Speedbooster question

davesrose wrote in post #18887825
In photography, you can have either positive or negative focal length in magnification

This is not about offsets; it is about scalars.

Certainly, one can ask "what is the magnification of the speedbooster", and one can reply "0.71x", but to mention "magnification" without stating that the factor is between one and zero can lead to misunderstanding.

TeamSpeed wrote in post #18887888
A teleconverter changes the effective focal length, as does a wide converter, ie speed booster.

A 35mm becomes a 25mm with a .71x, a 2x makes the view look like a 70mm. Neither is negative, the converters just change the effective lengths.

The crop factor of 1.6 then applies to this new effective FL.

That is all I am saying. I feel this description is more technically correct than some sort of negative versus positive effect. I hope that clears up matters. A picture would probably describe this even better for those just learning about speed boosters.

"A magnification of 0.71x" is a fair statement. However, without that number, the word "magnification" takes on its common meaning and connotation, which is to become greater. That is why I replied to daverose.

davesrose wrote in post #18887825
In photography, you can have either positive or negative focal length in magnification

To so-called 'negative magnification' that you mention is simply magnifican by LESS THAN ONE but greater than zero.

• 0.7x magnification of 100mm results in 70mm.
  
• If the 100mm lens was f/2 at max aperture, it still has a diaphram opening of 50mm as a 70mm FL. So the same 50mm aperture is 70/50 or f/1.4
  

I can understand that the practical side of this concept as one multiplication factor (and what you and John are stating as less than 1.0). Point taken that it's easiest to convey these terms, even though the underlying optics are more complicated.