Brief Scenes

Would an 18-55mm lens be okay to start out with?

I’m considering the pentax k-x which has 18-55mm DA-L lens kit (What does DA-L mean?)

The 18-55 zooms have become pretty much the standard kit lens for all brands. The focal lengths (i.e 18-55mm) pretty much covers the focal lengths most frequently used by the average photographer (particularly the hobbyist).

About focal lengths:

Don’t worry about the difference between the focal length of the 18-55 and your P&S. The P&S has a tiny sensor which means light only needs to be focused on a tiny spot. DSLRs generally have a bigger sensor which means longer focal lengths for a similar view, film cameras generally need even bigger focal lengths to manage similar views.

Basically focal length tells you how stongly light converges as it passes through the lens ( the more the convergence the smaller the number, the less the convergence the bigger the number). Which is why it’s not so useful as a comparison between cameras with different sensor sizes. Nowadays the focal lengths are sometimes given in equivalents of a 35mm film camera.

Oh yeah and focal length affects the apparent magnification and field of view. Here’s an explanation of focal, field of view.

http://www.dpreview.com/learn/?/key=focal+length

About depth of field:

Depth of field is rather complicated to understand unless you understand how a lens works and how focus works. I’ll try to explain it all the same. A lens can only focus on a point. Everything infront or behind that point is out of focus. Your eyes don’t have the level of detail necessary to see the tiny focus problems (which is a good thing because your eye also can only focus on a point. Just imagine walking around and only being able to see a point about 0.0000001mm thick, 1 meter ahead of you). Anyway that introduces the circle of confusion, which basically is how much out of focus things can be while you still see them as if they are in focus (ok it’s more complex than that but that’s the only thing you really need to know).

So let’s sum up what we have so far, Only a point is in focus, but when you look, you see some distance infront and behind that point as if it’s in focus, anything that is further than a certain distance from the focal point you’ll see as out of focus.

The next question is what is the distance from the focal point at which things still appear in fous. Answer, it varies. Next question, what makes it vary and how does it vary? Answer, a relation of focal length, subject distance and aperture (i.e how big the hole is). At longer focal lengths you get shallower depth of field (more stuff out of focus), shorter focal lengths give wider depth of field (more stuff in focus). Aperture is stated in f-numbers (the numbers refer to how much light the can pass through the hole). Bigger f numbers are smaller apertures and smaller f numbers are larger apertures. Larger apertures give shallower depth of field, smaller apertures give deeper depth of field. The last factor, subject distance is not that important since you can’t really change it most of the time, but it has two effects. The first is when the lens is focused on a point at say 2 meters, you have a shallower depth of field than when the lens is focused on an object at or near infinity. The second thing is the distance between the object you are focusing on and the object infront or behind it. The further away the out of focus object is from the in focus object the more out of focus it (the out of focus object) appears, and vice versa. There are a number of websites that explain this, and they have diagrams which make it easier, using google or reading the wikipedia pages will probably be easier to follow than my explanation.

About the 18-55 DA-L:

The 18-55 is definitely ok to start with. It’s not the best pentax lens you can find, but it’s cheap, covers a useful range, is small. It doesn’t really have any significant optical flaws.

The DA-L is pentax terminology and probably means something, unfortunately I’m a Nikon guy myself so I’m not to sure. From some quick googling, it appears Pentax has two 18-55mm lenses (optically they are the same, the glass and movement is exactly the same). The difference DA-L 18-55 is lighter, has a plastic mount and lacks “quickshift”* while the DA 18-55 II is slightly heavier, has a metal mount and has “quickshift”*. The DA-L is the standard pentax kit-lens.

* Quickshift means you don’t have to turn off autofocus before you can use the manual focus ring. You can just focus by turning the ring even when the camera and lens are set to autofocus.

Well I’ve tried to be as detailed as possible without making it too complicated, but after reading it myself I’m afraid I failed. It would be a pity to have completely wasted my time writing all this so I’ll post it anyway and hope you find it useful.

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