pentax to nikon dumb question

[Bossy]

Ok this question is incredibly dumb, but I'd love it if you could help me out.
I have a Pentax lens, a SMC pentax 16mm 2.8-22.
I have a Nikon D7k.
I tried to see if I could just hold it up to the opening and focus manually, and it didn't have a focus point unless I held it a couple inches from the subject. Is it still going to be like that if I buy an adaptor? Is it possible that I just can't use this lens with my camera and can only use it with a pentax?
When I do it with my 35mm lens I can still focus.

 

[480sparky]

I'm sure it has to do with two numbers: the physical diameters of the lens mounts, and the lens-to-focal place distances.

I think Pentax mounts are physically larger than Nikons. So it's too large to fit inside a standard F-mount Nikon. This means the lens must be further away from the sensor.... and why you cannot focus at infinity.

The only way an adapter will work is to not only have the mechanical mount to convert them, but optics as well. And adding the optics will most likely kill any IQ.

 

[KmH]

The focus points are in the camera, not in the lens.

The focus points are projected onto a part of the camera known as the focusing screen.
The focusing screen is in the top of the mirror box. When you take the lens off, you can see the 45° angled main mirror that reflects light up through the focusing screen , through the pentaprism, and out through the viewfinder.

The Pentax lens will not communicate with the auto focus module in your Nikon camera, so none of the focus points in the camera will be functional even if you use an adapter.
You will have to manually focus the lens regardless.

The Pentax K-mount flange focal distance (FFD) is 45.46 mm. The Nikon F-mount FFD is 46.5 mm. Flange focal distance - Wikipedia, the free encyclopedia
So an optical element would be needed in an adapter to maintain an infinity focus capability.
You will still be able to use the lens with an adapter that does not have a lens element in it, but you will not be able to achieve infinity focus with that adapter.

 

[Bossy]

Manual focus is fine, I use that often enough anyway. Is the optic glass that bad that it wouldn't even be worth it to adapt? Are we talking disposable camera plastic bad here or just a little lossy?

And thanks <3

 

[KmH]

Adapters of that kind are a low volume selling item.

Few of them that are made are quality pieces of kit.

If you can, sell the Pentax lens and get it's mid-1990's equivalent as made by Nikon. http://imaging.nikon.com/lineup/lens/specoalpurpose/fisheye/af_fisheye16mmf_28d/

http://www.nikonusa.com/en/Nikon-Pr...ses/1910/AF-Fisheye-NIKKOR-16mm-f%2F2.8D.html

For your crop sensor (DX) D7000 Nikon makes a 10.5 mm DX version - Nikon 10.5mm f/2.8G ED AF DX Fisheye Nikkor Lens

 

[Bossy]

I cant even find this lens online to see how much it'd be worth to sell (sentimental attachment aside).
its a SMC Pentax -A fish eye 1:2.8 -22 16mm. You offering to pay the overage?