Complete instructions on how to use Image J.

Brief instructions for ImageJ for this exercise to measure lengths of cells:

Open your measurement and calibration scale images:

1.  Open the image(s) you want to make linear measurements from that were take at a known magnification for which you have an internal scale.

2.  Open the image of the internal scale associated with this/these image(s).

NB.  If you need to make measurements on a bunch of images taken at different scales or magnifications, it is extremely useful to embed a calibration scale directly into the image using PaintShop Pro prior to making your measurements.  This saves you an extra image load step, and the mental effort in keeping tract of what calibration scale goes with what image - at least at the measurement step, since you can calibrate your measurements from your embedded scale.

Calibrate the Images for measurement:



Making Linear Measurements:

1.  Use the Analyze>Set Measurements command to specify what area statistics are recorded.

With line selections, the following parameters can be recorded: length, angle (straight lines only), mean, standard deviation, mode, min, max and bounding rectangle (v1.34l or later). The mean, standard deviation, etc. are calculated from the values of the pixels along the line.

2.  Select Straight Line, Drag and drop a line to correspond with region to be measured.

3.1  Then click Analyze / Measure.  Results are displayed in Results Window.

3. 2 - 3.end.  Repeat 2. and 3. for as many regions you want to measure.

NB.  I know this seems tedious, and not overly user-friendly - but what do you want for free?! 
Spend 4 - 10 K$ for a more convenient program that basically does the same thing, but with automatic data transmission!

4.  Once you have collected all your measurements, click File / Save As/ and save the data as an excel sheet. 

NB.  It might help if you edit the data in the spreadsheet to keep track of what your measurements were, before you forget!