f/16 Isn't Equal to f/16
Nov 25, 2018 07:33:21 #
Having taught physics (including optics) for 35 years, I find this discussion very interesting. I've run many a demonstration using ripple tanks with a variety of barrier configurations, Young's single and double slit experiment, and with lasers using collimated beams in various frequencies/wavelengths.... Even dabbled in producing some laser holograms (constructive/destructive interference patterns). All very informative and instructive.
Without dusting off the old slide rule (yes I still have mine, lol)...I do know that the effects of diffraction can be observed in photographs. Not so long ago, a member on this site posted a series of images taken through all lens apertures available for a particular lens.....everything else being equal. At some value for that lens/camera/shutter/aperture combination, the image quality did seem to degrade. If memory serves, it seems that by f/16, the image had noticeably changed. I also believe (perhaps superstitious knowledge??) that the sensor size, blade type and number, sensor type (film, digital, B&W/Color, etc) and light quality also come into play here. As in many complex physical phenomenon...there are way too many controlling variables to allow for a definitive calculation.
So, relying on practice... If you look at bench tests of lens resolution, you see that "resolution/sharpness" increases as the aperture decreases from wide open....reaching a maximum/optimum at generally 1-2 f-stops change...then decreases rapidly with successive decrease in aperture (increases in f-stop values). Different lenses with different design and construction (# elements/groups, ED, Fresnel, coatings) may peak at different aperture values.
IMHO: Seems to me like the image at f/32 on the old large-format graphic view camera using sheet film, is not the same as the image at f/32 on a modern 35mm DSLR. Anyway...my brain hurts now.
Without dusting off the old slide rule (yes I still have mine, lol)...I do know that the effects of diffraction can be observed in photographs. Not so long ago, a member on this site posted a series of images taken through all lens apertures available for a particular lens.....everything else being equal. At some value for that lens/camera/shutter/aperture combination, the image quality did seem to degrade. If memory serves, it seems that by f/16, the image had noticeably changed. I also believe (perhaps superstitious knowledge??) that the sensor size, blade type and number, sensor type (film, digital, B&W/Color, etc) and light quality also come into play here. As in many complex physical phenomenon...there are way too many controlling variables to allow for a definitive calculation.
So, relying on practice... If you look at bench tests of lens resolution, you see that "resolution/sharpness" increases as the aperture decreases from wide open....reaching a maximum/optimum at generally 1-2 f-stops change...then decreases rapidly with successive decrease in aperture (increases in f-stop values). Different lenses with different design and construction (# elements/groups, ED, Fresnel, coatings) may peak at different aperture values.
IMHO: Seems to me like the image at f/32 on the old large-format graphic view camera using sheet film, is not the same as the image at f/32 on a modern 35mm DSLR. Anyway...my brain hurts now.
Nov 25, 2018 08:11:13 #
Gosh, Gee Whiz, Guys,
What I don't know and will never try to understand does not get in the way of the minutes and hours I spend waiting for the right light and shadows.
Those minutes or hours spent framing and composing the photograph I hope to achieve are time well spent. I could easily apply my
knowledge of physics and wave lengths to competing for greater accuracy and honesty in your discussion. But that would be a superb waste of
time when I'd rather look for a fine image. My hats off to you for engaging in that esoteric discussion.
What I don't know and will never try to understand does not get in the way of the minutes and hours I spend waiting for the right light and shadows.
Those minutes or hours spent framing and composing the photograph I hope to achieve are time well spent. I could easily apply my
knowledge of physics and wave lengths to competing for greater accuracy and honesty in your discussion. But that would be a superb waste of
time when I'd rather look for a fine image. My hats off to you for engaging in that esoteric discussion.
Nov 25, 2018 08:17:34 #
If I had to worry about knowing all that stuff, I'd give up taking photographs!
rmalarz wrote:
Well, actually it is, or is it? br br Let's get o... (show quote)
Nov 25, 2018 08:19:53 #
It is 8:15AM, Eastern US time and I am reading and trying to digest this for the first time. I need a drink! Then I will take another look. (But seriously, folks: interesting discussion.)
Nov 25, 2018 08:28:40 #
Nov 25, 2018 08:37:03 #
rmalarz wrote:
"...is that diffraction probably plays a minor role," exactly my point.
--Bob
--Bob
Just how "deep" do you Peep at those pixels, anyway? I guess if it is an issue, one should shoot wide open and Focus Stack every landscape, but I am WAY to lazy for that. Good subject to think about, Bob.
Nov 25, 2018 08:37:20 #
rmalarz wrote:
If diffraction does occur, is it observable in our photographs?
--Bob
--Bob
Yes, it is - and as you allude, all F-16's are NOT all the same diameter - and this fact is almost ALWAYS glossed over in any discussion ! And, it IS the absolute diameter of the aperture that directly controls the diffraction - not relative f-stops which only have an indirect relationship.
Nov 25, 2018 08:38:14 #
Nov 25, 2018 08:39:26 #
Largobob wrote:
Having taught physics (including optics) for 35 ye... (show quote)
https://www.cambridgeincolour.com/tutorials/diffraction-photography.htm seems to be the standard reference for this.
I was looking for an article which gave airys disk sizes for different f-stops but couldn't find it.
Basically the smaller the aperture the greater the diffraction but the biggest problem is enlargement, with an 8 by 10 negative you probably will not enlarge at all where an iphone might be enlarging by around 48x our eyes resolve up to around 10 line pairs per mm if the diffraction is less than this we just don't see it. The iphone has an f1.7 lens which isn't diffraction limited for the typical 8 by 10 print size , f2 it probably would be
So as sensor size increases you can get away with more diffraction as long as you are not magnifying it to the point that we can see it, there isn't a problem. However if you crop the image you are risking diffraction becoming visible.
if you consider an aps-c camera where diffraction comes into play around f8 , then shooting at f5.6 should be fine but cropped to say a 1/4 of its full size and maybe its not going to look so good.
So in practical terms shoot within the diffraction limit of your sensor and fill your frame. Which perhaps makes it easier to use an iphones camera as you can't stop down, or an 8x10 since you can use pretty much any aperture without losing out to diffraction.
Nov 25, 2018 08:44:02 #
User ID wrote:
` br br br br You said: br "....... (show quote)
Actually, it does. The circumference = 2*pi*radius while the area = pi*r(squared). That would make a considerable difference as the apeture shrinks.
Nov 25, 2018 08:44:45 #
With ALL my cameras when I need to go to f16 I do not have any issues doing so. As a matter of fact, that degradation of image quality, at least for me it is not easy to see. Perhaps it does at a big enlargement but customarily I do not print beyond 13x19 inches.
Bob I am not very concerned with the physics of optics, perhaps because I do not understand it. I think it is more fun to grab any of my cameras, get out there and shoot what I like as my subject.
Bob I am not very concerned with the physics of optics, perhaps because I do not understand it. I think it is more fun to grab any of my cameras, get out there and shoot what I like as my subject.
Nov 25, 2018 08:47:10 #
PeterBergh wrote:
...{light} is at its fastest in vacuum and slower in any other medium...
Generally true for ordinary materials.
There exist some exotic materials which exhibit negative index of refraction. That means the speed of light appears faster than the speed in a vacuum. What actually happens is that the phase speed is faster than the vacuum speed but the wave speed is still under the vacuum speed. That preserves the relativistic requirement that a signal can't travel faster than light in a vacuum.
Nov 25, 2018 08:56:47 #
imagemeister wrote:
Yes, it is - and as you allude, all F-16's are NOT all the same diameter - and this fact is almost ALWAYS glossed over in any discussion ! And, it IS the absolute diameter of the aperture that directly controls the diffraction - not relative f-stops which only have an indirect relationship.
https://photo.stackexchange.com/questions/15924/does-focal-length-affect-diffraction-in-addition-to-aperture
Does focal length affect diffraction, in addition to aperture?
The reason I ask is that f/18 on a 24mm lens = 1.5mm, and f/18 on a 180mm lens = 10mm. I thought diffraction is due to the small physical size of the aperture, rather than the f-ratio, yet I only ever see mention of the f-ratio in discussions of diffraction.
Excellent question. It boils down to the nature of F-number, which is focalLength/physicalAperture, and the fact that longer focal lengths magnify more. Keep in mind that light projected through an aperture still has to travel from the aperture to the sensor. The greater the distance from aperture to sensor, the greater the magnification...including magnification of the airy disc. The difference between a 180mm lens and a 24mm lens is about 7.5x. To get the same amount of diffraction from a 180mm lens as you would from 24mm lens at f/18, the 180mm lens would need a physical aperture of about 11.25mm in diameter. Given that 180/18 = 10mm, the amount of diffraction present at the sensor is actually a little bit more than with the 24mm lens.
If this is correct and it seems to be a reasonable answer then it pretty much resolves the op's question a longer lens will have a physically larger aperture but at the same time enlarges the airy disc. In practice theres going to be a small difference that can be ignored.
Nov 25, 2018 09:07:16 #
gvarner
Loc: Central Oregon Coast
I limit my pondering to what the photo will look like if I shoot wide open or stopped way down. 😜😜
Nov 25, 2018 09:22:32 #
rmalarz wrote:
Well, actually it is, or is it? br br Let's get o... (show quote)
As a high school math and science teacher who looks to simplify complex ideas as much as possible for my students, I'll look to pretend I'm a photography instructor and this question is posed by a relatively new student. I'd look to explain the answer to your question in a three-fold fashion. Remember, I have no desire to go into quantitative computation other than a percentage here in a couple of sentences. To my mind, qualitative analysis will suffice. I'd first make sure you understand how a camera can fail to see a person walking through a scene when the exposure time is 15-30 seconds. If you understand the image formed is the sum of the information, and that 0.2 seconds of information on any particular pixel will we overwritten by the other 29.8 seconds of information presented to that pixel, then you are ready to ponder the aperture effects. With a wider aperture, a far smaller percentage of the light entering the lens actually interact with the edge of the iris than do so in a small aperture. Because the percentage of affected signal is higher for a smaller aperture, the amount of 'flawed' signal recorded goes up, and the effects (softness in focused areas) increases.
As for the physical dimensions of apertures at f/16 being different, the answer is, again, simple enough to skip a high level formulaic analysis. With a longer focal length lens, whose f/16 aperture is physically larger, one must remember that the physical distance that diffracted light (flawed signal) travels is greater than the distance traveled in a shorter focal length lens. Thus, the diffracted signal travels further off-line, creating an equally detectable softness with a smaller percentage of affected signal. Sure, the f/16 vs f/16 effects will differ somewhat for different lenses of equal focal length as well, and just like the old 'sunny 16' rule, this one generalized.
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