 Vincent Montalbano |
Why are my images soft when shot at wide settings? My wife and I use a Canon 20D with several zoom lenses, and I've noticed that most images shot at wide settings tend to be soft when viewed in tight. Is this a factor of the image sensor in the 20D not being a full size, or is it an issue with the zoom lenses? Typically, we shoot with a Canon 17-85mmIS F/4 and a Tamron SPAF28-75mm F/2.8 XRDi. Thanks
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Pete H |
Hello Vincent; All lenses have what some call the "sweet spot"..or the f/stop that gives you maximum sharpness. This is never wide open. Below are a couple of links that will provide an answer in a VERY detailed way. http://www.normankoren.com/Tutorials/MTF6.html http://en.wikipedia.org/wiki/F-number
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Vincent Montalbano |
Thanks Pete. Having made the transition from film to digital a few years ago has added a new dimension to our interest in photography. What might have gone less noticed with film has now become more obvious with digital. Although when viewed tight on the computer most of these suspect images show this "softness," all but a few still produce very acceptible enlargements. I'm sure this problem existed with film in the past, but was less noticed simply because we did not use computers to analyze and edit our work. By the way,this is probably a coincidence, but I went to high school with a Peter Herman...are you from Long Island originally?
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Pete H |
No..Must be a another one of me out there. That's a scarey thought LOL
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Alan N. Marcus |
Hi Vincent, While lenses have a “sweet spot”, I think what you are describing likely falls into a different category. As we zoom the camera to the “wide angle” position, the taking focal length becomes shorter. As a general rule, shorter focal length translates to greater depth-of-field and higher acuity. This is just the opposite of what you are describing. So let’s explore: When set to short focal lengths the angle of view of a lens increases at the expense of apparent image size. Stated another way, the image size of objects at the wide angle position is greatly reduced. In fact, the image size of an object taken at 25mm is cut in two (1/2) smaller than the same object shot at 50mm. When we view our images on our computer, wide angle shots depict objects as tiny. I think we have a tendency to securitize such shots using the zoom tool. Further I think the magnification we apply is much higher than normal under these conditions. So what I am saying is; this is not a lens issue but a viewing issue, comparing apples with oranges. None of the above rules out the possibility that the lens performance is substandard at wide angle. Also you should know that I have been wrong more than a 1000 times so please consider this as marginal technical advice. Alan Marcus
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Vincent Montalbano |
Thanks for your reply Alan. I pretty much know exactly what you and Peter stated above, but as I noted above to him, and you pretty much eluded to it,is it nothing more than now having the ability to zoom in very tight on the computer to see these "flaws" whereas in the past with film our labs were seeing that, and not us. However, images shot somewhere near that sweet spot on the lens are extremely crisp regardless to how tight you zoom on the computer. I also wonder if it's not a matter of the optical qualities of these lens, and although not cheap, certainly a far cry from lenses costing in the thousands. Can we also assume that a prime wide angle lens Vs a zoom lens would yield better results?? The bottom line...most of these wide shots still produce very acceptable enlargements up to 11 x 14 providing they're not heavily cropped
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Alan N. Marcus |
Hi again Cincent, A “prime” or “fixed” lens is one that is not adjustable as to focal length. The focal length engraved on the barrel of a lens is calculated based on the lens being focused on a point source at infinity. Considering all lens designs, the “prime” gives the best performance. All lenses generate flawed images. These are to name a few, astigmatism, chromatic aberration, spherical aberration, pincushion/barrel distortion, and countless more. Most lenses are figured to perform best at infinity. Exceptions are “process”, “projection”, and “macro”. These specialized lenses are figured for near focus. The macro is figured for unity (life size). A zoom lens required moving lens elements. Zooms are figured for infinity. Acuity over the entire zoom range is accomplished by a series of compromises. Normally a lens delivers maximum sharpness when focused at infinity. At maximum wide aperture (wide open) the periphery of the lens is utilized. The edges of the lens are where the strongest figure (curve) is located. Rays that transverse the edges are more likely to be misdirected, depth-of-field is shallow and acuity is reduced. As the lens is stopped down the depth-of-field span increases, so does sharpness. At about two stops down from maximum opening, the lens functions at its highest acuity. As the aperture continues to stop down, a higher percentage of rays must brush by the edges of the now tiny aperture restriction. Rays experiencing a near miss with the aperture are changed in direction and intensity. This is known as diffraction which degrades the image. Many falsely believe tiny apertures are the most sharp. This stems from the f/64 club of famous photographers. However they were using 8x10 view cameras. Now a giant camera functioning at f/64 will have a large aperture opening and thus immune to diffraction until stopped down to f/90 or more. Our 35mm and digitals experience degrading diffraction at about f/16. Keep in mind I am wrong a lot so consider this marginal technical advice. Best regards,
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Vincent Montalbano |
Well Alan, I'm not in a position to argue with your information, and it certainly sounds accurate and sounds like you know what you are talking about. My wife does children's photography..all available light, no studio, and I know she needs the capabilities that a medium zoom lens offers her. I also know she would not be happy flipping lenses from primes to zooms while chasing her little subjects around some field or garden. I also know we are not going to purchase a $2000-3000 dollar zoom lens that may, or may not improve this issue. Although arguably not the best, the Tamron 28-75mm with a fixed F2.8 across the focal range has been a pretty good compromise...especially at sub $400. Thanks again for your insight.
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