Diffraction or is it something else?

batmura

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Hello,

I have a question for you guys. When I take photos with a wide-angle lens, sometimes the buildings in the far distance or light poles nearer to me looked terribly crooked, and I was wondering if there is a way to fix that on Lightroom 4, as it's the only software I have. Also, what is this problem called? Diffraction?

Thanks!
 

480sparky

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It's either distortion.... bending of what in real life is a straight line, rendering it a curved line (either barrel or pincushion) or wavy line(complex) distortion.... or keystoning.

Post an example.
 

amolitor

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There are several possible effects in play here, none of them are diffraction ;)

- You could be seeing defects/features of the lens design. Not every lens renders straight lines as straight on the film/sensor -- some through poor design, some quite deliberately.

- You could be seeing optical effects caused by the extreme wide angle. The part of the telephone pole at camera height might much much closer to the camera than the top of the pole, so the top will of course be rendered much smaller. This is more or less a "railroad tracks" effect applied to pretty much everything in the frame. Straight lines are still straight, but they don't necessarily look straight.

- a variation of this is keystoning, which is pretty much the same railroad tracks effect but you're making it worse by pointing the camera up or down, so now stuff that is EVEN FARTHER away in the frame, and so you see things EVEN SMALLER when your brain tells you "but the telephone pole is the same diameter, more or less, all the way up".

There are probably technical terms all over the place here, but I am darned if I can recall any of 'em.
 
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batmura

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There are several possible effects in play here, none of them are diffraction ;)

- You could be seeing defects/features of the lens design. Not every lens renders straight lines as straight on the film/sensor -- some through poor design, some quite deliberately.

- You could be seeing optical effects caused by the extreme wide angle. The part of the telephone pole at camera height might much much closer to the camera than the top of the pole, so the top will of course be rendered much smaller. This is more or less a "railroad tracks" effect applied to pretty much everything in the frame. Straight lines are still straight, but they don't necessarily look straight.

- a variation of this is keystoning, which is pretty much the same railroad tracks effect but you're making it worse by pointing the camera up or down, so now stuff that is EVEN FARTHER away in the frame, and so you see things EVEN SMALLER when your brain tells you "but the telephone pole is the same diameter, more or less, all the way up".

There are probably technical terms all over the place here, but I am darned if I can recall any of 'em.
thanks. And how do Imfix that on L4?
 

Garbz

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I don't think you can. Perspective distortion correction seems to be something that isn't part of Lightroom's repertoire. The lens correction filter in Photoshop has a horizontal transform feature which corrects lines but you need to take care as the result can quickly look unrealistic.

You can get around this at the time of shooting if you have a tilt shift lens, or perspective correction as Nikon calls them PC-Nikkors.
 

bratkinson

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Distortion is a fact of life regardless of what lens is being used. When using wide-angle lenses, it becomes very apparent as vertical lines in the photograph are either bent or not vertical. With telephoto lenses, 'distance compression' makes whatever is physically further away and sqeezes it all together into looking like it's 'nearer'.

I'm no optics engineer, so I'll just use a fictional, one element piece of glass as a lens for this discussion. IÂ’m just trying to remember my high school science class from eons agoÂ…

Consider a wide angle lens. The convex shape it has (bulged out in the center) gets more and more bulged out the 'wider angle' it is. For convenience sake (not the real world), consider only the light rays that enter perpendicular to the point on the front side of the element. I'm visualizing a pincusion with 100s or even 1000s of pins stuck in the top half all at 90 degrees to the surface of the pincushion where it makes contact. Now then...the light rays entering have to be bent 'just right' (coming out the flat bottom of the pincushion) to hit the sensor (just below the bottom of the lens/pincushion) at a 90 degree angle also. What that means is the light from the 'widest' angle has to be bent the most to hit the sensor at 90 degrees. Light coming in about half way between the outer edge of the lens and the center would only have to be bent 45 degrees. Obviously, light coming right through the center of the lens already is at 90 degrees to the sensor, so no internal bending of the light ray is necessary. For what it’s worth, the same thing happens with our own peripheral vision…except that our brain ‘straightens things out’ for us and keeps the vertical lines seen out of the corners of our eyes appear to be vertical.

As for a telephoto lens, think of your old magnifying glass you used to fry ants with as a kid (I was a mean, widdo kid, huh?). It’s convex just like the wide angle lens, right? Think of the pincushion above being ‘upside down’ where the flat bottom faces the subject and the pins sticking out the rear. The difference is WHERE that lens is placed relative to the sensor, for the magnifying glass, the retina of your eyeball. Because it’s being used as a telephoto, for this simple discussion, all light rays enter in parallel. Think about a telephoto shot of the moon. For all intents and purposes, the light rays are coming in parallel. But as the parallel rays EXIT the lens, they EXIT at 90 degrees to the rear surface of the lens. What that does is to ‘widen them’, making them disperse. So, considering 2 parallel light rays coming in the front, one in the center of the lens and the other 1/10 inch away. On the way out, that 1/10 inch is now getting another 1/10 inch apart every 1/10 inch further away from the rear surface of the lens (45 degree angle). So, if the sensor was a mere 5/10 inch behind the lens, the object would be 5 times bigger than it was ‘going in’. The very act of moving your magnifying glass closer or further from your eye made the object smaller or bigger, as the distance to the rear element changes.

Now throw in perhaps 8-10 more lens ‘elements’ of varying shapes, to correct for the angular variations caused by light wave frequency (reds are very different than blues), color variations due to the chemical composition of the individual elements, the internal and external reflections that occur any time light passes from one surface to another (air/glass, glass/air, glass/glass), and the list goes on. Now make that a ZOOM lens moving groups of them concurrently and that’s where the computers REALLY have to take over to figure things out.

Needless to say, minor distortions caused by all the pieces of glass and whatnot are minimized as much as possible by the lens manufacturers, within the constraints of costs and market price point (eg, a $300 lens isn’t as ‘perfect’ as a $10,000 lens).

In essence, wide angle lenses have no choice but to bend/angle otherwise ‘straight’ lines to ‘squeeze everything in’ to fit the sensor. While it would be nice to expect the camera to behave as our eyeball and brain combination do to ‘fix’ what we see/perceive, until hard lenses become flexible as our eye lenses, and the computer in the camera as smart as our brains, don’t expect it to happen anytime soon.
 
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480sparky

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I don't think you can. Perspective distortion correction seems to be something that isn't part of Lightroom's repertoire. The lens correction filter in Photoshop has a horizontal transform feature which corrects lines but you need to take care as the result can quickly look unrealistic.

You can get around this at the time of shooting if you have a tilt shift lens, or perspective correction as Nikon calls them PC-Nikkors.

I just use the Perspective tool in GIMP.
 

Buckster

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Perspective correction in LR is achieved by going to the Lens Correction module/palette, check the Enable Profile Corrections check box, click on the Manual tab, then work the Vertical and/or Horizontal sliders. A grid will appear as you work it to help you with alignment.
 

Garbz

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In essence, wide angle lenses have no choice but to bend/angle otherwise ‘straight’ lines to ‘squeeze everything in’ to fit the sensor. While it would be nice to expect the camera to behave as our eyeball and brain combination do to ‘fix’ what we see/perceive, until hard lenses become flexible as our eye lenses, and the computer in the camera as smart as our brains, don’t expect it to happen anytime soon.

Bend yes, angle no. If you have a wide angle rectlinear lens and you shoot a square straight on you will get a square. The problem with wide angle perspective distortion is that if you are standing at the bottom of the square looking up then the light is bent exactly as you say, but that only applies if the piece of glass is parallel to the sensor plane.

Tilt shift lenses provide the ability to tilt or shift critical elements so you can change the relative position of light rays being bent. This allows you to adjust the angle of lines affected by perspective. A very ugly looking example of this is here: http://www.nicovandijk.net/pcnikkor.jpg Perspective distortion caused all vertical lines in the building to appear angled and converge at a point, but by tilting the elements in the sensor you can make all vertical lines appear parallel (which in this case looks crap).

Perspective correction in LR is achieved by going to the Lens Correction module/palette, check the Enable Profile Corrections check box, click on the Manual tab, then work the Vertical and/or Horizontal sliders. A grid will appear as you work it to help you with alignment.

You're right. Caveat because I was looking for this function earlier when I replied, it does not appear to work on JPEGs.
 

Heitz

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Yea you can 'deal' with it in light room but 'fix it' you cannot. It's an optical property of the lens. You just gotta frame correctly and have good glass
 

Buckster

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Perspective correction in LR is achieved by going to the Lens Correction module/palette, check the Enable Profile Corrections check box, click on the Manual tab, then work the Vertical and/or Horizontal sliders. A grid will appear as you work it to help you with alignment.

You're right. Caveat because I was looking for this function earlier when I replied, it does not appear to work on JPEGs.
It works with all formats on mine (LR 4.4 64 bit Win7), including JPGs.
 

Garbz

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Yea you can 'deal' with it in light room but 'fix it' you cannot. It's an optical property of the lens. You just gotta frame correctly and have good glass

You got that backwards. You can fix it in Lightroom, but you can't fix it by framing correctly. You don't need to have good glass you need to have highly specialised and very configurable glass.

It works with all formats on mine (LR 4.4 64 bit Win7), including JPGs.

Dafaq? Hmm maybe it's not a JPEG issue then but a small picture issue? The JPEGs are only 3.5mpxl. When I slide the distortion sliders around it stretches the image horizontally or vertically but doesn't keystone it like it does with my RAWs...
Or maybe my computer is just being illogical like computers often are :)
 

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