Originally Posted by hyogen
The bokeh I can get with the 17-50 f/2.8 is quite nice especially when zoomed in (probably to around 30-50mm.....so I guess the 30mm f/1.4 lens will be for shots where I can stand a little further from my subject and take portraits (like that of the little boy) and have a nice blurry background? And indoor shots when I can stand back far enough.. otherwise, taking pics of stuff like headphones -- that should probably be the least of my worries since the 17-50 2.8 does bokeh pretty well at around 30-50mm. It's labeled as a "MACRO" lens.....as is the 18-200mm.. the 18-200mm is not really ideal for macro I've found--you have to be too far from the object and need a tripod.
For the sake of the forum format and keeping it very simple: depth of field (depth-of-field is the portion of your photograph that is in focus--that which is out of focus we'll call bokeh or blur) is determined by several things. The most crucial of those things are focal length, aperture, and distance to subject.
The first thing to remember is that you have a certain distance in focus. Your depth of field will surround this area in focus and everything inside that field will appear in focus as well. For any given depth of field, roughly 1/3 of it is in front of the focus distance and 2/3 behind the focus distance. So, you will have twice as much in focus behind your subject than in front. So, if you focus distance is set at 10-feet and your focal length, and aperture combine such that you have depth of field 6 feet deep, roughly everything from 8 feet to 14 feet will appear in focus.
Now we look at focal length and aperture. If you divide the focal length by the maximum f-stop, you will get an indication of how much bokeh you can get (i.e. how much will be out of focus). A 35mm f/2 lens - 17.5. An 85mm f/1.2 lens ~71. A 600 f/4 lens ~150. The higher that number, the more blur you can get. So, inherently, wide angle lenses physically cannot deliver as much bokeh as a telephoto. Yes, you can still get blur but not as much. FWIW, the quotient in these calculations actually means something but it's too much detail for this discussion.
Now, finally, lets look at distance to subject. Let's say, for example, that your focus distance is set to 10 feet. So something that is 5 feet behind it is 50% farther away than the focus distance. Now lets say your focus distance is 50 ft: something that is 5 feet farther away is only 10% farther than the focus distance. To get the same 50% farther away than the focus distance, you're talking 75-feet. So, the farther that something is from the camera, the more around it will be in focus.
Ultimately, if you want Bokeh, you're talking telephoto lenses, big aperatures, and close distances (or very large difference in distance between in-focus subject and background). Wide angle lenses, even fast ones, just can't get the same job done.
A couple of examples:
I don't remember the exact settings but this was shot at a large aperture so there should be a pretty narrow depth-of-field but the focus distance is pretty far and it's a pretty wide lens so both of those minimize bokeh and pretty much everything is in focus:
This was shot with a 100mm Macro at f/2.8 so large aperture, somewhat long lens, and very close focus distance (this is about 90% of the original image. I did a slight crop to fix the composition and straighten it a bit).
You can see the OMEGA on the right is in focus but parts of the gear train and mainspring only millimeters farther away are already out of focus.
So, similar apertures on the two above, but different focus distances and focal lengths lead to very different depths of field.. I don't have it in my Photobucket account but I could show you an image shot at 300mm @ f/5.6 that has the creamiest, smoothest bokeh you'll ever see. Long lens, focus distance very close, and a background quite far away.
Long story short, remember that focal length, focus distance, and aperture all have an effect on bokeh.
Here's another example:
This was taken with the same 100 Macro @ f/2.8 but you can see that the focus distance is much farther away (but still quite close). Here you can see the depth of field getting larger, probably a 1-2 inches deep instead of millimeters even with the same lens and aperture. Again, farther focus distance, more depth of field.
Edited by leftnose - 5/29/12 at 8:12am