[troypiggo (Troy)]

Wanting to make some external apertures for a couple of my lenses to get rid of diffraction spikes when stopped down a little. I just need to check my logic in terms of sizing the diameters to get the f-ratio I want.

Take the 135mm f/2L. I want to the extl aperture to stop it down to f/2.8. I know that the internal blade aperture of the should be about 135/2=67.5mm. If you set the lens to f/2.8, the blades should be about 135/2.8=48.2mm.

But where I'm uncertain is when the aperture is external - not at 135mm from the sensor. I'm thinking I should use the distance from sensor to front of lens where the extl aperture would be?

Say it's 156mm from sensor to front of lens. To get f/2.8, by this logic, the aperture should be 55.7mm as opposed to the 48.2mm.

Any thoughts/comments?