Depth of field at the negative; requirements for sharp focus

[RobC]

when it comes right down to it, most images on film are resolution limited by the film itself combined with the lighting contrast ratio on it. Result is that actually achieving 100 lp/mm on film is extremely difficult regardless of whether your film or lens are potentially capable of much higher resolution figures cos they can't do it if conditions aren't right for it, which they aren't the vast majority of the time.

 

[wiltw]

when it comes right down to it, most images on film are resolution limited by the film itself combined with the lighting contrast ratio on it. Result is that actually achieving 100 lp/mm on film is extremely difficult regardless of whether your film or lens are potentially capable of much higher resolution figures cos they can't do it if conditions aren't right for it, which they aren't the vast majority of the time.

^
...which is why lens tests performed with Plus-X are not as high in resolution as those with Tmax
Kodak Plus-X has f50 = 100 lp/mm, Kodak T-MAX 100 has f50 = 125 lp/mm

 

[cliveh]

The OP mentions depth of field of the enlarger lens, but have others have mentioned it is depth of focus. But what on earth is this thread about?

 

[wiltw]

The OP mentions depth of field of the enlarger lens, but have others have mentioned it is depth of focus. But what on earth is this thread about?

...which is why I brought up the points in post 47.

Op said, "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?"

If the negative in the negative stages is considered to be the 'object plane', the negative itself and its flatness is considering the deviation of the subject from the Object Plane focus and Depth of FIELD, and the image is projected and captured on photosensitive material (the paper emulsion on the easel) where the Depth of FOCUS.
The term 'DoF' (ambiguous) was being applied in the earlier posts, and some of the posts talked about small distances which would be applicable to 'Depth of Focus' measurement. For example, "depth of focus is usually way more than required. For the example above it would be + or - 1 mm using my figures..."
If what I described as 'Context A' is indeed the correct 'school solution', I will, after the fact, admit to being to blame for bringing up Depth of FOCUS (although not at all the first in the thread to do so) and wrongly drawing the analogy of negative in enlarger to film at focal plane of camera. But if 'Context B' is the correct 'school solution', then I will stand by my first effort to call it 'Depth of FOCUS', not 'Depth of Field'.

 

[removed account4]

The OP mentions depth of field of the enlarger lens, but have others have mentioned it is depth of focus. But what on earth is this thread about?

it might be to see how many licks it takes to get to the center of a tootsie pop ?

 

[tedr1]

It is very simple. I began the thread to make a technical point which I did in post #1. RobC attempted to discredit my work and there was a long series of exchanges between him and me. Everybody joined in and a lot of people talked without anybody listening.

 

[Nodda Duma]

You guys. I'm an optical designer and the weaving path of this discussion is so nitpicky that it's almost silly.

OP asks a good question. My professional answer would be: Flat enough that the image of the grain at your baseboard is in focus from corner to corner. Don't waste time and angst trying to calculate it when you can measure it. You're not designing an enlarger, you're trying to use one. Check focus, adjust alignment, and go make focused prints.

 

[RobC]

It is very simple. I began the thread to make a technical point which I did in post #1. RobC attempted to discredit my work and there was a long series of exchanges between him and me. Everybody joined in and a lot of people talked without anybody listening.

That's a downright lie. You went on an unprovoked attack and tried to discredit what I wrote in the previous topic and now you expect me not to argue about it. Do you know what hypocracy means.
You said

Further to the discussion about depth of field (see post #71) I have researched the subject a little and concluded that information given in post #71 is incorrect. Rather than wander off-topic here I have begun a new thread on the subject here (there was a url link here which no longer exists)

You said

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.

You said

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

You said

Count on RobC to use rudeness inappropriately

If that's not an unprovoked attack I don't know what is. And you don't expect me to defend myself. And you don't think people can't see the deceit in you claiming its me doing the attacking. I've merely been defending myself. You're severely deluded.

 

[tedr1]

It is spelt hypocrisy. I have made no attacks, simply stated some facts. My statement in post #1 is available for checking by anyone. The photographic community understands the concept of circle of confusion. You chose to not understand it. That is where things stand.

 

[Tim Layton]

I offer that you make some prints at whatever settings you wish, and then view them at the proper viewing distance as part of your decision-making process. In the end, that is all that really matters.

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.

 

[Gerald C Koch]

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.

What I was attempting to say, I guess not too clearly, is that the depth of focus is determined by the aperture of the lens. However there is a limit as to just how far you can reduce the aperture before aberrations become a factor. Obviously this is also determined by the quality of the lens.

 

[Gerald C Koch]

[QUOTE="wiltw, post: 1780815, member: 28732[/QUOTE]
The term 'DoF' (ambiguous)

Not if it is read in conteXt.

 

[Old-N-Feeble]

Let's not forget the effective aperture becomes smaller as magnification increases. So the effective aperture with the lens set at f/5.6 and 4x magnification is different than the same lens set at f/5.6 and 8x magnification. Hmm... what's that... 2 stops difference? Close enough anyway.

 

[ic-racer]

The "m" magnification factor in the equation in post #15 takes this into account; it hasn't been forgotten.

 

[wiltw]

I have made no attacks, simply stated some facts. My statement in post #1 is available for checking by anyone.

Unfortunately in this NOT-face-to-face internet world, we can state a fact, perhaps even color or enlarge the text of key words or phrases (or not) and it can be interpreted by some reading it as an 'attack' and react to said post in a negative manner. I am dealing with such a reaction in another forum -- not related to photography -- myself right now! And some individuals tend to have this unnecessary interpretation and reaction more than others, for whatever reason, and I am dealing with such a reaction in that forum.

Folks, we need to relax a bit, and if there are not insults or responses like "that was a stupid" accompanying the comment, just probably the response was not intended to be viciously negative. Responding folks need to reread their posts just to see of a bit of negative attitude shows through even inadvertantly. But such is life.

 

[wiltw]

[QUOTE="wiltw, post: 1780815, member: 28732

The term 'DoF' (ambiguous)

Not if it is read in conteXt.[/QUOTE]

Unfortunately not necessarily true, context does not make a wrong term right...

Some folks talk about 'smaller aperture' while they thing of an f/stop like f/2 in the pursuit 'more 'bokeh'.
There we have TWO wrongly used terms, and it is clear what they are thinking but mispeaking (twice). And then there may be the question of what to change in order to make the statement right per convention:

• 'f/number' and not 'aperture', or 'larger' rather than 'smaller' ('smaller f/number', or 'larger aperture'...which one to change; in this case we know what normally is associated with 'shallower DOF', the 'larger aperture')
• 'depth of out-of-focus blur' and not 'bokeh'