Cost effective prime lenses for wide field imaging

[cuivenion]

3 hours ago, fwm891 said:

What's the 'hack' your doing? If your changing the back focus then that is probably why your not getting clean stars in the corners even when stopped down.

What is changing the back focus? My stars aren't clean in the corners even at f8 with a Carl Zeus Jena 135mm that I messed with to get focus.  There's info in this thread. 

Any help much appreciated. Sorry for the thread hijack. PMs are fine with me if this is too off topic.

[Stub Mandrel]

12 hours ago, cuivenion said:

What is changing the back focus?

This would  be anything that changes the distance from the lens element to the sensor when in focus, such as a lens, filter or just a glass sheet in between, or adjusting the spacing of the lens elements.

Just changing the travel of the lens will not have an effect as in prime lenses all the elements are fixed in place (with the possible exception of catadioptric lenses which have a moving focusing element. These tend to have extra travel anyway as their properties seem to be less predictable).

[AdeKing]

On 22/11/2018 at 16:39, happy-kat said:

The pancake 24mm is on offer at curries for £79 today

Thanks for that info, I've wanted one for a while and had been holding off, but I picked one up from Currys last night. ?

[happy-kat]

I hadn't realised the 24mm is only in EF-S flavour, the 40mm pancake I have is in EF flavour.

EDIT: but anyway it is reserved at local store so will give it a go at that price and the 40mm is a great lens so hopefully this will be too.

Must check if there is a Canon lens profile to download for it.