Tamron 28-50 f/3.5 loose front...

[ChrisR]

I've recently acquired a Tamron 28-50mm f/3.5 Adaptall zoom, with my "new" Pentax MX. The front element (the focussing ring) wobbles very slightly. I thought I'd found a Youtube video showing how to take the front element off, but it's a later f/2.8 lens, with a slot to help you lever off the retaining ring. There's no such slot on this lens; it looks as if the front screws out on the filter thread. There's nothing to give a purchase to unscrew it except two tiny slots 180 degrees apart. I tried using a screwdriver head and my nail on these, but no joy. Is there a special tool needed?

 

[Jao]

I think some of the more compact Tamrons they do require special tools.

I wonder are there any possible links to repair details from this site:

http://www.adaptall-2.org/

Is the wobble enough to impact in image quality? If not it might be best to live with the wobble.

 

[ChrisR]

Thanks for the link, but I can't find anything useful there. The wobble is quite small, so as you suggest I'm probably better off living with it.

However, another problem has arisen. I've just changed lenses on the MX from my SMC-M 50mm lens to the Tamron. The scene from my window meters at f/8 at 1/250 with the Pentax lens, but f/4 with the Tamron zoomed to 50mm. Two stops is quite a lot!

I did have a probem last night when I accidentally removed the Tamron lens from its Adaptall-2 mount. I've since found an instruction sheet online (http://www.gyes.eu/documents/adaptall-2_custom-mount_for-pka_manual.pdf), and I'm convinced I've now mounted it back properly. Do I have a faulty lens, or is there likely to be something I'm missing?

 

[ChrisR]

Anyone any suggestions as to why an Adaptall lens might meter two stops out? Anything weird I could have done setting it up? :shrug:

 

[donut]

i would check the exposure with some other means ,to see which lens is the one two stops out first

 

[donut]

( they might both be one stop out )

 

[freecom2]

( they might both be one stop out )

Very good point - I'd also check the relevant manuals for Adaptall-2 mounts to check it has been installed absolutely correctly, because I'd imagine if it was installed incorrectly then the sensing of the aperture selection might not work properly as a result, leading to false metering (although the camera might believe it is entirely accurate metering).

 

[s162216]

From a quick look around, Tamron only every made a 28 - 50 f3.5 - 4.5 so that must be what you have. With adaptall lenses variable apertures can be funny sometimes as up to the very late ones of the 90's they were unable to indicate their maximum aperture and the effect of the change in focal length, so the camera always read it to be constant but compensated for the drop in light read made by the focal length. When you actually take the picture it will come out fine and I've never had a problem with my adaptall 28 - 70 f3.5 - 4.5. Remember that the adaptall may actually not transmit as much as light as the prime as its a zoom (hence why motion camera lenses have T stops as well as F stops indicated to show the actual light transmission difference)

BTW, mounting an Adaptall 2 mount is very simple, you can only fit them one way round, just find the green dot on the lens and the mount and slot them together where the two dots meet then turn until it clicks shut and can't be removed (to remove hold down the little lever on the mount and turn the opposite direction)

 

[ChrisR]

Thanks everyone. I just came back to this thread after checking the two lenses again, this time with the filter off, and it definitely needs 1.5-2 stops wider. I had also dismounted and remounted the Adaptall mount. S162216, it hadn't occurred to me that the lens might be taking more light so the pics might be OK! :bonk:

I've got a few shots left on my second MX test roll, so I'll try it out. I find the lens a bit too heavy for this body, so I'll probably end of selling it, but didn't want to sell if obviously faulty!

Thanks again...