I just picked up a mint SMC Takumar 28mm f3.5 lens. From what I read people say it provides quite good image quality. And the things I read about my Canon T3 kit lens 18-55mm f3.5-5.6 IS, is that it is garbage.
So I was excited to do an A B comparison of these two lenses. And I could not really notice a difference. In some cases the takumar was sharper, but then another case would arise where the kit lens was sharper. Maybe my test weren't very controlled.
Either way, what should I be comparing when assessing image quality? And why does it vary?
Now in the case that parameters are held constant....
Can a lens ever vary the color of the image? If so why?
Could a lens vary the sharpness of an image? If so why?
What other attributes of image quality usually vary across lenses?
Answer
From what I read people say it provides quite good image quality. And the things I read about my Canon T3 kit lens 18-55mm f3.5-5.6 IS, is that it is garbage.
So I was excited to do an A B comparison of these two lenses. And I could not really notice a difference. In some cases the takumar was sharper, but then another case would arise where the kit lens was sharper.
Part of the difference between what you are reading about one lens and the other can be attributed to varying expectations. When reviewers say things such as, "It provides good image quality" about an older lens it usually implicitly includes for the price at which this legacy lens can be acquired. "Good" image quality, as used by reviewers, normally means less than "great", "excellent", or "superb" image quality.
On the other hand, when you read that kit lenses are "garbage" it is often just something that people who don't know any better repeat to sound like they know something. Either that or they seem to expect a cheap kit zoom lens to perform as well as a premium prime lens that costs ten times as much.
When cheap zoom lenses first replaced cheap prime lenses (e.g. a 40mm or 50mm f/1.8) as kit lenses for some of the earliest AF (film) SLRs in the 1990s many of them were fairly poor in the image quality they provided. That is often no longer the case. The difference between the older and more recent kit zoom lenses isn't necessarily due to variations in the optical formulae of the respective lenses as much as it is in improved quality control of the manufacture of the lenses, especially their optical elements or the precision of the alignment of those elements. Sometimes improvements to the optical design or other features such as stabilization are also in the mix.
My first digital SLR was a Canon Rebel XTi (400D). It came with an EF-S 18-55mm f/3.5-5.6 (pre-IS) that was pretty bad. There was a spot to the left of center that was always soft. Even when objects both closer and further from the camera were well focused, whatever was at that spot would be soft. I attribute it to a manufacturing defect in a lens element. Many other users have reported similar low quality examples with the pre-IS kit lenses from Canon and other manufacturers. When Canon improved their 18-55mm kit lenses to include IS, they also seemed to improve the optical quality of the lens by manufacturing it to better standards (or, at the very least, not shipping lenses which performed poorly when tested at the factory). The same type of thing has been reported for other camera makers.
Kit zoom lenses produced in the last few years tend to be better than the kit zoom lenses supplied with early AF film SLRs in the 1990s and the kit zoom lenses supplied with the earliest DSLRs in the 2000s. Yet many people still endlessly repeat the idea that today's kit lenses are no different than their predecessors from nearly three decades ago! At the same time, improvements in more expensive lenses over the same period are not ignored by many of the same people. Every minute difference in performance is noted and emphasized.
What should I be comparing when assessing image quality? And why does it vary?
Most assessments of lenses place the emphasis on acutance (a combination of sharpness and contrast) and how well they deal with common lens aberrations. The seven "classic" lens aberrations are: defocus, spherical aberration, coma, field curvature, astigmatism, geometric distortion, and chromatic aberration. The first six are monochromatic and not affected differently by the varying wavelengths of visible light. The seventh, chromatic aberration, is caused by the differing amounts of refraction to varying wavelengths of light that result from passing through the same lens element. Sometimes the way a lens design deals, or chooses to not deal, with one or more of the aberrations will affect what we might call the "look" of that lens.
In addition, lenses can render different color tints based in varying transmittance of the different wavelengths of light. No lens passes 100% of the light that strikes the front element all the way through to the image plane on the other end. Different materials used to make various lens elements vary with regard to their transmittance of different wavelengths. Improvements in lens coatings that reduce reflections (reflected light is "lost" light) have increased the ability of modern lens designers to create more color neutral lenses. Most lens designs in the digital era attempt to be as color neutral (and thus maintain maximum transmittance) as possible since color is almost infinitely adjustable in post processing when using the raw data from a digital sensor. In the film era this color flexibility was not so much the case and lenses were made and sometimes marketed as "warmer" or "cooler" based on whether they allowed slightly more of the red/orange/yellow light at one end of the visible spectrum or more of the blue/indigo/violet at the other end to pass through the lens to the film that has more rigid color response characteristics.
Although they aren't directly related to image quality, there are two other factors that may be even more important in the selection of a lens: focal length and maximum aperture. Focal length combined with sensor size determines the angle of view that gives a certain framing of a subject at a certain distance. Maximum aperture affects what minimum shutter time can be used at a specific ISO in lower light as well as how shallow the depth of field can be at that specific subject distance.
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