technical question regarding teleconverters and stopping down lenses.

[sequitur]

hi guys

here's the question

it's widely known that for most lenses, it's essential to stop down the lens 2-3 stops from their widest aperture. i.e. f/2.8 sharpest around f/5.6.

if i use a teleconverter (2x) with a f/2.8 glass, the effective aperture is now f/5.6. does that mean the lens is already at its sharpest aperture ? or do i still have to stop down 2 stops to get the sharpest image ?

 

[Zerstorer]

Most lenses are sharpest at f8-f11. Some fast lenses such as those f1.4 ones may have optimal sharpness at f5.6 and worse thereafter.

When you add a 2x teleconverter to f2.8 lens, it is still operating wide open at f5.6. You will need to step down to as usual.