stopping down hurts?

[arthurking83]

Ok I've seen a few images around here where the exif says ... f=22 or similar.

diffraction!

There's usually no issue and the images have appeared fine, but(!!) I've been seeing a lot of lens tests/comparisons where the tester has noted things like "diffraction causes problems from f/16, as is to be expected..." or words to that effect.

It appears to afflict certain cameras worse than others, they all do it, but to what degree?

As I've read many lens tests proclaim it to be a problem, I tend to use a min f/11-13(f/16 on some occasions) for any landscapes where there's excessive light... many at f/8.

I'm wondering if any folks here are aware of this 'problem'?

I finally found a technical link that many folks can "visually understand" (meaning I finally see what they're on about :p).

THE TECHNICAL LINK! :D (http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm#)

the site is interactive, so you can see what diffraction does to an image, and explains technically what's happening within you camera(the sensor).

OK here's the catch! The sharper the lens the WORSE the diffraction appears!! :crzy:

It actually makes sense really, if your lens is sharp as a tack(hey Dowden, you still got that 70-200? :p) then you will notice any blurry abberation more than you would if the lens was already softish ;)

[Knawful]

Hmm, gunna have to experiment tomorrow me think's.
I usually shoot f2.8 - f11 with the 70-200 and havent really noticed to be honest.

[arthurking83]

I'd like to see some samples of a brick wall at f8 and f16(and smaller) just to see.

f/5.6 is supposed to be the sharpest setting on that beast..

I don't think I have a lens sharp enough to really make a definitive example...but all those MTF tests and info like that link can't be wrong :confused013

brick wall(maybe a paling fence?) is good because of the texture, and focal plane!

ps I'll stop posting tonight, and post my 1000th post tomorrow :action53: .... headache/cold is wearing me out... and it's only 12:30! :sad68:

[dejavu]

Diffraction is noticable from around f16 and up (exclusive). Now, the thing is why do you want to shoot at f22+?

The increase of DoF from f22 to f32 is absolutely minimal, even in macro. There are much more effective methods of achieving higher DoF without resorting to bumping the aperture up that high.

Be careful reading some of those image test online though. Unless the tests have been conducted with the strictest standards, the test will biase against high aperture settings since the longer exposure will be more susceptible to vibrations.

[xpantz]

Diffraction is noticable from around f16 and up (exclusive). Now, the thing is why do you want to shoot at f22+?

There are much more effective methods of achieving higher DoF without resorting to bumping the aperture up that high.



Got tips? :-)

[xpantz]

Got tips? :-)

hey I got one of my own. :-)

poking around looking for technique to increase DOF I came across a couple of pieces of softare to help you do the job.

it's like HDR for DOF

Helcion Focus
http://heliconfilter.com/pages/index.php?purchase#heliconfocus
and
CombineZ5
http://www.hadleyweb.pwp.blueyonder.co.uk/CZ5/combinez5.htm

I used Helcion to create this image... perfectly in focus from back to front.

http://img218.imageshack.us/img218/4751/20061030095926r8s4kj4.jpg

Created from these 6 images.. focus moving from back to front in steps.
All shot at f/11
http://img158.imageshack.us/img158/2802/img0021wf4.jpg
http://img209.imageshack.us/img209/2219/img0022wf1.jpg
http://img212.imageshack.us/img212/6366/img0023de8.jpg
http://img124.imageshack.us/img124/397/img0024uk9.jpg
http://img209.imageshack.us/img209/4544/img0025vs0.jpg
http://img53.imageshack.us/img53/2599/img0026ee0.jpg

I've only used the Halcion software so far but I believe the CombineZ5 does the same thing and is free. Halcion costs 30$ for the basic version and 70$ for the pro which can clone and copy bits from any of the source images to the processed image and a few other tricks... you can just live with the lite version I rekon.

Really easy to use.. just take a few shots with focus ranging from one side to the other... open them up in the Helcion softare and hit the process button.

if you can get your bug to sit still long enough you can have him from focus from eye to tail.

kinda feels like cheating but hey :-) it works!

[Knawful]

Cheer's X...Top info....:th3:

Now my puter will be even slower.
I'm like AK with his gadget's, only my vice is software.

[Knawful]

Shed wall pictures here (http://dowdenphotography.com/potw.html)

[TassieSnapper]

Since I am relatively new to photoshop CS, does it have a feature where you can combine different DOF images or different focus planes automatically or manually?

[arthurking83]

Diffraction is noticable from around f16 and up (exclusive). Now, the thing is why do you want to shoot at f22+?

.....

The point I was trying to establish, for anyone coming from film (me!) when shooting landscapes, I was 'taught' to stop down for DOF.

I suppose p&s(digital) folks probably never had this issue ... (dunno.. :confused013 .. I've never had one)

I have seen some folks shooting at f22, I felt compelled to point this phenomenon out.


My main concern with those MTF charts and lens comparisons are more about sample variation... more so, than controlled environments...
I have no reason to suspect (nor seen evidence to the contrary) that they are not done to a standard.... ie. consistent environments.
Further to that, many lens tests seem to report similar findings anyhow ... that Lens X is generally sharper than Lens Y, yet Lens Y may have better color or CA control, or it may feel less robust.. etc.... a pattern does emerge (only from what I've read on the lenses that I have any interest in!)

[xpantz]

Since I am relatively new to photoshop CS, does it have a feature where you can combine different DOF images or different focus planes automatically or manually?

not to my knowledge.... no... there may be a plugin somewhere but I havnt seen it.

photoshop does image stacking for HDR and there are plenty of methods for creating fake DOF blur (bokeh) in photoshop.

AFAIK if you want image stacking for DOF (to create sharp images... usually from macro shots) you need to use one of the programs I posted earlier.

both are easy to use and extremely effective.

[dejavu]

I guess without arguing over triviality (as some people on some forums do), DoF is DoF regardless of whether it is film or digital. The crop factor of the digital sensor changes that a little, but not much. When you shoot landscape, the proper way of getting the DoF you want is setting your lens to its hyperfocal distance, and composing the shot such that foreground interest is placed at the start of the hyperfocal distance.

Shooting at the hyperfocal distance gives foreground to inifinity sharpness. Well it appears to our eyes that everything is in focus. View cameras takes this concept further by allow shift & tilt to greatly increase foreground DoF. You can do similar thing with the Canon TS lenses.

In macro, to increase the DoF, try shooting with your camera's sensor parallel to the subject. While this is not strictly "increasing DoF", it ensures that the interesting parts fall within the focus plane.

Take this shot for example:
http://users.tpg.com.au/adsl98di/images/Insects/images/crw_1742.jpg

I could have shot the spider from an angle, but I wanted both eyes to be in the plane of focus. If you look carefully, you can see exactly where the plane of focus is. This is not always possible, since sometimes "paralleling" the sensor to the subject would result in crappy composition, but it's something to keep in mind, especially if that particular shot is of a documentary nature.

Anyhow, rules are there to be broken, but you need to know the rules to know how to break them properly. At the end of the day, if the shot looks good to you, then that's all that matters.... unless you are shooting for a client :confused013

[TOM]

diffraction is dependant on two variables

1. the distance fromt he lens, of the object affected by diffraction
2. the size of the lens being used

one cannot simply say, diffraction kicks in at fxx.

[Tannin]

~~ Huh?

Neither of those have anything at all to do with diffraction. Zip. Nothing. Nada.

[Kym]

Diffraction can work for you! The 'star' effect is due to diffraction.

f/16, 30 seconds, ISO 100, 43 mm

http://farm4.static.flickr.com/3427/3912178684_b5f6d83cf2_o.jpg (http://farm4.static.flickr.com/3427/3912178684_b5f6d83cf2_o.jpg)

[TOM]

Neither of those have anything at all to do with diffraction. Zip. Nothing. Nada.

the amount of diffraction is determined by lens size, and the amount of affect that diffraction has on part of your image, depends on how far that part of your image is from the lens. so for an image with a film camera (not that sure with a Digital Camera), i work on s=D/1600d where s is the diffraction limited size of object, D is the distance from the lens in feet, and d is the lens size (focal length/f stop). so if i am photographing the on the Sturt Reserve in Murray Bridge with a 35mm lens at f11, and the twin bridges are 1km in the background, i know that anything that is just over a half a foot in size or larger will be rendered sharply in my image. So as you can see, by using a larger aperture, we get better resolution and less diffraction, however our dof is limited. But just over half a foot (16) allows me to render the basic structure of the bridge, but rivets and the shape of the beams are not detectable.

so beams of light travel ina straight line, but when they are squeezed through a diagraphm opening, they divert. they still travel forward, but now at an angle, so obviously the further away an object is from the lens, the more that diffraction will affect said object. now with a Digital Camera, you can just chimp, but if you ever graduate to a larger format camera, you may not have that luxury. the above calculation doesn't have to be worked out every time, you will become familiar with these figures. i know that for max dof for most of my scenic shots, that f9.5 on my 35mm will give me great results.

as for lens size, Ansel Adams named his school "f64". this was an f stop that he often used. the lens that he was quite partial to was a 300mm lens, and he used this for alot of his work. Lens size = 4.6mm

[Steve Axford]

the amount of diffraction is determined by lens size, and the amount of affect that diffraction has on part of your image, depends on how far that part of your image is from the lens. so for an image with a film camera (not that sure with a Digital Camera), i work on s=D/1600d where s is the diffraction limited size of object, D is the distance from the lens in feet, and d is the lens size (focal length/f stop). so if i am photographing the on the Sturt Reserve in Murray Bridge with a 35mm lens at f11, and the twin bridges are 1km in the background, i know that anything that is just over a half a foot in size or larger will be rendered sharply in my image. So as you can see, by using a larger aperture, we get better resolution and less diffraction, however our dof is limited. But just over half a foot (16) allows me to render the basic structure of the bridge, but rivets and the shape of the beams are not detectable.

so beams of light travel ina straight line, but when they are squeezed through a diagraphm opening, they divert. they still travel forward, but now at an angle, so obviously the further away an object is from the lens, the more that diffraction will affect said object. now with a Digital Camera, you can just chimp, but if you ever graduate to a larger format camera, you may not have that luxury. the above calculation doesn't have to be worked out every time, you will become familiar with these figures. i know that for max dof for most of my scenic shots, that f9.5 on my 35mm will give me great results.

as for lens size, Ansel Adams named his school "f64". this was an f stop that he often used. the lens that he was quite partial to was a 300mm lens, and he used this for alot of his work. Lens size = 4.6mm
This doesn't seem right to me. The diffraction works from the iris to the sensor (film or digital), not from the subject to the iris. Am I missing something?

I find that f18 is the limit for my 180mm macro (at close distances). For my MP-E it is f16, which is the maximum. It is worth remembering that the effective aperture is also dependent on how close is the subject (angle of incidence to the lens). This really is irrelevant for most lenses, but for the MP-E it can change the f16 to f64 at 5x magnification. For some of the other lenses it may be a little less, maybe f13. This is for the 1Ds Mk3, but it would be less for a camera with higher pixel density. You really need to experiment a little to find what is best for your lens/camera combinations.

[DAdeGroot]

The point I was trying to establish, for anyone coming from film (me!) when shooting landscapes, I was 'taught' to stop down for DOF.


Diffraction existed in the film world too - on a 135 format (full frame in the digital world) body, you'll start to get diffraction from f/16. The issue with film is most people weren't shooting high quality, low grain film in order to notice it.

On an APS-C body, diffraction creeps in from f/11 onwards. You can usually get away with f/16 and not notice too much, but beyond that, and with a sharp lens you'll see softness compared to a wider aperture.

So while stopping down will get you more depth of field, you will lose sharpness past the diffraction point of your camera system. It should be noted that the point at which diffraction becomes noticeable is related to the size of your film/sensor and the lens construction. Thus, large format shooters using 8x10" film can happily shoot away at f/32, while small format (135 format) users cannot without getting a fairly soft image.

[arthurking83]

...

On an APS-C body, diffraction creeps in from f/11 onwards. You can usually get away with f/16 and not notice too much, but beyond that, and with a sharp lens you'll see softness compared to a wider aperture.

.....

in many cases this may be 'correct' but from what I've noticed, diffraction is more about lens design(design brief??) and less about the size of the sensor.

Tammy 70-200/2.8 is sharp at f/2.8 and starts to diffract at about f/8. Not very noticeable, but on a resolution chart(the cheapo downloadable version with warnings not to print it up on your home printer version :p) I've noticed a slight loss of very fine detail with this lens at f/8, compared to f/5.6 at least, and it looks closer in sharpness/detail at f/8 as it is at f/2.8.
Not an overly concerning issue tho, as the loss is very slight and not noticeable in a real image anyhow.

But I think Steve is on the right track, as someone(can't remember who now) pointed out to me a while back. When I use my old manual Tamron 300/2.8 it gets sharper from f/4 to f/8 and maintains that sharpness level at f/8 up to f/16(there goes the f/11 rule :p) but when I add the Tamron specific teleconverters, there's very good sharpness right up to f/32 where I simply ran out of light!(at 30sec exposure, and I was too lazy to delve any deeper into the curiosity). So as the TC's are added and stacked, it was mentioned that the iris is being moved further away from the film/sensor plane, and the 'diffracted image is now being relayed to the sensor via more lenses(hopefully not diffracting or altering the image any more).. but the images produced were definitely sharper at f/22 than at f/11, using various combinations of TC's on this lens. Subject to lens only decreased by a few inches at most, maybe 75mm, over a focus distance of 5-6 meters, so the subject to lens difference in distance could be regarded as irrelevant (or 0.015% difference). But the iris to sensor plane difference was huge with the addition of the TC's.

Then there's the design of the lens to be taken into account too.
Diffraction doesn't necessarily have to exist at smaller apertures, but I think must be a design compromise that the engineers have to deal with as best as they can :confused013
If the lens is of a smaller aperture design(such as those found in the MF/LF format, where a 135mm f/8 or 300mm f/8 prime lens is not uncommon, then the engineers don't have as much of a compromise to deal with as they would if the lens was an f/2.8, or f/1.4 type. There's obviously a very good reason why that 50mm f/1.4 lens only stops down to f/16, and not f/22 or f/29 as most f/2.8 lenses do.
I doubt very much that a fast lens couldn't be designed to delay the onset of diffraction till much later than the normal f/11 or whatever, but at what cost? Who's willing to pay $10k for a fast prime that's still as sharp at f/22 or f/29? I suspect no one really, as the lens's primary purpose would be for it's speed.

[Tannin]

Hoolie doolie that's a hard way of thinking about an easy subject Tom! To summarise, you are saying that you can .... no, that's too hard for me. My brain hurtz when I try to summarise what you just wrote. Let's do it the other way:

Diffraction depends on your f stop.

Wasn't that easier? And it also has the advantage of being 100% correct. Diffraction increases as aperture gets smaller; the higher the f/ number, the more the diffraction. For any given camera, no other factor is relevant. Diffraction depends on your f stop. End of story.

[DAdeGroot]

in many cases this may be 'correct' but from what I've noticed, diffraction is more about lens design(design brief??) and less about the size of the sensor.

Partially. Go back and look at the technical link you posted.

Where diffraction is occurring if the film plane is closer to the aperture plane you'll end up with less noticeable diffraction (but it's still there). If you move the film plane further away from the aperture, the light spreads more and thus you get more diffraction effect.

[Tannin]

Where diffraction is occurring if the film plane is closer to the aperture plane you'll end up with less noticeable diffraction (but it's still there). If you move the film plane further away from the aperture, the light spreads more and thus you get more diffraction effect.

Yes, but if you move the film plane you are altering the focal length. In fact, for every doubling of the distance between the film plane and the aperture stop, we have an increase in the f number of one stop unless we also alter the size of the physical aperture stop. (Technically, focal length is measured from the the rear nodal plane, but that detail need not bother us in this context.)

In other words, DA, if you move the film plane, you are by definition changing the aperture.

The moral of the story is: forget all the complicated stuff, the sums all cancel out - diffraction depends on your f/stop. Nothing else.

[arthurking83]

Yep! it's definitely related to both sensor resolution(more than size) and lens design.

I haven't fully tried to see any differences with the 70-200 Tammy between the D300 and the D70, and I may have erred with the f/8 shots I took.. MLU, multiple shots. remote(when it was working :() and focus confirmed with Live view.

But the 300/2.8 with TC's is the spanner in the works, and I suspect that other may see similar results with stacked TC's on their lenses too.
The F/22 shots on the Tammy with a 1.4x and 2x TC stacked are definitely crisper than the f/16 shots were.

What may cause confusion to members that haven't fully tested their gear, is that there is a general consensus that f/11 is the limit for an APS-C sensor, or a sensor with a x.yz pixel pitch, or whatever.. but that doesn't take into account the lens used, and as Steve mentioned, his macro lenses work very well up to and beyond f/11(his example of f/16 and f/18 are noted). I've seen similar results from the 105VR micro too.. where f/16 and even f/22 don't look overly diffracted.... as, say, the 10-20mm Siggy does.
I max my Sigma 10-20 at f/11 by default(exif in tact and you'd notice that 99.9% of my images from the 10-20 are at f/11. If I'd remembered to check camera settings :rolleyes: my last image taken with the Tammy 28-75 at f/11 would never have happened.. I generally stick to f/8 as maximum(or minimum depending on your preferred terminology) on that lens.

That's why it's important as Steve said, for those that think it's of value to them, to test their lenses(chimp it as TOM reckons :D) to see what variables they need to get their lenses work effectively.

I remember reading a quoted line of text by a very professional photographer and tester, where he said that the theory is only valid once the practice proves it to be true. Until then it's only theory(translated to mean useless in the real world if not proven to be true :p)

DOF and diffraction are two of those elements that haven't actually been proven by any formula... because a lens engineer can easily design one, to make them look as silly as they appear to be. What may work for one lens, may not work for another.

[TOM]

Tannin, lens size is most certainly the more accurate way to talk. F11 on a 35mm lens is a smaller aperture than f11 on a 75mm lens. lens size is lens size, no matter the focal length, no matter the format.

Okay, a quick summary...for landscape or shots where max dof is required, use a lens size between 3 and 5mm.

[Steve Axford]

Tannin, lens size is most certainly the more accurate way to talk. F11 on a 35mm lens is a smaller aperture than f11 on a 75mm lens. lens size is lens size, no matter the focal length, no matter the format.

Okay, a quick summary...for landscape or shots where max dof is required, use a lens size between 3 and 5mm.
True, but the diffraction effects are constant, irrespective of lens size. Of course this does not explain why some lenses show diffraction effects at lower fstops than others??

[TOM]

it's funny how there is this mystique surrounding some Leica lenses and the way that the images from these lenses have a "glow" to them. This is known as the "Leica glow", but this glow is caused by light bouncing off the diagraphm blades. It was something unique to these lenses, and it is highly unlikely that it was unintention, at least not the first lens designed with the inherant property.

[Tannin]

Ouch! First, "lens size" is a ridiculous term to use when actually you seem to mean physical aperture - most people would assume you are talking about focal length. Second, physical aperture ("size of the hole") doesn't control depth of field or diffraction, it is physical aperture in combination with focal length.

So either you keep two different factors in your head and do sums with them .... OR you recall that, as it happens, there is a very convienent single number already calculated that tells us all we need to know about diffraction. It's called the f/ratio.

[TOM]

lens size is a term that has been used for decades, and i didn't make it up. it is a logical term that everyone should be familiar with. if you use f16 with a 15mm lens, and f16 with a 35mm lens, diffraction limitations/affects will be different.


doesn't control depth of field or diffraction, it is physical aperture in combination with focal length.

?? that is exactly what lens size is...focal length/aperture=lens size. a 3mm lens size on a 300mm lens is exactly the same size as a 3mm lens size on a 35mm lens.

[Tannin]

Bahh. Focal length divided by aperture .... that's what an f-ratio is. That's how you define it.

[Kym]

The diffraction works from the iris to the sensor (film or digital), not from the subject to the iris. Am I missing something?

Concur 100%


diffraction depends on your f/stop. Nothing else.

Again I concur 100%


Bahh. Focal length divided by aperture .... that's what an f-ratio is. That's how you define it.

And we have a good write up here: http://www.ausphotography.net.au/forum/showthread.php?t=24079

The reason that both the halving and doubling and the smaller numbers mean more light things make sense is that the f/stop is a ratio. The ratio is between the diameter of the aperture in the lens and the focal length of the lens. The focal length is generally measured in millimetres (mm). On a 50mm lens, f/2 is saying that the diameter of the aperture is 25mm. The ratio is this 50/25 = 2.

And http://www.ausphotography.net.au/forum/showthread.php?t=24050

f/stop is a representation of area being the amount the lens is open to light. So f/1.4 is twice the light of f/2, which is twice the light of f/2.8 etc.
(*Warning* Maths: Approximately... 1.4 squared is 2; 2 squared is 4; 2.8 squared is 8 and so forth - so the term stop means half or double the area and therefore amount of light; each stop number is multiplied by 1.4 of the lower previous number; this is because 1.4 (approx) is the square root of 2).

[TOM]

Bahh. Focal length divided by aperture .... that's what an f-ratio is. That's how you define it.

i don't disagree with that, but it is a ratio of a given focal length. so diffraction will not affect lens x at f16 the same as it will affect lens y at f16 (given that x and y are different focal lengths), just as dof at f16 will differ on lens x and y. all i'm saying Tannin is that with a given focal length of a lens, a 3 to 5mm lens size will usually give the best the best compromise between dof and diffraction. so a 15mm lens at f5 will give closer results (concerning diffraction) to a 35mm at f11, than both lenses both at f16.

[Darvidanoar]

I've just finished reading "Understanding Exposure" by Bryan Peterson and noted throughout the book he constantly stopped down to f/22 when shooting an image requiring a large DOF (eg. landscapes). Perhaps that's where a lot of this is coming from?

I mostly shot landscapes at f/8 to f/11 previous to reading this. I am now starting to go up to f/16 but am still unsure about the validity of shooting at f/22. Especially when many lenses perform best from around f/8 to f/11 ... :confused013

[arthurking83]

.....lens size is a term that has been used for decades, and i didn't make it up. it is a logical term that everyone should be familiar with. if you use f16 with a 15mm lens, and f16 with a 35mm lens, diffraction limitations/affects will be different.

?? that is exactly what lens size is...focal length/aperture=lens size. a 3mm lens size on a 300mm lens is exactly the same size as a 3mm lens size on a 35mm lens.

Someone reading that will now assume that the limits of diffraction will therefore be the same for two lenses of the same focal length using the same aperture, and going by the theory, which could be true. But the theory doesn't take into account the way in which the engineer intended the lens to work.

if you're a regular reader of Bjorn Rorslette's writings, you may be interested in doing the tests yourself to confirm that.. and once again, it goes back to lens design.

I don't have a 50mm f/1.8 to compare against, but I do have other lenses of similar focal lengths to compare with, and most of them have very visible differences at which aperture value they begin to show signs of diffraction.

Of the two Tamron zoom that I have I know that at 35mm and 50mm the longer 28-75mm version is better at f/11 than the newer design(digital only) 17-50mm lens. So setting both lenses to the same focal length(in my case 35mm and 50mm, I get more diffraction at f/11 with the 17-50 than I've seen with the 28-75mm. So your reading or those decades of theory have all amounted to nothing(in my non professional opinion).. and, once again, as Steve Axford said.. it's kind of more important that each lens is tested to see how they each perform if this issue is going to be of concern to the operator.

I now have 5 lenses that cover the 50mm focal length, and if I could find the energy to test them at various apertures to demonstrate this effect, I would.
But I feel it's a useless exercise too, as this would only be pertinent for either the D300 or D70s, as there is also the different sensors to take into account and their designs too with the various filters used over the sensor, I suspect that the AA filter may also have a say in how each lens works for a specific sensor design.(ie. a D70/D70s may have different characteristics to a D100, D40, or any other Nikon camera built around the D100 sensor, as the AA filter is surely differently specified).

For partial confirmation of 'my theory' :D have a read of Bjorns ratings of the 58/1.2 noct against the 55/1.2 lenses. The 58/1.2 is an excellent performer at f/1.2 and begins to decline at f/4 suffering visibly at f/8, whereas the 55/1.2 is the opposite, where it only begins to start working well at f/2.8 and works well at up to f/8, where it begins to decline past that point. Two seemingly similar lenses(similar enough, for it not to make any difference at least) yet they work totally differently at similar lens sizes. Obviously the design brief for each lens was different, of which introductory pricing is the key issue.

That's one of the reasons I now hate the theory of photography, and why I never saw any benefit in using hyperfocal distances too, and other stuff like that.
Theory is great for those times when there's nothing else to do but sit and reflect. But when it's clearly proven to be wrong because it doesn't take into account a human factor.. it only makes for humorous reading.

Photography is not about theory... it's more about getting out there and just getting an image formed onto some medium that is capable of recording one .... whatever way you can.

[Tannin]

Nice rant, Arthur, but there are two possibilities here:

1:You are wrong. Whatever you are seeing with your various assorted lenses, it is not diffraction, because diffraction does not behave like this.

2: The science of optics as presently understood is wrong, as is our scientific understanding of the quantum physical behaviour of photons. Throw away your textbooks, and expect all sorts of items from electronics through to motor cars to stop working.

[swifty]

i don't disagree with that, but it is a ratio of a given focal length. so diffraction will not affect lens x at f16 the same as it will affect lens y at f16 (given that x and y are different focal lengths), just as dof at f16 will differ on lens x and y. all i'm saying Tannin is that with a given focal length of a lens, a 3 to 5mm lens size will usually give the best the best compromise between dof and diffraction. so a 15mm lens at f5 will give closer results (concerning diffraction) to a 35mm at f11, than both lenses both at f16.

Been following this discussion with interest.
Tom: though I follow what you're saying, that you're trying to describe aperture in absolute terms rather than a ratio (f-number) which would vary the actual size of the aperture diaphragm depending on the focal length of the lens. This does logically makes sense since we're talking about squeezing photons through a small hole and the absolute size of that hole should matter, however at least according to this:
http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm
scroll to the technical notes at the bottom, the larger actual aperture opening of a telephoto lens is offset by the greater distance the light must travel in the longer focal length lens so it seems it's appropriate to use the ratio aperture approach and not actual aperture size as it wouldn't matter.
Unless I missed it I'm also wondering why no one mentioned pixel pitch vs airy disc on this discussion. It seems fundamental when talking about diffraction.

AK: I wonder how much of what u mentioned about lens design and performance at different apertures is just that - lens characteristics, and not diffraction related. At least in theory it seems diffraction will affect a lens designed to perform best at wide apertures as well as one designed to perform best stopped down equally. But what diffraction robs will be relative to that lens and the normal gains with dof as u stop down and IMO cannot be compared to another lens of the same focal length.

[TOM]

Photography is not about theory... it's more about getting out there and just getting an image formed onto some medium that is capable of recording one .... whatever way you can.

i agree with that 100%, at the end of the day it is the print hanging on the wall that matters. but you need to know how to get there, and if you have a Digital Camera and you blast away you have a good chance of getting the right shot, provided you have time to do it. but if you upgrade to a larger format, you may not have the luxury to chimp. Plus, some people, and i'm one of them, do enjoy the craft of taking an image as much as i do looking at the final image. if i am armed with the theory to allow me to work out how to maximise any effect that i wish to acheive in an image, then where is the harm. i am home most evennigs looking after my children, so i can choose to read, or watch Deal Or No Deal. I like to read, but because i read, it doesn't diminish my love for getting out and making images either. if i shoot with film and digital, or i shoot hyperfocal and infinty, i then can make an informed decision on what works best for me.

of course a lens is not a projector and diffraction affects from the diaphragm to the film plane, but the further away an object is, the less ability that the lens has, due to diffraction, to render those objects acceptably sharp. the diffraction limited spot size increases the further away from the lens you get. in another post on here, i posted two pics where one is focused at inifinity, and the other at hyperfocal. the image focused to inifity should really render the object (about two kilometres away) sharper, but due to diffraction, the object was rendered about the same in both images. as a film shooter, it is important for me to know what i am going to get before i press the shutter, especially when it can cost about $5 a shot.

swifty, i get the point regarding the telephoto. my calculations however are usually made working on infinity focus, as i almost alway am at infity when at smaller apertures where diffraction is more an issue.

so why do I find "actual lens size" an important figure:

compose the image

look at the front of the lens, and observe the size of the aperture. move the aperture ring so that the aperture is no small larger than 4mm. (SLR users will have to stop the lens down manually, or just do the maths in your head)

focus to infinity

That's it. everything in your image that is 4mm or larger, front to back, will be acceptably sharp. now, using the same aperture, shoot using the hyperfocal scale on your lens and compare the results. the infinity focused shot should be better. however this process does not take diffraction into account, and the further on object is from the lens, the more it is affected by diffraction (the greater the diffraction limited spot size). with a lens size of 3 to 5mm, diffraction should be well controlled.

there are variable where this process is not ideal, but it should work most of the time.

EDIT: BTW, this can be used with any lens, even zoom lenses with variable apertures. it applies to small, medium, and large format, wide angle, normal, or telephoto lenses.