Need help on lens I found

**UncleVanya** · 09-25-2016, 07:52 AM

Originally posted by Aslyfox

I think it says - Without proper "spacing", proper focus is impossible if I read it correctly

T mount 55.00

Pentax k mounts 45.46

difference of 9.54

T mount is "set" farther away so you can't get proper focus???

Did I get it right??

Or am I wrong??

I think you are still confused.

Let's try walking through this slowly. If I tried to put a lens into my k-3 with a shorter registration distance than 45.46, I would not get infinity focus without an optical adapter acting like a teleconverter which will change the field of view and degrade the optical quality. This is why a Canon FD lens won't work on a Pentax but the same lens works great on a mirrorless Sony NEX. The Sony has a short registration distance and can add a mount adapter between the lens and mount to match the originally designed distance - giving normal optical results.
T - mount starts out at a long distance. But also of importance is that it is designed to have a removable mount adapter. Without the adapter installed the lens isn't complete. The adapters are sized so that they provide the correct spacing for the camera they adapt the lens to. One for a particular brand may be longer or shorter as needed to make the total flange focal distance correct.
The upshot of this is that a t-mount with the adapter has proper infinity focus. Longer flange focal distances adapt easier to shorter mounts. Adapting short flange focal distances is what is difficult.

---------- Post added 09-25-16 at 10:54 AM ----------

Oh and I agree with earlier thoughts about this lens. Not a great choice. I have a 400 f5.6 Sigma in the marketplace that runs circles around lenses like this.

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
I Need Help Getting Good Exposure on Backlit Birds	Setter Dog	Pentax K-30 & K-50	11	09-24-2016 08:17 PM
Found a lens need input	photolady95	Pentax SLR Lens Discussion	13	08-23-2016 04:11 PM
Need HELP using & understanding my K3 & I can't find info on Youtube etc/need suggest	CeciProAm	Pentax DSLR Discussion	19	03-02-2015 03:57 PM
Found a lens ... need help	Thoran	Pentax SLR Lens Discussion	10	04-08-2012 09:23 AM
Sold lens ... now need advice on what I need	jpzk	Pentax SLR Lens Discussion	35	11-29-2009 10:32 AM

09-25-2016, 07:52 AM	#16
UncleVanya Loyal Site Supporter Join Date: Jul 2014 Photos: Gallery \| Albums Posts: 28,401	Originally posted by Aslyfox I think it says - Without proper "spacing", proper focus is impossible if I read it correctly T mount 55.00 Pentax k mounts 45.46 difference of 9.54 T mount is "set" farther away so you can't get proper focus??? Did I get it right?? Or am I wrong?? I think you are still confused. Let's try walking through this slowly. If I tried to put a lens into my k-3 with a shorter registration distance than 45.46, I would not get infinity focus without an optical adapter acting like a teleconverter which will change the field of view and degrade the optical quality. This is why a Canon FD lens won't work on a Pentax but the same lens works great on a mirrorless Sony NEX. The Sony has a short registration distance and can add a mount adapter between the lens and mount to match the originally designed distance - giving normal optical results. T - mount starts out at a long distance. But also of importance is that it is designed to have a removable mount adapter. Without the adapter installed the lens isn't complete. The adapters are sized so that they provide the correct spacing for the camera they adapt the lens to. One for a particular brand may be longer or shorter as needed to make the total flange focal distance correct. The upshot of this is that a t-mount with the adapter has proper infinity focus. Longer flange focal distances adapt easier to shorter mounts. Adapting short flange focal distances is what is difficult. ---------- Post added 09-25-16 at 10:54 AM ---------- Oh and I agree with earlier thoughts about this lens. Not a great choice. I have a 400 f5.6 Sigma in the marketplace that runs circles around lenses like this.