strange pixel count

[Kristermacher]

photoshop CS2

I am using photoshop for a research project that involves counting pixels in order to determine lichen growth rates (fascinating, I know), but I have been having problems with larger images.

For each picture, I trace the lichen and copy it and put it into a new window and use the histogram to count pixels. I then do the same for a sticker of known area that we put in the photo for size reference. This has worked very well except for when the lichens are larger. When I paste the picture into a new window, the window says 50 or 66.67% and the count is clearly off. (when I compare it to the dot it will be about 10 times, when just eyeballing it, it is at least 20) I tried just changing the size back to 100 or changing the dot to the same percentage as the lichen, but it never works out right. Why is the pixel count changing? Is it changing the pixel aspect ratio? Does anyone know what photoshop does to pixels when it scales down an image?

Thanks,
Kristi

[Tips]

There are numerous explanations of resolution in this forum as well as the Photoshop Tips & Tricks department at DTG.

If I understand your problem correctly:

You may be pasting into a file of a different resolution

Therefore, Photoshop either eliminates or invents pixels to make up the differences.

For Scientific work such as you're proposing, I don't recommend Photoshop. Counting pixels is fairly un-scientific.

However, using the method you are using, to keep 100% accuracy and consistency, I would advise keeping ALL of the elements of the experiment exactly the SAME from one sampling to the other.

My method would be to simply cmd/J or cntrl/J which is the "Float" command and COPY your selection to a new layer. Now, you can turn OFF the previous layer, and count your pixels.

On the next sampling, assuming the camera is in the exact same location -- exact same picture size and resolution, you should be able to simply DRAG Sampling photo #2 into Sampling #1 image -- where it will appear on a new layer...
Make your selection
Float it to a new layer
Count your pixels, then
Move the Sampling #1 Cut-out layer ABOVE Sampling #2 cut-out, and you'll be able to SEE visual growth.

Does this help???

If not, then I don't understand the experiment.