Pentax to Nikon lense adapter

[Todd Cavanagh]

Hi,

I have been given a Sigma AF 100-300mm f/4.5-6.7 DL with a Pentax mount that I want to attach to a Nikon D40 DSLR Chassis. From what I have read on the internet I should be able to do this with a teleconverter. I have been to a couple of camera shops and have been that it is impossible to do this. Is this true? Please advise as I am getting very confused.

Todd

[danic]

Welcome to the forum Todd.

A Teleconverter is used to extend the focal length of a lens. So in your case, a X2 Teleconverter will extend your lens from 100-300mm to 200-600mm. Does that make sense?

You can buy adapters to mount different lenses to bodies, but I'm unaware of a mount that will allow a Pentax Lens to fir a Nikon body.

[another view]

As danic said, a teleconverter will covert a lens's focal length - not convert one brand of lens to another. They're usually 1.4x or 2x or maybe something inbetween, so multiply that number by the focal length and that's what you'll get.

However, like with anything else in the world, there is no free lunch. A 1.4x TC will cause your lens to lose one stop of light (so it'll be a 140-420mm f6.3 to about f9). Pretty much all autofocus systems need a lens that's f5.6 or faster (lower number) to work, so you'd have to manually focus for best results. Autofocus may work in some conditions, but it'll be inconsistent and very slow if it does. TC's can also have an effect on sharpness, especially with zooms.

[Todd Cavanagh]

Thanks for the information.

Bit more clarification please. I have found different types of lense adapters on the internet but am difficulty in determining what type of Pentax mount the sigma lense has. There seems to be many different types but I can't find out what type it is. Could you please assist by identifying the lense mount type and then advise if an adapter is available to mount onto a Nikon D40 Digital SLR. I have attached a photo of the mount.

Thanks for your help.

Todd:mad2:

[Dylan8i]

oh and btw im not sure (but most likley- depends on the lens itself, and then the adaptor you get) but you probbably won't retain af function either. since the d40 requires af-s and that lens probbably isn't compatable.

[deckcadet]

Almost no lens adapters have any sort of AF compatibility at all anyways, so the D40's AF-S only requirement is a bit of a moot point.

No standard adapters will work, as the flange focal distance of the Nikon F mount is longer than almost any other SLR mount, including the Pentax K-mount (in this case the Kaf variant). Any lens mount adapter would be unable to achieve infinity focus without an additional optical element in the adapter that adds to the cost, complexity, might influence light transmission, and typically degrade optical quality, sometimes severely.

[Frog]

You might be better off selling it and getting a Nikon compatible lens.