Turn Off The TTL And Turn On The Light
Nearly all the flashes I own are TTL except for the ones that are not. And fortunately the ones that are can be switched back to manual control.
This may puzzle the newcomer to the sport - after all the Nikon, Pentax, Canon, Olympus, Fujifilm, Sony, and Leica people all make a great deal of telling us that their various flashes are TTL - indeed in most cases this mode is the default one when you switch the speed light on. There are lots of occasions upon which it is exactly the right thing to do and many people can rely upon it to give them what they want.
One of the joys, in fact, of demonstrating a modern Nikon or Canon flash to a first-time flash buyer is to put an SB700 or 430EXII onto their camera, glance at it to see that it is set to Auto or Program, TTL and AF, and then just point the thing at them and rip off half a dozen portraits right there. The TTL balances whatever the shop lights are doing and nearly everyone looks good. Done right, it is a sure seller.
Have a look at this picture taken at York on Sunday. I was in Brigg's York Motor Museum while the rest of the townspeople and tourists were out watching belly dancers and eating ice cream cones. No interfering crowds and plenty of time for experimentation. The car was a small Austin parked in front of a picture window with greenery outside.
It is nowhere near as good as the heading image - yet this darker view is taken with the flash on TTL and supposedly working in harmony with the camera. What I think it is doing is quenching itself too early based upon the backflash from the cowling of the Austin and thus not getting enough illumination out to the mid ground.
The heading picture was taken with the same shutter speed - this is dictated by the top synch speed of 1/180 sec on the Fuji X-T10 and whatever the outdoor illumination is going to yield for an f stop...and then the flash set to either full or half-power manual dump. The Fujifilm EF-42 is easy to control for this sort of selection - the Nikon SB 700 as well.
The top one is the picture I want. There are many of these sorts of situations - half-illumination with reflection - where the TTL mechanism does what the designers have told it to do but not what you would like it to do. Here's another pair - this time a Maybach-engined GP racer.
The top picture is the TTL version and the bottom the manual flash version. Note the plastic chain bears the same exposure in each but the manual setting has allowed more oomph to get further back.
Thanks to the York Motor Museum for the pictures and a fun afternoon's visit - rendered all the more enjoyable for being available on a senior's card discount.