Somebody told me that there is a difference in sharpness between a zoom lens vs. prime lens. A 58-200mm will give sharp focus in the center of the frame but blurry performance at the corners, while in the meantime a prime 200mm will give a sharp definition to the whole frame of the photograph. Is this true?
As with any blanket statement, it's not true in every case that primes will give consistent sharpness across the field and zoom lenses won't. To take one specific example, the Canon EF 50mm f/1.8 II has much worse performance in the corners than the centre at f/2.8, whereas the Canon EF 24-70 f/2.8L II USM is pretty consistent across the frame at 50mm, f/2.8. Now of course, this isn't really a fair comparision as the 50mm can be picked up for about £65 and the 24-70mm will set you back about £1400 - but it does go to show that it's perfectly possible to design a zoom lens which has better corner performance than a (cheap!) prime.
No, that's not generally true.
You might look at the DxOmark website for actual measurements on different lenses. Sharpness at different points across the field is one of the things measured in great detail, and graphed using color to indicate sharpness.
The sharpness varies not only with the specific lens, but varies with the zoom setting on zoom lenses, and with the aperture setting!
So maybe your fast prime will be more blurry wide open than your kit zoom at the same focal length, only because it doesn't go wide open but only opens up to f/3.5, for example.
Maybe a zoom will show degradations in the corners at very short focal lengths, and a prime at that length is 5× the price but sharp.
There are many tradeoffs.
Lens sharpness is fairly complex topic as there are many variables that dictate what makes an image sharp and what does not. Here I will try and keep it as basic as possible with a just a few areas that can be considered regarding sharpness.
It is generally true that Prime Lenses are sharper than Zoom Lenses. The reason for this is due to a prime Lens not having the extra lens elements to correct for diffraction as do zoom Lenses.
As a result, and this applies to even the cheapest of the Prime lenses, they are the masters of just one focal length with just one job to do, and generally, they do this fine, even the cheaper lenses.
Whereas, a zoom lens, has to get the sharpness correct over a much larger focal range. The bigger the range, the tougher the job and more scope for errors, but that is not to say that they are not able to correct for any errors and still produce a relatively sharp image.
A $100 50mm f/1.8 may result in very sharp, edge to edge images at say f/8, but a $300 70-200mm f4.0 may actually end up being sharper edge to edge at 80mm and f/16, better known as the lens' sweet spot.
On the other hand, a $2000 Zoom Lens may end up being way sharper throughout the entire focal range of 70-200mm, it can all be relative.
In Zoom lenses, to keep the overall lens size down and to keep focal issues at bay, correction has to be made by introducing a further lens element just behind the main lens to reduce the distance to the sensor. This second lens has to be positioned just right so that it does not produce any chromatic aberration. Several of these elements are positioned within these lenses and each group is there for the correction of CA, reflections, contrast and sharpness.
Therefore, we have to take note that each group is also creating its own set of challenges, and the subsequent groups, along with keeping the size down, are also trying to correct the previous groups reflections and errors.
However, we also have to assume that the more expensive zoom lenses, will have better quality glass, with higher quality elements and grouped more efficiently than say a budget Zoom, and therefore, yield a sharper edge to edge image.
As further research, you maybe interested in reading about MTF, Modulation Transfer Function and how this measure is used in determining sharpness of a lens
Performance in terms of acutance of any lens varies in a lot of ways, but throwing in the variable focal lengths a zoom lens is capable of adds to the complexity of things. Even a prime lens with a fixed focal length can vary in terms of center sharpness from one aperture setting to the next. How much that sharpness is degraded from the center to the edges can also change at different apertures. Adding the extra variables of a zoom lens creates an even more complex comparison.
In general the gist of what you have been told is basically true, especially when comparing prime and zoom lenses that are both in the same or similar price ranges. If you compare cheaper prime lenses with high end zoom lenses, though, all bets are off in terms of this generalization.
There are also some very high end prime lenses that are specifically designed to soften up on the edges and in the corners. The unique look of the Canon EF 85mm f/1.2 L is due to uncorrected field curvature that makes the edges of a flat target soft when the center is correctly in focus. It's not so much that the edges can't be sharp with such a lens, it's just that the edges of a flat target will be sharp at a different focus distance than the center of the flat test target.
With consumer grade telephoto zoom lenses, the longest focal lengths typically tend to be the least sharp, even at the center of the frame. As the lens is zoomed to the maximum focal length all of the aberrations of the lens are magnified. So if the center sharpness decreases a little as the focal length is lengthened, then the edges will usually soften up even more.
A camera lens is composed of many lens elements. Each element bends light in a different way. Some elements need to be made of a different material that bends some colors more or less than others (especially at the edges). It is difficult to manufacture elements that are of unusual shapes (aspherical) or that require much finer tolerance.
A zoom lens has many more elements than a prime lens, including entire groups that move with the zoom ring. All of the errors and imperfections from each individual element can stack up pretty quickly.
Many of the newer options can offer better performance than older designs, due to improvements in materials, coatings that reduce glare and flare, the arrangement of elements, the computers used for design, and the equipment that grinds the lens elements.