image circle wide open?

[false_Aesthetic]

Hey0,


Regularly we see the image circle @ f22.

But how do I know if I'll be "ok" wide open?

[vinny]

look at your ground glass.

[false_Aesthetic]

yeah kind've but depending on gg it can be hard to tell how much fall off there is and if its hitting the corners.

[ic-racer]

The thing with 'wide open' is that the image circle will be very indistinct and fall off gradually in the final print. In reality, as you stop down, the far edges of the circle stay about the same intensity but the center gets darker to match the edges. Giving the effect that the image circle got bigger. In reality, the angle of view does not really change as you stop down. You can see this through the clipped corners. So the 'hot spot' on a standard ground glass, coupled with bright crescents on the view through the clipped corners makes make it difficult to predict how much darker the corners are going to be in the final print when you are shooting wide open.

[Leigh]

Many modern datasheets give the IC wide open as well as @ f/22 (sometimes f/16).

There can be quite a difference.

For example, the Nikkor SW 120/8 IC @ f/22 is 312mm, but only 200mm wide open.

- Leigh

[Dan Fromm]

Discerning coverage on the GG is very hard 'cos the GG has low resolution.

We batting two different coverage concepts around here. Circle illuminated and circle of sharp coverage.

The circle illuminated can grow a little on stopping down as the lens' exit pupil gets smaller if this reduces mechanical vignetting. Very wide angle lenses put less light in the corners than at the center (cos^4, sometimes as little as cos^3, depending on the lens), which is why there are center filters (denser in the center than at the edge) for them.

The circle of sharp coverage grows on stopping down 'cos stopping down reduces the effects of some aberrations off-axis. And it grows until diffraction dominates.

[false_Aesthetic]

Many modern datasheets give the IC wide open as well as @ f/22 (sometimes f/16).

There can be quite a difference.

For example, the Nikkor SW 120/8 IC @ f/22 is 312mm, but only 200mm wide open.

- Leigh

Oh holy crap. I didn't ask the question but you answered it for me. I guess I need something different than a Nikkor SW 120. That just saved me $500-$600 bucks.

[Leigh]

Let's see if this helps:

This drawing is meant to illustrate a concept, and is not an accurate ray trace nor a description of the optical design.

The Image Circle is defined by the area that's fully illuminated by all light coming through the diaphragm opening (between the blue lines).

When the aperture is large, a significant amount of light is shadowed by the end of the rear cell, which reduces the usable IC.

As the aperture is made smaller, less vignetting occurs and the usable IC diameter increases.

- Leigh

[Dan Fromm]

Leigh, you're talking about the circle illuminated, not the circle of good definition. Mechanical vignetting, with a nice diagram, not cos^4 falloff or the effect of aperture on some off-axis aberrations.

I think the last couple of posts in this thread were all being composed at the same time and that the posters weren't aware of what was about to be posted.

[Leigh]

Dan,

If you'll read my post, the drawing was meant to illustrate a concept.

It was NOT an optical analysis of the system, which is what you're discussing. That fact was clearly stated.

You'd do well to read a post before you criticize it.

- Leigh