Going over to the dark side: beyond f/16

[Steve Axford]

This lens to lens variation may not be surprising as the effects of diffraction are proportional to the diameter of the lens. A larger diameter lens will produce a tighter diffraction pattern allowing closer pin-points of light to be resolved into 2 distinct points whereas a smaller diameter lens will not be able to split the 2 point sources (Dawes Limit or Rayleigh Criteria). Note that in astronomy, aperture means the diameter of the main objective lens or mirror of the telescope and not the focal ratio or F stop.

Have a look at this nicely presented article on the resolving power of a telescope.

http://www.rocketmime.com/astronomy/...vingPower.html

Cheers

Dennis

So isn't the iris size the same as the objective lens size?

[Tannin]

They would be equivalent in this case, Steve, or so I understand it. A camera's aperture diaphragm, for practical purposes, makes the diameter of the lens larger and smaller.

Arthur, I'll buy that theory, at least as a starting point.

Imagine a simple lens. A light beam at the top of the glass is bent such that it passes through a central point and then continues until it strikes the bottom of the film. A ray passing through the bottom of the lens also goes through that central point and winds up at the top of the film. Let's say it's an f/4 lens.

Now, suppose an aperture diaphragm occluding 50% of the light to put us at f/5.6. It will be, essentially, a disc with a hole in the middle. If we place the disc close to the glass at the front of the lens, it will be a large one. Or we could place it close to the central point where the light rays cross over, and it could be quite small. Is this taking us in the right direction?

[ameerat42]

...Note that in astronomy, aperture means the diameter of the main objective lens or mirror of the telescope and not the focal ratio or F stop...

Cheers

Dennis...

Note that EVERYwhere "aperture" means the diameter [...being used of the main objective lens or mirror...] and not the focal ratio or F stop.

Just had to pick you up on that one, Dennis

[Tannin]

Hmmm .. I've just been reading up on what the modern approach is in tese days of high-resolution digital sensors. Apprently, it is as follows:

"Any good modern lens is corrected for maximum definition at the larger stops. Using a small stop only increases depth; beyond a certain point definition is actually impaired"

Some chap called Adams. I'm told he is quite good.

[nardes]

So isn't the iris size the same as the objective lens size?

Hi Steve

Only when wide open, but not when stopped down to e.g. F16.

The difference in the size of the Airy Disc is based upon the Aperture of the lens, either wide open or stopped down. But, as we can see from the screen capture below, a 400mm lens at F16 has an effective aperture of 25mm whereas a 50mm lens at F16 has an effective aperture of only 3.13mm, a significant difference.

This is one of the reasons that astronomers crave larger diameter (aperture) telescopes; they want the ability to resolve more detail.

Cheers

Dennis

[Steve Axford]

Dennis, I think that when you stop down then that is the same thing as reducing the size of the objective lens.

The reason for the difference in resolving power in astronomical and camera lenses, is that we are measuring different things. In the case of a astronomical lens we have a fixed object at a fixed distance. This object is effectively, infinitely small and the bigger the lens the more effectively we will be able to distinguish it from its neighbours. In the case of a camera lens, we are looking at a variably sized object at variable distances. The resolving power is a measure of how much detail we can see per field of view. So, we can compare a 50mm lens looking at an object at 10m, and a 500mm lens looking at the same an object at 100m (not sure if those distances are right, but you get the idea). This means that the 50mm lens can have more resolving power than the 500mm. This would make no sense in astronomy where a bigger lens always has more resolving power than the smaller (provided the lenses are roughly equal technology).

[nardes]

Note that EVERYwhere "aperture" means the diameter [...being used of the main objective lens or mirror...] and not the focal ratio or F stop.

Just had to pick you up on that one, Dennis

Hi AM

Not quite sure about this Am, as often when we refer to the shooting Aperture as e.g. F2.8, F5.6, F16 etc. for the Aperture Setting e.g. I shot this at a Shutter Speed of 1/250th sec, Aperture at F8 at ISO400, the use of "Aperture" does not describe the native wide open Diameter of the lens, but the stopped down Focal Ratio of the lens?

I never say I shot this at a lens Aperture of 72mm, or 52mm which is the lens Diameter value?

However, I guess we all understand the use and meaning of the term Aperture in the context of astronomical telescopes and photography lens settings so no big deal?

Cheers

Dennis

- - - Updated - - -

>snipped stuff here.

Another thing that I have noted, and now had confirmed by internet articles, is that different lenses have different diffraction profiles.

>snipped stuff here.

Dennis, I think that when you stop down then that is the same thing as reducing the size of the objective lens.

The reason for the difference in resolving power in astronomical and camera lenses, is that we are measuring different things. In the case of a astronomical lens we have a fixed object at a fixed distance. This object is effectively, infinitely small and the bigger the lens the more effectively we will be able to distinguish it from its neighbours. In the case of a camera lens, we are looking at a variably sized object at variable distances. The resolving power is a measure of how much detail we can see per field of view. So, we can compare a 50mm lens looking at an object at 10m, and a 500mm lens looking at the same an object at 100m (not sure if those distances are right, but you get the idea). This means that the 50mm lens can have more resolving power than the 500mm. This would make no sense in astronomy where a bigger lens always has more resolving power than the smaller (provided the lenses are roughly equal technology).

Hi Steve

Not sure if we are discussing the same issue in this overall topic, but I was responding to your post above (QUOTE: "is that different lenses have different diffraction profiles"). In this case I assumed that you meant different lens diameters and not just design, # of elements, etc. and as the diameter of a lens is one of the critical factors related to the diffraction profile of the lens, which is borne out by the formula in the attached screen capture, then yes, different lenses (as in different lens diameters) will have different diffraction profiles.

However, if your meaning of "different lenses" referred to lenses of the same diameter but different design, # of elements, etc. then there is no issue.

Cheers

Dennis

[Steve Axford]

I did mean lenses of different designs, but I think you may be missing the point with camera lenses. Your yellow highlighted point is true, but it is only the whole story for astophotography. With a camera, resolution is measured differently. See my previous note.

[nardes]

I'm still not sure what point I am missing Steve?

The relationship between the size of the Airy disk and the diameter of a particular lens is a property of the lens and is the same for visual observing as well as for astrophotograhy with the lens/telescope. It matters not whether there is an eyeball, 35mm film or a digital image sensor collecting the light, as the Airy Disk is a property of the lens opening and wavelength of light, not the sensor gathering the light.

Is this just a simple communication issue, i.e. I am talking about diffraction and you are talking about resolution?


Cheers

Dennis

[Tannin]

I thought fetishes were things we did that we wouldn't tell our mother about.

Oh I don't mind telling my mother about my fetishes. I just try to be a bit vague about how much they cost.

"Is that a new lens, Dear?"
"No Mum, it's the same one I had last week."
"That's funny, it looks about a foot longer."
"Oh, they often swell up like that. It's just the humidity."


Fun aside, that's a great point you make, Tom. But is it not wise to be aware of the technical limitations before (quite possibly) disregarding them them?

[Steve Axford]

I'm still not sure what point I am missing Steve?

The relationship between the size of the Airy disk and the diameter of a particular lens is a property of the lens and is the same for visual observing as well as for astrophotograhy with the lens/telescope. It matters not whether there is an eyeball, 35mm film or a digital image sensor collecting the light, as the Airy Disk is a property of the lens opening and wavelength of light, not the sensor gathering the light.

Is this just a simple communication issue, i.e. I am talking about diffraction and you are talking about resolution?


Cheers

Dennis

With astophotography we talk about the absolute amount of diffraction
With normal photography we talk about the amount of diffraction compared to the size of the image

In one case we have D (where D=diffraction)
in the other we have D/I (where I=image size)

Image size has no meaning when looking at a distant star and it can be considered a constant

[The dingo]

Oh I don't mind telling my mother about my fetishes. I just try to be a bit vague about how much they cost.

"Is that a new lens, Dear?"
"No Mum, it's the same one I had last week."
"That's funny, it looks about a foot longer."
"Oh, they often swell up like that. It's just the humidity."


Fun aside, that's a great point you make, Tom. But is it not wise to be aware of the technical limitations before (quite possibly) disregarding them them?

Wise? Who is wise? They say wisdom comes with age. I'm still waiting.

Having lived through 70 years of technology growth I recall the days when cameras were simple devices and I only needed to concern myself with pointing the camera in the right direction, which often proved to be a difficult task for me.
My mentor at the time was a man my age now. He taught me how to estimate exposure by just looking at the scene. I still do that from time to time when things are tricky and I know the light meter in the camera is confused.

I learnt a great deal about the technology while studying science at uni, especially such things relating to optics. It was interesting enough but it didn't influence the way I approached my pictures. CONTENT and CONTEXT were the prime concerns.

The biggest change for me was when autofocus came about. I no longer needed to rely on my poor eyesight. And it was fast!

The digital age and the internet have brought about a lot of changes. One of them is the number of experts there are on any one topic.

The science behind diffraction is interesting, important and relevant. How relevant to photography is often assumed with little understanding of what is happening and how it might influence a photograph.
Correcting for it, as Canon propaganda suggests, they can deal with it simply.
What they are doing is endeavouring to overcome its effects. They do this by 'sharpening'. Its not really sharpening. Its altering the contrast at high contrast points.

So. understanding the technicalities of diffraction is one thing; being obsessed about 'sharpness' is another. Anyone standing that close to a print in a gallery will be told to get their nose off the glass.

[ameerat42]

'Ts OK Dennis. I know people use terms like "an aperture of f/8" wrongly, as you point out.
The reason for replying is to alert "newbies" et al that there is a proper terminology.

I am with you 101% on your point, and I was just expanding on it.

[jim]

101% is impossible, am.

[ameerat42]

Not in the world of HYPE, in which we are immersed

[jim]

You might be immersed in a world of hype. I just glance idly in its general direction every now and again.

[ameerat42]

You might be immersed in a world of hype. I just glance idly in its general direction every now and again.

Ahh! - Life on Mars! [/Dreamy gaze]

[nardes]

With astophotography we talk about the absolute amount of diffraction
With normal photography we talk about the amount of diffraction compared to the size of the image

In one case we have D (where D=diffraction)
in the other we have D/I (where I=image size)

Image size has no meaning when looking at a distant star and it can be considered a constant

Sorry Steve – I still “don’t get it”.

I get the formula describing the size of the Airy Disc (based on the diameter of the lens and wavelength of light) which defines the diffraction profile of that lens (lens, telescope, microscope, etc.) irrespective of the sensor; eye, film or digital.

But image size (I) – what does that mean, it appears to have many interpretations:

• A 10”x8” print viewed at arm’s length, or
• A full screen image viewed on a 32” monitor from 25 CMS distance, or
• A 28 MB, 6000x4000 pixel digital file, or
• The apparent pixel dimensions of a blade of grass or insect leg in the frame, e.g. 200 pixels long by 4 pixels wide, or
• The size of the sensor cells, e.g. 5.6 um in the camera?

However, I’ll stop here for now and take this off line to look at other sources of information on the topic of “Diffraction Limited Aperture” to nut this one out.

Cheers

Dennis

[Steve Axford]

No matter. If I can't explain it, then that is my problem.

[arthurking83]

What a curious feature. It does seem like it does something, though there are clearly limits to how much it can do. Unfortunately, it only seems to work with the Canon RAW processor or with jpeg, so anyone who uses 3rd party converters (eg Lightroom) can't access it. The 3rd party converters are so much better in other respects that this limitation negates any advantage. I would guess that Canon have done this because they are concerned that the code is not effectively patentable, in which case why doesn't everyone do it? Perhaps some do?
I note that this feature has been around since early 2016. Why so little media comment?

My inherent cynicism of all things magic(like that diffraction correction tool) leads me to think it's just a high pass filter or USM routine catering for specific camera bodies.
That is, for each camera body with their respective pixel densities require different amounts of sharpening to produce more contrast, but not also introducing haloing effects as the downside.

It wouldn't be too hard for Canon to get their software folks to test varying amounts of high pass filter or USM settings for each camera, and produce a tool that reads the camera data, reads the aperture used and apply a specific sharpening routine that produces zero haloing.

Other than a need to have a starting point for a sharpening routine, I doubt any overly complex mathematics have been used to create this tool.

As for using or not using manufacturer's software on raw files:
Using ACR is akin to using DPP.
Photoshop in itself can't read raw files, so you NEED to use a raw file converter first.
There are advantages and disadvantages to either method, but in effect they are still the same.
(using the common ACR workflow to describe any thirdparty raw converter)

Using ACR, you get the option to pass it directly into photoshop and in what adobe seem to think is the best format for photoshop .. which is psd(unless you specify otherwise). What you don't get tho, is access to specific camera maker raw data that reads the file as per the way the camera shot the image.
My understanding of psd is that it's a lossless compressed tiff file(I'm sure I read that somewhere).
You could just as easily use any other thirdparty raw file converter and if it allows the use of the format, you could still pass it through to photoshop as a psd file.
I don't know of any thirdparty converters that can create psd files other than PaintDotNet. Then again PDN is basically a free photoshop type software that doesn't (natively) read raw files itself!
I know of a few that can read the format tho.

I can't really imagine thirdparty software makers producing a similar diffraction neutralising too as such, simply due to the complexity and scale of amassing all the necessary data required for the sharpening routine ... remembering that each lens would need to be measured at each aperture setting, and matched to each camera body(ie. pixel density) .. for how many manufacturers!!

I don't know if folks prefer to use high pass or USM, they each have their own subtle differences in how they sharpen, but you could try a very mild USM routine to minimise the softening effect of diffraction.

Another point of note regarding diffraction:
It's not always a bad thing having diffraction.
Some cameras(ie. referring to very high pixel density cameras with no AA filter) can produce moire where very fine details are misinterpreted by the camera sensor used with a very sharp lens.
The two methods to eliminate moire, without resorting to software trickery ... is to either slightly defocus the lens so that the detail isn't recorded on the sensor with so much precision, or to stop the lens down past the point of diffraction(ie. same effect.. less sharp .. but still sharp enough).