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What did I do wrong?


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Cosmos57

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I am trying to figure out what magnification my camera lens is shooting at, so I took a silver oxide battery and measured it with my digital calipers. It read out at 7.76 millimeters. I then took a picture of the battery with my lens at close focus and used a macro focusing rail to make minor adjustments to the focus and opened up the image in Photoshop CS6. I set the units to millimeters and used the ruler tool to measure the battery in the image. It said the battery was 660 millimeters. I am sure I did something wrong in the PS settings. I have included the image here. Would somebody please measure the battery in the image and tell me if I did something wrong? Thank you in advance for any help you can give me!

_MG_1964_crop.JPG
 
Hi Cosmos57,

As Photoshop is pixel based editing software you cannot really use it to measure linear distances.

The DPI setting for an image only has effect when printing the image to paper, it has no effect on the image when being edited in PS.
Whilst the DPI does in fact determine the size of the printed image the DPI can be set to anything.....you can make the image as big or as small as you like.

I'm not convinced that you can actually use a photo in this way without some other reference in the image with which to compare, and wont the distance from the camera lens to the object have a major influence on things?

Here's one idea..... you must have seen those police photos of 'murder weapons' etc....right?
What do they all have in common?
They always place a ruler next to the item!

If you did the same you could then use your photographed 'ruler' as the control for any measuring you needed to do.

I still don't see how this will help determine the magnification of a lens though, unless you do two images one at minimum zoom, and one at maximum zoom.....or a series at known intervals?
You'd need to keep everything absolutely in the same place between shots especially the camera distance from the object, but from the sounds of it that's not a problem.

I would think you could then calculate the actual 'magnification' using those two extremes, the size of the ruler in the image and the distance of the camera from the object.

You could then use PS to measure the differences between shots, the actual measurement (in mm) is irrelevant as its just the relationship between shots that's important....ie, the battery 'measures' 660mm at 100% zoom and 120mm at 0% zoom.

I dunno for sure but hopefully you can see where I'm going with this.

I'm sure someone here will know of a more 'exact' way.

Regards.
MrTom.
 
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I agree with Tom,,,, use a calibrated device to show proper size. Years back we did this in silver images..Macro , tubes, closeup lenses and even a bellows was used.

The issue is too many variables to shoot a 1:1 and size it without a scale.

I suspect that the CSI plug in has all of this taken into the algorithms....:rolleyes::bustagut:
 
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