Lens Review Question

[Cecelia James]

Hi, Newbie here. The following is a review for the Canon EF-S 18-200mm f/3.5-5.6 IS. I have three questions listed after the review. Thanks.

Canon finally gives us a 10x zoom for the EF-S mount cameras (T1i, 7D) cameras. With the newest generation, 4-stop equivalent IS this provides an excellent one-lens solution for vacation or walk-around shooting. The lens definitely has a bit of barrel distortion at the wide end, and while it has excellent center sharpness gets a bit soft around the edges. In other words, its about what you’d expect from a good 10X zoom.

Let’s face it, a ‘superzoom’ is kind of like a moped: its practical, reliable transportation and can be a lot of fun, but you feel a little embarrassed when a Harley pulls up next to you at a stop light. This is a practical, reliable and extremely convenient lens with pretty decent image quality. I take it on vacation with a T1i quite a bit for a one-lens, one-camera solution. And then I cringe when someone stands next to me with a 5DMkII and a 70-200 f2.8 IS looks over at me with that smug look. I try really hard not to say something stupid like “I left my 500 f4 at home this trip”.

#1 -- Is 'superzoom' due to going from 18-200, rather than only 70-200? Meaning, you lose lens quality if the range is too much? I guess this is the point of prime lenses. So then, what is a good maximum range?

#2 -- Referring to "while it has excellent center sharpness gets a bit soft around the edges", does that mean the edges of the photo such as the part you could crop off, or does it mean, for example, the person's face is clear but the edges/outline of the person is blurred?

#3 -- What is the most pivotal factor in a cheap lens versus a good lens? Anything over $500? Anything over $1000? Anything with IS or VR?

Thanks for helping!

[Cadwell]

#1 -- Is 'superzoom' due to going from 18-200, rather than only 70-200? Meaning, you lose lens quality if the range is too much? I guess this is the point of prime lenses. So then, what is a good maximum range?


Anything with more than about an 8x zoom (divide max focal length by min to get x-factor) is usually classed as a "superzoom". The 18-200mm is about an 11x zoom so well into the superzoom / hyperzoom category.

As a general rule the longer the range of focal lengths a lens covers the worse the optics of the lens are. This stems from the fact that it is easy to design a lens with optics optimised for one focal length but almost impossible to do it for one that covers a wide range of focal lengths. Lenses which transition from wide angle (@ < 50mm ) to telephoto (@ > 50mm) are particularly hard to design as wide-angle and telephoto require some quite different optical designs.

My personal "maximum zoom" tolerance is about 4x.



#2 -- Referring to "while it has excellent center sharpness gets a bit soft around the edges", does that mean the edges of the photo such as the part you could crop off, or does it mean, for example, the person's face is clear but the edges/outline of the person is blurred?


It refers to the difference in lens sharpness between the centre of the frame and the outer edges of the frame. Centre sharpness is relatively easy to achieve. Designing a lens which can maintain uniform sharpness across the whole frame is very difficult.

Frankly, lenses which vary in sharpness across the frame are a nightmare to deal with. A certain degree of uniform softness can be dealt with by sharpening in post-processing but that doesn't work so well when the softness varies. If you get the edges sharp the centre becomes over-sharpened, if you avoid over-sharpening the centre the edges are too soft.



#3 -- What is the most pivotal factor in a cheap lens versus a good lens? Anything over $500? Anything over $1000? Anything with IS or VR?


Image stabilisation has no relationship to image quality, it merely compensates for camera shake. You can achieve the same by using a tripod, monopod, leaning against a wall or drinking less coffee.

Good optical design is key to lens performance.

Price is not always a guideline, there are some excellent lenses which can be had for relatively modest expenditure and there are some expensive howlers out there.

Additionally, people's tolerance and acceptance of optical flaws varies. I've heard many people describe as "excellent" lenses that I wouldn't give the time of day to. If you've come from a point and shoot camera with a 24x zoom on it you might think a Canon 18-200mm is optically superb. If you're used to shooting L series primes you might think it poor.