|
Using the same photo, I've split the channels into red and green to explain more about focus and achromatic refractors.
You will see in the red channel the stars are all bloated and look like blobs. They are out of focus.
With the green channel the stars are all tiny and well focussed. Why? Well human eyes have peak sensitivity at green, so I focussed my camera for my eyes.
If I had somehow managed to focus the red channel properly, (which I could have by taking short exps to test) then the green channel would be out of focus.
Such is the life of a fast (F5) achromatic refractor. They improve as the F ratio rises, say to F15, or if special glass is used to make the objective. Hence why everyone loves APO's.
Edit, an achromatic refractor can still take good pics with all colours focussed, but you have to shoot each colour separately and combine them later in software. Or be happy with monochrome.
|