worth keeping?

[zackesch]

Yesterday I picked up a used Rokinon 80-200 F4.5 with a pentax K mount, manual, for 10.00. I didn't notice until I got home that it is stuck wide open. Is it worth holding onto at that point?

Although I am still learning my shooting technique and photographic style, very rarely do I go above 6.5 or so. My initial intention for this lens is to try my hand at railroad photography. I do have a 50mm prime, but it would be handy to have something longer.

 

[Ian Grant]

At that price I'd try and fix it myself, I have done similar in the past. It's not one of the good 3rd party makes but it might be OK.

Ian

 

[zackesch]

I popped the back off last night but I am not sure where exactly the pressure is coming from to close the aperture.

 

[mrred]

It might not be a pressure problem per-se. It may just need some lube, in the appropriate spots. Maybe just a cla vs repair.

 

[zackesch]

For what it is, F4.5 through its zoom range, its worth a shot. At the very least if it is a poor lens, I have one to learn about optic repair on. With rail photography, itll be hard to get everything tack sharp with a fast moving object. Looking at the F stop in my copy of "The art of Railroad Photography" most of the photos are f4 to 5.6. There are a few non-train but railroad related photos with a higher F stop, but those are done from 50mm and wider.

mrred, where can I find information on how to lube lenses properly?

 

[E. von Hoegh]

One thing zooms are good for - pay attention to which focal lengths you like to use it at, then get good Takumar primes in these lengths.

W.F. Nye sells optical damping grease for the helicoids. The aperture mechanisms are usually best cleaned and left dry - it may be a problem of lube or separated grease creeping into where it doesn't belong.

 

[Tom1956]

Whenever a zoom lens comes with an old camera deal I picked up, I throw it in the garbage. And to think how much I wanted one as a teenager back in the early 70's.

 

[AgX]

The aperture mechanisms are usually best cleaned and left dry - it may be a problem of lube or separated grease creeping into where it doesn't belong.

But typically the diaphragm is closed by outer force and opened by internal spring-force. Thus grease related problems show up during opening, not closing.

 

[E. von Hoegh]

But typically the diaphragm is closed by outer force and opened by internal spring-force. Thus grease related problems show up during opening, not closing.

Yes on a screwmount Pentax. Not on the Nikon I have in front of me. I don't know about the Pentax K mount.

 

[zackesch]

What I did notice when I had the back off, I can move the ring that operates the aperture blades, and when they are moved, they feel and react slow and stiff. It must be from old grease. What I can do, Im able to operate the aperture after repositioning the ring away from fully open, it functions fine at a high f stop "f32 and down". When getting to the lower "5.6 and higher" the aperture will change its value, but with a delay. It only gets stuck when fully open. I know the quick fix would not be to open it all the way, but I do want to use f4.5.

 

[vsyrek1945]

Yes on a screwmount Pentax. Not on the Nikon I have in front of me. I don't know about the Pentax K mount.

The diaphragms of K-mount lenses are sprung to close when dismounted from the camera, the same as Nikon-mount lenses.

Regards,
Vince

 

[John Koehrer]

1. It's not too little grease but that the grease in the lens has migrated to the aperture housing.
2. It's gonna need the housing and blades cleaned. There is NO lube on blades or housing.

OK then.....It's possible the linkage you can access may have grease where it rotates.
If you want, a couple of drops of old fashioned lighter fluid or naptha may free it up. If not it's got to be taken(really taken) apart.