Calculating 'real image width' was possible only in strict horizontal direction.
Now direction does not matter.You could now do 'real image width' with image like this: Doh I doubt you will. The reason I did this is: as demonstrated on this image:

• 1.You take your photo (The upper cricket forinstance), and you emideately see it is to big to get it into photo
  at your favourite focusing distance (minimal distance as it is cutom for macro (-:)
  with which you have no problem remembering & you alread have 'real image width' stored in NPMH prog.
• 2.Your idea is to do body measure later with NPMH, without remembering new focusing distane needed to get whole cricket into picture.
• 3.So you take a shoot at your favourite focusing distance (Photo 1), adjust the lens, take Photo 2 and that is it.
  No need to remeber anything, as you know how you took Photo 1.
• 4.Load Photo 1 into NPMH and take two well seen/defined points, like those marked on Photo 1.
  Measure distance between those two points as normaly.
• 5.Load Photo 2 into NPMH, select "calculating real image width" option, drag between same two point as under 4.
  Input distance which you have measured under 4 when asked for it. "Real image width" gets filled for you !
• 6.Switch to "Measuring object" option, and here you are, you can measure your cricket body length.

Added small magnifying glass, where part of original image is displayed at 1:1
You are measuring on smaller image than original,hence there is accuracy gap
You can measure same way as at previous versions if satisfied with accuracy,
that is click, drag, release or (to mesure at highest acuracy possible):

• 1.Click, hold down mouse button (don't move mouse)
• 2.With keyboard arrow buttons adjust starting point of measure (look at 'magnifying glass')
• 3.Drag to endpoint of measure, keep holding mouse button down
• 4.With arrow buttons adjust end point of measure (look at 'magnifying glass')
• 5.Release mouse button, measuring done.

Added marked button. Opens a dialog with some sample data that clarifys what is all about.
You can save there your lens & camera & focusing distace combination & Real image 'width' data for later use.
Double click to select certain Real image 'width'.

You need to know how big the object on photo realy is. Forinstance: you took the shoot of a insect
that was running aroud fast and you could not measure it, catch it, ...
And you are allso to lazy to carry around a ruller to measure each bug, record each measure, ....
Realy all this is pointless to do and big pain in the arse.
And even if you would caught it, if you would want to know how long the antenae are, how would you measure that ?
The accuracy of measurement is not questionable,it is bad. Also mybe you want to let it be (the bug)

• This is simple app, and enables you to do precise measurement of bugs, flowers,...:
  1.Input how wide complete area that you took photo of (at certain focusing distance) is.
  2.Copy/paste photo(taken at same focusing distancae as at 1, of forinstance an insect into application .
  2.Now you can click/drag in any direction/release left mouse button and see how long that part of insect is .
  

• The upper image has 2 images combined, taken at same focusing distance (with same camera & lens), so we can estimate bug body lenght on first sight.
  That estimate is very inacurate, useles to do any serious determination of bug!
  To measure bug body length to 1/100 of mm exact follow next procedure (described for upper images):
• 1.get original photo of bug (not cropped or modified in any way) into program by copy/paste from any image viewer. I do prefer Irfan view.
• 2.Input actual width of ruller on image as we see it (say that is approx. 23 mm) into "Real image 'width'" field;
• 3.Left click at start of bug head,hold, drag to the end of bug body, release mouse button and you will get measurement of bug body length in mm in "Real measurement of object" field.
• 
• 4.That is it, for greater accuracy read "Long instructions".

• Your camera could have stored some information on focusing distance into EXIF. And yes some, rare, do that. My Nikon D300S does that but accuracy is to 10 cm exact which is useless.
  Besides calculating the real size of bug from that(if it would be to, at least 1 mm, accurate) & sensor size & lens data & .... is not realy possible in real life.
• The real option is to remember the focusing ring position while you took the shot of a bug. That should be no problem as in our case, bug hunting, it is mostly set to minimum focusing distance for all images.
• Then, when you get home, you can take a shoot of ruller(at same camera settings as you took shoot of bug ), and there you are, you can 'see' the size of your bug. Comparng shoot of ruler & shoot of bug.
• Doing that for each image manualy is nerwe cracking,time consuming and inaccurate , hence I made this app.

• In upper screenshot you see UI & loaded image. Image is a photo of random ruller that I found laying around.
  Photo took with my D300S & SIGMA 150 EX DG MACRO atached, at minimum range (0.38 m).
  You will do that with your own image ofcourse. You can load image into this prog with copy / paste from any image viewer.
• Ok, we can see the width of area shot is about 22-24 mm. But we will measure that with greatest accuracy possible.
  1.select "calculating real image 'width'" option in upper left corner
  2.The measuring is nov set.
  3.Click, hold forinstance in the middle of line above 4, drag horizontaly to the middle of line above 5, release mouse button.
  4.Input will pop up, you will input 10, as you dragged across 10 units (mm in this case).
  5.Confirm, and you will see in "real image 'width'" field, how wide your 'image' is in nature.
  That is it, from now on, you know what your camera & lens at given focusing distance produces, you don't need to measure that again.
  Next time, running this program you can input "23.57" (your will be different except if you have same camera & lens & focusing distance) manualy.

• Now switch to 'Measuring object' option.
• Paste your image (of an insect,flower or whatewer) from wahtewer image viever you like. I prefer Irfan view
• Click, hold forinstance at start of bug head, drag to the end of bug body,release mouse button. The measure appears in "Real meaure of object:". In mm in my case.