|HP 15c (from the Wikipedia entry)|
This post is not about photography.
Hewlett Packard has created apps for both Android and the iPhone that replicate the HP 15c Scientific Calculator. On my phone the app looks and acts like the original calculator — for a fraction of the price and without carrying around another device. The app is not a substitute for the real thing when there's heavy use, of course; but it's great for the once-in-a-while.
In the early '80s I bought a Hewlett Packard HP 15c. It wasn't my first HP calculator; and, because my 15c was in my shoulder bag when it was stolen from my office almost 20 years later, it wasn't my last. (The 15c was out of production by then, so I had to buy a newer model.) And, I'm confirming my geekiness by admitting that I've kept my 15c owner's manual for the decade following my 15c's "departure."
HP calculators were a cut above other brands. As a small example, you never had to worry about the numbers on the tops of the keys rubbing off — because the numbers were injected molded. They went all the way through each key. Keys might get a little shiny with use, but they would never fade.
The algorithms in the 10 series calculators were the product of renown mathematician, Prof William Kahan who (Wikipedia reports) was the architect of the IEEE 754 standard for floating-point arithmetic.
HP's "10 series" calculators had/have almost cult followings. The 15c was the choice amongst engineers and maths folk, and the 12c (financial calculator) amongst the money men. (There wasn't a "Master of the Universe" on Wall Street in the '80s who didn't have an HP 12c.)
There were three other, less famous, 10C series calculators: The 10c, a basic scientific; the 11c, a more sophisticated scientific; and the 16c, a specialist computer programming calculator.
HP calculators were distinguished by their input notation. They were not "algebraic" (with parentheses and an "=" key), but instead had an "ENTER" key. Equations were input in a different way, called Reverse Polish Notation (RPN). Briefly, instead of adding 2 and 2 as "2 + 2 =", in RPN that simple calculation is entered as "2 ENTER 2 +".
So, with a slightly more complex calculation like 6 x (2 + 3), the algebraic key entries would be "6 x ( 2 + 3 ) =" (Eight keystrokes). In RPN it would be "2 ENTER 3 + 6 x" (Six keystrokes). The more complex the formula the greater the savings in both keystrokes and time using RPN.