grain focuser questions

[Terry Hayden]

dof/sharpness

Bob,

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?


Later,
Terry

 

[Bob Carnie]

Hi Ed

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.

 

[Ole]

...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:

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!

 

[Claire Senft]

Other considerations

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.

 

[Ed Sukach]

Hi Ed

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.

Not to worry. I DON'T plan to do all that again.

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.

 

[Bob Carnie]

Ed
Thankfully , I agree changing the enlarging apeture has nothing to do with Depth of Field.

 

[Ed Sukach]

Ed
Thankfully , I agree changing the enlarging apeture has nothing to do with Depth of Field.

Oh boy. Where is my latest strip of Patented Flavored Inner Tube... This month - TURI Estonian Vodka.

Not what I said, Bob. What I said was depth of field / focus is the same for both enlarging and taking lenses; and that any change in sharpness you described was probably NOT due to change in the depth of field.

f/stops DO have an effect on depth of field. Tried to show it by the math...

 

[Bob Carnie]

Ed

I am going to go home now to drink 5 Old Milwakkees and try not to think of this thread for a 19hr stretch. If I can add a few shots of Bourbon I will. Tommorow is another day see you then.
best regards
bob

 

[gainer]

Time for another pair of feet to jump in:

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!

You finally struck the right chord. Depth of field can in fact cause focusing errors because the sharpness of the image changes more gradually when the depth of field (or focus) is large. That is why there is such a thing as depth of field (or focus).

A perfect lens or even a very good one may be sharpest at maximum aperture. Diffraction has nothing to do with lens design, only with diameter of the aperture. The formula for diffraction does not contain any elements that have to do with lens design. ANY practical lens shows an increase in resolution as it is stopped down, and ANY lens shows an increase in diffraction when it is stopped down. Where the two cross over is the optimum aperture for resolution. A poor lens may not show much change in resolution over its entire aperture range. Don't buy one like that.

For focusing, use the widest aperture. If your lens does not have a flat field, you will have to stop down to flatten it. The same will apply to most of the other possible aberrations. There are many old wives tales connected with lenses and their use. If you can dig up the article on "Hazards of the Grain Focuser" that I wrote fot Photo Techniques, you will see the experiments that you can do for yourself to resolve the questions that have arisen here. I'm not bragging, just telling the truth.