How do I use a SB-24 on a D-7100?
The SB-24 cannot work in TTL mode, because it is film TTL, not digital iTTL mode.
It can work in flash Manual mode M or Auto mode A.
Auto mode A acts a little like TTL mode, in that it is auto flash exposure. The flash sensor meters this, so it normally meters a wider view than the lens sees.
In Auto mode A, it cannot do todays CLS communication with the hot shoe, so you will have to set the current ISO and f/stop values into its menu.
Manual mode M is straight forward, except it is only settable in full stop steps. It has no slave mode. I frequently use a SB-24 on a hot shoe cord in a umbrella, in Manual mode. Works fine.
But, truth is, it is dated, and a $100 flash like a Yongnuo YN565EX will run circles around it today.
See
Review of the Yongnuo YN565EX Speedlight
