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09-28-2009, 01:39 AM   #1
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Lenses - terms used

I've been visiting the lense section in the forum for quiet a while now and i couln't help but wonder about some of the terms used/mentioned. i tried searching for the description but im really not sure if i understand what it would exactly. It would be helpful for us newbies if some pros could demonstrate a photo and point ut out

here's the list: (add up if theres important terms lacking)
1. chromatic abberations
2. Vignetting
3. Flare
4. Distortion, barrel distortion
5. cyan fringing,
6. Aperture blades

Lense features
(ex. from DA50-135mm)
1. ED,
2. IF,
3. SDM
(ex. from Tamron 17-50mm)
1. Aspherical
2. XR
3. LD
4. IF
5. DI

09-28-2009, 03:03 AM   #2
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For lenses have a look at this site. There is a glossary there too.

Welcome to Bojidar Dimitrov's Pentax K-Mount Page

Have you tried a google search for the other terms?
09-28-2009, 03:48 AM   #3
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thx for the help in the lenses. il try searching again later for some pictures about the terms.
09-28-2009, 07:29 AM   #4
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1. chromatic aberrations - Chromatic aberration - Wikipedia, the free encyclopedia
2. Vignetting - Vignetting - Wikipedia, the free encyclopedia
3. Flare = Lens flare - Wikipedia, the free encyclopedia
4. Distortion, barrel distortion - Distortion (optics) - Wikipedia, the free encyclopedia
5. cyan fringing, - see chromatic aberration
6. Aperture blades - Diaphragm (optics) - Wikipedia, the free encyclopedia
... also see
Bokeh - Wikipedia, the free encyclopedia
Shutter (photography) - Wikipedia, the free encyclopedia




09-28-2009, 07:41 AM   #5
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Terms

QuoteQuote:
here's the list: (add up if theres important terms lacking)
1. chromatic abberations
2. Vignetting
3. Flare
4. Distortion, barrel distortion
5. cyan fringing,
6. Aperture blades
1. Chromatic aberration. If you've ever used a prism, you know that, when light passes through, different colors are bent to different angles. A lens is a special kind of prism, but as the various colors of light pass through the individual lens elements, their rays are bent differently. This means that different colors of light focus at different points. This causes the light to be separated on the image, rather than focused at the same point. The best lens designers go to great lengths to design CR out of their lenses. This is part of the reason that good lenses cost a lot more than lousy lenses.

2. Vignetting: Any lens throws a circle of light onto the image sensor or film plane. The rectangular image we are used to is only part of that circle. If the image is larger than the circle, then the corners or edges will be dark, with no discernable image. Filters or lens hoods can cause vignetting, as well.

3. Flare: Light coming into the lens from odd angles, not in the image, can bounce around the internal surfaces of the lens elements. Eventually, some of this light reaches the sensor, causing glare and reflections on the image. This is why lens hoods are useful. As long as they don't block the image and cause vignetting, they will block those stray rays of light.

4. Barrel distortion: This is a type of image distortion, in which the straight vertical and horizontal sides of the image bow outwards from the center, sort of like the staves in a barrel. The opposite is pincushion distortion, in which the lines bow inward.

5. Cyan fringing. For some reason, some sensors, under certain conditions, cause the cyan color to be separated from the rest. This happens around sharp edges in the image. It shows up as a purple or cyan fringe around the edge.

6. Aperture blades: every lens, except long mirror lenses have an aperture, or iris to reduce the amount of light passing through. This iris consists of thin metal blades that close down as you turn the aperture ring. Cheaper lenses may have only six blades. Some lenses have twenty blades. This shows up in the out of focus areas of the image, something that is referred to as "bokeh". Point sources in the image that are out of focus will show up as polygons with as many sides as there are aperture blades. The more blades, the closer those polygons come to being circles. This is important because the out of focus areas can add or detract from the overall image. Generally, the more blades there are, the more pleasing most people find the bokeh to be.
09-29-2009, 03:57 AM   #6
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Nice! thx...
09-29-2009, 08:13 AM   #7
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QuoteOriginally posted by rustynail925 Quote
Lense features
(ex. from DA50-135mm)
1. ED,
2. IF,
3. SDM
(ex. from Tamron 17-50mm)
1. Aspherical
2. XR
3. LD
4. IF
5. DI
Since no has has addressed there, I though I'd make a quick stab. Primarily to point out that most of these aren't "features" - they are just descriptions for the type of glass or type of lens design used. They are not really relevant - they don't change the behavior of the lens. That is, if they didn't tell you a lens was ED (or LD - both mean the same thing, reduced dispersion), there would be no way to tell. That is, being ED doesn't let you do somethign you couldn't if it weren't ED, and just because a lens uses ED/LD glass doesn't automatically make it better.

The only acronyms from this list that actually affect the *behavior* of the lens are IF and SDM. IF is internal focus, menaing the lens does not change length when you turn the focus ring. SDM means it has a focusing motor within the lens itself, as opposed to (or in addition to) the more traditional AF method whether the motor in the camera focuses the lens by turning a screw within the lens. SDM lenses focus more quietly; in theory they might also be able to focus faster, but that isn't actually case much of the time.
09-29-2009, 09:14 AM   #8
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Aspherical

I'll take a stab at another one: aspherical.

Most lens elements are spherical sections. That means that the surface of the glass might be cut out of a large sphere of glass. This also means that the cross-section of the element is a circular section.

In an aspherical lens element, the surface is NOT a spherical section. It is a parabaloid. A cross section would be a parabolic section.

Spherical elements have traditionally been used because they are cheaper and easier to grind, but, in reality, a parabola is the proper shape to use. It has to do with the way the angles of the light rays are bent as they pass through the glass. The light rays at the edge of the glass need to be bent differently than those that pass through the center. I'd have to consult a text on optics, but suffice it to say that a parabola does a better job of focusing the light rays than a circle does.

In most camera lenses that are labelled aspherical, only one or two elements out of the possible seven or eight are aspherical. The rest are spherical.

09-29-2009, 12:37 PM   #9
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just wanna ask this with regards to the aperture blade.
more blades the better?
09-29-2009, 01:31 PM   #10
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QuoteOriginally posted by rustynail925 Quote
just wanna ask this with regards to the aperture blade.
more blades the better?
Yeah well you could look at cheaper lenses like the $120 Canon f1.8 50mm. It has 5 diaphragm blades and thus the resulting bokeh (or background blurring) looks unnatural and polygonal to an extent.

So yes, the more blades the better.

Think of a pinhole camera. The pinhole is great because it's a circle, rather than a pentagon. The closer you can get to a circle the better... ie. the more obtuse the angles between each blade is, the better.
09-29-2009, 11:23 PM   #11
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Thx you guys bout the infos and stuffs!
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