Circle of coverage at small apertures?

[richard_deimel]

Published circles of coverage for LF lenses are apparently standardized at f22. We know that the area of coverage increases as the lens is stopped down, but by what factor? Would it be a percentage of the dismeter? If so, what would the percentage be, or is there a formula, or what?

[shambrick007]

It depends on the physical obstructions to the light path caused by

the lens barrel.

[michael_mahoney1]

Richard, that's a good question, I assume that the maximum circle of

good definition occurs at f22 for most lenses, and then remains

constant as you continue to stop down, otherwise the manufacturers

would probably let us in on the other f stops data as well. This is

just my assumption, don't know for sure. I've never seen the maximum

circle of definition shown at anything but f22, so it could also just

a standardized comparision point. You could of course take the test

shots yourself at smaller stops and see if there's any difference

beyond f22. Anyone know the real answer?

[pete_andrews]

Sheldon's answer is correct. It depends on the lens construction, and

has very little to do with the circle of acceptable definition. Most

modern lenses will vignette before the point where the image quality

is totally unacceptable. Their designers engineer them to do

that.<br>If you look through a lens from the back, set to a mid

aperture, and slowly rotate it about an axis parallel to the iris,

you'll see that part of the iris aperture becomes obscured by the

perimeter of the lens at some angle. If you now stop the lens down

further, that angle becomes greater before the aperture is obscured.

This is the reason why coverage increases with stopping down, and not

that the area of best definition carries on increasing.<br>The point

at which vignetting occurs varies according to the optical and

mechanical construction of the lens, so there's no fixed formula that

can be applied.

[richard_deimel]

OK, you have convinced me that the expansion of the circle of

coverage is limited by the physical design of the lens, but that

really doesn't answer my question. As we stop down the lens, the

circle expands. Are you all saying that there's no logic to its

expansion, but that in each lens it expands at a different rate,

determined by the physical design of that particular lens?

[david_goldfarb]

I thought it depended on design factors other than physical

obstruction designed into the lens, for instance that Dagors have

sharply increased coverage at small apertures, while Heliars are

supposed to have a more evenly illuminated circle of coverage

throughout the range (or so I thought, I haven't actually tested for

this).

[erik_ryberg]

You guys are making this too hard. Determining area of coverage is a

simple physical problem, not an optical one. If you look at your (for

example) f4.5 12 inch lens wide open and hold it at an angle so you

can just barely see light through it you will be looking at a tiny

sliver shaped aperture, pointed on the ends and bowed in the center.

This is, obviously, what a circle looks like when viewed from an

angle. Wherever your eye is at this point is the farthest extent that

light makes it out on the edges. Even though the lens is wide open,

only this sliver-shaped aperture is available way out on the edge.

So, while an image is being produced out there, in comparison to the

image produced in the center the light fall-off is severe. (And the

image may not be too good even taking account for the light fall

off.) An image properly exposed in the center will have no exposure

to speak of way out in the extreme edges.

 

<p>

 

Now close the aperture down to f22, 64, whatever, while you are still

looking through the lens from an angle. Notice that your sliver of

light becomes smaller, and more like a circle (an oval, actually).

But notice that now the aperture you have out on the edge is not

really that much smaller than what you have in the center. So, at

smaller stops the light fall-off will be less in comparison with the

center than it is at larger stops.

 

<p>

 

A lens doesn't actually give you more coverage at smaller stops, it's

just that the fall-off becomes less severe relative to the center the

more you stop down. f22 is an arbitrary point the lens makers have

chosen to define where their arbitrary standards for light-fall off

are met, and also happens to be an aperture where lf lenses tend to

perform fairly well. The only way to determine what the image circle

is for you is to establish your own arbitrary standards.

 

<p>

 

Optical quality suffers on the edges depending on the lens design, but

the point where it suffers has little to do with aperture. That is,

you'll have a better image at f22 than f4.5, but light will still be

cast on the circle's edges at either aperture, and again you need to

establish your own arbitrary standards for when a lens makes an image

that suits your purposes.

[neil_poulsen1]

I've always assumed that f22 is the f-stop at which manufacturers

quote image diameter, because it's this f-stop that many lenses become

diffraction limited.

 

<p>

 

An interesting example that may pertain to this thread is the Fujinon

250mm f6.7 versus the Fujinon 250mm f6.3. Both lenses are plasmats,

the f6.3 is wider than the f6.7, yet the f6.7 is quoted to have the

larger of the two image circles. It would appear that image circle

depends, at least in part, on the design of the lens.

 

<p>

 

Certainly, design can have a large impact on the image circle.

Compare a Symmar to a Super Angulon, or to a double-Gauss design.

[richard_deimel]

Erik's is the first explanation that really makes sense to me. It's

not that the actual circle of coverage varies (that would be limited

by the optical design), but that the USABLE circle of coverage will

vary. However, having gotten that far, the original question still

remains: does it vary by any measureable factor?

[pete_andrews]

Measurable? Yes.<br>Predictable, without any knowledge of the optical

construction of the lens? No.<p>Someone mentioned Dagors and Heliars

earlier. These are old fashioned lenses, where the coverage is left to

fend for itself, so to speak. Modern lenses have the circle of

coverage more tightly controlled by design, since there's no point in

having a large circle of light that's unusable due to poor definition.

This just increases flare and lowers image contrast needlessly.<br>A

well-designed lens will have its image circle clipped by internal

baffles or by barrel design, at the point in the image circle where

quality falls below a certain standard.

[david_goldfarb]

I know that's the modern engineer's view, but personally, I'd rather

have a soft corner and a little extra flexibility in movement than a

vignetted corner.

[w._browning]

Erik's fine (and finally, understandable!) explanation, it seems to

me, implies that a data chart might be constructed which would compare

a lens' light transmission both in the center and in the corner, as

the aperture is reduced. This chart (or its implied graph) would

illustrate the lessening edge falloff as the aperture shrinks.

 

<p>

 

Does this falloff approach zero as the diaphragm size also approaches

zero?