MTF Charts for f/4 Lenses with 2xTC

[AlanF]

As some posters have written that their f/4 600mm II is soft with a 2xTC, I checked the Canon MTF charts. But, I am now mystified. The black lines on the charts are for wide open and the blue stopped down to f/8. For the f/4 500 and 600s with a 2xTC the wide open is f/8. So, why are there still separate blue and black lines?

 

[Loswr]

I had noticed that before. I suspect it's because their 'model' is accounting for light passing through the optical elements of each part of the system separately. IOW, the model takes the (theoretical) output of the bare lens wide open or at f/8, then passes that output through the (theoretical) teleconverter. Not the right way to model it, of course. If true, that also applies to 1.4x TC data and also to faster lenses (which are stopped down further than you'd expect for the f/8 curves).

 

[Mt Spokane Photography]

I think Neuro is right, the f/4 and f/8 is the setting of the lens. When you add a TC, it reduces the light by 2 stops, but it does not change the aperture of the lens itself.

 

[AlanF]

I think Neuro is right, the f/4 and f/8 is the setting of the lens. When you add a TC, it reduces the light by 2 stops, but it does not change the aperture of the lens itself.

I don't think neuro meant that. A 2xTC definitely doubles the f number. The f number = focal length/diameter of lens. The 2xTC doubles focal length and leaves diameter unchanged. What neuro is saying that the data they are presenting in the MTF charts are for the undoubled apertures. So, the blue lines are for a true f/16, not f/8.

 

[Loswr]

Adding a TC doesn't affect the physical iris diaphragm diameter, but since it increases the focal length by 1.4x or 2x, it must decrease the f-number (focal length / iris diaphragm diameter) by one or two stops.

But I think we're all saying the same thing - the 'f/8' MTF for the f/4 lens plus TC is with the lens 'set' to f/8 (because it's a virtual/theoretical lens, it's set independently of the TC). So, 'wide open' and 'f/8' curves are really f/5.6 and f/11 with the 1.4x, or f/8 and f/16 with the 2x. I assume that concept applies to the f/2.8 lenses, etc., as well.

 

[AlanF]

Adding a TC doesn't affect the physical iris diaphragm diameter, but since it increases the focal length by 1.4x or 2x, it must decrease the f-number (focal length / iris diaphragm diameter) by one or two stops.

But I think we're all saying the same thing - the 'f/8' MTF for the f/4 lens plus TC is with the lens 'set' to f/8 (because it's a virtual/theoretical lens, it's set independently of the TC). So, 'wide open' and 'f/8' curves are really f/5.6 and f/11 with the 1.4x, or f/8 and f/16 with the 2x. I assume that concept applies to the f/2.8 lenses, etc., as well.

The charts are thus misleading for when TCs are involved. Nowhere on the Canon site does it explain the situations.