Depth of field at the negative; requirements for sharp focus

[RobC]

exactly.
But depending on how big you want to print, then with LF, magnification factor is often much smaller than with 35mm which I'm sure you're aware of. However, on film resolution of LF is usually less than can be obtained with 35mm so how much you can enlarge from LF is limited by that if you are wanting high resolution close up viewing. It's swings and roundabouts depending on final viewing conditions/distances.
But the basic principle of aligning your enlarger as accurately as you possibly can always holds true and since the tools are available to do it very accurately there is no reason not to.

 

[RobC]

infact the figures I gave are based on the size of the airy disc at the effective aperture. i.e. virtually no margin for error without compromising print resolution (even if you think you couldn't see it). i.e. the COC = Airy Disc. Anything else is based on subjective choices.

 

[tedr1]

I hope you get your wish. When that happens don't presume the alignment of your enlarger, check it for yourself.

 

[tedr1]

A circle of confusion is a bunch of photographers sitting around discussing depth of field

but we photographers have to understand it so has to be studied, you can even read about it in books.

 

[removed account4]

but we photographers have to understand it so has to be studied, you can even read about it in books.

sorry to interject here, tedr1 ..
but my answer to your statement is, no, not really.

people can study anything they want, or choose to ignore whatever they want.

i really couldn't care less about a lot of things photographers obsess about
uber-sharpness, uber-resolution, uber-DOF, and a handful of other things i consider to be time-wasters to obsess about.
and i have had no problems at all since i was given my first camera in about 1970.

 

[DREW WILEY]

If you want practical results, buy the best machined enlarger you can afford, the best enlarging lenses you can afford, the most precise glass carriers
you can afford. Bolt everything down just like we do here in earthquake country. Then get a true machinist's level or at least a good German carpentry level (not the kind of thing they sell at Home Cheapo), or scrounge for one of those Salthill front-surface mirror leveling devices, which are
sorta a poor man's optical collimator. Barring that, lasers can be used, but I generally find them woefully innacurate for this kind of purpose. I combined a professional one costing about two thousand bucks (easy for me to borrow since I sell em) and bounced that beam back and forth thru
the mirrors, resulting in a correction line equivalent to about a mile long! Ridiculous overkill; but it was a fun project. The point being, whether you
use a bubble level or laser device, make sure it is really level first. After that, it's up to you whether you like playing games with the math or just
prefer to ignore all that. It wouldn't make an iota of difference to the precision of my own work if I had never even heard about "circle of confusion".

 

[RobC]

resolution in print has nothing to do with CoC. Its all about diffraction limit at effective aperture. i.e. airy disc. CoC is for ball park good enough workers.

 

[tedr1]

A remarkable assertion for someone that isn't able to calculate DOF.

 

[RobC]

you can tell me I know nothing all year if you like. But what you aren't capable of, is understanding that the figures I quoted are not mine but are those produced by the software written by an optics expert and not someone who has done a little reading. Once again, if you don't like the numbers then take it up with the Dr Geoff Adams.

 

[tedr1]

If you want practical results, buy the best machined enlarger you can afford, the best enlarging lenses you can afford, the most precise glass carriers
you can afford. Bolt everything down just like we do here in earthquake country. Then get a true machinist's level or at least a good German carpentry level (not the kind of thing they sell at Home Cheapo), or scrounge for one of those Salthill front-surface mirror leveling devices, which are
sorta a poor man's optical collimator. Barring that, lasers can be used, but I generally find them woefully innacurate for this kind of purpose. I combined a professional one costing about two thousand bucks (easy for me to borrow since I sell em) and bounced that beam back and forth thru
the mirrors, resulting in a correction line equivalent to about a mile long! Ridiculous overkill; but it was a fun project. The point being, whether you
use a bubble level or laser device, make sure it is really level first. After that, it's up to you whether you like playing games with the math or just
prefer to ignore all that. It wouldn't make an iota of difference to the precision of my own work if I had never even heard about "circle of confusion".

I can recommend simple and inexpensive tools for alignment. Two are needed, a straight edge and a height gauge, both available from your local hardware store. For the fine tuning a set of feeler gauges is helpful in quantifying the errors found and a second set of feeler gauges provides the shims sometimes required to align the column and negative stage as required.

Film base thickness is around 0.005in (120) and 0.008in (35mm and 4x5) and a worthwhile aim is to get the baseboard and negative carrier aligned across the width of the format diagonal in use within the thickness of the film base. The lens is then made perpendicular to the baseboard to complete the alignment. Some enlargers include adjustments for these alignments, some require improvisation of adjustment, it varies very much between brands of enlarger. The first step is to check that the baseboard is level, the other measurements require this as a starting point.

 

[tedr1]

RobC, when you posted the link to that software originally on the other thread, you did so with the caution that it is necessary to learn how to operate it. It seems quite possible the error lies not in the software but with the human operator.

 

[RobC]

you've just made that up clutching at straws.

 

[wiltw]

According to a text written by faculty of RIT in 1990, the Depth of Focus is indeed based upon the Circle of Confusion size.

Depth of Focus distance = Permissible Circle of Confusion size * f/stop * 2

The fact that the permissible Circle of Confusion is not an agreed-upon size (there is the manufacturer definition of CofC which assumes wrongly that the human eye is pretty bad in its ability to perceive detail, compared to what optometrists know that the eye can perceive!) adds uncertainty in the calculation of the above value.

And RobC is right, that ultimately it is the Airy disk which limits resolution. CofC simply is an additional level of 'close enough to be acceptable' imprecision, but ultimately it is the Airy disk which defines detail resolution achievable at any aperture, in the optically ideal and perfect (and nonexistent) lens design.

 

[RobC]

According to a text written by faculty of RIT in 1990, the Depth of Focus is indeed based upon the Circle of Confusion size.

Depth of Focus distance = Permissible Circle of Confusion size * f/stop * 2

This disucssion is about Depth of Field at the negative. What is permissible as CoC is purely subjective. The real number of interest is the diffraction limit radius (or diameter). Anything bigger than that for acceptable CoC is a loss of print resolution whic is termed by ball park workers as acceptable to them. Aim for the airy disc size and you might not hit it but your have a bigger margin of error to work within than aiming for a bigger CoC which leaves you none. i.e. Get your alignment as good as you can without worrying about CoC. I just happened to use the numbers to illustrate the importance of enlarger alignment and tedr1 has decided to pick an argument about it for some pointless reason.

 

[wiltw]

This discussion is about Depth of Field at the negative. er: 7069What is permissible as CoC is purely subjective. The real number of interest is the diffraction limit radius (or diameter).

'Depth of Field at the negative' IS 'Depth of Focus', and the amount that the negative can deviate from the perfect plane in the enlarger or camera is analogous to the amount that the Subject can deviate from the focus plane

Anything bigger than that for acceptable CoC is a loss of print resolution whic is termed by ball park workers as acceptable to them. Aim for the airy disc size and you might not hit it but your have a bigger margin of error to work within than aiming for a bigger CoC which leaves you none. i.e. Get your alignment as good as you can without worrying about CoC. I just happened to use the numbers to illustrate the importance of enlarger alignment and tedr1 has decided to pick an argument about it for some pointless reason.

I do not disagree with your point about loss of print resolution and acceptable CofC correlation.
If CofC is Y and Airy disk is X, as Y approaches X, the ultimate resolution is defined by aperture of the lens. The way I view things, CofC merely is the more practical side where the Airy disk is usually not considered as the ideal optic does not exist.

 

[RobC]

'Depth of Field at the negative' IS 'Depth of Focus', and the amount that the negative can deviate from the perfect plane in the enlarger or camera is analogous to the amount that the Subject can deviate from the focus plane

Wrong. Depth of Field is at the Object(subject) and not the image(negative or print). An enlarger is a camera taking a picture of a negative. The resulting image is the print. Depth of focus is at the print when enlarging and Depth of Field is at the negative being photographed.

 

[RobC]

'Depth of Field at the negative' IS 'Depth of Focus', and the amount that the negative can deviate from the perfect plane in the enlarger or camera is analogous to the amount that the Subject can deviate from the focus plane



I do not disagree with your point about loss of print resolution and acceptable CofC correlation.
If CofC is Y and Airy disk is X, as Y approaches X, the ultimate resolution is defined by aperture of the lens. The way I view things, CofC merely is the more practical side where the Airy disk is usually not considered as the ideal optic does not exist.

all lens WILL produce an airy disc. How much that is altered by other lens abberations and mis alignment we could argue about all year. But as I pointed our in original topic, I was quoting theoretical limits. Aim to get as close to those as possible and you maximise your potential. Aim futher away from that and you minimise the potential. Its common sense. Or at least it should be.

 

[RobC]

but there is no depth of field at the negative. We're talking about the thickness of the emulsion layer on a negative of only .12mm thickness which is mostly the film base and not the emulsion. What is the thickness of the emulsion, 0.02mm maybe. And depth depth of filed outside of that just means less than optimal enlarger alignment if its being used.
Most people will align as accurately as they can and NOT to a lesser standard becasue they think it doesn't matter. Picking a CoC with an overly large built in margin of error doesn't make sense becasue you could do better if you are prepared to align properly. The lens will then do what it can do but not bothering to get alignment as accurate as possible makes zero sense.
We all know getting it perfect isn't likely but unless you try to get it perfect you are likely to be far off perfect.

 

[DREW WILEY]

Michael - your idea might work with very big film, but is relatively imprecise compared to other SERIOUS methods. You could take a sharp pin or knife blade and make distinct scratch marks in the corners of your mock negative which will be sharper than anything printed, just to double check
corner focus. When I'm aligning things I'm often looking at the image under a degree of enlargement equivalent to a print eight or ten feet across,
if one factors in the magnifier itself. Of course, no enlarger even comes with a baseboard of a machinist's notion of actual flatness. A few formerly
very expensive vacuum targets for big copy cameras came close. The idea is not theoretical perfection, but ironing out all the problems until visual
perfection by your own definition is achieved. If you ever get into a serious optics catalog you will see all kinds of devices for aligning image planes
that go vastly beyond what darkroom workers typically recognize, and even did so in the 1920's.

 

[wiltw]

Wrong. Depth of Field is at the Object(subject) and not the image(negative or print). An enlarger is a camera taking a picture of a negative. The resulting image is the print. Depth of focus is at the print when enlarging and Depth of Field is at the negative being photographed.

We are in violent agreement, I believe!

Depth of focus is a lens optics concept which measures the tolerance of placement of the image plane (the film plane in a camera) in relation to the lens. In a camera, depth of focus indicates the tolerance of the film's displacement within the camera, and is therefore sometimes referred to as "lens-to-film tolerance."

In the context of the Camera

• If the subject in front of the camera deviates from the Object Plane of focus, then 'Depth of FIELD' is the principle of whether our eye continues to perceive it to be 'in focus' (due to the CofC fooling our eye+brain into seeing a blur disk as a point).
• But if the film at the focal plane of the camera deviates from the Film Plane focus (perhaps due to something on the pressure plate) this is the 'Depth of FOCUS" which determines of that object is still sufficiently 'in focus' on the film. We care about whether or film is all planar within the Depth of Focus.

Switching to the context of enlarging, we have two contexts (one more 'proper' than the other)...

Context A

• The silver grains embedded within the emulsion of the film are essentially our 'subject' at the Object Plane of focus, and we employ film flattening mechanics to bring all grains into plane and Depth of FIELD is not a necessary consideration
• Our paper on the easel (analogous to the film in the camera), is subject to 'Depth of FOCUS' consideration, but generally gravity and amospheric pressure helps with flatness, although vacuum easels exist for holding things absolutely flat and in place.

Context B

• But if we consider to think of the negative in the film stage as analogous to the film in our camera, the enlarger glass carrier is somewhat equivalent to the pressure plate in the camera, and the 'Depth of FOCUS" applies in this context, and we care about whether or film is all planar within the tolerance of the Depth of Focus.
• And our paper is suffienctly flat due to gravity and air pressure that we do not have to worry about 'Depth of Field' on our flat field.

I cannot say with certainty which of the two contexts is the 'school solution' how RIT or Brooks would teach it (few discussions of enlarging expound about this point in print)...I tend to think that Context A is more proper than Context B, in the application of the theory of Object Plane vs. 'Film' Plane...where our enlarging paper is receiving the image its flatness (or deviation) employs Depth of FOCUS to help us achieve acceptable results in spite of lack of perfect circumstances (just as we hope our sheetfilm is flat in our film carrier. If the sheetfilm camera is aimed downward we know the film in the holder is bucking away from the flat plane, particularly with a very large piece of film like 16x20)

 

[DREW WILEY]

My brother went to Brooks. During that era at least, which was still large format and film dependent, darkroom methodology was, well, comparatively Medieval. They taught the basics. My own mantra is to achieve the shallowest depth possible which still allows optimum lens performance. That way one is focused on the emulsion, and not on scratches of dust on the backside of the film or carrier glass etc, or on
grain from attached masks.

 

[gone]

Way too much theoretical woo woo talk here. In the real world, an enlarging lens stopped down to f8, which is optimal on most lenses, will cover a multitude of errors in focus and base board alignment, whether using LF or other formats. A very few work best at f5.6, but all of them work well at f8 and more. Try it yourself sometime. On the same subject, I never saw a dime's worth of difference in sharpness on my prints using a glass neg carrier or a glassless one, but I did have to do a lot more spotting w/ the glass printed photos. I also saw no difference when using "inferior" lenses like Companars when compared to the lenses w/ more elements. So theory is one thing, printed photos are another.

Does anyone use hyperfocal distance focusing on their cameras? A camera lens is no different from an enlarging lens, the enlarging lens just has a flatter field of focus. You can set a camera lens to a distance that is far from infinity this way and still get a tack sharp image by stopping down the recommended amount. I once had a Leica lens that had a bad adapter and it couldn't reach infinity w/ that adapter. It only got sharp to about 50 or 60 feet, and the infinity shots were blurry, but stopped down it made sharp shots at infinity. We're talking a difference of about 100 FEET here, all by stopping the lens down enough. So just focus well on your paper and stop down to f8 and see what you get.

 

[Gerald C Koch]

Check the resolution of your camera and enlarging lenses. Some of the large format lenses are diffraction-limited: at their best settings they deliver the maximum resolution allowed by the laws of optics for their focal length. If the resolution of either lens is not that great as compared to say 35mm then all this fuss about DOF may be a tempest in a teapot.

 

[wiltw]

Check the resolution of your camera and enlarging lenses. Some of the large format lenses are diffraction-limited: at their best settings they deliver the maximum resolution allowed by the laws of optics for their focal length. If the resolution of either lens is not that great as compared to say 35mm then all this fuss about DOF may be a tempest in a teapot.

But ALL lenses are 'diffraction limited'...they all follow the identical rules of Physics about light bending around small openings, the amount will vary by aperture used, and the visibility of the diffraction is determined by the enlargment magnification value...135 neg is enlarged 4x more than 4x5 sheet film neg, which is the reason why 135 is diffaction limited by a 4X larger aperture than large format lens aperture.

 

[DREW WILEY]

If is someone is stopping down an enlarger lens so far that diffraction comes noticeably in play, they might think about some other way to achieve
neutral density. Doing so merely to achieve focus means that something else is terribly out of whack.