I am taking photos of sea urchins with a GoPro at a fixed distance (1 meter) and I need to determine the sizes of urchins in photos using Photoshop. I do have a scaler in my photos, so I know exactly how long 1 cm through 20 cm actually is. In order to find the lengths of my sea urchins, should I measure the distance in pixels, then convert that to cm? In other words, I can measure the number pixels in my 1 cm scaler, then apply that to how many pixels are in the length of one urchin. Is this the correct method, or is there a more accurate/efficient way to do this?
You'd still need a reference. If your distance from the object is a constant and accurate enough for your purposes, then photographing a ruler would give you that reference. Your photographs would obviously need the lens axis to be perpendicular to the objects being photographed, and the lens to have a fixed focal length, but you knew that already.
Then, providing you are using Photoshop CC, or the Extended version of CS6 or earlier:
Excellent - thank you!