Nikkor 90 F8 iris Diaphragm questions

[Serge S]

I just purchased a Nikkor 90mm F8 lens & had it serviced as the shutter ring was a little stiff.

A question for those who own this lens, the diaphragm when set to F8 does not open completely as it does usually on my other lenses?
It functions properly as it does close down when different apertures are selected (f16, f64 etc).
Is this how this particular lens functions...not sure if the iris should open completely or if partly open at F8 is the right diaphragm setting for F8.
Hope this is clear

Thanks!

Serge

[neil poulsen]

I just checked my Nikon 90mm f8 lens.

With the dial set at exactly f8, I can just barely make out the edges of the six diaphragm blades. (But only, juuuust barely.) When I move the dial further to fully retract the blades, these just barely detectable edges disappear, and the opening becomes a circle. That said, I would have to say that there's no appreciable change in the amount of light that passes at f8, versus when the blades are fully retracted.

If it appears that your lens lets in appreciably more light when you fully retract the aperture blades, then your lens is operating differently from mine.

In doing this check, I looked on axis through the lens to a well-lit white background, so that it would be easy to discern the shape of the opening at various apertures.

[mpirie]

If the lens has just been CLA'd, then i suggest getting back in touch with the service centre and ask them to confirm they've done it properly?

After all, the idea behind a CLA is to get the lens back in better condition than it was, not worse.

Mike

[Serge S]

Hi Neil,

Thanks for checking.
Unfortunately, the blades go significantly in at f8
I will send it back to have him address the issue.


Thanks again!

Serge

I just checked my Nikon 90mm f8 lens.

With the dial set at exactly f8, I can just barely make out the edges of the six diaphragm blades. (But only, juuuust barely.) When I move the dial further to fully retract the blades, these just barely detectable edges disappear, and the opening becomes a circle. That said, I would have to say that there's no appreciable change in the amount of light that passes at f8, versus when the blades are fully retracted.

If it appears that your lens lets in appreciably more light when you fully retract the aperture blades, then your lens is operating differently from mine.

In doing this check, I looked on axis through the lens to a well-lit white background, so that it would be easy to discern the shape of the opening at various apertures.

[Serge S]

Yes it's going back now that Neil confirmed the issue with the iris.

If the lens has just been CLA'd, then i suggest getting back in touch with the service centre and ask them to confirm they've done it properly?

After all, the idea behind a CLA is to get the lens back in better condition than it was, not worse.

Mike

[mpirie]

It sounds like the aperture control is 180 degrees out from the aperture blade mechanism.

It's a common mistake when reassembling the shutter/aperture.

Mike

[Jim Jones]

I'm not familiar with that particular lens and shutter. Often a shutter is designed to be useable with faster or longer lenses that require a larger diaphragm opening than for your lens. This opening can be restricted to the maximum diameter for the particular lens that is installed, or like Neil's example, can open wider. It doesn't affect lens performance if it does open wider. The important thing is for the aperture scale to be accurate. For an approximate check of this, measure the apparent diameter of the aperture while looking through the front of the lens. The eye should be some distance from the lens to reduce parallax error. At f/8 the aperture diameter should appear to be about 11.25mm (90/8) or a little larger as the eye is closer to the lens. If the focusing scale is accurate at the maximum opening, it will usually be accurate at smaller openings.

[Serge S]

Hi Jim,

I measured it and it appears close to your measurement.
Maybe it is OK
What do you think?
This photo shows the shutter at F8


Thanks

I'm not familiar with that particular lens and shutter. Often a shutter is designed to be useable with faster or longer lenses that require a larger diaphragm opening than for your lens. This opening can be restricted to the maximum diameter for the particular lens that is installed, or like Neil's example, can open wider. It doesn't affect lens performance if it does open wider. The important thing is for the aperture scale to be accurate. For an approximate check of this, measure the apparent diameter of the aperture while looking through the front of the lens. The eye should be some distance from the lens to reduce parallax error. At f/8 the aperture diameter should appear to be about 11.25mm (90/8) or a little larger as the eye is closer to the lens. If the focusing scale is accurate at the maximum opening, it will usually be accurate at smaller openings.

[mpirie]

No, i still think it's wrong.

If the maximum aperture is f8, then i'd expect the aperture to open as wide (or pretty close to) the maximum diameter.

I think the aperture mechanism is 180 degrees out (don't ask me how i know)

Send it back.....you paid for a service and haven't gotten it!

Mike

[JimL]

Looks about right to me... but it would be good if you took a similar picture with the front element installed and the lever set to f8. Neil and Jim's posts both describe how the aperture should behave when looking through the front lens element. For that lens at f8, the aperture should not be open to the full opening of the shutter (which is 24mm). Unless the lever is installed upside down (which it's not), the only other thing that could be wrong that I can think of is if the adjustment ring was not engaged properly with the internal pins of the mechanism. But then your aperture wouldn't open and close properly.