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Old 12th July 2017
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Diffraction confusion!

Well I thought I understood the diffraction phenomenon but this link Dave recently posted has me a bit confused.

http://www.cambridgeincolour.com/tut...ensor-size.htm

There is a diffraction limited aperture estimator and if you follow the diffraction limits and photography link there is a diffraction limit calculator.
So my dilemma,
I tried the diffraction limit calculator and for a 4/3 slr then it says that f11 is not diffraction limited but f16 is. Now that is pretty much what I was working to from a workshop I did where the diffraction point was estimated on my EM1 at around f12/13, hence I've usually tried to use around f11 for my highest resolution and dof landscape shots when I have sufficient light.
OK so far
However, when I looked at the advanced version of the calculator or the simpler Diffraction limited aperture estimator on the original link they both give the opportunity to input the sensor resolution in Megapixels. Now I thought that, to quote Cambridge in colour themselves
"Diffraction thus sets a fundamental resolution limit that is independent of the number of megapixels, or the size of the film format. It depends only on the f-number of your lens, and on the wavelength of light being imaged."

So why input the megapixels? and if you do the calculators give a very different answer for the diffraction aperture limit.
Using the simpler Diffraction Limited Aperture Estimator for a 20 Megapixel input I get f6.5 as the diffraction limit and for a 16 Megapixel input I get f7.3....?
hence I'm now a tad confused.
So
- does my new EM1 Mk2 have a bigger aperture diffraction limit to my old Mk1? especially if diffraction is supposed to be independent of the number of megapixels?
- should I be working at f6.3 for my landscapes rather than f11 for max resolution? (assuming dof isn't an issue)

Any help clearing this up would be much appreciated.
Thanks.
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Old 12th July 2017
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Re: Diffraction confusion!

The physical degree of diffraction isn't affected by resolution/sensor size, but how soon that diffraction might be noticeable is. If diffraction is only blurring your image by the width of a pixel it won't be noticeable however you output your image.
If it's covering 5 pixels but your reducing your image 6 fold to publish on the web the diffraction won't be an issue, but if you're cropping your image and want a sharp print it may well be that anything larger than the pixel size on your new super megapixel camera will be an issue...

When pixel peeping the effects of diffraction will be seen sooner on your new Mk2. It probably won't be an issue if just printing to A4 or using on-line. It may not be an issue printed at A3.
DOF is another factor that depends on the final output. What looks perfectly focused on a 6x4 print might look quite soft on an A3 version especially if viewed from the same distance.

I believe the 'circle of confusion' is used in the actual formula for both DOF & diffraction to define how much things can be softened before it becomes noticeable. The name has always struck me as very appropriate as it seems to cause no end of confusion! It can be used at the pixel size to determine when it will be noticeable on pixel peeping, of at a number suitable for the output print & allowing for the degree of enlargement in producing that print. Many sources just set a standard output such as 10x8 viewed at arms reach, & so remove the variable from their equations.
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Old 12th July 2017
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Re: Diffraction confusion!

Diffraction is a very recent invention. It's only purpose is to give internet "experts" and trolls something to annoy forum users with.















Actually I seem to remember covering it when studying photography. But in those days it was in reference to using large format and really, really tiny apertures IIRC. And surely, if that Cambridge in Colour quote is correct the number of megapixels will matter as the smaller the pixel pitch the more effect the varying wavelengths of light will have.
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Old 12th July 2017
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Re: Diffraction confusion!

I have to say, way back in the day of film, I used the aperture most conducive to the image I wanted to create.

A philosophy I still pursue today.
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Old 12th July 2017
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Re: Diffraction confusion!

Quote:
Originally Posted by Michael Sewell View Post
I have to say, way back in the day of film, I used the aperture most conducive to the image I wanted to create.

A philosophy I still pursue today.
And always the right one, unless that creates too much diffraction softening & focus stacking can be used for the subject to give the DOF required without the softening.

Outside of the field of macro I don't think those exceptions are likely to occur.
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Re: Diffraction confusion!



E-M1 mkII 1/250th sec ISO64 12-40mm f16
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Re: Diffraction confusion!

She must be about to fall over,..................

Well defracted I think.....

(joke)
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Old 12th July 2017
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Re: Diffraction confusion!

In the film days people didn't pixel peep as they do today.

Diffraction is a digital phenomenon but it does exist. You can take action to minimise it or choose to ignore it. In most cases its not noticeable but not all.
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Old 12th July 2017
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Re: Diffraction confusion!

Quote:
Originally Posted by Phill D View Post

So why input the megapixels? and if you do the calculators give a very different answer for the diffraction aperture limit.
If you specify the the pixel count and the sensor format, the calculation can determine the linear size of the pixel and hence using the CoC value yield the F value for the diffraction limit.
Quote:
Using the simpler Diffraction Limited Aperture Estimator for a 20 Megapixel input I get f6.5 as the diffraction limit and for a 16 Megapixel input I get f7.3....?
hence I'm now a tad confused.
These are indeed the values produced by most DoF calculators for 4/3 sensors with the stated (Mega)pixel counts.
Quote:
So
- does my new EM1 Mk2 have a bigger aperture diffraction limit to my old Mk1? especially if diffraction is supposed to be independent of the number of megapixels?
- should I be working at f6.3 for my landscapes rather than f11 for max resolution? (assuming dof isn't an issue)

Any help clearing this up would be much appreciated.
Thanks.
Diffraction softening isn't a binary phenomena - i.e below the the stated limit everything is fine, whilst at F stops above it the result is rubbish. The effect is progressive and the CoC being used is an (industry standard) subjective value.

Some types of photography/photographer require more attention to diffaction limiting than others, and so some Field of View/Diffraction calculator apps allow the user to select 'tighter' or 'looser' CoC values.

I suggest that for most landscape images you wouldn't find noticable diffraction softening until beyond F8 when using your EM1M2.
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Old 12th July 2017
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Re: Diffraction confusion!

Quote:
Originally Posted by Michael Sewell View Post


E-M1 mkII 1/250th sec ISO64 12-40mm f16
I can just imagine the bride saying, "That's a wonderful picture but it's a shame it's diffraction limited, I reckon he must have shot at f16."
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Old 12th July 2017
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Re: Diffraction confusion!

Quote:
Originally Posted by Petrochemist View Post
The physical degree of diffraction isn't affected by resolution/sensor size, but how soon that diffraction might be noticeable is. If diffraction is only blurring your image by the width of a pixel it won't be noticeable however you output your image.
If it's covering 5 pixels but your reducing your image 6 fold to publish on the web the diffraction won't be an issue, but if you're cropping your image and want a sharp print it may well be that anything larger than the pixel size on your new super megapixel camera will be an issue...

When pixel peeping the effects of diffraction will be seen sooner on your new Mk2. It probably won't be an issue if just printing to A4 or using on-line. It may not be an issue printed at A3.
DOF is another factor that depends on the final output. What looks perfectly focused on a 6x4 print might look quite soft on an A3 version especially if viewed from the same distance.

I believe the 'circle of confusion' is used in the actual formula for both DOF & diffraction to define how much things can be softened before it becomes noticeable. The name has always struck me as very appropriate as it seems to cause no end of confusion! It can be used at the pixel size to determine when it will be noticeable on pixel peeping, of at a number suitable for the output print & allowing for the degree of enlargement in producing that print. Many sources just set a standard output such as 10x8 viewed at arms reach, & so remove the variable from their equations.
I think you nailed it Mike! If you get past all the physics of it and look to the practicalities as it applies to photography then the essential points are:

- Diffraction increases as a function of F stop. The focal length of the lens is irrelevant.

- The apparent loss of sharpness will increase as the magnification of the original image to final image increases. This means that smaller sensors will be more impacted by diffraction effects.

- As pixel pitch decreases (e.g. by packing more pixels into the same area) then the impact of diffraction will become more evident simply because the higher resolution of the sensor has the potential to show more detail.


Personally, I consider f8 to be my limit in u43. I might push it to f11 if I need the extra DOF or maybe to pull the shutter speed down.
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Old 12th July 2017
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Re: Diffraction confusion!

Quote:
Originally Posted by Gwyver View Post
Diffraction softening isn't a binary phenomena - i.e below the the stated limit everything is fine, whilst at F stops above it the result is rubbish. The effect is progressive and the CoC being used is an (industry standard) subjective value.
I agree. My understanding regards CoC is:

A point is only in focus on a single infinitely thin plane, where it will appear as a point. Either side of that plane, the point is out of focus and resolves as a fuzzy disc. However, the fuzzy disc will still appear to be a point to our eyes, as long as the disc is smaller than a certain size - the 'Circle of Confusion'.

Diffraction is a nasty phenomenon, where the dual nature of light rears its ugly head. For much of our camera optics, the particle theory of light works just fine, but breaks down with diffraction, which is a wave phenomenon.

Jim
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Old 12th July 2017
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Re: Diffraction confusion!

Quote:
Originally Posted by wornish View Post
In the film days people didn't pixel peep as they do today.

Diffraction is a digital phenomenon but it does exist. You can take action to minimise it or choose to ignore it. In most cases its not noticeable but not all.
You get diffraction with film, it's just that there was no internet for the "experts" and trolls to tell people they were shooting film wrong.
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Re: Diffraction confusion!

Diffraction gets more attention today because the ability to produce images of the highest technical quality has never been so easy:

- Excellent lenses
- Image stabilisation
- High resolution sensors
- Software to post-process raw files

Whereas in the days of film it would require high expertise throughout the while photographic process to produce a situation where diffraction was apparent, now almost anyone can get into situations where diffraction is the limiting factor. But I agree - it's mostly a pixel-peeking thing. Unless you're printing mega huge than even f11 on u43 is unlikely to get you into trouble.
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Old 12th July 2017
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Re: Diffraction confusion!

Quote:
Originally Posted by Jim Ford View Post
...A point is only in focus on a single infinitely thin plane, where it will appear as a point. Either side of that plane, the point is out of focus and resolves as a fuzzy disc....
All true but nothing to do with diffraction.

Quote:
Originally Posted by Jim Ford View Post
...Diffraction is a nasty phenomenon, where the dual nature of light rears its ugly head. For much of our camera optics, the particle theory of light works just fine, but breaks down with diffraction, which is a wave phenomenon.
This is the crux of the matter. When light passes between a slit in an opaque object it becomes diffracted or spread out. This is caused by the interaction of the light wave with the edge of the slit. The larger the slit the less the light gets diffracted, the smaller the slit the more the light gets diffracted. This is often demonstrated in the school laboratory by passing light through a thin piece of glass covered in graphite in which a line has been scribed to produce a thin slit. With lots of slits the light waves from each slit spread out and interact with each other to form a diffraction pattern.

Diffraction can also demonstrated in water, usually with a vibrating arm to product waves and two blocks which can be moved diffrent different distances apart to vary the "slit" diameter from a few millimeters to a few centimeters. The wave can be seen to curve out at an angle at the edge of the block at small gaps.

With light, the slit (aperture) has to be much smaller to get diffraction but it only has to spread out beyond a pixel width by the time it reaches the sensor for it to become noticable so it can happen at quite modest apertures. The smaller the pixel (the higher the pixel density) the more it is a problem.
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