Chemistry

f/36 | 6sec. | iso400 | 70mm

f/4.5 | 1/3s | iso400 | 70mm

f/4.5 | 1/3s | iso400 | 70mm

f/2.8 | 1/15s | iso400 | 17mm

I shot these as color and depth of field examples.  

-High/Small aperature/f-stop= Longer depth of field | e.g. the first pic shot at f/36

-Low/Large aperature/f-stop = Shorter depth of field | e.g. the second pic, shot at f/4.5

-More zoom makes depth more noticeable even when combined with small aperature | e.g. the first pic, shot at 70mm, f/36, less blur, but

-Less zoom makes depth/blur less noticeable no matter your aperature, but depending on how close your subject, is where you will find depth of field. Mainly only if you're shooting macro.

Generally, higher focal length means greater depth of field.  This can be cut down to a degree by using a smaller aperature.  Also, when your focus is on a near subject, but you want some detail in the background, you have to use the smallest aperature possible because it's hard to do a macro shot and still salvage a detailed environment.  Higher focal length also can allow the camera to use a smaller aperature than at a lower/closer focal length.  

Photography is chemistry.