Viewing post #703960 by dirtdorphins

Frustrated by my inability to produce pictures of some pretty flower where all of it was in perfect focus I asked, innocently enough, "how do I get the whole thing in focus?" and then I got a lesson about how cameras work, in some kind of practically foreign language and a demonstration.
At this point, I actually do understand much better than I can articulate, so I will share with you the demonstration because it was extremely valuable to me in learning to understand the language used to describe how cameras work.

Here is the set up: 14 batteries arranged diagonally across a table spanning about 13 inches


Here are photographs of the batteries at a 45degree angle coming straight across the table from about two and a half feet away from the center of the line of batteries. The only variable manually manipulated is the f-stop (The camera is auto-adjusting the other parameters accordingly; auto focus in the center; ambient light/no flash; tripod mandatory for experiments like this!)

Note that the depth of field increases, or that the entire line of batteries will come into focus as the f-stop increases

f/5.6; 1/15sec; ISO-800

f/10; 1/5sec; ISO-800

f/20; 0.8sec; ISO-800

f/29; 1.6sec; ISO-800

f/40; 3sec; ISO-800

Note also that the ISO is relatively 'fast' and constant-- the light was low and the camera was trying to minimize exposure times by using the upper end of its preset bracket. The bracket was preset at 100-800 to avoid graininess.
Most importantly, check out those increasing exposure times with the increasing f-stops and ask yourself how long you can hold a camera perfectly still. I can hold still for about 1/250sec if I don't breathe...
Honestly, I blame the wind and I blame the cats (both are to blame on occasion), but 99.9% of my unfocused pictures are either because I moved (I don't lug the tripod around the yard) or because I actually focused in front of or behind my subject somewhere...

The same principle applies for close up shots as well, but the really confounding thing is that the closer you are to the subject the more compressed the field becomes such that the area that is in focus with a particular aperture/time/sensitivity combination is necessarily a smaller 'slice' of the whole.
Here again, with the ruler now, looking down at it from about a 30degree angle and closer to it (about 15 inches away)

the dust specks, numbers, hash marks, and scratches on the table are all in focus at about the 10 and 1/2 mark with f/5.6 in the first photo and in each subsequent photo with increasing f-stops (and, necessarily, exposure time) more of the whole comes into focus - both in the foreground and in the background. If the photos were taken exactly perpendicular to the table and ruler then everything would have been in focus because it's all in the same plane with no foreground or background.

But plants are, of course, three dimensional...so it's good to figure out what you want to focus on and how big of a slice you need in focus (versus how long you can hold still).
f/13; 1/400sec; ISO-200
And then there is the ever important consideration of background. Sometimes it can be very critical to blur the neighbor's boat, for example, into a non-distracting, unrecognizable assortment of color and texture with a shallow depth of field.