In case you had not noticed, Mongo has been testing and calibrating the 300mm f4 over the last couple of moths or so. From the various posts, you will see that Mongo has tried this lens with just about everything ! ---- except the Nikon 17E11 (because he does not own one). However, Sar has been kind enough to lend him one and now, Mongo can post his findings.
This combination give a very light 500mm f6.7 equivalent.
First, let Mongo say that he had read a lot of nonsense on the net generally as to how unhappy some have been with the 300 f4 in combination with the 1.7 converter. A few have said its OK..........Mongo thinks its bloody amazing !!! See for yourself.
These 2 images are 50 % crops of the originals (shot at slightly different distances). Converted from RAW to Jpeg, ever so slightly processed (as it simply was not necessary with these) and resized for posting.
This one was taken at f11 (that is, about 1.5 stops down from wide open with the converter)
This was taken at f6.7 (that is, wide open with the converter)
Mongo sees no measurable difference between them. So, if you are not looking for DOF and would prefer extra shutter speed with no loss of sharpness, you may as well shoot wide open again with this combination.
So far, all teleconverter combinations have been as good wide open as stopped down. The only exception is the 16A converter which required 1 to 2 stops. However, in fairness, when Mongo did that test, he was not fine focus tuning the AF. He has been doing so since then . So, it may be that even the 16A will give excellent results with no stopping down or with less stopping down than previously thought if fine focus tuning is carried out. The above examples were shot with fine focus tuning of +6. Either way, the results are impressive.