Saturday, June 16, 2007

Adjusting depth of field with F-Stops

Ok, I'll admit it. I'm not a professional photographer, but I'm going to blog like one one the internet. Or at least make an honest attempt. F-Stop, sometimes called relative aperture, adjusts your depth of field in a photograph. Your depth of field is what is in focus in your photograph. Let me explain a little more. You're at a family gathering and they decide they want a portrait taken. You take a the photo of them against a wall. Both the wall is in focus and the family. There really isn't alot of distance separating the family from the wall, the depth of the picture is relatively shallow. Now they want some pictures taken outside. You set up a shot of them in a field, behind them off in the distance is a line of trees. It's about 1200 yards out or so. You take a picture and this time the family still comes out sharp, but the background is all fuzzy. Your depth of field between hasn't been changed on the camera, and it's still relatively short.

Part of what controls your depth of field is your f-stop. It's an adjustment usually found on most new digital cameras. The camera that I am using, is a Canon Rebel XT. This camera is a digital SLR (Single Lens Reflex), it has interchangeable lens. The lens I am using has the one that comes with the kit for this camera. It is a 18-55mm lens, and has an f-stop of 3.5 to 5.6. F stop numbers tell you the ratio of the amount the aperture is open in comparison with the length of the lens. It's semi-confusing to understand but bare with me. The larger the F-number, the smaller the opening in the aperture of your lens. Likewise, the smaller the number, the greater the opening.

Let me break out the visuals on this. I set my f-stop to 3.5 and took this picture.

You're going to want to click on the image ot enlarge it, or you will not see what I am talking about. See how the background is relatively fuzzy?
I took this picture at my max f-stop (5.6) and adjusted my shutter speed slightly, or the photo wold have come out too bright, more on that later.

Again click on it, see how the background is a little sharper? It's really hard to see at this small of a photo, and the range of my lens. I zoomed in on the image and isolated a chunk of the fuzzy photo, or the 3.5 f-stop, in the sharper 5.6 f-stop photo.

Again, click on the image so that you can get a better idea. You see the difference? If you have a lens with a greater f-stop range, you can make the change even more dramatic. It makes for a more interesting photo in some cases. There is more than one way to adjust depth of field. Theres more than one way get a lot of effects, but at least this gives you a better idea of what's what.


Steph said...

Hi Eric. Looks like I am your first student. :-) I have the same camera you have but basically I leave it on the auto focus, point-and -shoot setting. To achieve this "depth of field" effect, do I first put it on the manual setting (M)? Also, I can see the f-stop (5.6) in the display window but how do I change it?

Matt said...

Good explanation, Thanks!