Three New Lenses~~~

[Astro]

for once an interesting question but none seem to be able to answer it gothmoth.

people here prefer to talk endlessly about switching to nikon or why 36MP are so much better then 22MP. :

 

[neuroanatomist]

D in the case of an camera is the diameter of the lens aperture
the aperture size (for example f4) is the same on a 70mm lens .. no matter how big the front element.

for a telescop D is the diameter of the telescope's objectiv:

so maybe someone with more experience can shine a light on that?

You've actually got the answer in your post..."D in the case of an camera is the diameter of the lens aperture," while, "for a telescop D is the diameter of the telescope's objectiv."

The resolution of a lens is determined by it's optical aperture, which is not necessarily the smallest diameter part of the system, nor the largest.

In photography, there is a desire to have the ability to reduce the amount of light reaching the image plane, thus, the aperture of a camera lens is an adjustable diaphragm that can be closed down, and it is placed in the optical path such that the diaphragm is the optical aperture of the lens. Thus, the iris diaphragm is located at the principal focal point of the lens (the place where all the light rays 'come together'), and that aperture diameter becomes the determining factor for resolution. For a telescope, OTOH, there's no need to 'stop down' the lens, so there's no diaphragm at the focal point, and thus, resolution is limited by the diameter of the primary (front) lens or the primary (rear) mirror, for refracting and reflecting telescopes, respectively.

 

[Gothmoth]

You've actually got the answer in your post

i thought so but was not 100% sure.

thought it could be possible that the elements before the aperture collect more light with a bigger front glass, bundle/bend the light and send it to the same sized aperture.
but form the math that would not make the lens resolve more... only collect more light.

i mean there has to be a reason for the bigger front element... but im not that firm in lens design to answer that question.

 

[neuroanatomist]

i mean there has to be a reason for the bigger front element... but im not that firm in lens design to answer that question.

Generally, the relatively larger front element is to help reduce aberrations when shooting wide open. Telescopes are usually in the f/5-f/10 range, not f/2.8 and faster like many lenses.