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:
http://www.mayadate.org/pix/ImageCircle1.png
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

[Dan Fromm]

Leigh, I didn't criticize your post. It is a nice depiction of mechanical vignetting by the rear barrrel.

It shows very well why my little 60/14 Perigraphe, which is stuffed into the front of an Ilex #3, can't reach its rated coverage. At about 45 degrees off axis the rear of the shutter's barrel completely occludes the exit pupil.

You didn't mention what's sometimes called optical vignetting, otherwise our friend cos(theta)^4 (sometimes less than 4). Like mechanical vignetting, it has nothing to do with the circle of good definition; unlike mechanical vignetting, stopping down doesn't affect it.

I presented and explained the alternative definition of circle covered, viz., circle of good definition.

Your definition of circle covered is "The area that's fully illuminated by all light coming through the diaphragm opening (between the blue lines)." This definition is not the one currently used by lens manufacturers. At least most of the time.

Re manufacturers' practice, if you look at the MTF curves they publish for their lenses, you'll see that they usually place the circle covered's edge at the distance off-axis where MTF falls to zero. There are exceptions, e.g., the 75 Rodagon D, whose MTF curves are pretty flat to the limit of coverage; they look like those of a wide angle lens with a field stop.

Cheers,

Dan

[Leigh]

Dan,

I was trying to present a simplified example of a principle.

I'm sorry it did not meet your criteria for technical precision.

Would you prefer I post a complete Zemax analysis of some specific lens, to totally confuse the OP?

- Leigh

[r.e.]

Hey0,


Regularly we see the image circle @ f22.

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

While these guys are arguing about the arithmetic, it might be worth pointing out that image circle is actually expressed as f/22 at infinity. At 1:1, the image circle will double, and it also increases as one stops down.

Also, there is an exception for macro lenses, for which the image circle is expressed as f/22 at 1:1.

Also, as Leigh has pointed out, the manufacturer sometimes says what the image circle is wide open. Off the top of my head, and without checking, I think that my brochure on Nikkor lenses provides this information.

[Dan Fromm]

Dan,

Would you prefer I post a complete Zemax analysis of some specific lens, to totally confuse the OP?

- Leigh
Yes, of course, please do it.

[r.e.]

Something that I want to try in the next while is to compare the actual results of two Nikkor macro lenses that I have, the 120mm and 210m, on 4x5 and 8x10 cameras at 2:1, this magnification being roughly necessary, at f/22, for the 120mm lens to cover 8x10.

[rdenney]

If you remove the ground glass and sight through the corner of the image frame, or look through the corner notch in the ground glass if there is one, you'll only see the diaphragm blades all the way around the aperture when the lens is stopped down. When you open it up too much, you'll start to see the lens barrel. If you can see the lens barrel, then the aperture is no longer defining the amount of light getting to tfe film. Usually, the aperture will be shaped like an American football, providing only some fraction of the opening visible in the center of the image.

That's how you tell. If you can see any opening, then you'll get some exposure. If the opening you see is only half what the center sees, then you'll get half the light, in addition to the cosine effect (which is caused by the oblique angle).

Rick "waiting for Leigh's anlysis" Denney

[false_Aesthetic]

Thanks everyone.

Uhm. Since I failed algebra 1 and then majored in sculpture + philosophy I'm gonna nod my head and smile about the math. I think I get the drawing though.

Thanks for the tip rick.

It's saturday night, why am I on this forum?

[Dan Fromm]

false, the drawing is nice but irrelevant. I don't know what's gotten into Leigh, but he's fixated on a coverage concept that makes sense only for contact printing.

[rdenney]

It's saturday night, why am I on this forum?

Because you have no life, like the rest of us.

Rick "duh!" Denney

[ic-racer]

Your definition of circle covered is "The area that's fully illuminated by all light coming through the diaphragm opening (between the blue lines)." This definition is not the one currently used by lens manufacturers. At least most of the time.

Re manufacturers' practice, if you look at the MTF curves they publish for their lenses, you'll see that they usually place the circle covered's edge at the distance off-axis where MTF falls to zero. There are exceptions, e.g., the 75 Rodagon D, whose MTF curves are pretty flat to the limit of coverage; they look like those of a wide angle lens with a field stop.

Cheers,

Dan


4. Why does the size of the image circle in a Large Format lens change as the lens is stopped down?
Well . . . it doesn't actually. What actually happens is that when the lens is wide open, the center of the image is very bright and the corners of the image are VERY dark. So dark in fact that they do not expose the film (When the film is properly exposed for the center of the image). As the lens is stopped down, uniformity of illumination improves and therefore the circle of usable illumination increases.
From the Schneider website.

[Armin Seeholzer]

The SW Nikkor 150mm has at f8 only 253mm and at f22 400mm
But my old Zeiss Jena f3,5 250mm does cover 8x10 full open but its an old lens! Without shutter only with the Sinar shutter behind!
I think a modern 240mm should also work at around f8/11 but for example the 300mm Nikkor W does at f5.6 with 346mm.

Cheers Armin

[Dan Fromm]

ic, I found the URL. https://www.schneideroptics.com/info/faq/photography.htm#q4

It was written by a poorly-briefed marketer, not by an optical engineer.

For a lucid discussion of optical vignetting (not what Leigh posted about, I've mentioned it), mechanical vignetting (what Leigh posted about), and the circle of good definition (what I've been insisting on), see S. F. Ray, Applied Photographic Optics, pp. 132 - 137. The circle of good definition's diameter increases "as the lens is stopped down and residual aberrations decrease."

http://toothwalker.org/optics.html gives a clear explanation of aberrations and their dependence, if any, on aperture and distance off-axis.

Every so often someone reports here that a 4" tessar type lens taken from an old 6x9 folder can be used as a wide angle lens on 4x5. Yeah, sure, as long as image quality outside the lens' ~ 100 mm circle of good definition doesn't matter.

[johnielvis]

you know--you'd think that by now, with all the lens testing the people seem to do on their own...who knows how many times a lens was tested for this or that --same lens, buy different people--all with subjective opinions about what is sharp enough....

and still there are theoretical arguements....

and the SAME arguements....hard facts countered by some learned gent's practical experience--one contradicts the other...still....2012....lenses from many many many years ago and STILL there is no agreement.

Can maybe one of you lens boys that's into lensbooks and theory and stuff, maybe devise a system that can be adopted here that gives objective results---say a standardized target that can be downloaded and printed...one for each distance of interest...every time someone wants to sell a lens, say, the standard of quality will be a shot of the test target...people can request this before buying...then these tests all eventually go into a database here....the methods will be standard enough I think to provide guidance withOUT arguement....AND if enough of the same kind of lenses are tested, you can get a "best performer" average performer etc....

we have the capabilities...AND this will also settle the question of whether them nicks and separation really don't or do make a difference...and if they do..is it acceptable compared to a perfect glass but bad performer...stuff like that. EVEN xref with serial numbers or something/owners...sort of like what they do with cars...

it seems to me that there should be a way to do this...

bahhhhh....

nobody will cooperate with this....it's more fun to argue and attack,right guys?

now lemme have it

[ic-racer]

I use falloff of MTF as a guide to image circle when shooting stopped down, and I would not be surprised to find other photographers doing the same and would not be surprised to see a lens manufacturer basing published image circle specs on some minimal MTF when [when stopped down.] However, I have never read where a manufacturer actually states this on a web site or lens brochure.

In terms of wide open image circle [on a typical LF lens], there is nothing to argue over since usable image circle wide open is always going to be very operator specific. I also suspect the lens manufacturers care less about this than those posting here since the lenses are not optimized for wide open use. An analogy would be automobile engine manufacturers stating torque and HP figures at idle.

If there is a problem with what Schneider posts on their site, I'd contact them. Maybe they will 'fess up' and let us know how they really get the specs for wide open image circle.

[Dan Fromm]

ic, I have no problems with Schneider's marketers. They're marketers, can't be trusted. Its the nature of the beast.

I do have problems with people who lay down the law without thinking first, also with people who don't communicate well in the common language.

[ic-racer]

A little off topic but to direct things in a constructive direction. We as photographers tend to check and test things to verify manufacturer's specs. With respect to image circle this is not an easy task because we don't have a universal definition of where the image circle ends. What we need is a criteria that gives image circles at at defined MTF. For example a manufacturer should state image circle where it drops to 20% of maximum MTF, etc. Kind of like audio equipment manufacturers stating power output at a specified THD.

Too bad it is too hard to determine a specification like that with home testing. Otherwise I'd gladly contribute image circle data, to the forum here, for lenses that I test.

It also seems to me that the many battles over lens coverage are like arguing with someone about their favorite color. If someone gets results they are happy with using a lens 'proven to not cover the format' why try to force them to admit they are not happy with the results.

[ic-racer]

Now this may be way off topic but since we are discussing image circle, here goes. One of these days I'm going to start a good instructive thread on how to do this (shown in the picture below). The lens manufacturers would lead you to believe that a super XXL lens and massive front rise provide the only solution to the problem. However, the correction shown below allows the camera taking lens to use the central patch, and all the good things that go along with that. For example, there is no MTF falloff on one side of the negative and all 4 corners are evenly illuminated on the negative.

On the paper, the difference in magnification of grain and enlarging resolution on paper between the top and bottom of the print are not visible.

Also not appreciated in the picture, the enlarger lensboard not only has swing, it also has shift. So the operator can maintain the projected image centered on the enlarging lens image circle.

http://img.photobucket.com/albums/v670/ic-racer/DSCF4801.jpg
http://img.photobucket.com/albums/v670/ic-racer/2011/corrected.jpg

[Dan Fromm]

What we need is a criteria that gives image circles at at defined MTF. For example a manufacturer should state image circle where it drops to 20% of maximum MTF, etc.

Absolutely. Image quality in the final print's corners does it for me.

Maybe. Wander over to the current Rodenstock site and take a look at the 75/4 Apo Rodagon D-1x' published MTF curves. They're here: http://www.rodenstock-photo.com/mediabase/original/e_Rodenstock_Printing_CCD_43-62__8230.pdf Fantastic, just fantastic. Flat as can be.

I got one really cheap a couple of years ago. Tried it out, sold it. It illuminates and covers 6x6, illuminates nothing larger. I shoot 2x3, couldn't use it. Obscenities!

One point that's been dropped from this discussion is how large one intends to enlarge. We sometimes make negs that contact print at least well enough but that can't be enlarged much. If the goal is contact prints, fine.

Another is how important what's in the corners is for the final print. Les Newcomer and I have been batting this back and forth for years, in the context of how good Wollensak Ser. II Raptars are or aren't. In Les' work, what's in the corners is not of interest. When you know that, his position on Raptars makes good sense.

Cheers,

Dan

[Leigh]

I do have problems ... with people who don't communicate well in the common language.
Look in the mirror, Dan.

In my post with the drawing I quite clearly stated that it was an illustration of mechanical vignetting, NOT an optical analysis.

Apparently you don't understand common English.

- Leigh

[rdenney]

For those not into rigorous testing, but who want to know what works for them with their setup and usual technique, there is a reasonable strategy.

Image quality absolutely sucks for the parts of the image occluded by the lens barrel (unless Zone 0 is your desired tone placement, in which case optical performance doesn't matter), so the first thing to check is occlusion. Do that by sighting through the lens with the ground glass removed, or through the notches, as I suggested earlier.

It you can see daylight, there will be some exposure. If you can see the entire aperture as formed by the diaphragm blades, the falloff will be limited only by the optical design and the shallowness of the angle (which is itself largely dictated by the focal length in relation to the format for typical large-format lenses). Then, just take some pictures. If the edges and corners are unacceptably sharp compared to the center, you have your answer. Sell the lens and try something else.

Optical performance can be quantified to the nth degree, but that is not likely to be much help in making decisions for the non-scientifically-inclined.

Rick "whose experience is that most modern lenses are designed to perform well to the mechanical limits of coverage compared to historical designs" Denney

[Dan Fromm]

Look in the mirror, Dan.

In my post with the drawing I quite clearly stated that it was an illustration of mechanical vignetting, NOT an optical analysis.

Apparently you don't understand common English.

- Leigh
Leigh, reread post #8 in this thread. You said no such thing.

You seem to disappear from this forum from time to time. You could have been away, you could have been banned. I haven't asked any of the moderators whether you've been banned, doubt any of them would tell me more than that is none of my business. It isn't, so you shouldn't tell me either. And I haven't discussed your posts with any of the moderators or reported them as abusive. I do know that abusive posters are sometimes banned.

I fear you're on the road to exile, hope I am mistaken.

Please read the references I gave in post #22 in this thread. You persist in confusing mechanical vignetting with reduction in off-axis aberrations on stopping down.

[johnielvis]

no...I meant just post the full ILLUMINATION coverage with targets shot at the entire illumination field---then you can see for yourself and judge for yourself what's acceptable, you know....just like on nice hi-res scan of targets all from the center, 25mm out 50mm out....to the edge of the illumination circle---see...then you have that information and you can see what it actually does "out there" and whether or not that "covers" to YOUR specifications...don't try to make some mathematical average determination of "where to draw the line"....this IS the problem...nobody can agree on that--but you don't have to if you have that kind of info available, right?

[Leigh]

Leigh, reread post #8 in this thread. You said no such thing.
Dan,

Grow up.

The very first sentence following the drawing is:
"This drawing is meant to illustrate a concept, and is not an accurate ray trace nor a description of the optical design."

Learn to read, and get off your high horse.

As for "disappearing", I'm on here every day, and post almost every day.

- Leigh