Published I C for Nikkor SW 90 mm f8

[David Casillas]

Just got this lens again, and looking how much rise I could get with the lens wide open, I got 67 mm of rise that will make for a I C of around 260mm. that is a lot more than the published 154 mm. A 254 mm of I C will get about 64 mm of rise.
So it could be that the brochure is wrong?
When closing down the the lens to f 22 the image circle seems to stay the same.
With 235 mm I should get about 53 mm of rise and 47 mm of lateral shift.
So, What really means 235 mm published I C? Am I misunderstanding something?

[Gem Singer]

The published image circle of 235mm for the Nikon/Nikkor f8 90SW means that at f22, that lens will project a circle of light 235mm in diameter onto the film.

It might be capable of lighting up a slightly larger image circle on a ground glass.

However, if you expose and print that image, the corners will show vignetting due to light fall off at the outer area of the circle.

[Jan Pedersen]

Am I misunderstanding something?


Perhaps. You may see a larger image circle than Gem's explanation to Nikon's definition of acceptable IC.
Your measured 260mm IC that is what most will call image illumination which is different than an IC stated by a lens manufacturer. The outer part of the illumination will not have definition that satisfy the manufacturers quality goals.
Quality standards would be a better term i think.

[David Casillas]

Ok, that means that the circle of good definition for nikkon standards is only 235 mm, at f 22 and the same would apply for 154 mm at f8. Is this correct?

Thankyou for your answers

[Gem Singer]

Yes.

[Jan Pedersen]

Sounds about right but i don't have the Nikon data at hand so can't confirm. I have the SW90 4.5 which has very similar data to the f8 and it needs to be stopped down to create a sharp IC to cover 6x17 which is the format i used that lens for.

[Leigh]

It's all a question of quality, as Gem said.

While the lens may illuminate a circle larger than 235mm @ f/22, the quality of the image in the peripheral area is not up to Nikon standards.

The IC spec for the f/8 lens is 235mm @ f/22, which is the same as the IC for the f/4.5 lens @ f/16. Both are rated to cover 5x7.

- Leigh

[EdSawyer]

Actually the nikon specs are at f16, so it will cover more than spec at f22. The 90 f8 is better than the nikon 90mm f4.5. It's basically the best 90 ever made by anyone.

[Leigh]

No.

The specs for the f/4.5 lens are at f/16.

The specs for the f/8 lens are at f/22.

Both have ICs of 235mm at the respective apertures.

- Leigh

[David E. Rose]

You are focused on infinity?

[David Casillas]

Yes focused at infinity, I used raise a lot of rise to see the image cut off, then measured from de edge of the image circle to the edge of the 4x5 frame. Then lowered the front standard the amount of that distance so the 4x5 groun glass was fully covered and then I checked the toatal of rise of 67 mm.
Also i found these pictures on this past thread http://www.largeformatphotography.info/forum/showthread.php?t=56620&page=3
It is clear these pictures are focused closer than infinity, but the image circle here appears to be greater than 260 mm as it should be since it is focused closer.

[Leigh]

The IC is what the datasheet says it is.

You need to check at the corner of the frame, not at the edge.

And the check needs to be made on an optical bench with a microscope, not on a bunch of trees.

- Leigh

[David Casillas]

Leigh:
That is what i was trying to say I measured the distance along the edge of the frame from the image cut off to the corner of the frame. For a diference of 25 mm between the published diameter of the IC and what i see on the ground glass I don´t need a microscope, a loupe withe a scale will be enough. At the end I just wanted to share that I found the Cirlce of illumination was bigger than the published IC as Jan pointed out.
I am not saying I am giving an accurate measurement, if so I would be giving you at least tenths of milimeter.
I understood and agreed that the usable IC is what the data sheet says since The first Gem Singer reply. He was very helpful and clear about it.

[Leigh]

I found the Cirlce of illumination was bigger than the published IC as Jan pointed out.
No, David,

You found that your measurements don't agree with the actual IC.

I was trying to be polite, but if you insist on continuing this, I'll be less so.

- Leigh

[David Casillas]

Ok, Leigh I FOUND that my measurement don´t agree with the actual IC. And thats true, as I say I just was trying to share this. And discussing anything else is pointless.

[Leigh]

That's an accurate statement.

Thank you.

- Leigh