MRJohn Posted November 19, 2016 Share #1  Posted November 19, 2016 Advertisement (gone after registration) Recently I went to a local Jazz club and took the SL with the Summilux 28 mm with me. Since I was late I did not get a seat with a good view but nevertheless decided to take a few souvenir shots. - So this is a shot from where I was sitting. Apart from the scene itself, you can also see a lamp post (a solid metal ca 1-2 cm round tube painted white) which was about 25-50 cm from me on the table partially obstructing the view.  As you can notice, there is quite a bit of diffraction going on caused by the lamp post (I guess the opposite effect vs a pin hole)  leading to the headstock of the bass being visible (an unusual version with 5 tuners), as is the sheet stand, despite being obstructed. I was quite amazed to see this. Solid things further away are not „transparent“, such as the fence obstructing the view. The fence probably must cause diffraction as well, but it does not get focused onto the sensor.  My question: is this particular diffraction effect only caused by the object width and the distance to the sensor or do the lens characteristics play a role as well? Welcome, dear visitor! As registered member you'd see an image here… Simply register for free here – We are always happy to welcome new members! Link to post Share on other sites Simply register for free here – We are always happy to welcome new members! ' data-webShareUrl='https://www.l-camera-forum.com/topic/266630-question-about-diffraction/?do=findComment&comment=3152108'>More sharing options...
Advertisement Posted November 19, 2016 Posted November 19, 2016 Hi MRJohn, Take a look here Question about diffraction. I'm sure you'll find what you were looking for!
michaelwj Posted November 19, 2016 Share #2 Â Posted November 19, 2016 It's not diffraction, it's just out of focus. Looks cool though! Link to post Share on other sites More sharing options...
pico Posted November 19, 2016 Share #3 Â Posted November 19, 2016 The fence looks hazy by the back-light due to flare. Link to post Share on other sites More sharing options...
MRJohn Posted November 19, 2016 Author Share #4  Posted November 19, 2016 It's not diffraction, it's just out of focus. Looks cool though!  it is out of focus, but this should not make it "transparent" - so you think this effect is due to focus? I thought it was something like this Welcome, dear visitor! As registered member you'd see an image here… Simply register for free here – We are always happy to welcome new members! Link to post Share on other sites Simply register for free here – We are always happy to welcome new members! ' data-webShareUrl='https://www.l-camera-forum.com/topic/266630-question-about-diffraction/?do=findComment&comment=3152242'>More sharing options...
MRJohn Posted November 19, 2016 Author Share #5 Â Posted November 19, 2016 The fence looks hazy by the back-light due to flare. Â the fence is twisted black steel, probably normal on the image, I just referred to it as an opposite example Link to post Share on other sites More sharing options...
spydrxx Posted November 19, 2016 Share #6 Â Posted November 19, 2016 It doesn't look like diffraction to me, agree with it being severely out of focus. Link to post Share on other sites More sharing options...
lct Posted November 19, 2016 Share #7  Posted November 19, 2016 Advertisement (gone after registration) Where's the pencil? Just OoF  Welcome, dear visitor! As registered member you'd see an image here… Simply register for free here – We are always happy to welcome new members!  Link to post Share on other sites Simply register for free here – We are always happy to welcome new members!  ' data-webShareUrl='https://www.l-camera-forum.com/topic/266630-question-about-diffraction/?do=findComment&comment=3152312'>More sharing options...
michaelwj Posted November 20, 2016 Share #8 Â Posted November 20, 2016 it is out of focus, but this should not make it "transparent" - so you think this effect is due to focus? I thought it was something like this To get the effect you're talking about the numbers just don't add up. The object doesn't have a hard edge and its dimensions and distance from the image plane are not sufficient to get the light bending far enough. As a reference, think about diffraction from using a small aperture: for a 28mm lens, we're talking about a hard edged hole that is 28/16 ~ 2mm diameter. We then see some light from the edge of the airy disc spill into the neighbouring pixel, ~20um(?) away after propagating ~4cm. Â If lct's example provided the intermediate step we would see a semi-transparent pencil, as you see with the lamp. Link to post Share on other sites More sharing options...
jaapv Posted November 20, 2016 Share #9  Posted November 20, 2016 Two types of diffraction are being confused here: Narrow slit diffraction and obstruction diffraction, which follows the Huygens Principle. For a clear explanation:  http://www.mike-willis.com/Tutorial/diffraction.htm  Math warning, however, one of the diagrams makes the effect clear.:  Welcome, dear visitor! As registered member you'd see an image here… Simply register for free here – We are always happy to welcome new members!  If this effect were not there, mirror optics would show a black spot, and we would not be able to "remove" a mesh in front of our lens by moving up close. Link to post Share on other sites Simply register for free here – We are always happy to welcome new members!  If this effect were not there, mirror optics would show a black spot, and we would not be able to "remove" a mesh in front of our lens by moving up close. ' data-webShareUrl='https://www.l-camera-forum.com/topic/266630-question-about-diffraction/?do=findComment&comment=3152417'>More sharing options...
michaelwj Posted November 20, 2016 Share #10 Â Posted November 20, 2016 More like two manifestations of the same diffraction just that with a hole or slit, it happens one both sides. In photography we have a clear meaning for the term diffraction, the disc in a mirror lens is just out of focus. Link to post Share on other sites More sharing options...
jaapv Posted November 20, 2016 Share #11  Posted November 20, 2016 Yes, not seeing because out of focus is an apt description of the effect. Now for the cause...  Maybe the best comparison for the different effects is waves passing through the gap of the doors of a nearly closed dock gate vs. around a pole in the water. The wider the pole the bigger the triangle of calm water behind it. The narrower the dock gate the more pronounced the circle of waves behind.  Astronomy mirror telescopes create 228.000 hits in google for obstruction diffraction.   Interestingly the obstruction in a mirror lens becomes visible in the OOF areas - doughnut highlights. Link to post Share on other sites More sharing options...
willeica Posted November 20, 2016 Share #12 Â Posted November 20, 2016 I use this effect with pictures of animals in zoos and birds in cages. With the lens wide open (anything from 1.4 to 2 and even 2.8 with a long lens) and with the front of the lens close to the cage bars or mesh, the bars or mesh disappear and you get a clear shot of the animal or bird. Sometimes you get swirly effects in out of focus areas, but I like that sort of thing. Â William Link to post Share on other sites More sharing options...
michaelwj Posted November 20, 2016 Share #13 Â Posted November 20, 2016 Yes, not seeing because out of focus is an apt description of the effect. Now for the cause... Â Maybe the best comparison for the different effects is waves passing through the gap of the doors of a nearly closed dock gate vs. around a pole in the water. The wider the pole the bigger the triangle of calm water behind it. The narrower the dock gate the more pronounced the circle of waves behind. Â Astronomy mirror telescopes create 228.000 hits in google for obstruction diffraction. Â Interestingly the obstruction in a mirror lens becomes visible in the OOF areas - doughnut highlights. The invisible object in the foreground out of focus area is a combination of a few things I think. It's not purely diffraction, or you would see the effect of it in the image, as in there would be distortions as the light bends - where the object was would be filled in by the adjacent background, but it's not. You'd also still see the object as it is reflecting its own undiffracted light towards the sensor. Now, there must be an upper limit of the size of the pencil (in lct's example) where it cannot be made invisible. I assume it has something to do with the size of the lens opening as there must be a point where there is too much light coming from the pencil relative to the background. Â For what it's worth, I work in X-ray diffraction imaging, but we don't use complex multi element lenses, and I have no optical imaging experience beyond a simple single lens. Link to post Share on other sites More sharing options...
jaapv Posted November 20, 2016 Share #14  Posted November 20, 2016 If you go into the astronomy sites you will find that obstruction diffraction effects are seen. It is just that they don't impact our photography significantly.  http://www.telescope-optics.net/obstruction.htm   http://www.hoflink.com/~mkozma/obstruction.html    http://www.beugungsbild.de/diffraction/diffraction.html Link to post Share on other sites More sharing options...
MRJohn Posted November 20, 2016 Author Share #15  Posted November 20, 2016 If you go into the astronomy sites you will find that obstruction diffraction effects are seen. It is just that they don't impact our photography significantly.  http://www.telescope-optics.net/obstruction.htm   http://www.hoflink.com/~mkozma/obstruction.html    http://www.beugungsbild.de/diffraction/diffraction.html  So the effect we see is a result of a) obstruction diffraction and being out of focus, correct? Because if it is in focus it would obviously not diffract. Do I understand you correctly? Link to post Share on other sites More sharing options...
jaapv Posted November 20, 2016 Share #16 Â Posted November 20, 2016 I think so Link to post Share on other sites More sharing options...
MRJohn Posted November 20, 2016 Author Share #17  Posted November 20, 2016 I think so   thanks, and do you think the lens characteristics play a role as well? in other words, would I get the same effect with 4 different 50mm lenses at the same aperture, focus and distance (if it were to occur in at least one of them)? Link to post Share on other sites More sharing options...
pgk Posted November 20, 2016 Share #18 Â Posted November 20, 2016 Â thanks, and do you think the lens characteristics play a role as well? in other words, would I get the same effect with 4 different 50mm lenses at the same aperture, focus and distance (if it were to occur in at least one of them)? Â I think that you may be straying into the realm of nuances of nuances here. There will no doubt be very marginal differences which you might, with a great deal of study and determination, be able to perceive, but ..... Link to post Share on other sites More sharing options...
earleygallery Posted November 20, 2016 Share #19  Posted November 20, 2016 My advice is to get to the jazz club earlier next time! Link to post Share on other sites More sharing options...
MRJohn Posted November 21, 2016 Author Share #20  Posted November 21, 2016 My advice is to get to the jazz club earlier next time!  to avoid serendipity? Link to post Share on other sites More sharing options...
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