One of the important things when you talk about real lenses is for the photographers among you, you know about this, for those of you who are new to it, it's cool, it's called depth of field. And depth of field is, and, looks at the question of, okay I've got some depth in space that's exactly in focus, how much does focus change as we get. A little bit off of that, of that distance, right, so if I move a little bit of the point that's precisely in focus, how much does the focus change, and what that's controlled by actually is the aperture. And the reason is, you can see here, so here we have a wide aperture, and with a wide aperture, these, these rays diverge quite a bit, if my film plane instead of being here, okay, was actually here, okay, the rays from this point would have been spread out a lot, so it would have been out of focus a lot. But on this smaller aperture, okay, where things were supposed to be in focus here, if I actually, am off by a little bit there that is that my film plane has moved a little bit, you see that the spread is much less, and that's why the depth of field is much better. That's illustrated here, these are some pictures from Wikipedia, so here's a picture of a flower. You'll notice the flower itself is beautiful and sharp, and the background is totally out of focus, now, it says f/5.6. Now. This is f for aperture, not f for focal length. I apologize, those of you who buy lenses, you'll buy a 50 millimeter lens, f/1.4, the f is about the aperture, it means the, the smallest aperture, the, the largest aperture it can get. Also, fs are inverted, the bigger the number, the tighter the value, so 5.6 is a relatively larger aperture, compared to say f/32. So here, same flower taken from the same position, but you'll notice now that the background is in sharp focus. So if you're a photographer and you, you want to separate out your foreground from your background, you want to have a wide-open aperture. If you're trying to look at a landscape and you want to see everything in focus, you want a very tight aperture. Here is a very nice example of depth of field, the focal plane obviously is somewhere in the middle of the phone, so the stuff that's in front and the stuff that's in back is out of focus.