Canon nFD 300mm 2.8 L - iris problems

Fonphoto · Joined: 04 Oct 2014 Posts: 5

Hi all - my first post here. Hopefully it makes sense!
I have an old nFD 300/2.8L, which has been gathering dust for a while but now I'd like to get it back into use. I've noticed however that there appears to be a problem with the iris. A couple of blades appear to be "out of sync" with the rest - so at wide open they are visible rather than fully retracted. Stopping down a bit makes this more obvious:

Stopping down further gives an odd-shaped aperture - it's not very clear in the photo but hopefully you can see what I'm getting at, on the left-hand side of the aperture:

The iris itself appears to operate OK. There's no oil or anything on the blades that I can see, and it opens and closes as you would expect it to.
The questions I have are as follows:
Firstly - should I even be concerned about this? To my mind it would affect metering, especially at smaller apertures, and might also affect bokeh - but if I'm being oversensitive and it won't affect anything then I will leave it alone Smile

If it *is* something that needs fixing - has anyone seen this before? Is it likely to be something that cleaning the iris as a single unit will fix, or am I going to need to disassemble the iris itself? I'm reasonably confident working with small / delicate mechanisms, but from what I've read here and elsewhere disassembling and reassembling irises can be very tricky - it's not something I've tried before.

I welcome any comments or advice people may have - thanks!

Fonphoto · Joined: 04 Oct 2014 Posts: 5

The photos don't appear to show up unless I put in a reply - I guess because it's my first posting. Hopefully this works Smile

Lloydy · Expire: 2022-01-01

I have no experience of this Canon lens, but I have got a few different lenses where the iris is unequal in its operation.

ZoneV · Expire: 2011-12-02

I have opened the iris on a Canon FD 500 f:4.5 L. See on the webpage for several images.
From what I remember and from the images it was not very hard - those deep inside a tube and with more blades are much more challenging. But I have some experience in lens repair.

sceptic · Joined: 01 Jun 2013 Posts: 255

Reassembling aperture blades is, in theory, easy when you know how. In practice, it can be quite a challenge. The blades are ridiculously delicate and light, practically like paper.

The blades all have a "knob" on them which run in separate, curved groves. The groves are cut out from a ring placed on top of the blades, which is turned when turning the aperture ring on the lens. The ring is held in place by the plate seen in your photo (with the six screws).

With the iris fully open, all "knobs" should be at one end of their grove; fully closed they should be at the other (give or take a little space at either end of the grove).

Your "odd" blade(s) could have their "knob" placed a little out of sync with the others, a little too far from either end point when fully opened and closed.

This can, possibly, be fixed by making sure all blades are equally opened when the aperture is set to fully open.

In your first photo, the blade at 11 o' clock is clearly a little "ahead of the others" (already a little closed when it should be opened). By CAREFULLY pushing it back to make a "perfect" circle of the blades, it might get back in sync with the others. I've done exactly that, using a pencil held paralell to the lens inside the iris opening and carefully pushing the blade outwards with the side of the pencil.

As always, you're doing this at your own risk.

Fonphoto · Joined: 04 Oct 2014 Posts: 5

Thanks to everyone for the replies. I was hoping ZoneV would read this post - it was your posts about your FD to EF conversions that led me to get the FD lenses in the first place Smile

So does everyone agree that this is a problem that should be fixed, and not something to ignore?

Thanks to sceptic's description and ZoneV's photos, I think I understand how the iris should work. I wonder maybe if there is some dirt, or corrosion, or something at the end of one of the grooves that prevents one blade from retracting fully?
I'll try very gently pushing the affected blade and see what happens...

Gerald · Joined: 25 Mar 2014 Posts: 1196 Location: Brazil

I'm hesitant to say that the iris mechanism has a problem that deserves a disassembly, especially as you say that the iris appears to operate OK. When there is a more serious problem, for example, a blade pin out of its hole/slot, the corresponding blade stops rotating.

It seems that the irregular shape of the aperture is caused by normal tolerance of mechanical dimensions. In particular, the two pins riveted on each iris blade may have been mounted in positions and angles slightly different from the nominal ones. Inaccuracies of fractions of tenths of a millimeter, or tenths of a degree, are enough to produce a visible irregularity in the shape of the aperture. The slightly irregular shape of the aperture is quite common in lenses, including the professional ones. The effect on bokeh or exposure is usually negligible.

Fonphoto · Joined: 04 Oct 2014 Posts: 5

Thanks for this. I guess from your comments and Lloydy's earlier that having a slightly uneven aperture is not necessarily a huge problem.
I tried very gently pushing the affected blade but without any real improvement, and I'm wary of applying too much force. Plus, I'll mainly be using the lens in stopped-down mode anyway, so the camera will be metering the actual light coming in and any discrepancies will be compensated for.

Thanks to everyone for the help. I think I'll leave it alone, stop worrying about it and just go and take some shots Smile

Next I have to deal with the sticky focusing on the other FD lens I have...

ZoneV · Expire: 2011-12-02

I think to leave it untouched is a wise decision.
In case you don�t like the bokeh form you could think about this repair later - probably with a bit more lens repair experience.
Repair the atixky focus FD lens, and learn there a bit.
And use your FD 300/2.8 Smile

Fonphoto · Joined: 04 Oct 2014 Posts: 5

I will be reading your website for some help - the sticky lens is a 500 / 4.5 which I know you have worked on Smile

Himself · Expire: 2013-05-30