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10-04-2012, 06:42 AM   #1
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lens aberrations category

what are the characteristics of lens being referred to when, in the lens reviews on this site, the category is 'Aberrations' ? what does that mean ? thanks. (and, are there various different kinds of aberrations it is referring to ? )

10-04-2012, 07:05 AM   #2
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these are flaws in the optical design.

the usual aberrations are with respect to color, i.e. chromeric aberrations, wither lateral (where you see red banding / shading on one side and green on the other side of the subject) or longitudinal, (where you see green halo around the entire subject when it is behind the plane of focus, and violet halo around the entire subject when it is in front of the plane of focus) Chromeric aberrations are caused by the light not converging at the same point for each color.

but aberrations may also refer to lens distortions such as barrel distortion (parallel lines bend outwards as you move from edge to middle of the frame like a fisheye) pincushion distortion where parallel lines move together in the middle of the frame (the opposite of barrel distortion), Mustache distortion where parallel lines are wavy, etc.......

i hope this helps
10-04-2012, 07:10 AM   #3
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I don't remember the majority of my optical theory from over 35 years ago, so I can't give you many specifics, but the glass in lenses affects what happens to light in a variety of ways.

Glass itself either refracts (bends) or reflects light. This is what happens in a prism that lets you see the different colors. The different colors refract at different speeds and angles, so after going through a prism you can see the different wavelengths. The curvature of the surface of the lens determines how much it refracts light. When you put two or three slabs of glass together to create one composite lens, the different layers have different effects as well. So you get what are called aberrations. Aberrations basically means abnormalities.

With binoculars, the different size objective lenses, the outside lens facing your subject, can result in bending of the extreme outside edges of the view, more or less field of view, pincushion effect, and others. With my 18-55 if I take a picture of a building in town at 18mm I notice the edges and the street lights lean inward. That's an aberration. I think my 28mm might do the same, but haven't tried it, I use it mostly for landscapes.

You can also get color fringes, another form of aberration. One of my lenses gets me a purple fringe around everything if I shoot in bright daylight. Called chromatic aberration. Fisheye lenses exploit an aberration to the extreme, pincushion I think, making for some pretty interesting pictures. Unfortunately most people tend to over use them and I get tired of it pretty quick.

I'm sure there are others I don't even know about, or remember, and some of the other more technically minded guys here will have a lot more and better information.

EDIT: I see Lowell already corrected me without trying...barrel distortion, not pincushion...
10-04-2012, 07:18 AM   #4
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If you consult this page on Wikipedia there is a handy little box to the right on the page near the top that gives you further links to most of the essentials:

  • distortion
  • spherical aberration
  • coma
  • astigmatism
  • field curvature
  • chromatic aberration
All something, Mother Nature has invented to keep lens designers busy.

What it means in practice for us as photographers is, that no single, spherical lens element can produce nice, clean pictures and lens designers have to resort to multiple element designs with different glass types and thus, refractive index, aspherical lens curvatures etc. etc. And THAT again means that quality lenses cannot and will not ever be really cheap.........

10-04-2012, 04:01 PM   #5
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thank you guys, lotta stuff to consider then , with the aberrations, i do notice a lot, aside from the other things you mention, try as i do to get a photo straight in the viewfinder, when i look at it later , it's hardly ever straight the same as i had it , not so much curved as tipped one way or other, maybe the fifty mm lens i have is the best for straightness , the wider 35 is more prone to this un-straightness, and the 30 even moreso.
10-04-2012, 07:13 PM   #6
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QuoteQuote:
try as i do to get a photo straight in the viewfinder, when i look at it later , it's hardly ever straight the same as i had it , not so much curved as tipped one way or other,
If you mean what I think you do, that's still one of my biggest problems. Take a landscape I think is straight, look later and find out it's tilted 5 one way or the other so the horizon slopes off to the bottom corner. Only cure for that is practice, and I suppose I'll never get enough practice, I still do it after 30 years...The only way I can get a landscape straight is to use a tripod.

For nature shots, birds, insects and such it's usually not such a problem, often you can't tell it but with things like a duck swimming around, it's not hard to see. And I can't use a tripod for that kind of thing.

Fortunately even the simple photo editing software I use, Irfan View, has a correction for that. I can set it at any angle I want, including parts of a degree by simply making it a decimal. So if I need to tilt it 3.6 it's no problem and as if by magic it's a straight picture. Then I have to crop it because the entire picture is cockeyed.

If that's what you're having trouble with, the only cure I know of is practice. And like I said, after 30 years I still do it regularly.
10-04-2012, 08:16 PM   #7
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QuoteOriginally posted by glinda Quote
try as i do to get a photo straight in the viewfinder, when i look at it later , it's hardly ever straight the same as i had it , not so much curved as tipped one way or other,
This can also be caused by the viewfinder and sensor being out of alignment, as it is on my bodies. Easy enough to fix in Elements by dragging a line across the image which you want horizontal. I should also mention distortions such as barrel distortion (bulging out), pin-cushion (edges curving in), keystoning (narrowing to the top or side when the subject, such as a building, is shot from an angle) and vignetting can also be corrected in the "filter" menu.
10-05-2012, 06:29 AM   #8
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QuoteOriginally posted by glinda Quote
try as i do to get a photo straight in the viewfinder, when i look at it later , it's hardly ever straight the same as i had it .
As mentioned there may be a mechanical reason, but your brain and sense of balance has a lot to do with it too. For example, I recently took my first sailboat ride. The boat was at a 20+ degree lean to horizontal as we headed toward shore. Naturally I was at a nearly 20 degree lean also. I figured it would make a great shot. I did my best to use the shore as 'level' as I shot through the mast and rigging. The picture had all the dynamics I wanted - only that shore was far from horizontal. In spite of consciously trying to put shore at equal points on the frame sides, that lake had nearly a 7 degree slope in my captured image. I had to sacrifice a bit of my composition to fix matters in post processing.

A bubble level affixed to the flash shoe can be a necessity in hilly terrain - but of course, you will also need some kind of support to make use of it.

10-07-2012, 10:56 AM   #9
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QuoteOriginally posted by glinda Quote
thank you guys, lotta stuff to consider then , with the aberrations, i do notice a lot, aside from the other things you mention, try as i do to get a photo straight in the viewfinder, when i look at it later , it's hardly ever straight the same as i had it , not so much curved as tipped one way or other, maybe the fifty mm lens i have is the best for straightness , the wider 35 is more prone to this un-straightness, and the 30 even moreso.
Since you mention the wider lenses being more "unstraight" - I wonder if you are looking at how lines converge / diverge from vertical towards the sides of the frame. If you have the camera pointed slightly up or down the vertical lines will angle to the sides, even though the horizontal lines dont, and this effect is more obvious with wider lenses, When I started using a 20mm lens on full-frame I had to pay careful more attention to this; especially since I use it on a rangefinder camera where you don't see the image through the lens until you process the film. (One case where the LCD on a digital is of use.)
10-07-2012, 11:08 AM   #10
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The levels are a problem for me as well, and always have been, so I exchange my focus screen for one with lines that I can use (when I remember ) to keep things level or vertical. They also help when reducing keystone. I can back off or zoom out to leave room for the correction and subsequent cropping.
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