To Kit or not to Kit – Three Pentax Kit Lenses
Everyone says MyBrandX kit lens is the best, producing the nicest colors and is sharper than BrandY’s kit lens, but once they buy into the system they upgrade after a few days, months, or years, and they start saying “Never buy the kit lens, its junk!”. So what’s the real deal?
I am comparing 2 Pentax kit lenses and the mid priced standard zoom upgrade. The two kit lenses are the SMCP DA 18-55mm f/3.5-5.6 AL and the 18-55mm AL II, and the 16-45mm f/4 is the upgrade lens, although it has more recently been replaced by the 17-70 f/4 SDM which I don’t have to compare. The AL II optics make it into all current versions of the kit lens including the WR and DA-L models.
So if I buy the camera, which kit lens do I buy? Is it worth the money to upgrade to the f/4 zoom? It all depends on your needs, as you will see from the test charts, lenses have design trade offs. Just because a lens is more expensive, it doesn’t make it better in all regards. The best lens optically might weigh a ton and cost a small fortune.
I will jump straight to the charts after a quick note. As you resolve finer and finer lines, the contrast between a white area and a black area fades to 0 (they blend to gray). The first charts show MTF 50, which is the resolution where the contrast is still 50%. A high contrast lens will appear sharper than a low contrast lens, but might not resolve any more fine details. So MTF 50 is a good indicator of how punchy an image will be right out of the camera.
The next measure, MTF 20, is resolution where the contrast has dropped to 20%. This is a measure of how fine or lines a lens can resolve, but contrast between the details is low. This might require contrast boost in post processing, but is a good measure of how much detail a lens will capture. This figure will be important if you shoot subjects with a lot of detail, like birds, etc.
The central sharpness for both MTF 50 and 20 is an arithmetic mean of the partway and central resolutions I present in my individual lens tests and represents a majority of the frame, including the top and bottom edges, so it is a good representation of overall lens performance. The second chart for resolution is for the extreme corners which represent the left and right edges of the frame and are important for those with subjects running to the extreme edges of the frame.
Next we have Chromatic Aberration (CA). This comes in several flavors, but is basically that different colors of light are not focused the same so they separate out leaving color fringes. The two primary types are longitudinal CA (the lens has different focal lengths for different colors of light) that shows in out of focus regions and lateral CA (different colors of light have the same focal plane, but different magnifications) that shows as color fringes the get larger as you go towards the edge of the frame. Lateral CA generally cleans up nicely in post processing, but longitudinal does not. Since this test is of a flat chart in excellent focus, it tests lateral CA.
Finally there is distortion. The sensor is flat, but the world is 3D. The lens tries to project a flat slice of the world onto the sensor, but no lens is perfect and it doesn’t always end up with a flat slice. This happens because the edge of the lens has either more or less magnification than the center of the lens. If a lens has less magnification near the edges, it causes the image to become squeezed in the corners and gives a circular look (barrel distortion), and if it has more magnification near the edges it becomes pinched in the center (pincushion). Some lenses have a combination of the two at a given focal length and this is called mustache distortion. Distortion isn’t important for many subjects, but if your subject includes straight architectural edges, concentric circles, etc, near the edge of the frame it will become readily visible.
Read the next pages for test charts, or skip to the last page for conclusions.