Depth of field at the negative; requirements for sharp focus

[tedr1]

For prints that are sharp from edge to edge we require film flatness and the alignment of the enlarger negative stage, baseboard and lens to be satisfactory. What precision do we need? Here is some information I found relevant to the alignment of my LPL4x5 enlarger, which I am using for medium format enlargements.

The critical factor is the depth of field (DOF) of the enlarging lens. This varies with lens aperture and the degree of magnification. For calculation of DOF a choice is required for the circle of confusion (CoC) value, in the case of enlarger optics this is the CoC of the image on the baseboard, which will be reproduced in the finished print. Recommended values for CoC fall in the range one thousandth to one fifteenhundredth of the diagonal of the finished image. Taking as an example the negative format 6x7cm and 6x enlargement we get a print size of roughly 12x16in, and for a print this size the diagonal is about 20in (500mm) so the CoC value is about 500/1000 or 0.5mm.

The depth of field (at the negative) may be calculated using a simple formula that is precise within a few percent:

DOF = 2Nc (m+1/m^2) where N is the lens aperture, c is the CoC, m is the magnification.

inserting values for our example print and a lens aperture of f5.6 we get

DOF = 2 x 5.6 x 0.5 x (6+1/36) = 5.6 x 0.194 = 1.086mm

Rounding this to 1mm we now know that for our 6x7 negative, with a lens aperture of f5.6 and an enlargement of 6x, to be in sharp focus, the permissible deviation from flat and square between the film and the lens is 1mm across the 6x7cm negative.

In my case I use a glass negative carrier so that film flatness is assured. In order to find the alignment accuracy of the negative carrier relative to the lens I used commonly available straight edge and ruler. I began with a test of the flatness of the baseboard, progressing thru measurement of the deviation of the negative carrier from being parallel to the base, and finally to adjustment of the lens so that it is perpendicular to the base.

In another recent thread on the subject of anti-newton glass (here (there was a url link here which no longer exists) see post #71) figures were given for calculated DOF that were approximately 0.06mm for the same conditions. These figures are incorrect and imply requirements for film flatness and alignment that are too stringent by an order of magnitude and which are not required for high quality work.

 

[RobC]

The figures I gave in post #71 at earlier topic were calculated/taken from the software i gave link to.

http://www.winlens.de/index.php?id=70

This software was written by Dr Geoff Adams and Rodenstock used to provide the software free at their website. I have no idea what his connection to Rodenstock was/is but he or his company still seems to have some dealings with Linos. He now seems to do work for QIOPTIQ and the software used to be downloadable from there. It is now downlaodable from above link and an older version from

http://www.opticalsoftware.net/index.php/how_to/lens_design_software/predesigner/

If you have a problem with his calculations and/or the formula he uses, then I suggest you contact him direct.

Furthermore, if you choose to use a less stringent target for your work towards the limit of borderline acceptable quality, then you leave no margin for error. That's your choice. Personally I prefer to target the absolute finest detail obtainable (even if I can't see it) becasue I know there will be errors in my setup but in the knowledge there will be some margin for error even if it is very small.

I'm not going to go into the formula used in yor example becasue I trust the formula in the software. But I know the standard DoF software is not accurate for macro work where the formula required become more complex.
Note that further in topic I have explained that with macro work the effective aperture is much smaller. In your example the effective aperture would be F28 and this would give an approx result of 5mm which we all know is wrong. I'm afraid that all you have done is to illustrate your lack of understanding of optics and optics formula. You must research formula for use in macro work if you want to get anything approaching correct result. There was some stuff on web about it yeras ago but I have no link to it. I'm just going to assume Dr Geoff Adams knows what he's doing in his software.

And bear in mind that these are all theoretical values which take no account of errors or specific lens performance.

 

[tedr1]

The figure I used of 1/1000 diagonal is accepted (0.3mm on an 8x10 print), 1/1500 is more stringent and when inserted into the formula would give 0.66mm DOF, that still leaves us an order of magnitude apart, too much for issues of personal choice in circle of confusion.

The formula used is correct for macro work where lens to negative distance is small compared to lens to image distance as it is in enlarger work.

Quoting figures an order of magnitude smaller than the truth spreads disinformation and leads people astray. This thread puts the record straight.

 

[RobC]

you are very confident that you know better than Dr Geoff Adams software predicts. I know who is spreading mis-information and whose work I trust.

And a quick check in wikipedia says that for macro work that formula you have quoted is suspect becasue it depends very much on the lens design and pupil magnification.

Again, the figures I gave are for theoretical finest resolution possible (visible or not).

 

[tedr1]

The concepts of "circle of confusion" and "theoretical finest resolution" are not compatible. The formulae for DOF accept the reality of limited resolving power of the eye, this is not a disadvantage it is an advantage, it allows us to make images with cameras and enlargers that are mass produced in factories because the DOF of the optics permits leeway in optical alignment that would be prohibitively expensive were it necessary to eliminate it. It isn't necessary, circle of confusion and depth of field are real.

I am not a lens designer however the last time I read the specification for an enlarging lens I understood this to be pretty simple optically, four or six element, close to symmetrical design, just the kind of thing the formula I quoted works for.

Note that depth of field depends only on the value chosen for the circle of confusion, the magnification, and lens aperture, the lens focal length doesn't enter the equation, at least until the accuracy of calculation required exceeds a few percent error, then focal length does enter the formula. We aren't in the realm of needing errors smaller than a percent, we are addressing an order of magnitude difference.

 

[RobC]

I really can't be arsed to argue the point. You use your own figures. I will trust the software and what it tells me is optimal and not marginal.

 

[Sirius Glass]

What is the necessary depth of focus [at the negative, (redundant)]? At the most practical level when the grain focuser can see focused grain at the center and each corner.

 

[DREW WILEY]

"Circle of confusion" was an expression invented either to confuse people, or by people who were themselves confused. I'm a practical type. I use the
best grain magnifier I can afford (which is the best made), capable of seeing into the corners of the image. Then I make a print, using a vac easel if
necessary for precision. If I can't see any significant degradation in the corners of the print at my working magnification, I'm done. But otherwise, I
try to use any given enlarger lens at the SHALLOWEST possible depth of field so that ONLY the emulsion grain itself is in focus, that is, within that
zone of recommended apertures where the lens is truly sharp and provides relatively even illumination. Charts and tables and calculations might be
fine for those who enjoy such excursions. I find them merely a distraction and of little consequence to the final result. Our own eyes are the most
sophisticated instrument available.

 

[RobC]

depth of focus is usually way more than required. For the example above it would be + or - 1 mm using my figures and + or - 18mm using above 1mm Depth of Field example.
The higher the resolution in the print the better it will look? The smoother is will look. The sharper it will look. Why would anyone target what is less than is optimal when the could target optimal which they may miss by a fraction.
Why would anyone not try and get their neg truly flat and in the optimal plane and instead rely on a margin of error to get it there.

 

[removed account4]

hi tedr1

my instructors and mentors always told me to focus wide open,
use a magnifier and stop down 3 clicks past wide open which
is usually where it's optimized /the sweet spot.
I don't bother with computer programs or theories
but I put my faith in experience.
too many people get all wound up in formulas and equations and the minutia
and probably spend a lot of time theorizing about stuff that doesn't matter anyways.
rather than taking a computer program or wiki entry's word for it
why not just make a print?
good luck!

 

[RobC]

What is the necessary depth of focus [at the negative, (redundant)]? At the most practical level when the grain focuser can see focused grain at the center and each corner.

and remeber that the CoC is the product of the effective aperture. Its not something you can set any other way. So the mere fact of using F5.6 with 6X enalrgement gives an effective CoC which is only 0.038mm. Anything bigger and you are you are throwing away print resolution. Even if you think you can't see it, your print will look better for having that extra resolution.

It all depends on the standard you want to work to. Optimal or good enough.

 

[michaelorr]

my instructors and mentors always told me to focus wide open,
use a magnifier and stop down 3 clicks past wide open which
is usually where it's optimized /the sweet spot.

I am not the OP but i sure appreciate this comment John. Great stuff. I am hoping to start doing some enlarging soon, and am going to rethink the apertures i choose for the formats i use. Thanks, again. >michael

 

[tedr1]

We all use our enlargers to produce optimally sharp images, that is a given.

My purpose is to find out the precision required for the alignment of negative, baseboard and lens. I wish to know this because I wish to form my own judgements about whether the precision given to me by the manufacturer of the enlarger is adequate, this is weighed with measurements I make of the alignment of the particular machine at hand. For this purpose hard data is needed on depth of field at the negative, and the answer is about 1mm, not 0.06mm, I prefer to not work with bad data.

 

[RobC]

I prefer to align my enlarger as accurately as possible. Parallel is parallel and not almost parallel is good enough in my book. And perpendicular is perpendicular and not almost perpendicular is good enough.
I'd like to know how do you intend to measure within 1mm. i.e. not parrallel but almost parrallel and know its within 1mm.

And once again you have dissed the figures given by the software as being wrong. They aren't. They are the most accurate you are likely get unlike yours which are ball park made up by you and throwing in rounding to make it easy for you to do your inaccurate calculations. According to your figures you can use F22 and not F5.6 becasue you will still produce a print with sufficient resolution. i.e. more than 10 lp/mm. But everyone (except you) knows that's not the thing to do. Your figures are based on what was considered acceptable pre WWII. What is considered normal in 2016 is a conssderably higher level of print resolution. But not if you're still workig in the 1920s

 

[ic-racer]

Of course as the lens is stopped to smaller apertures the circles of confusion get smaller but the Airy disks get bigger. The method of Hansma can be use to obtain an optimum. (http://www.largeformatphotography.info/fstop.html)

N_max ~ 20 / (1 + m) sqrt(dv)

N_max = maximum tolerable F number
m = magnification
20 = constant for circle of confusion about 0.15mm on the print
dv = millimeter distance on the enlarger column between good focus on the highest and lowest portions of the curved or askew negative. (if 'dv' is zero then your negative is not curved or enlarger/baseboard/easel is not askew)

Just a clarification, the easiest way to obtain the variable dv in the equation above is to focus the enlarger by moving the head up and down and reading the scale on the enlarger column.

 

[tedr1]

Count on RobC to use rudeness inappropriately

 

[RobC]

Your 1mm DoF figure is plucked from thin air. Perhaps you should tell us how you intend to set your enlarger so that you get a 1mm DoF. Fact is a lens will give a CoC according to the aperture you set and its resulting affective aperture due to lens extension and that will also determine what the actual DoF will be. CoC and DoF are not some fixed values conveniently plucked from the air to suit what you would like it to be.

 

[tedr1]

Go back to the top of the thread...........

 

[RobC]

yeah and? you've plucked 0.5mm out of the air becasue you don't know what the actual figure is. Why don't you calculate ist. A lens doesn't pluck its CoC out of the air. The CoC is variable depending on the lens and aperture and the figures I gave are calculated using that variability. The CoC won't be 0.5mm. It will theoretically be 0.038mm for your example and that will make the DoF much smaller than 1.0mm. What you are suggesting is ball park good enough based on 80 year old standards for acceptability which have moved forward a long way since then. But the old literature doesn't tell you that.
Anyone with any sense who is trying to align an enlarger WILL get the negative flat. Will get the negative stage parrallel with the baseboard and WILL get their lens perpendicular to both the negative stage and the baseboard. Laser alignment tools are available for this and if used accurately your system will approach the level of accuracy the figures I quoted. Anything else is sloppy alignment. i.e. only ball park good enough. Take your pick but don't tell me the figures are bad information becasue they are not. Your figures are bad information because they don't help people maximise the potential of their system.
But what should we expect from someone who just did a little research and suddenly knows it all.
And just to remind you, they are not my figures, they are the figures produced by software written by an optics expert.

 

[RobC]

And don't forget that if DoF is as great as you are suggesting, then AN glass grain would be in focus and show in the print which several people in previous topic say they have never seen.

 

[removed account4]

I am not the OP but i sure appreciate this comment John. Great stuff. I am hoping to start doing some enlarging soon, and am going to rethink the apertures i choose for the formats i use. Thanks, again. >michael

pleasure's mine michael !
i've found its best to keep things simple
have fun printing

john

 

[RalphLambrecht]

hi tedr1

my instructors and mentors always told me to focus wide open,
use a magnifier and stop down 3 clicks past wide open which
is usually where it's optimized /the sweet spot.
I don't bother with computer programs or theories
but I put my faith in experience.
too many people get all wound up in formulas and equations and the minutia
and probably spend a lot of time theorizing about stuff that doesn't matter anyways.
rather than taking a computer program or wiki entry's word for it
why not just make a print?
good luck!

Amentheoretical calculations are good;making a print or a negative is better.A circle of confusion is a bunch of photographers sitting around discussing depth of field

 

[tedr1]

Keep in mind that the greatest influence on DOF at the negative is the enlarging ratio because it is the square of the enlarging ratio that determines DOF. The example given at f5.6 is good for 6x enlargement ratio however at around 18x enlargement ratio the DOF is only one tenth as much as at 6x. This situation is more likely to arise with big enlargements from small format negatives. Reducing the working aperture two stops doubles the DOF.

 

[Jim Jones]

Sometimes we think too much about the wrong things. We can consider topics like DOF at the easel and optimum lens aperture diameter as appropriate subjects for technical analysis, or we can directly tackle the more complex problem of making the best possible prints of a variety of subjects from negatives with widely different characteristics.

None of the mathematical formulae in the above posts address the relationship of Airy disc to film grain size, although this may be a useful tool. None even mention the relationship of the subject matter to resolution in the print. Edward Weston produced some prints that were severely diffraction limited, but who cares? He may not have bothered with the math, but he could get results. Some of Eugene Atget's early images were captured with dismally inadequate lenses, but remain masterpieces. Math and theory can be entertaining diversions, but in photography it is the ultimate image that really matters.

 

[Old-N-Feeble]

If/when I ever build another darkroom it will be strictly for very large prints from 4x5in or 6x12cm. Since the effective aperture at high magnifications is greatly decreased, I'll be printing wide open or only one stop closed to minimize diffraction effects. That's why I opted for a f/4 Apo lens rather than a f/5.6. I'm just going to assume I have near zero DoF and get everything as precise as is practicable... so 'acceptable' DoF and CoC are basically moot for me.