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Old May 9, 2009, 6:07 AM   #1
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Default Sigma Lenses, what is the difference

Hi, can anyone help me. In laymans terms, what is the difference between a 70-300mm f/4-5.6 DG Macro sigma and a 70-300mm f/4-5.6 APO DG MACRO Sigma. I have looked at their website but its all gobbledegook to me. Thanks
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Old May 9, 2009, 6:30 AM   #2
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APO stands for apochromatic, meaning the lens has special coatings to correct for chromatic aberrations thus providing better color and contrast as well as a sharper image. Here is Sigma's definition:


An Apochromat lens, which keeps color aberration to a minimum, using Special Low Dispersion glass is called an APO series lens at Sigma. As the refractive index of glass depends on the wavelength of the light, color aberration occurs when different colors form images at different points. This problem often occurs with telephoto lenses. In the case of normal optical glass it can only be corrected for two primary spectral colors by combination of low dispersion convex and high dispersion concave lenses. Sigma Apo lenses use SLD (Special Low Dispersion) or new ELD (Extraordinary Low Dispersion) glass to offer superior sharpness, high contrast and color correction by minimizing the chromatic aberration.

I only have one sigma lens and can't really speak to if it's worth the extra money. I do know, the higher level Nikons, such as the D300 automatically correct CA.
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Old May 16, 2009, 7:59 PM   #3
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Originally Posted by rjseeney View Post
APO stands for apochromatic, meaning the lens has special coatings to correct for chromatic aberrations thus providing better color and contrast as well as a sharper image. Here is Sigma's definition:


An Apochromat lens, which keeps color aberration to a minimum, using Special Low Dispersion glass is called an APO series lens at Sigma. As the refractive index of glass depends on the wavelength of the light, color aberration occurs when different colors form images at different points. This problem often occurs with telephoto lenses. In the case of normal optical glass it can only be corrected for two primary spectral colors by combination of low dispersion convex and high dispersion concave lenses. Sigma Apo lenses use SLD (Special Low Dispersion) or new ELD (Extraordinary Low Dispersion) glass to offer superior sharpness, high contrast and color correction by minimizing the chromatic aberration.

I only have one sigma lens and can't really speak to if it's worth the extra money. I do know, the higher level Nikons, such as the D300 automatically correct CA.
The first part is not correct.
Coatings do not modify refractive index or dispersion characteristics of lens elements.

In a perfect world, we would only need one glass element for a perfect lens. However, because different colors of light have different dispersion characteristics through glass(they bend by different amounts), the same colors don't focus on exactly the same point. In order to compensate for this, two different types of glass are used: "Crown glass" for convex lens elements, and "Flint glass" for concave elements. These two different types of glass have a different refractive index and different dispersion. When used in combination in a lens, depending on the design, the red and blue light will focus more or less on the same point. This is called an achromatic doublet. In order to correct for green as well, one or more extra low dispersion lens elements can be used, which along with the other two different types of glass, are called apochromatic lenses. The aim is this case, is to focus red, blue AND green on the same point. However, because of the cost, most lenses don't have the low dispersion lens elements.

Also, lens elements are also mostly ground to a spherical shape(the ideal is parabolic), so additional compensation must be designed to reduce this problem(extra glass elements). In some cases, some aspherical lens elements are used.

So far as the 70-300mm f/4-5.6 DG Macro sigma and a 70-300mm f/4-5.6 APO DG MACRO Sigma are concerned, it means that the APO lens should have much better color correction for GREEN, and both should have about the same color correction for red/blue.

~~~~~~~~~
I'm not sure about CA correction by the D300, but if this is actually the case, then the camera would require the CA characteristics for each Nikon lens, probably obtained as firmware data from each lens when it is on the camera. This means that it would not correct CA for older lenses, and probably not at all for any non Nikon lenses.

Last edited by dnas; May 16, 2009 at 8:05 PM.
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Old May 21, 2009, 5:56 AM   #4
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I believe the D300 automatic CA correction works from the various color channels in the image irrespective of the lens:
-> I use a Tokina 16-50 f/2.8 which is exceptional except for its CA...

http://photo.net/nikon-camera-forum/00OT3O
http://www.dpreview.com/reviews/nikond300/page21.asp
http://www.cameralabs.com/reviews/Ni...0/sensor.shtml
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Old May 21, 2009, 7:52 AM   #5
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Ok that's interesting....

So it looks like it's processor in the camera, and not related to preconfigured lens data.
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Old May 24, 2009, 11:07 AM   #6
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I believe the D300 automatic CA correction works from the various color channels in the image irrespective of the lens: ...
That is a totally different class of CA than what is being discussed in this thread.

The CA being discussed in this thread is that which is produced by the lens itself and would affect both digital and film. It manifests itself by there being different points of focus for different colors and hence different image sharpness. Absolutely nothing can be done in post processing to correct for this optical aberation.

The CA that is adjusted by the D300 and by RAW converters is a chromatic aberation in the micro-lenses placed in the front of each photosite on most digital sensors. This CA manifests itself purely as a radial displacement based on color and can be adjusted in software. The amount of displacement is influenced by many of the optical characteristics of the camera lens, but not that lens' chromatic abberations.

BTW, originally achromatic meant that the lens was corrected perfectly for two colors and apochromatic meant perfect correct for three. These days "APO" means "apochromatic-like performance" where the overal chromatic correction is very high, but perfect correction may still only exist for two colors. In "olden days" true apochromatic lenses were critical when doing color separation work and early three-shot color photography. In the "real world" its not so important how many colors for which the lens is perfectly corrected, but how close to perfect the correction is on average across the whole photographic spectrum. o

Last edited by dwig; May 24, 2009 at 11:14 AM.
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Old May 24, 2009, 9:50 PM   #7
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Originally Posted by dwig View Post
That is a totally different class of CA than what is being discussed in this thread.

The CA being discussed in this thread is that which is produced by the lens itself and would affect both digital and film. It manifests itself by there being different points of focus for different colors and hence different image sharpness. Absolutely nothing can be done in post processing to correct for this optical aberation.

The CA that is adjusted by the D300 and by RAW converters is a chromatic aberation in the micro-lenses placed in the front of each photosite on most digital sensors. This CA manifests itself purely as a radial displacement based on color and can be adjusted in software. The amount of displacement is influenced by many of the optical characteristics of the camera lens, but not that lens' chromatic abberations.

BTW, originally achromatic meant that the lens was corrected perfectly for two colors and apochromatic meant perfect correct for three. These days "APO" means "apochromatic-like performance" where the overal chromatic correction is very high, but perfect correction may still only exist for two colors. In "olden days" true apochromatic lenses were critical when doing color separation work and early three-shot color photography. In the "real world" its not so important how many colors for which the lens is perfectly corrected, but how close to perfect the correction is on average across the whole photographic spectrum. o
I'm not sure which is correct, but one of the links at least, indicates that it's correction for CA due to the lenses.

"The in-camera reduction of chromatic aberrations varies in effectiveness depending on the lens in question. Our example shows there’s some reduction, but others have reported an eerily complete removal of fringing from lenses which regularly suffer from the problem. We understand the feature looks for CA in the image, irrespective of the lens used, so it should hopefully work with non-Nikkor optics; we will report back with details as we find them."

Now, I suppose they could be talking about three things:
1. CA due to the lens alone.
2. CA due to the micro lenses in the sensor not receiving the light 90 degrees to the sensor. e.g. Older lenses (non telecentric) sending the light to the sensor off axis. This property of the lens in combination with the micro lenses in the sensor, can produce CA.
3. A combination or 1 & 2, which means that it could correct ANY CA from either (or any other) source!!!

Not sure of the real reason, and would be interested to find out!!!!!

~~~~~~~~~~~~~~~~~~~~~~
Oh, and by the way, I already mentioned achromatic(red & blue correction) and apochromatic(+green correction) in an earlier post.

Last edited by dnas; May 24, 2009 at 9:53 PM.
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Old May 25, 2009, 6:49 AM   #8
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Quote:
Originally Posted by dwig View Post
The CA that is adjusted by the D300 and by RAW converters is a chromatic aberation in the micro-lenses placed in the front of each photosite on most digital sensors. This CA manifests itself purely as a radial displacement based on color and can be adjusted in software. The amount of displacement is influenced by many of the optical characteristics of the camera lens, but not that lens' chromatic abberations...
Does it really matter where the CA is from as long as it is fixed in the image ???
-> http://www.kenrockwell.com/nikon/d300.htm

"The D300 has an amazing automatic ability to fix lateral color fringes. The D300 actually makes lenses look better than they are! This gives better, sharper results. There's no need to activate this, it just works, with AF and even old manual focus and fisheye lenses.
I kid you not: the 10.5mm DX and manual focus 8mm AI-s fisheyes somehow lose their color fringes and snap back into perfection in the corners. The 24-70mm f/2.8 has a little LCA at 24mm on the D200, and it also goes away with the D300. Ditto with the 18-200mm VR at 18mm: no more color fringes on the D300!"


This CA could be in the
micro-lenses OR the lens, but it get fixed regardless:
http://www.flickr.com/groups/photosh...7606054207237/


Last edited by NHL; May 25, 2009 at 7:05 AM.
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