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Loswr
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Image quality (IQ) is what you make of it. 
Seriously, it's the quality of an image delivered by the lens, and that's affected by several parameters. Many people equate IQ with sharpness, but it's important to recognize that sharpness is only one characteristic of IQ, and sharpness itself has multiple meanings. Other characteristics include contrast and color rendition. There are various distortions and aberrations that affect IQ - geometric distortion, spherical aberration, chromatic aberrations (lateral and axial), vignetting, flare, etc.
The recording medium also affects IQ - a lens does not function in isolation (although there are ways to test lenses in isolation, those are practically meaningless, since as a lens user, you need a camera to use the lens with). Higher MP sensors result in an increase in perceived sharpness, but also sometimes magnify the effects of aberrations.
Post-processing is another factor, involving yet more tradeoffs. Many of those aberrations listed above can be corrected by software (especially geometric distortion, lateral CA, and vignetting). However, some of those corrections result in a loss of sharpness in the corrected areas.
So...I guess what it boils down to is that a lens with great IQ takes pretty pictures.
I'd recommend not getting too hung up on lens sharpness - there are a lot of things you can do to increase the IQ of your images that don't depend directly on the lens. For example, if you're shooting static subjects, using a tripod helps enormously. Using a lens hood can increase contrast and color saturation in your images compared to the same lens without a hood. Etc.
Or, if you prefer, go buy yourself an ISO 12233-type chart that costs more than some L-series lenses, and go nuts... Heck, it works for me! ;D
Seriously, it's the quality of an image delivered by the lens, and that's affected by several parameters. Many people equate IQ with sharpness, but it's important to recognize that sharpness is only one characteristic of IQ, and sharpness itself has multiple meanings. Other characteristics include contrast and color rendition. There are various distortions and aberrations that affect IQ - geometric distortion, spherical aberration, chromatic aberrations (lateral and axial), vignetting, flare, etc.
The recording medium also affects IQ - a lens does not function in isolation (although there are ways to test lenses in isolation, those are practically meaningless, since as a lens user, you need a camera to use the lens with). Higher MP sensors result in an increase in perceived sharpness, but also sometimes magnify the effects of aberrations.
Post-processing is another factor, involving yet more tradeoffs. Many of those aberrations listed above can be corrected by software (especially geometric distortion, lateral CA, and vignetting). However, some of those corrections result in a loss of sharpness in the corrected areas.
So...I guess what it boils down to is that a lens with great IQ takes pretty pictures.
I'd recommend not getting too hung up on lens sharpness - there are a lot of things you can do to increase the IQ of your images that don't depend directly on the lens. For example, if you're shooting static subjects, using a tripod helps enormously. Using a lens hood can increase contrast and color saturation in your images compared to the same lens without a hood. Etc.
Or, if you prefer, go buy yourself an ISO 12233-type chart that costs more than some L-series lenses, and go nuts... Heck, it works for me! ;D
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neuroanatomist said:Image quality (IQ) is what you make of it.
Seriously, it's the quality of an image delivered by the lens, and that's affected by several parameters. Many people equate IQ with sharpness, but it's important to recognize that sharpness is only one characteristic of IQ, and sharpness itself has multiple meanings. Other characteristics include contrast and color rendition. There are various distortions and aberrations that affect IQ - geometric distortion, spherical aberration, chromatic aberrations (lateral and axial), vignetting, flare, etc.
The recording medium also affects IQ - a lens does not function in isolation (although there are ways to test lenses in isolation, those are practically meaningless, since as a lens user, you need a camera to use the lens with). Higher MP sensors result in an increase in perceived sharpness, but also sometimes magnify the effects of aberrations.
Post-processing is another factor, involving yet more tradeoffs. Many of those aberrations listed above can be corrected by software (especially geometric distortion, lateral CA, and vignetting). However, some of those corrections result in a loss of sharpness in the corrected areas.
So...I guess what it boils down to is that a lens with great IQ takes pretty pictures.
I'd recommend not getting too hung up on lens sharpness - there are a lot of things you can do to increase the IQ of your images that don't depend directly on the lens. For example, if you're shooting static subjects, using a tripod helps enormously. Using a lens hood can increase contrast and color saturation in your images compared to the same lens without a hood. Etc.
Or, if you prefer, go buy yourself an ISO 12233-type chart that costs more than some L-series lenses, and go nuts... Heck, it works for me! ;D
Those charts cost that much?
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Flake
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You would perhaps benefit from visiting photozone.de and taking a look at how they test the lenses, and what those tests translate to in real life.
One measure which I do find useful is resolution across the frame, especially on full frame cameras, a lens which gives very high resolution in the centre whilst giving poor resolution at the borders & corners will not be as useful to me as a lower resolution more evenly spread across the frame. Many lens failings such as distortion or chromatic aberations can be removed with software, others cannot & it's important to know the difference!
There are other factors which are worth considering for instance, build quality, it would be pointless to have a lens which gave the best IQ but was so poorly made that it lost the advantage over time, or quality control issues which make getting a good copy a lottery.
Of course technique will be needed to acheive the very best results from any lens, and it's worth while learning how to use the equipment you have to its best.
One measure which I do find useful is resolution across the frame, especially on full frame cameras, a lens which gives very high resolution in the centre whilst giving poor resolution at the borders & corners will not be as useful to me as a lower resolution more evenly spread across the frame. Many lens failings such as distortion or chromatic aberations can be removed with software, others cannot & it's important to know the difference!
There are other factors which are worth considering for instance, build quality, it would be pointless to have a lens which gave the best IQ but was so poorly made that it lost the advantage over time, or quality control issues which make getting a good copy a lottery.
Of course technique will be needed to acheive the very best results from any lens, and it's worth while learning how to use the equipment you have to its best.
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Penn Jennings
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From time to time I have had images that I thought were soft or out of focus or otherwise lacking even though I was positive that the the image was focused when I took it. For years I just kind of ignored the problem since it was a small percentage of images. One day though I started wondering why my images weren't are sharp or contrasty as it should be or other images that I see. After stepping back and thinking things over I started to wonder about my lenses. Most of the time when I would ask people about lens I get an answer but not a detailed and clear answer. The people at the stores were some of the worse, their basic answer is "You get what you pay for". Thats true but what does that mean. After doing a lot of reading (Hopefully what I read wasn't crap) I kind of boiled things down in my own head like this, and remember, I'm by no means an expert.
Image (Lens) Quality consists of:
All these make up image (lens) quality, this all ignores your body and shoot habits. However, a great lens and decent body will often yield better results than a decent lens and great body. An prime L lens and a T2i may easily out perform a middle of the road zoom and 1D. A lot of these though will be personal preference.. or client perference.
If someone thinks that I got anything wrong, please tell me. I'm still learning the stuff too.
Below are some links that might help:
http://www.luminous-landscape.com/tutorials/understanding-series/understanding-mtf.shtml
http://www.canonrumors.com/tech-articles/how-to-test-a-lens/
http://www.photozone.de/
http://www.expertphotography.com/prime-vs-zoom-lenses
http://www.edmundoptics.com/products/displayproduct.cfm?productid=1665
Thats all probably more than you wanted... Sorry LOL
[/list]
Image (Lens) Quality consists of:
- Resolution - This typically how sharp the image is. It can vary greatly between the center of the frame and the corners and depending on aperture. With more expensive lenses, you usually get better corner performance.
- Focusing speed/precision - If your lens doesn't focus accurately or fast enough... well, the rest is pointless
- Speed (Aperture) - A fast lens may improve image quality by allowing you with a higher shutter speed. Beware though, I usually try to avoid shooting wide open if I can because it introduces several other factors that reduce image quality.
- Distortion (Usually barrel or pincushion) - This is the distortion that you are more likely to see when shoot ing with zoom. This is sort of hard to explain in 1 or 2 sentences
- Vignetting - This is light fall of at the corners, usually occurs when shooting wide open
- Chromatic Aberrations - This is color distortion. This can occur in areas out of focus or areas of transition.
All these make up image (lens) quality, this all ignores your body and shoot habits. However, a great lens and decent body will often yield better results than a decent lens and great body. An prime L lens and a T2i may easily out perform a middle of the road zoom and 1D. A lot of these though will be personal preference.. or client perference.
If someone thinks that I got anything wrong, please tell me. I'm still learning the stuff too.
Below are some links that might help:
http://www.luminous-landscape.com/tutorials/understanding-series/understanding-mtf.shtml
http://www.canonrumors.com/tech-articles/how-to-test-a-lens/
http://www.photozone.de/
http://www.expertphotography.com/prime-vs-zoom-lenses
http://www.edmundoptics.com/products/displayproduct.cfm?productid=1665
Thats all probably more than you wanted... Sorry LOL
[/list]
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There are good posts in here so I will try to avoid rehashing it. Instead I'll try to simplify the categories and weigh their relative importance (to me):
(The first three points are described more fully and clearly by Michael Reichmann at Luminous Landscape.)
- Microcontrast "sharpness." Also known as resolving power: The ability to resolve small details. Almost all lenses transmit acceptable levels of detail for full scenes in smaller prints or for compressed-size web use, but some photographers have stricter demands on their lenses (astronomers, bird photographers, and anyone who finds themselves cropping much of the image away to get a small portion of the scene).
- Whole-scene contrast (also appears as "sharpness"). Also known as acutance - how "acute" the difference between regions at a border appears. This is important because you want vivid scenes to appear vivid, not gray.
- Boke (short language note: A lot of people - probably most - add an "h" to the end; having studied Japanese, it is my opinion this promotes drawing out the word, or stressing one of the morae, the syllables): What I sometimes call "defocus area quality." The quality of the blurred area not in focus. Typical descriptions of boke are "nervous," "creamy," "distracting," and "smooth." Like any other optical effect, any type of defocus area quality can be useful in some situations and not useful in others, though the general preference is for boke to be smooth, continuous, and not have a spherical pattern (the blur area of some lenses is a bit like looking into a well, or having the centers closer to you). One thing is certain, however: Boke is the most reliable tool available to photographers to add structure to an unstructured scene, as it merely sketches out, in big blobs of light, the parts of the scene you wish to isolate from the subject (you hear a lot about the "subject isolation" capability of large aperture lenses). Can't reposition your camera to line up with architectural elements, or the scene is completely chaotic and you have to emphasize a relationship or downplay the chaotic totality of the scene? Get a wide lens and open it wide. Additionally, shooting in very low light is improved with wide aperture lenses. Almost always, photographers prefer to have lenses that do not detract attention from the elements in focus, while retaining the correct color of background elements.
- Control over optical aberrations. Some of these aberrations affect blur quality, so we are focusing on other effects. Some common problems are purple fringing (sometimes I see this blamed on recent DSLR sensors) and green fringes. Some fringes are relatively easy to correct; others are more difficult. In the same vein, the ability of a lens to render straight lines as straight, without any distortion, is also important, though its cause is different. Different types of glass, lens coatings, and lens designs minimize these effects - one of the most sought-after designs is the apochromatic lens, which is designed to ensure that different wavelengths of light focus at the same point at the image sensor (no simple task actually).
- An even less typical criteria usually considered in lens IQ, but vitally important, is the presence or hopeful absence of focus shift. In some lenses, when the aperture is reduced, the area which is rendered in sharp focus changes. Since DSLRs and even mirrorless cameras generally meter and focus wide open, this presents a real problem if you focus on a subject with the lens aperture wide open for a bright viewfinder and quick AF, but it turns out that the area in focus has changed after the aperture has closed for the shot (and you don't get any feedback suggesting this until you review the shot).
A couple other points: The susceptibility of the lens to flare is important, especially for wide lenses, and in that case the type and quality of the lens flare spots can be considered, just like with boke. There are a number of effects tracing their origins to the aperture inside the lens, as well - some render "sunstars" from relatively bright light sources when closed down, and the number of points changes depending on the aperture construction. The out-of-focus circles (boke quality) is degraded if the aperture is not round - many older lenses with straight aperture blades render polygonal blur circles. And DO (diffractive optics) lenses, which operate on the same Fresnel lens principle as a lighthouse's lens, also can have unsatisfying blur quality (like a bullseye) and haloing - but they don't have to.
(The first three points are described more fully and clearly by Michael Reichmann at Luminous Landscape.)
- Microcontrast "sharpness." Also known as resolving power: The ability to resolve small details. Almost all lenses transmit acceptable levels of detail for full scenes in smaller prints or for compressed-size web use, but some photographers have stricter demands on their lenses (astronomers, bird photographers, and anyone who finds themselves cropping much of the image away to get a small portion of the scene).
- Whole-scene contrast (also appears as "sharpness"). Also known as acutance - how "acute" the difference between regions at a border appears. This is important because you want vivid scenes to appear vivid, not gray.
- Boke (short language note: A lot of people - probably most - add an "h" to the end; having studied Japanese, it is my opinion this promotes drawing out the word, or stressing one of the morae, the syllables): What I sometimes call "defocus area quality." The quality of the blurred area not in focus. Typical descriptions of boke are "nervous," "creamy," "distracting," and "smooth." Like any other optical effect, any type of defocus area quality can be useful in some situations and not useful in others, though the general preference is for boke to be smooth, continuous, and not have a spherical pattern (the blur area of some lenses is a bit like looking into a well, or having the centers closer to you). One thing is certain, however: Boke is the most reliable tool available to photographers to add structure to an unstructured scene, as it merely sketches out, in big blobs of light, the parts of the scene you wish to isolate from the subject (you hear a lot about the "subject isolation" capability of large aperture lenses). Can't reposition your camera to line up with architectural elements, or the scene is completely chaotic and you have to emphasize a relationship or downplay the chaotic totality of the scene? Get a wide lens and open it wide. Additionally, shooting in very low light is improved with wide aperture lenses. Almost always, photographers prefer to have lenses that do not detract attention from the elements in focus, while retaining the correct color of background elements.
- Control over optical aberrations. Some of these aberrations affect blur quality, so we are focusing on other effects. Some common problems are purple fringing (sometimes I see this blamed on recent DSLR sensors) and green fringes. Some fringes are relatively easy to correct; others are more difficult. In the same vein, the ability of a lens to render straight lines as straight, without any distortion, is also important, though its cause is different. Different types of glass, lens coatings, and lens designs minimize these effects - one of the most sought-after designs is the apochromatic lens, which is designed to ensure that different wavelengths of light focus at the same point at the image sensor (no simple task actually).
- An even less typical criteria usually considered in lens IQ, but vitally important, is the presence or hopeful absence of focus shift. In some lenses, when the aperture is reduced, the area which is rendered in sharp focus changes. Since DSLRs and even mirrorless cameras generally meter and focus wide open, this presents a real problem if you focus on a subject with the lens aperture wide open for a bright viewfinder and quick AF, but it turns out that the area in focus has changed after the aperture has closed for the shot (and you don't get any feedback suggesting this until you review the shot).
A couple other points: The susceptibility of the lens to flare is important, especially for wide lenses, and in that case the type and quality of the lens flare spots can be considered, just like with boke. There are a number of effects tracing their origins to the aperture inside the lens, as well - some render "sunstars" from relatively bright light sources when closed down, and the number of points changes depending on the aperture construction. The out-of-focus circles (boke quality) is degraded if the aperture is not round - many older lenses with straight aperture blades render polygonal blur circles. And DO (diffractive optics) lenses, which operate on the same Fresnel lens principle as a lighthouse's lens, also can have unsatisfying blur quality (like a bullseye) and haloing - but they don't have to.
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