Don't be too upset about getting pissed. Just don't do it on us. Let me tell you a story. One day when working at NASA, a colleague and I were in a conference room engaging in an attempt to solve a technical program. We would get quite excited about some point or another and our voices would rise. Outside the room the secretary was thinking, as I found out later, that we were about to engage in fisticuffs. Then we went down to the Coke machine, flipped a coin, drank our cokes, came back and went at it again.
Originally Posted by Bob Carnie
Just pretend we are together in the same room having it out about some technical point.
With regard to the technical point, I doubt that you will be able to focus as exactly as you think with the lens stopped down. With most lenses nowadays, the resolution will go down from 4 to 8. I can't see how the grain will look as sharp at 8 as at 4 with a 2.8 lens. I can tell the difference between 4 and 2.8 with my 50 mm APO Rodagon. 5.6 is about as good as it gets.
Each lens is designed to be optimum at a particular magnification. Most 50 mm enlarging lenses are optimized at 10, I believe. Try your experiment at 10X magnification. Focus on a nice, grainy Rodinal negative wide open and watch the definition of the grain as you stop down. It should get sharper, then fuzzier as you pass through the aperture where diffraction takes over.
Actually I am not getting pissed just I find this DOF issue with enlarging lenses and some of the statments made go against the grain and I humbly have to disagree .
I have been taught to focus wide open when printing and then close down to two stops . I have been told by my instructors as well as the lens manufactures that this is the proper way of focusing for critical sharpness.
I have found out that by practical experience that the grain is the same at both f stops. I rarely focus down 3-4 stops but when I do I use the above method and I always find my images grain sharp.
I should state what I find most frustrating is the issue that if you close down the lens you will get the same effect as DEPTH OF FIELD when one takes a photograph.
When enlarging we are focusing on an emulsion that is lying flat much like the easal we are projecting onto. If I find that the grain is sharp at different apetures at a given magnification with out refocusing , I believe it is not attributed to DEPTH OF FIELD but to a perfectly aligned enlarger in conjunction with using glass carrier to hold the negative flat and also using a Apo rodogan lens that is a nice piece of glass.
I am not arguing that DEPTH OF FOCUS is not present in this scenerio.
If I am wrong and as you suggest that the grain is less sharp at different apetures, with all of the above in place(glass carrier, exact same magnification, aligned enlarger) and as you say it is hard to focus at the lower apetures, how on earth would you suggest to focus the enlarger ???
In your senerio I would have to work at a wide open apeture as as I close down the focus shifts, as I would not be able to go to a lower apeture to focus because lack of light.
I am now going to have a coke and wait for you to insert your coins into the machine and join me on this discussion
There is a lot of confusion here in definitions. Resolution is related to, but not the same as "depth of field / focus". Image quality is related to, but not the same as depth of field / focus. Diffraction is something else entirely.
It is true that *every* lens - not exclusively enlarging lenses - has an "Optimum" f/stop. However, optimum does NOT mean that all others are useless. If this were true, all lens manufacturers, of both camera and enlarging lenses would not equip them with iris diaphragms.
I'm tying to think of a way to define depth of focus/ field concisely, without first going into an extensive "course" in basic optics. I'll try...
Depth of focus is the distance separating of two planes perpendicular to the optical axis, deviating in either direction from an ideal focus, where acceptable resolution is maintained. Not "the best possible resolution", but acceptable. An example: A given lens is perfectly focused at 10cm. Using a finite f/stop, and lessening the focusing distance to 9cm, the resolution (~ squinting, approximately - call it "sharpness") is still "acceptable", that is, it does not exceed an arbitrarily chosen value for resolution. Moving in the other direction, it is still "acceptably sharp" at 12cm.
The depth of focus here is 3cm - between 9cm and 12cm.
I hope this conveys the idea. For better, more polished information, I can only suggest reading a book on Basic Optics.
Ed Sukach, FFP.
would this acceptable area on both sides as you suggest be called the *circle of confusion*.
being the small area of depth of focus that the human eye will accept as sharp.
I suggest we are both talking about the same thing here, My only concern is too not confuse the issue by considering * depth of field * as an applicable method of setting up an enlarger because the 1/3-2/3 idea is unrealistic when focusing on a flat playing field.
I am going back quite a few years from when I studied optics at college so maybe my memory is rusty, but from focusing thousand's of negatives my practical knowlege tells me that when focused at f4 you will get the same level of sharpness as if you stop down to f11 and adjust your printing time accordingly. (using a glass carrier and a aligned enlarger)
We are not talking about the same thing.
Originally Posted by Bob Carnie
A circle of confusion has been defined as: "When we try to focus all the rays of light from a lens into one point, we cannot. The best we can do is to form a circle of light. That circle's deviation from an infinite point - is the "Circle of Confusion". It can be reduced by eliminating some of the rays toward the outside extremities of the lens - restricting the image to the central rays... in other words, using a smaller f/stop."
Formula for determining Depth of Focus / Field:
First calculate the "Hyperfocal Distance - the distance where the lens, when focused at infinity, will produce an acceptably sharp image of the nearest object.
Hyperfocal Distance = ((FxF)/f) x (1/C)
F= Focal length of the lens
C= Circle of confusion
f = f/stop = diameter of aperture / Focal length of lens
An example = A 50mm lens at f/8, with a Circle of Confusion of 1/100mm:
H= ((50 x 50) / 8) x (1 / (1/100))
H= 312.5 x 100
H= 31250mm; 31.25 meters
Everything from 31.25 meters to infinity will lie within a Circle of Confusion (acceptably sharp) of 1/100 mm.
It is obvious that reducing the f/stop (increasing numerically) will decrease the Hyperfocal distance.
Now, Depth of Field:
For the nearest sharp plane:
(H x D) / (H + ( D - F))
D = Distance focused upon
H = Hyperfocal distance
F= Focal length of lens
and the farthest sharp plane:
(H x D) / (H - ( D - F))
Working the same lens, same aperture and and the same circle of confusion; focused at 25 meters:
Np = (31.25 x 25.0) / ((31.25 - (25 - .05))
Np = 12.4 meters
The Farthest plane:
Fp = (31.25 x 25.0) / ((31.25 - (25.0 - .05))
Fp = 124.0 meters.
Someone check my math!!!
I haven't cracked those books for one hell of a long time!!! I just realized HOW long!!
Ed Sukach, FFP.
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Maybe I just noticed somehing that can help clarify this.
I, and others, have been talking about the effects of depth of focus in
relationship to aperture.
It seems that maybe we took a mis-cue from the originally posted note.
In a recent post you stated that you understood the relationship between depth of foscus nad aperture size.
You further stated your experience regarding grain magnifiers and stopping down once sharply focused at maximum aperture.
Herin lies the divergence - no ( other than the effect of optimum aperture ) increase in sharpness results from the depth of focus effect of stopping down.
It should increase due to particular lens design, but that is not related to D.o.f.
If you re-read my original post ( that started this divergent thread) you will see that I was talking about d.o.f. and stopping down. In it I stated that the plane of sharpest focus was not a true plane - that there was a bow, or dome to it. To compensate for this, we needed to focus at a given point on the easel and stop down to increase the d.o.f. to allow the "domed" image to be sharp all the way across.
As was pointed out - circle of definition does not directly relate to this - it does, however, relate to the resolution of the optic system.
So - cokes all around?
Ok , I bow down to this onslaught of mathematic formulas, but have we solved the original question that seems to be still in my mind?
I think that grain is sharp at same mag , for all fstops when enlarging,
I do not think that closing down the lens will make the image sharper.
If you come back with more math , I will have to resort to changing to my mother tongue which is Moronica and then we both will absolutely not get anywhere on this thread.
Time for another pair of feet to jump in:
Originally Posted by Bob Carnie
If sharp at full opening, it should be sharp at all apertures. But since no lens is perfect, "sharp" is a relative term. The plane where sharpness is best will not be perfectly flat, but slightly curved. So best sharpness in the center doesn't mean the corners are as sharp as possible.
So we need more DoF to make the corners acceptably flat. We get this by stopping down the aperture.
A theoretical lens will have maximum resolution at maximum aperture, decreasing due to diffraction as the aperture is closed down. But since weære dealing with real-life bits of glass, our enlarger lens is not perfect. So there will be some point where DoF is sufficient to get a good sharpness over the whole easel, while not yet losing definition to diffraction.
Incidentally I just received four old lenses in the mail (I bought them for the shutters). They seem to have been used in some sort of repro setup, or perhaps printing. On one of them the aperture was taped stuck at f:8 - a reasonable value for the optimum working aperture for an f:4.5 lens in reproduction work!
-- Ole Tjugen, Luddite Elitist
Obviously the size of the negative, the inherent qualities of the negative, and the quality of the lens used as well as the magnification, as well as other factors all effect the print quality of the end result. There are other factors involved in the flatness of field. If a lens of the same design is doubled in focal length the level of abberations will increase by fourfold at the same aperture. If my 63mm el Nikkor N has a particular level of correction then the same formulation at 6 time the focal length then such as lens used at the same f stop would have 36 times the abberation level. If I was using my lens at f4 because that is its sharpest aperture and the longer lens was used at f8 then I would estimate that the longer lens still has well
over 10 times the abberations. A second consideration is that 9X9 negatives are capable of resolving extremely small detail and doing it so well that what is satisfactory in a 6 1/2x9 3/4 inch print from the abberations of my35mm negative might not be so satisfactory in a similar magnification of a 9"x9" negative. It is hard to make a high quality enlarging lens for 35mm and the difficulty increases with focal length.
One also should keep in mind that variations is to be both expected and embraced in any manufactured product with each lens being different. This is the basis for statiscal process control as stated by Deming and as used by Taguchi in quality. As an aside, this is a very good reason for getting a period of time that a lens can be returned after purchasing it. I do not believe by any stretch of the imagination that every example of a product. even those by the most reputable manufactures, are going to be fine examples that will satisfy the dicerning user.
How do I know that my lens is satisfactory at the edges when the center is sharply focused? All I have to do is stop the lens down more to see if an improvement results. This improvement does not happen. I hope that nobody gets the idea that I am touting Nikon's lens as being superior to other brands because such is not my intent.
I was trying to quote the Photo Lab Index by Morgan & Morgan in my statements above regarding abberations and I see that the section on photographic optics is missing. Since no one else uses this reference that I own, I know exactly who to blame. If you have a copy of this book and you check your photographic optics section you will have an oportunity to check the memory of an old geezer.
Not to worry. I DON'T plan to do all that again.
Originally Posted by Bob Carnie
I'm trying to illustrate that Depth of Field / Focus is a different characteristic than simple "sharpness". Changing an aperture of a camera or enlarging lens, MAY increase or decrease "sharpness" but that in itself has nothing to do with depth of field.
Ed Sukach, FFP.