PierreD

Here is an interesting (and rather long) article on this subject: https://photographylife.com/what-is-diffraction-in-photography

As I understand it, one conclusion is that the aperture at which there is the least amount of diffraction is not necessarily the widest aperture....

bleirer

I think it is saying that there is diffraction in physics every time light passes through an opening of any kind, but the larger the opening the smaller the 'Airy disk' that results. When the Airy disk is smaller than 1-2 pixels on the sensor it is not noticeable. That's how I understood it anyway.

larryepage Loc: North Texas area

If you recall the Hubbell Telescope problems from a few years ago, you will recognize how easy it is to make a lens that is not perfect. Of course, the corollary to that is how difficult it is to make a lens that is perfect (or sufficiently close to perfect that the difference canot be detected). It is even more difficult to mass produce perfect lenses. So from an optics perspective, I think the big thing that gets omitted in this discussion is what physically happens in the lens when a small aperture is used with a lens that is not really perfect.

The purpose of the aperture diaphragm in a lens is to control the portion of light rays from the subject that actually pass completely through the lens and land on the sensor or film. It does that by blocking some of those rays by forming itself into a hole that is smaller than the maximum passage available. For ease of design and implementation, it does that by blocking the rays at the outer margins of the passage and allowing those closest to the axis (the center) to pass through. Now if you will recall mentally the diagrams of lenses you have seen in brochures or textbooks or wherever else you might have seen one, that means that the light rays that remain to be allowed to pass through the aperture to the sensor all become closer and closer to the axis (the center line) as the diaphragm closes down more and more. That means that not only are there fewer and fewer rays of light available, but all of the rays that are available have been refracted through smaller and smaller angles by the various lens elements. So even a small deviation from the path that an individual ray was intended to take will cause a bigger error after the rear elements do their job of spreading everything back out to cover the entire sensor area. So a small error in any of the elements will produce a big error in the final image. And there are fewer and fewer other "correct" rays of light to mask the error from the rays that end up in the wrong place.

None of this is diffraction. It is just error. Error that can't be masked out by a lot of "correctness," because there is not a lot of anything else, wrong or correct, at small apertures.

I'm not saying that lenses don't get soft at smaller apertures. I'm not even saying that none of this softness is caused by diffraction. I am saying that lens design is complicated. It's complicated for any lens, and it's particularly complicated for any lens with four elements or more. It's complicated for single focal length lenses, and it's complicated for zoom lenses. I am also saying that every complex system that you have ever owned performs best at one combination of operating conditions. I had a 1980 Camaro that I really liked. It cruised best at 55 miles per hour with its V6 engine and 3-speed automatic transmission. That was perfect when the nationwide speed limit was 55 mph. But when that law was repealed and speed limits were increased, "that was when the trouble started." The cooling system didn't perform correctly, gas mileage tanked (pardon the pun), and all sorts of other things happened.

When I took my photography classes, the first thing we did was calibrate our cameras' film speeds and determine our lenses' optimum performance zones. It's just part of photographic life. Maybe a little less so now, since we have preview screens and histograms and other helps and repair tools. the nature of lens design and performance is just part of life. Sure, some are better than others. But they all have limits, just like cameras have limits on their high ISO performance. We just need to learn those limits and either decide to work within them or use them to our artistic advantage. What a better time than now, when we are so limited as to other things we can do?

CO

I'm trying to find the individual photos to look at the settings of each shot. This triptych I made in Photoshop has only some of the metadata. I remember that I raised the ISO some between shots to keep the shutter speed up. The camera was on a tripod. Using the mirror up mode would definitely be a good.

DirtFarmer Loc: Escaped from the NYC area, back to MA

So I'm stuck indoors. It's raining too hard to enjoy working in the garden.

I believe I understand diffraction, having worked in optics for 50 years. But I'm an experimentalist so I thought I'd try to quantify it.

So I took my D800e and 105 Micro and a very small awl and a flash. Put the speedlight on the camera and aimed it at the awl. Of course the light hit the awl and gave specular reflections. So I arranged a piece of cardboard between the awl and the speedlight so it wouldn't get direct light but the speedlight would light up the wall behind it. I had a couple awls, one bent, so I placed them so I could get one pointing horizontally and one vertically.

I then took a series of shots at various f/ numbers. The awl wasn't purely in shadow so there was some reflection from it but one side presented a good sharp edge. So, having a set of images (raw) I converted them to tif because there is a Python app that will read a tif and convert it to an array of numbers. Once I had an array of numbers I selected one line of pixels across one of the awls. The tif is a 3 color array so I took all 3 colors and combined them into one [sqrt(R**2+G**2+B**2)]. That gave me 100 numbers representing pixels at one level across the awl (second figure).

I then took the numbers and, using only the shady edge of the awl, differentiated the signal across the edge. That gave me a curve of the slope of the signal at the edge (red dots in the first figure). I fit those points to a gaussian and found the width of the resulting gaussian. I then plotted the width of the gaussian as a function of f/ number (third figure).

This was a quick and dirty test to see how diffraction would affect the sharpness of an image. It was only in the center of the image. Only for one lens. The edge width had a minimum at f/8 but it wasn't bad between f/5.6 and about f/18.

Always nice to have something to do when stuck indoors on a rainy day. If I get another rainy day like this one I might try to fill in some of the points and get larger samples. Maybe even try some other lenses.

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NCMtnMan Loc: N. Fork New River, Ashe Co., NC

Thanks. I understand that diffraction exists, but to what degree is diffraction and what is the result of movement/vibration or other factors. Just had my curiosity up.

lamiaceae Loc: San Luis Obispo County, CA

There is so much incorrect in this question I'll leave it to others to try to answer it. Though, shutter speed, really? No shutter is faster than the speed of light. The initial comment makes no sense. There are sources of correct information about diffraction with digital and film photography, hopefully someone posted them for you.

Paul Moshay Loc: Los Angeles, CA

[quote=camerapapi] I simply cannot understand how a "professional" could say that a camera with a high number of megapixels needs a high shutter speed for sharp images. What any camera needs, regardless of megapixels, is sound photographic techniques.

Yes, I agree with the above. Megapixels vs fstops has nothing to do with sharpness. Proper camera technique is the only way to have sharp pictures. No so-called "professional" would make that statement.

Doc Barry Loc: Huntsville, Alabama USA

First of all, photographic lenses are not diffraction limited at full aperture. The size of the Airy disk (diameter of first zero values) is about 1.22 x F/# (microns) assuming the wavelength of the light is 500 nm. This is a reasonable estimate even though the light is actually polychromatic. For low F/#, aberrations limit the resolution (or MTF) and as the F/# increases, the diffraction blur begins to overwhelm the geometric aberrations of the lens. The sweet spot (best MTF) of most lenses is typically in the range of F/5.6 to F/11 with F/8 rather common. Note that the diffraction blur is not determined by just the aperture diameter or the focal length, but by their ratio, i.e., the F/#. See https://www.ephotozine.com/article/nikon-af-s-nikkor-50mm-f-1-8g-lens-review-16746 for a resolution plot vs. F/# for a Nikon AF-S Nikkor 50mm f/1.8G. Many other lenses can be viewed on this site.

Again, the size of the image of a point source (say a star) is determine by the F/# (geometric aberrations and diffraction). High-end cameras have pixels on the order of 4-10 microns. At least one new camera takes multiple images where the images are shifted by 1/2 pixel in both directions. These are combined to produce an image having four times the pixels of the original single image and indeed the new image contains additional information about the scene.

DennisC. Loc: Antelope, CA

Because there is always camera shake when hand holding a camera. Higher the mega pixel count means more detail and magnified camera movement. It’s there with lower megapixel cameras just not as apparent since there is less detail that is visible. For best results use a sturdy tripod.

bleirer

I don't know if this was posted yet, but here is a very detailed look with a lot of practical examples.

https://backcountrygallery.com/lens-diffraction-in-photography/