There are two types of lenses that are “optimized” for digital SLR cameras. The first type, often denoted as “optimized” for digital, feature additional coatings, better known as SLD [super low dispersion] lenses to minimize optical aberrations caused by light striking the image sensor in any other direction than straight on. The second type, usually marketed as being designed “for” digital DSLR’s, in addition to the coatings mentioned before, also differ in the size of the rear lens barrel which is smaller in diameter to more closely match the size of the imager also to minimize the effects of stray light striking the image sensor. The first type, “optimized”, will work on 35mm film SLR cameras. The extra coating benifits all types of cameras and improved image quality. The second type, “designed for” cannot be used on 35mm SLR cameras due to the smaller rear lens barrel and will cause a deep barrel distortion at wider focal lengths. The best example I can give you is with Sigma lenses, branded DG lenses have the SLD glass and can be used on all cameras, DC lenses can only be used on digital cameras. I will link you to their website, it’s a page that explains the difference.
Hope my input helps.
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