The first test to do is very simple on this lens.
We will check the return of the aperture iris.
Close the lense down by rotating the aperture ring to f22 , turn it around so you can see the back.
You will see the iris blades closed in ,,now press the pin in as shown above and you will see the iris open up all the way.
Now quickly release the pin and you will see the iris close down again.
What we want to see is the iris close down FAST.
It should snap back instantly ,,not sluggish or slowly.
It should also be consistent in any lens position , , check this.
If the lens does not close down fast enough , over exposure will occur.
You may initially blame an over exposure fault on the camera , but if this test shows a slow diaphragm , then this is the cause.
If the iris is slow to close down in your lens , it may just be from grease being too thick or in the wrong place in the lense , or maybe no lubricant at all.
Do not try and squirt oil inside to lubricate it , chances are it will get onto the optics and then you will have a real BIG problem.
If you see black oil on the iris blades this will need cleaning off.
Not only can this affect iris speed .. but some say the oils gloss may introduce a flare problem.
The best repair for these problems is disassembly , cleaning and light lubrication of the mechanism.
next page in this Simple Lens Check article