The only lens I own for my Nikon D200 is a Nikon “AF-S DX VR Zoom-Nikkor ED 18-200m F3.5-5.6G(IF),” also known as “the 18-200mm.”
I really love it as a fantastic all-around lens, but at f/3.5-5.6, I find that it is sometimes a bit slow (that is, it doesn't let in enough light for me to use a fast-enough shutter speed). So, I've been thinking of getting a faster (allows in more light) lens for use in lower-light situations.
Because of its “18-200mm f/3.5-5.6” name, I know that my lens is f/3.5 at 18mm, and f/5.6 at 200mm, but what is it in between? Without knowing, it's hard to compare how much faster a different lens might be. For example, how much faster is Nikon's f/1.4 50mm lens than the 18-200mm at 50mm?
So, I set the camera to wide open and turned on the high-speed continuous drive, and held down the shutter while sweeping the lens through its full range of focal lengths. I then plucked the data from the MakerNotes in each image and plotted it.
So, compared to the 50mm f/1.4, my lens is 31/3 stops slower, which is a lot. That's the difference between a shot at 1/15th of a second, in which even mild subject movement is blurry, and one at a much more snappy 1/160th of a second.
I'm thinking of something like Nikon's 17-55mm F2.8, but can't see shelling out $2,000 for only a 2/3rd gain at 18mm and a 11/3rd gain at 55mm.
Isn't there at least an f/2 (or better) in this range?
UPDATE: I ended up getting the 17-55 f/2.8 and love it.
The main graph above is logarithmic on its Y (f-stop) axis, and linear on its X (focal-length) axis. Perhaps not surprising, the plot appears as a straight line when the X axis is also logarithmic.
The graph on the right shows the same data in a log/log plot.
Despite the 18-200mm's smooth zoom throughout its range, the Exif data and the MakerNotes data have only a limited number of points along that range that they'll report, and oddly, the two sets of data are slightly different. I've shown the MakerNotes' data in the graphs.
The full data from both are shown in the tables below.
From the MakerNotes Data
From the EXIF Data