PDA

View Full Version : diffraction limits...again



Kirk Gittings
21-May-2011, 11:37
In the middle of contemplating the rapture last night, a thought struck me (why then I have no idea-but that is another thread perhaps).

I am well acquainted with how diffraction limits effect my negatives when shooting (and plan accordingly-I have tested this on many of my lenses and know where it strarts to influence sharpness).

But I have never noticed its effect when enlarging and sometimes I stop way down purposely to get long exposures to give me time to do complicated burning and dodging. Hmmmmm.....

Mark Woods
21-May-2011, 12:03
Hello Kirk, what is the smallest stop on your enlarging lens?

Greg Blank
21-May-2011, 12:51
I have to tell you that my opinion is that: On all modern lenses I have used diffraction,... I don't see the so called problems these limits impose. I also firmly believe these issuses that are upon by hand wringing people that can't find enough reasonable excuses for not being able to make pictures. I have used my 4x5 enlarger lens wide open an stopped all they way down- wide open can certainly effect sharpness if your enlarger is not aligned well....stopped down effects sharpness sometimes "in a positive way". You might be able to see the problems through a microscope between f16 and f45 ;)



In the middle of contemplating the rapture last night, a thought struck me (why then I have no idea-but that is another thread perhaps).

I am well acquainted with how diffraction limits effect my negatives when shooting (and plan accordingly-I have tested this on many of my lenses and know where it strarts to influence sharpness).

But I have never noticed its effect when enlarging and sometimes I stop way down purposely to get long exposures to give me time to do complicated burning and dodging. Hmmmmm.....

Noah A
21-May-2011, 13:04
I haven't used an enlarger in years, but it seems to me that when you shoot a negative, any diffraction-related sharpness loss will be magnified when you later enlarge that negative (or inspect it with a powerful loupe).

When you make a print, it's possible that any sharpness loss is just not noticed since you're probably not further magnifying the print or even inspecting it under a loupe for that matter.

ic-racer
21-May-2011, 13:31
But I have never noticed its effect when enlarging and sometimes I stop way down purposely to get long exposures to give me time to do complicated burning and dodging. Hmmmmm.....

???
Very noticable with rollfilm. Perhaps your LF negatives have magnification ratios too small to see the effects on the grain.

With a grain magnifier, throw a 35mm TRIX negative in your enlarger with a 50mm lens. It is very informative. The grain in the center is sharpest wide open (usually, depending on your lens) and keeps getting worse as you stop down. Most usually F8 will be the farthest you can go without being annoyed by it. At f16 it is just a blurry mess.

With an 8x10 negative making a 11x14 print, indeed, the above effects are very hard to see because the magnification ratio is so small and the grain is so small.

What exactly is your setup?
Grain magnifier type?
Lens?
Format?
Enlargement size?

Henry Ambrose
21-May-2011, 16:04
It'll only matter if the resolution that the paper and negative are capable of is greater than the output ability of your lens at a given aperture and reproduction size. At 1:1 you may never see any loss no matter the aperture, at 1:10 you may.

Also consider your ability to see at the viewing distance to the print. Across the room from the 1:10 you may not see any loss.

Some really smart person will, no doubt, calculate this for us. Otherwise if your prints look great, they look great!

rdenney
21-May-2011, 21:33
Hmmmmm.....

Hmmmm indeed. In my Peak grain focuser, I could clearly see where the crispness of the grain improved as I stopped down from wide open, and then degraded again as I kept stopping it down. Maybe you need a better grain focuser.

Rick "wondering what penance in Hell is reserved for being presumptuous with the likes of Kirk" Denney

Oren Grad
21-May-2011, 21:55
It depends...

It's easy to see the effect of stopping down through my Micromega. But some years back I also did an experiment in which I made a modest enlargement from a half-frame 35mm negative made on Tri-X with my Pen FT and 40/1.4 Zuiko - I think the final image size was around 3x4", which is about a 4x enlargement. I made a series of prints, bracketing through the entire aperture range of my EL-Nikkor. I think I used the 50/2.8, though it may have been the 80/5.6.

I have the prints buried away somewhere. But as I recall, even with my nose in the prints it was very difficult to see any degradation in image clarity until I got down toward the tail end of the aperture scale - I think it was f/16. But at that point it was clearly apparent. With a greater degree of enlargement, I expect the degradation would have been visible sooner.

GPS
22-May-2011, 03:59
Diffraction is the same for all lenses for the same aperture number given a light wavelength. Everything else you can ascribe to the alchemy of perceived sharpness - there lies your hidden answer.

Greg Blank
22-May-2011, 04:44
That would equate to all lenses being made from the same materials. Which they are not. Differing materials produce differing degrees of diffraction, an example would be a Flourite lens versus a standard. Or an HM lens versus a standard.


Diffraction is the same for all lenses for the same aperture number given a light wavelength. Everything else you can ascribe to the alchemy of perceived sharpness - there lies your hidden answer.

GPS
22-May-2011, 05:03
That would equate to all lenses being made from the same materials. Which they are not. Differing materials produce differing degrees of diffraction, an example would be a Flourite lens versus a standard. Or an HM lens versus a standard.

Which surely is not the case of the OP, reverend hair Splitter...;)

Helen Bach
22-May-2011, 05:12
That would equate to all lenses being made from the same materials. Which they are not. Differing materials produce differing degrees of diffraction, an example would be a Flourite lens versus a standard. Or an HM lens versus a standard.

Greg,

Have you confused dispersion with diffraction?

There's nothing magic about enlarging that avoids the effects of diffraction, of course. The effective f-number is greater than the marked f-number in the same way as when doing close-up work - so a marked f/11 can easily be an effective f/64 when considering diffraction, but that is offset by the large-ish maximum acceptable diameter for the Airy disk - which is typically going to be no less than that for a 10-8. Others have already said this perfectly well - I'm simply putting it another way.

Best,
Helen

Greg Blank
22-May-2011, 06:41
I see your point.....

You mean this right (see below)? Read the last sentence....and think about grain focusers.

One typically does have to refocus at a smaller aperture - if that is the result of diffraction I stand corrected.

If one is using a modern enlarger lens one will not see a difference between prints made at f-11 or at minimum apertures. If you are "seeing" diffraction your camera lens or enlarger lens is garbage or your grain focuser is. Its all needless hand wringing.

Richard Feynman [4] says that
"no-one has ever been able to define the difference between interference and diffraction satisfactorily. It is just a question of usage, and there is no specific, important physical difference between them."

Diffraction-limited imaging
Main article: Diffraction-limited system

The Airy disk around each of the stars from the 2.56 m telescope aperture can be seen in this lucky image of the binary star zeta Boötis.
The ability of an imaging system to resolve detail is ultimately limited by diffraction. This is because a plane wave incident on a circular lens or mirror is diffracted as described above. The light is not focused to a point but forms an Airy disk having a central spot in the focal plane with radius to first null of

where λ is the wavelength of the light and N is the f-number (focal length divided by diameter) of the imaging optics. In object space, the corresponding angular resolution is

where D is the diameter of the entrance pupil of the imaging lens (e.g., of a telescope's main mirror).
Two point sources will each produce an Airy pattern – see the photo of a binary star. As the point sources move closer together, the patterns will start to overlap, and ultimately they will merge to form a single pattern, in which case the two point sources cannot be resolved in the image. The Rayleigh criterion specifies that two point sources can be considered to be resolvable if the separation of the two images is at least the radius of the Airy disk, i.e. if the first minimum of one coincides with the maximum of the other.

Thus, the larger the aperture of the lens, and the smaller the wavelength, the finer the resolution of an imaging system. This is why telescopes have very large lenses or mirrors, and why optical microscopes are limited in the detail which they can see.

http://en.wikipedia.org/wiki/Diffraction




Greg,

Have you confused dispersion with diffraction?

There's nothing magic about enlarging that avoids the effects of diffraction, of course. The effective f-number is greater than the marked f-number in the same way as when doing close-up work - so a marked f/11 can easily be an effective f/64 when considering diffraction, but that is offset by the large-ish maximum acceptable diameter for the Airy disk - which is typically going to be no less than that for a 10-8. Others have already said this perfectly well - I'm simply putting it another way.

Best,
Helen

Paul Fitzgerald
22-May-2011, 08:33
"But I have never noticed its effect when enlarging and sometimes I stop way down purposely to get long exposures to give me time to do complicated burning and dodging. Hmmmmm....."

Could be this all unnessary hand wringing ;)

Could be that the paper sees the image differently than human eyes :eek:

Could be you're viewing the image on the photograph thru a gelatin overcoat.

Could be that a grain magnifier shows more than the paper can and sees an arial image, not there and back thru the overcoat.

Could be that the 'optimal aperture for center resolution' is a mis-leading red herring. All lenses show LCA and a curved field, stopping dowm to f/11 seems to minimize these, also seems to improve contrast and corner to corner resolution.

"With a grain magnifier, throw a 35mm TRIX negative in your enlarger with a 50mm lens. It is very informative. The grain in the center is sharpest wide open (usually, depending on your lens) and keeps getting worse as you stop down. Most usually F8 will be the farthest you can go without being annoyed by it. At f16 it is just a blurry mess."

Why would you focus in the center and not 1/3 out to the corner? This would only used DOF to one side of the focus point. :confused:

Helen Bach
22-May-2011, 08:46
I see your point.....

You mean this right (see below)? Read the last sentence....and think about grain focusers.

One typically does have to refocus at a smaller aperture - if that is the result of diffraction I stand corrected.

If one is using a modern enlarger lens one will not see a difference between prints made at f-11 or at minimum apertures. If you are "seeing" diffraction your camera lens or enlarger lens is garbage or your grain focuser is. Its all needless hand wringing.

Greg,

That's not really what I meant. I was suggesting that your mention of fluorite (sic) and other types of glass had more to do with dispersion than diffraction. The type of glass has nothing to do with diffraction.

Diffraction is not usually helped by refocusing, - the size of the Airy disk grows in direct proportion to distance, it never decreases. Refocusing at smaller apertures/higher f-numbers is usually required because of other aberrations, not diffraction.

One is more likely to see the effects of diffraction with high resolution lenses than with soft lenses, not vice-versa. (See Henry's earlier comment, for example.)

Best,
Helen

paulr
22-May-2011, 08:58
I noticed it with my 150mm apo componon hm lens. The center of the image was always sharper at f8 than at f11. But the corners were much softer. So I settled on f11 except for the odd cases where there wasn't important detail in the corners. By f16 everything was noticeably softer.

Greg Blank
22-May-2011, 09:09
Paragraph 1) Well you are wrong. If you are seeing diffraction there is something wrong in your system. Or your eyes are a lot better than mine.

Paragraph 2) Which ones.

Paragraph 3) I agree with Henry, like I stated needless hand wringing.


Greg,

That's not really what I meant. I was suggesting that your mention of fluorite (sic) and other types of glass had more to do with dispersion than diffraction. The type of glass has nothing to do with diffraction.

Diffraction is not usually helped by refocusing, - the size of the Airy disk grows in direct proportion to distance, it never decreases. Refocusing at smaller apertures/higher f-numbers is usually required because of other aberrations, not diffraction.

One is more likely to see the effects of diffraction with high resolution lenses than with soft lenses, not vice-versa. (See Henry's earlier comment, for example.)

Best,
Helen

Greg Blank
22-May-2011, 09:17
In every LF negative that you print, using the same camera lens for every picture. Using the same camera aperture for every exposure tested?

Condensor or Dichrioc enlarger>? And the enlarger is perfectly aligned?

The list of unanswered questions gets longer the more the point is tried to be proven- live with it I say. Its needless hand wringing and I am done with it.



I noticed it with my 150mm apo componon hm lens. The center of the image was always sharper at f8 than at f11. But the corners were much softer. So I settled on f11 except for the odd cases where there wasn't important detail in the corners. By f16 everything was noticeably softer.

Drew Wiley
22-May-2011, 09:18
My enlargers are exceptionally well aligned, and I can easily see the effects of diffraction using a grain magnifier, but with 8x10 film at least, the degree of enlargement is generally so modest that the difference can't be seen in the actual print. A 20x24 fiber-based print made at f/45 looks just like one made at f/11. With color work on polyester, like Cibachrome, I generally shoot at a very shallow aperture just so no associated masking grain will come into focus, but even a 30X40 is not enough magnification to make diffraction a significant issue. When shooting in the first place, I'm a lot more careful however, and prefer not to go below f/45 if the shot is intended for a 30X40, or otherwise not below f/64; with 4x5, I don't like to go below f/32. I've got all kinds of high-end enlarging lenses, so feel fairly comfortable about my practice in this respect.

aduncanson
22-May-2011, 10:33
Helen is exactly correct.

As an engineer and a photographer, I like to use the photographer's rule of thumb (with all of its limitations and assumptions) that maximum resolution (in line pairs per millimeter) at a given F No. is given by 1600/F No. I find this easier to apply than thinking in terms of the size of the airy disc. The question is how does this formula apply when enlarging.

Practitioners of close up, macro and micro-photography correctly warn that one needs to use the effective F No. considering "bellows extension" when magnification is significant. When enlarging, the correction for effective aperture due to bellows factor (lens to paper distance divided by focal length) can be quite large. Lens to paper distance equals (1+ print magnification) * focal length.

Skipping the derivation, the rule of thumb considering effective aperture becomes:


Diffraction limited resolution = 1600 / ( F No.x ( 1+ M ))

Another, more intuitive way to think of this is to recognize that 1600/F No. gives the diffraction limited resolution when the subject is at infinity and the image at the focal length (that is when magnification equals 0) and this resolution is reduced when the image is magnified. When the magnification cannot be taken as zero, resolution is reduced by the factor 1/(1+m).

This phenomenon is not frequently seen when enlarging large format negatives because print magnifications are relatively small and because the standard for a sharp enlargement is low. Ten line pairs per millimeter (10 lppm) is a fairly stringent standard for prints. When making a 20x24 inch print from 4x5 (a print magnification of 5X), you can stop down to a marked F/22 aperture (effective aperture F/135) and still deliver greater than 10 lppm. Clearly, when making a 12X enlargement from a small negative, the problem is more likely to come into play, and a powerful grain focuser is also likely to reveal the phenomenon even if it can't be seen in the print.

- Alan

Greg Blank
22-May-2011, 11:42
There is a special place in hell for engineers. Try rolling a rock up hill for a few thousand years.



Skipping the derivation, the rule of thumb considering effective aperture becomes:
This phenomenon is not frequently seen when enlarging large format negatives
- Alan

BetterSense
22-May-2011, 13:15
I did a test for myself the other night.

I routinely print 5x7 from 35mm at a marked f/22 on my 75mm enlarging lens. The idea is that In need some dodging/burning time, so f/22 gets me to about 15 seconds, and a 5x7 print is small enough that diffraction shouldn't matter.

After I made my final 5x7 print, I tried one at f/8, which is only about 3 seconds exposure time. I have to admit that upon close inspection with the naked eye, and when comparing directly with the print made at f/22, the grain was sharper on the print made at f/8. It is a subtle effect but it's the first time I have ever tested and seen diffraction effects. I'm still not worried about it but it's on my radar as something that can make a subtle difference.

Armin Seeholzer
22-May-2011, 13:17
I see almost no diffraction limit at f 128 some lenses just start to get SF lenses;--)))):D

Cheers Armin

Greg Blank
22-May-2011, 13:46
You of course know that printing a 35mm negative with a longer focal length restricts the image field don't you, and could be a contributing factor to perceived diffraction as a result of the image reflecting off the internal barrel of the lens - you know that - right?


I did a test for myself the other night.

I routinely print 5x7 from 35mm at a marked f/22 on my 75mm enlarging lens. The idea is that In need some dodging/burning time, so f/22 gets me to about 15 seconds, and a 5x7 print is small enough that diffraction shouldn't matter.

After I made my final 5x7 print, I tried one at f/8, which is only about 3 seconds exposure time. I have to admit that upon close inspection with the naked eye, and when comparing directly with the print made at f/22, the grain was sharper on the print made at f/8. It is a subtle effect but it's the first time I have ever tested and seen diffraction effects. I'm still not worried about it but it's on my radar as something that can make a subtle difference.

Bob Salomon
22-May-2011, 14:12
I did a test for myself the other night.

I routinely print 5x7 from 35mm at a marked f/22 on my 75mm enlarging lens. The idea is that In need some dodging/burning time, so f/22 gets me to about 15 seconds, and a 5x7 print is small enough that diffraction shouldn't matter.

After I made my final 5x7 print, I tried one at f/8, which is only about 3 seconds exposure time. I have to admit that upon close inspection with the naked eye, and when comparing directly with the print made at f/22, the grain was sharper on the print made at f/8. It is a subtle effect but it's the first time I have ever tested and seen diffraction effects. I'm still not worried about it but it's on my radar as something that can make a subtle difference.

It would be even better with a quality 50mm enlarging lens.

BetterSense
22-May-2011, 16:39
I have a 50mm El-Nikkor, but I usually use my 75mm so the enlarger head is higher.

Honestly the bit about 35mm 'restricting the image field' and the image 'bouncing off the lens barrel' doesn't make any sense to me at all.

tgtaylor
22-May-2011, 17:06
I replaced my 75mm El Nikkor with an 80mm El Nikkor last month. The 80mm is a 6 element 4 group design and will cover 6x7 whereas the 75mm is a 4 element design. I managed to acquire a mint 80mm w/bubble and box off an ebay auction for $61 including shipping.

Thomas

rdenney
22-May-2011, 20:53
Seeing it in a grain focuser isn't, of course, the same as looking at the print, unless the magnification of the print is similar to the grain focuser, which it usually isn't with large-format negatives.

And diffraction is less likely to contribute to visible fuzziness than faults, such as curvature of field, curvature of the negative, or misalignment of the enlarger. If stopping down overcomes the effects of those faults, then diffraction is a small price to pay.

But I took Kirk's statement to mean that the effects of diffraction under the enlarger were invisible, not that they were unimportant. They are visible enough given sufficient magnification. But for our low enlargement ratios, diffraction is unimportant. So, if that's what Kirk meant, then I agree.

Rick "who sees high-magnification fuzziness from Epson scans, too, but can't detect it in 16x20 prints" Denney

Jim Jones
22-May-2011, 21:08
[QUOTE=ic-racer;729330. . . With a grain magnifier, throw a 35mm TRIX negative in your enlarger with a 50mm lens. It is very informative. The grain in the center is sharpest wide open (usually, depending on your lens) and keeps getting worse as you stop down. Most usually F8 will be the farthest you can go without being annoyed by it. At f16 it is just a blurry mess. . . .[/QUOTE]

This is my experience, too. I think mushy grain is more unpleasant than the same amount of unsharpness in the negative.

Vaughn
23-May-2011, 07:24
I have a 50mm El-Nikkor, but I usually use my 75mm so the enlarger head is higher.

Honestly the bit about 35mm 'restricting the image field' and the image 'bouncing off the lens barrel' doesn't make any sense to me at all.

Does not make sense to me either. Just seems like if there is a problem enlarging a 35mm neg with a 80mm lens, then the same problem would happen to the 35mm sized center portion of a medium-format negative enlarged with a 80mm lens.

I usually enlarge my 6x6 negs with a 135mm lens if the print is 10"x10" or smaller. I use a Omega D5-XL enlarger and figure I am using the "sweet spot" of both the condensors and the lens for sharpness and even illumination.

Vaughh

Drew Wiley
23-May-2011, 09:05
Lens performance is also related to eveness of field, performance off center, etc. So
I routinely use "longer than normal" enlarging lenses for best results. For example, for
6x7 medium format I'll use my 150 Apo Rodagon, often wide open, and get better results than any dedicated MF enlarging lens I've ever tried. My "normal" for 4x5 is 180
to 240. Eveness of field can be fine-tuned by having on hand an assortment of custom
diffusers, but if you've got the headroom, half the battle is won by using longer lenses
in the first place.

E. von Hoegh
25-May-2011, 07:27
In the middle of contemplating the rapture last night, a thought struck me (why then I have no idea-but that is another thread perhaps).

I am well acquainted with how diffraction limits effect my negatives when shooting (and plan accordingly-I have tested this on many of my lenses and know where it strarts to influence sharpness).

But I have never noticed its effect when enlarging and sometimes I stop way down purposely to get long exposures to give me time to do complicated burning and dodging. Hmmmmm.....

I have an old Wollensak 4 3/8(?)" dialyte type enlarging lens that I use on 2 1/4". Looking through the grain focusser, I can see the grain become less sharp as I stop down past f16-22.