A very basic question.

I have a Nikon Achromatic Finite Conjugate Objective 10x mounted on a bellows and my Nikon D300. I have the bellows extended so I have 150mm distrance from the base of the objective to the sensor. So I am assuming I'm achieving 10x magnification. If I reduce the bellows extension or extend it more, how do I calculate the magnification. Or should I only use it with the 150mm extension.

Is there a simple formula to calculate magnification based on a finite objective mounted on a bellows?

I looked at http://tirpor.com/macro/macro_DOF.htm but I don't know exactly what values to input. I have a Nikon D300 and a D800 and I have the 10x finite objective and bellows extension. Where do I find the objective numerical aperture (NA) and the circle of confusion for my D300 and D800?

Also, is there a calculation for example of the field of view in um when I use my 10x objective with 150mm of extension?

If you have addressed this question hundreds of times already, would you please point me to a reference. Thank you.
Bruce Leander

rjlittlefield
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Post by rjlittlefield »

With that objective it is OK to change the extension to get different magnification. If you reduce it too much the corners may go bad, and if you increase it too much the whole image will get a little soft from diffraction (but no softer than if you used 150 mm extension and cropped).

The best way to determine actual magnification is to measure it by imaging a stage micrometer. Your sensor width is 23.6 mm, so just divide 23.6 by the measured field width in mm, to get your magnification. At 150 mm extension, you should be getting very close to 10X magnification so 2.36 mm field width.

If you don't have a stage micrometer handy, then you can get pretty close by assuming 10X at 150 mm extension, photographing some flat patterned subject at 150 mm and any other extension, comparing the images, and computing from there.

To estimate the magnification, you can use the formula that magnification = 10 + (extension-150)/16. This assumes that the focal length of the objective is 16 mm, which undoubtedly is not quite right but it may get you close enough, depending on how much accuracy you need.

For DOF, the safe approach to estimation is to use the formula/calculator at http://www.microscopyu.com/tutorials/ja ... index.html and make e as small as possible. (e=0 would be a perfect sensor; the applet allows e=4 which is close for both of your sensors.) The resulting value is around 9 or 10 microns. I generally use a step size of 10 microns for non-critical work on my T1i, which is about the same as your D300. For critical work, again you should measure for yourself. The best step size for stacking is the largest value that never produces visible focus banding even in the toughest cases such as fine regular structures or an oblique smooth fiber.

Is this what you were looking for?

--Rik

leander
Posts: 68
Joined: Sun Jun 19, 2011 8:39 pm
Location: USA

Thank you!

Post by leander »

That was perfect. I'm going to order a stage micrometer. Thanks, Rik.

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