PKA Extension tubes.

Hello

I am hoping someone out there can help with a couple of questions, I have seen advertised on the internet some Vivitar PKA extension tubes.

Would PKA extension tubes be able to communicate between my kit lens with no aperture ring and the camera or my Pentax-a standard lens?

Also is there a formula for calculating how much the focusing distance will be reduced by using various sized extension tubes?

Thanks

Paul

I have 2 sets of Jessops tubes one is marked with PK and the other with PK/A. The reason I purchased both is because they both have the contacts so I can use my Pentax A 50mm and control it from the camera, no auto focus though, I almost always use Catch in Focus. On both occasions the advert contained photos where you could clearly see the contacts. Try contacting the vendor and ask if the contacts exist.

As for a formula I would also find this useful, I tend to guess, and add/remove segments as required depending on how close I think I can get to the subject and how much I want of it in the frame. e.g. you need less tube length to get a butterfly in the frame but to get a full face close up of an ant would require more and sometimes some of the second set.

This shot was taken with one full set - 65mm. It is surprising how quickly you get to the point of knowing which tubes and how many you will need.

Formula;

The magnification ratio of any tube-lens combination is calculated by dividing the tube size by the length of the lens. e.g. 50mm tube on a 100mm lens allows you to focus to .5 life size.

But you will suffer from light loss, which is dependent on the tube-lens used.

Hope this helps

PKA extension tubes should give you control of the aperture - some need the some of the paint removed to get electrical contact before they work, from what I hear.
If they don't have the contacts they are only PK mount not PKA.

Note Auto extension tubes generally only have the mechanical aperture linkage which is of limited use for DSLRs

Instead of PKA tubes, I use deglassed PKA TC's. Such TC's are more common and less expensive than tube sets I've seen, and removing the glass is easy.

Magnification formula is: M = (TE-FL) / FL where TE is Total Extension and FL is focal length.

A 50mm camera lens at infinity focus has 50mm of extension built-in.
If you add 25mm of tubes (like one 2X TC) then TE is 75mm. M= (75-50=25)/50 = 0.5x.
If you add 100mm of tubes (like 2 PKM tube sets) then TE is 150mm. M= 100/50 = 2x.

Yes this is true. The circuit is completed by the body of the camera and body of lens making electrical contact. Some brands of PKA tubes have been made by people who a dumb to this specs and anodised the tube bodies. you need to grind/sand off this anodisation ( or paint ) to get them to work.

I bought some Jessops tubes 2nd hand off ebay that had this problem. I suspect that's the why the original owner sold them

Indeed. This applies to TC's also. I filed off the paint on the bayonet flags of a de-glassed Promaster 2X TC (makeshift macro tube) to complete the circuit. Beware plastic lens mount!

Thanks for all the help everyone, a great response with lots of helpful info.

I followed some advice and emailed the vendor and it turns out it was being incorrectly advertised and were PKM which I was a bit annoyed about.

I am having another think and might dip my toe in by getting some manual tubes, and then teaming them up with my PKM standard lens or my SMC 105 2.8 M42 lens with an adapter, I hear these great things about the green button I might use it.

You really could lose a lot sleep worrying about these things.

Thanks everyone

Paul

Was knocking about with a similar set up a few days ago.

Sigma 105mm with Vivitar tubes (covered in black paint, no contacts) with the paint taken off (can't remember what length), camera to manual on the green button. I think lens was set to f16. I was just playing so its just an idea of what you'll get. Just keep your eye over the eyepiece or have it covered otherwise your exposure will be wild. I was out in the sunlight so the light is a bit harsh.

[IMG]http://i740.photobucket.com/albums/xx49/df8/1ef0a086.jpg[/IMG]

Just in case, as I didn't get the whole thing in shot, its a strawberry. Not a fresh one.

Really like that, thanks.

Paul