You can easily check this by referring to your flash guide number or flash-to-subject distance which is shown on flash LCD panel with most models at most f-stops. Lets say you are using a 200mm f-2.8 lens with a 2X teleconverter.
I really wish some camera manufacturer would make a feature that links the focus distance to the aperture based on the flash guide number. That would save me a ton of time when shooting. Doesn't it seem like an incredibly logical idea?
The Excel worksheet "Flash Guide Numbers" lets you explore how a flash's guide number is determined and how that number affects the aperture you can use and the flash to subject distance.
Calculating a Guide Number Section ...
This guide opens with in-camera flash basics, then moves to topics such as exposure flash readings, detachable units, flash guide numbers, bounce flash, and fill flash, then covers advanced methods for using off-camera flash, flash brackets, ...
This integrated flash system irons out the major problems photographers face with an external flash such as the flash guide number calculuation for the flash-to-subject distance, the film speed and the aperture setting.
The loss of light also depends on the shutter speed you use, thus the flash guide number used in manual calculations in 1.) above changes with shutter speed! You lose most of the light at the faster speeds and lose less at slower speeds.
In other words, if you double the lamp distance, light spreads over a larger area and illumination drops to 1/2 - 1/2 = 1/4 of its previous value. Forms the basis of flash guide numbers and closeup exposure increases.
By measuring the amount of light that actually reaches an object or scene, you can obtain a numerical value that can be converted directly into a flash guide number.
 See also: Subject, Photograph, Light, Guide number, Camera
  
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