Can someone here explain to DSG why his theory of diffraction is backwards.

sanity back. Me I just want to get out and take pics. As I've said before great photographers take great pics on almost anything and bad tradsmen always blame their tools
best
--
Geoff Roughton

'Always look on the bright side life...'
http://www.photo.net/photos/Geoff_R

Zone8 said:

If the camera + lenses can produce the quality you want for the
images you want to pursue, just get on and enjoy taking them.
Life's too short to get so involved in extreme technicalities. No
one will ever produce the perfect technical exercise, let alone
perfect technical image that pleases.

Click to expand...

The start of the link to the Puts article and this crazy diffraction discussion came from docmass and his mtf testing of LF lenses with his movements body. I had mentioned that I thought it would take a better group of lenses than regular LF lenses to fully resolve the SD_. At that point I threw some numbers around based on my observations of what it takes to fully resolve the SD, and also from the kodak formula on the Puts link.

The only reason I even brought it up in the first place is most LF lenses are just not sharp enough to fully resolve the SD and I felt docmass should know that before laying out more $ for lenses he would not be happy with in the end, especially when the future cameras come out and are even more demanding on lenses. After our back and forth conversation is about the time when you know who jumped in and started questioning what I was talking about and questioning the puts article.

Personally I like to have a good feel for where my gear will operate the best, but for me its several film cameras, LF, MF, dslr, 35 rangefinder etc etc.

The fact is a 35mm Leica or Hexar and microfilm can match a 645 with acros, and can beat even 6x9 when comparing a 35mm sharp 5400 dpi scan to a 6x9 epson scan.

Its all a system approach and that is what I was trying to explain to docmass. Just because you have a 54lp/mm lens and a 54lp/mm sensor or film does not mean you will resolve a final 54lp/mm . Its more like 35-40 depending, and even sharp lenses will degrade sharpness according to how much they are stopped down.

All of that malarky above combined with standard enlargement rules just helps me to know what the limits of my different systems are in a way that if I know what my end goal is in print, and how I expect to shoot, I have a better feel for what camera to grab.

Its almost like you know the sweet spot of a certain lens, but doing these sort of analysis allows me to sort of have a sweet system for particular conditions and goals.

Still you are right. Go out and shoot and enjoy.

Zone8 said:

It's akin to a letter to the editor of a computer magazine I read
several years ago. The reader/writer said he was an engineer who
serviced both PCs and Macs and he had come to the conclusion that
Mac users bought their machines to use, whereas the majority of PC
owners spent most of their time trying to get them to go faster.
(I use a PC and that made me stop, think and amend my ways!)

Think about it!

--
Zone8
The photograph isolates and perpetuates a moment of time: an
important and revealing moment, or an unimportant and meaningless
one, depending upon the photographer's understanding of his subject
and mastery of his process. -Edward Weston

Click to expand...

--
http://www.troyammons.com
http://www.pbase.com/tammons
http://www.troyammons.deviantart.com

I well know that images on a 5x4 (which I used for many years and still have) do not stand up to same magnification compared to say the 2.8 Elmar on a M3 Leica but of course, the ability to use movements, get under the dark-cloth for a snooze (or away from the midges, although about 4,000 joined me once on Dartmoor, so I gave up as ran out of hands to scratch whilst trying to focus) but one knows the 5x4 only needs a 4X enlargement to make a 20"x16" print.

The reason I now use the SD9 for all work, even though I still have my film cameras (5x4 field, 6x9 Mamiya) is that as I exclusively make prints via Epson inkjets, with pigmented MIS inks, there is no tangible difference in prints originating from 5x4, 6x9 or the Foveon sensor inside the SD9. I am happy for them to be exhibited alongside each other.

So, have given up worrying and use the Sigma SD9 + PhotoShop 7 (where I can also simulate movements if so wished) to now concentrate on producing my prints. Time, for me, is of the essence. I have wasted too many years and time worrying about technical niceties - however, don't assume that I do not achieve the highest quality - years in the darkroom plus teaching at high level put me in the right position to adopt digital techniques, after initial deep practical research.

IMHO, those producing the best quality are themselves experienced in the wet darkroom - they don't play about with digi techniques - they simply adopt them to replicate what they would have done in the darkroom - meaning they know what they want and only need to translate. Some advantages include being able to work on Highlights, Midtones and Shadows independently (although there are some overlaps as its a straight line in reality).

Nuff said - back to my printing
--
Zone8

The photograph isolates and perpetuates a moment of time: an important and revealing moment, or an unimportant and meaningless one, depending upon the photographer's understanding of his subject and mastery of his process. -Edward Weston

I have done all sorts of photography, but its mostly combined with digital printing now. I got carried away with digital for quite a while like everybody else, but now I am back into the tactile feel of film light table developing etc and especially back into B+W film. Digital is just not quite there compared to B+W film IMO.

I wet enlarged a lot about 20 years ago, but got distracted for quite a while. Recently I have gotten into all sorts of alternative processes and find that more interesting I guess because i am into art/painting mixed media etc more than just photography. With that the typical approach of an inter negative, ortho film for the contact negative is a PIA. I have recently found a digital transparency negative works best and you can create those from a dslr or a scan of any sort. Luckily I have a friend with a huge printer and keeps a large roll of transparency film around.

Link......

http://unblinkingeye.com/

--
http://www.troyammons.com
http://www.pbase.com/tammons
http://www.troyammons.deviantart.com

In the past I have been there, done that and got all the T-shirts too . I developed (pun intended) specialist formulae and specialist products as somewhat inventive sort of nature, so I do understand the interest in things like aspects of optics and suchlike technical matters. Delved into them over many years.

Interestingly, when I was into Platinum Printing, yonks ago, I used a certain Watercolor paper, which I now use for my pigmented ink prints. Things come round in a circle all right!

When I was much younger, Bromoil and such processes were considered by the vast majority to be "Not photography but muck oil, Laddie". I have pulled up photographers deriding inkjet prints and lauding "traditional" Bromoil prints and revealed to them some home truths. In reality, they are the same in the end. What a strange world we live in where truly, nothing is new.

I think I could reasonably claim to be a master photographer in the darkroom and that's why keen enthusiasts came to me for hands-on tuition for over 30+ years. Nowadays, I use digital, for which I have translated (through LOTSsssss of practical research) - that's my reasoning to just now get on with personal picture making.

I encourage anyone to use whatever they will - and will help them if so required to achieve the best quality without having to do all the years of research I put into photography. Principles remain the same, whatever the media chosen.
--
Zone8

The photograph isolates and perpetuates a moment of time: an important and revealing moment, or an unimportant and meaningless one, depending upon the photographer's understanding of his subject and mastery of his process. -Edward Weston

Its some neat stuff too. Go to the artist inspiration pages.

http://www.lazertran.com/artists/lisa_undercoffler.htm

Interesting that there is so much out there now, but in reality i am drawn more to the old processes. Right now I am into the new cyanotype process (Ammonium Oxalate) and gum prints. A little van dyke too. I enjoy gum printing the most, and have made several attempts to do color sepraration prints combining cyanotype and a couple of gum color, but have not quite gotten there. Its a long process and some part of daily life always gets in the way.

--
http://www.troyammons.com
http://www.pbase.com/tammons
http://www.troyammons.deviantart.com

Stopping down may decrease diffraction but focus s determined by the convergence of rays on the focal plane. regardless of f stop, the plane of focus (barring aberrations and other phenomena that have particular traits, such as lens flares etc) should still be in tight focus because those are rays that are hitting the focal plane straight on. If f2.8 and less equals soft focus then what good is it? I have used leicas, hasselblads, pentax, nikon, & canon film cameras and I never felt that I couldn't shoot at 2.8 because my focus would be soft. The sigma seems to give me soft images at times. A little too often for my taste, as anything over ISO 100 is treading into deep alligator water. On the plus side, it does some great things as far as information capture that film could never do, (with film, a flash blowout is just that, but with my SD10 I have had flash blowouts and then after doctoring and making the pix B&W, i have gotten startling detail).

Kerplunk said:

On the plus side, it does some
great things as far as information capture that film could never
do, (with film, a flash blowout is just that, but with my SD10 I
have had flash blowouts and then after doctoring and making the pix
B&W, i have gotten startling detail).

Click to expand...

Try telling that to some of the guys on the open talk forum...And you'll be called a Sigma zeolot!...Those Bayerites just dont have a clue what their missing!

Regards

DSG

--
http://sigmasd10.fotopic.net/

Here is my grade school take on this - it may shed some light on the subject, pun intended.

Diffraction is defined as the slight bending of light as it passes an edge of an object. The edge at issue is the aperature blades - the light bending causes distortion or a lack of sharpness. At times the diffraction of a light wave is cancelled out by the diffraction of another equal but opposite light wave. All lenses have some level of diffraction, and usually a lens fully stopped down has lots of diffraction, relative, and that is why resolution is lost. The bending light waves are interfering with one another when they hit the focal plane, leaving mush, for lack of a better term. The more a lens is stopped down, the fewer light waves that hit the plane. Less light goes through a smaller hole. Both a wide open lens and a stopped down lens have diffraction (distortion) but the fewer light waves amounts to less resolution. Think of it as more mp equals more data and less mp equal less data. Then throw in the diffraction distortion.

Now to the other issue going on - a lens wide open has more light waves going through more glass at different speeds experiencing more abberations. More abberations, like more diffraction, results in a less perfect image being formed at the focal point. This also results in less resolution, because the light waves hitting the focal plane are also mush, not being perfectly aligned.

It is my understanding that the center portion of a quality lens of appropriate design suffers from the least amount of abberation. As I understand it - it is because the speed of the light waves remains more consistent, truer, hence less abberations and more resolution. The light waves hitting the focal plane are closer to being perfectly aligned. Therefore, by stopping a lens down one excludes abberation.

In a perfect world, assuming all abberations can be corrected a wide open lens will have the least amount of material diffraction, but as the lens is stopped down the significance of diffraction becomes more material. Now we know a lens w/o abberations is not possible. Lens designs and now coatings have been created to limit abberations like coma, spherical, cromatic, curvature ... this comes at a price and weight factor as well as other trade offs.

In summary, in a real world one has to compromise between minimizing abberations and diffraction. So, for most lenses stopping down increases resolution to a point, that caused mainly by abberations, but after the sweet spot the increased diffraction effect results in loss of resolution.

You are both right. Remember, this is my grade school take on the subject.

With a BS in Physics, an MS in Microwave Enginering, and 20 years as an infrared and microwae test engineer (both subject to diffraction), I will try to give a layman's view that should help explain why DSG is wrong.

As Mg has said, light passing by an edge is bent. Think of light as being attracted to the edge when close, so that as light passes the edge, the light closest to the edge will go off in a slightly different direction.

For the sake of visualization, we will say that the light within 2 wave lengths of the edge of an aperture is bent. This may be too big or small, and cetainly is a graduated effect, but for our purposes of visualization, it will work fine. So, the bent light is in an annulus at the edge of the aperature, and has an area that is determined by the circumference of the aperture and the 2 wavelengths of annulus width. For an F8 aperature, this area is a very small, insignificant percentage of the total area of the aperture.

As the aperture is is reduced, f number incresed, the annulus percentaage of the total area gets larger, until the area of the annulus is a major percentage of the light pasing thru the aperture. At this point, the image now has lots of light falling on the focal plane that is not where it should be, and hence, soft immages.

DSG, you sound an awful lot like Susie/Jarvic/........
--
Cynthia, Prescott, AZ

AdmiralSchnitz said:

As the aperture is is reduced, f number incresed, the annulus
percentaage of the total area gets larger, until the area of the
annulus is a major percentage of the light pasing thru the
aperture.

Click to expand...

Did I read anus ???

(Maybe that is why my photos look like s* t ???)

--
Thanks for reading .... JoePhoto

( Do You Ever STOP to THINK --- and FORGET to START Again ??? )

Kerplunk said:

Stopping down may decrease diffraction ...

Click to expand...

You've got that backwards.

Kerplunk said:

... but focus s determined by
the convergence of rays on the focal plane. regardless of f stop,
the plane of focus (barring aberrations and other phenomena that
have particular traits, such as lens flares etc) should still be in
tight focus because those are rays that are hitting the focal plane
straight on.

Click to expand...

"Rays" are not a useful model when diffraction comes into play. You need "waves".

j

is here: http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm

--
'Do you think a man can change his destiny?'
'I think a man does what he can until his destiny is revealed.'

AdmiralSchnitz said:

With a BS in Physics, an MS in Microwave Enginering, and 20 years
as an infrared and microwae test engineer (both subject to
diffraction), I will try to give a layman's view that should help
explain why DSG is wrong.

Click to expand...

Er...I'm not wrong and your explanation is simply just stating what I have been saying all along, but in a far more elequant way.

AdmiralSchnitz said:

As Mg has said, light passing by an edge is bent. Think of light
as being attracted to the edge when close, so that as light passes
the edge, the light closest to the edge will go off in a slightly
different direction.

Click to expand...

As a Physisist I'm sure you know that diffraction even effect things in the non-micro world...For instance every time you walk through a doorway your direction of travel through the door is diffracted, only by a very, very small amount, but you are diffracted!

AdmiralSchnitz said:

For the sake of visualization, we will say that the light within 2
wave lengths of the edge of an aperture is bent. This may be too
big or small, and cetainly is a graduated effect, but for our
purposes of visualization, it will work fine. So, the bent light
is in an annulus at the edge of the aperature, and has an area that
is determined by the circumference of the aperture and the 2
wavelengths of annulus width. For an F8 aperature, this area is a
very small, insignificant percentage of the total area of the
aperture.

As the aperture is is reduced, f number incresed, the annulus
percentaage of the total area gets larger, until the area of the
annulus is a major percentage of the light pasing thru the
aperture. At this point, the image now has lots of light falling
on the focal plane that is not where it should be, and hence, soft
immages.

Click to expand...

You are simply stating the obvious, but I love the way you put it...Makes my explanation look incomprehensable!

AdmiralSchnitz said:

DSG, you sound an awful lot like Susie/Jarvic/

Click to expand...

Not guilty!...Just because Suzie once had Sigma is one of his (her?) many user names is no reason to associate me with him (her?).

Regards

DSG

--
http://sigmasd10.fotopic.net/

JohnClif said:

is here:
http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm

Click to expand...

Thats is a good article!...I particulary like the excellent diagrams.

But it seem he has totally neglected to mention how lens abberations effect lens sharpness at wider apertures and therefore the point where a lens attains its sharpest performance (the "sweet spot") is almost always a compromise, between abberation and diffraction and to reach it nearly always means stopping the lens down...However exactly how much you need to stop down to reach the sweet spot varies from lens to lens, depending on how well abberations are controlled in that particular lens.

AFAICT though it is virtually impossible to build a lens that has no abberations whatsoever and this being the case, it makes virtually all lenses sharpest when stopped down at least by one stop but often several, and not WO as the article above suggests.

Regards

DSG
--
http://sigmasd10.fotopic.net/

You sound like Susie/Jarvic/.... because you both insist on hanging onto absurly rediculous ideas. There just arn't many people out there that are like that.
--
Cynthia, Prescott, AZ

AdmiralSchnitz said:

You sound like Susie/Jarvic/.... because you both insist on hanging
onto absurly rediculous ideas.

Click to expand...

What absurdly rediculous ideas???...Your "ideas" were exactly the same as mine in your earlier reply so surely your not now saying that your ideas are absurdly rediculous too?

AdmiralSchnitz said:

There just arn't many people out
there that are like that.

Click to expand...

Well you must be like that for a start...As your ideas are the same as mine!

Regards

DSG

--
http://sigmasd10.fotopic.net/

DaSigmaGuy said:

AdmiralSchnitz said:

You sound like Susie/Jarvic/.... because you both insist on hanging
onto absurly rediculous ideas.

Click to expand...

What absurdly rediculous ideas???...Your "ideas" were exactly the
same as mine in your earlier reply so surely your not now saying
that your ideas are absurdly rediculous too?

DaSonyGuy said:

There just arn't many people out
there that are like that.

Click to expand...

Well you must be like that for a start...As your ideas are the same
as mine!

Click to expand...

Hi! I think Dom's explanation a bit back was very to the point:

'there are two limiting factors here, aberations that can generally be described as a wavefront error, this usually improves as you stop down.

On the other hand there is diffraction that obviously gets worse by stopping down and will also limit resolution.

It should be easy to show that there is a f/stop where both of these effects are equally limiting to the lens resolution, this would be the sweetspot of the lens. Wider open aberation will be limit, stopped down beyond diffraction will be the limit.'

This says it all, doesn't it? Anyway all I need to know to understand 'the sweet spot' - then all I have to do is find it on my particular lens :-7

Ole
--
http://www.pbase.com/thofte

Sweet spots are not always the same though.

On top lenses most are as sharp WO as they are at F4 and by F8 you are losing some rez although you would never see that loss on a SD. Basically the sweet spot is the entire range from WO to F8.

Lenses like the latest Leica 35mm asph lens, some contax, the latest zeiss lenses, digitars etc. are a few

Superior optics rules in my book.

Now Sigma needs to step up to the plate with some upgraded optics.

--
http://www.troyammons.com
http://www.pbase.com/tammons
http://www.troyammons.deviantart.com

DaSigmaGuy said:

But it seem he has totally neglected to mention how lens
abberations effect lens sharpness at wider apertures

Click to expand...

The only "total neglect" was you failing to read the article carefully to the end. (Hint: look in the section called "Notes on Real World Use in Photography")

DaSonyGuy said:

virtually all lenses sharpest when stopped down at least by one
stop but often several,

Click to expand...

Here is where Alf-speak leaves the more pedantic among us reaching for Dom's headbanger gif. The common definitions of "often" and "several" don't seem to match your usage.

DaSonyGuy said:

and not WO as the article above suggests.

Click to expand...

The article suggests nothing of the kind. In fact, it's primary purpose is how to find the point where diffusion begins to dominate even DOF. Even once you are past the len's optimum aperture, you may wish to stop down more to get additional effective sharpness at points other than exact focus point. But there is a point where stopping down more doesn't even buy effective DOF because you are now diffusion dominated.

--
Erik