Help with Depth of Field and ISO...

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Permalink Reply by Natasha on June 8, 2008 at 9:45pm

Here is a really good article on Depth of Field and ISO Speeds.

Permalink Reply by Maeda on June 9, 2008 at 12:25am

Thanks for sharing the site =]

Permalink Reply by David Wollum on June 9, 2008 at 3:42am

There are a couple hundred links you could click on for answers, but in a nutshell, the basic definitions go like this:

Depth of Field - Beginning with the outer edge of your lens and stretching out to the farthest thing you can see through the lens is everything that can possibly be in focus in that particular shot. Generally you will have a subject somewhere in that shot you hope will be in focus. From the point closest to the camera where things are in sharp, clear focus to the farthest point where they begin to blur again and are no longer in focus is what is referred to as the Depth of Field. Take a look at the attached shot, DSC_0010.JPG. This is just a test shot I took to see what kind of depth of field I was getting with a close subject. Notice the lens cap is in focus, but my pant leg at the bottom of the frame is out of focus, as are the pants at the top of the frame. To the right of the frame, the corner of the instruction manual for my camera is very much out of focus. All the areas in this shot that are in focus, mostly the lens cap and the jeans near the cap, define the Depth of Field for the shot.

More important than knowing what it is, is knowing how you control the depth of field, which is a function of the aperture, or f/ stop settings. The aperture is simply the size of the opening in your lens through which the light passes each time you press the shutter button. Sometimes you want a large, deep depth of field, as in most landscape shots where everything in the shot is in crisp focus. Other times you want a fairly narrow depth of field, as in a portrait where just the person in the shot is in focus and the background is blurred on purpose to direct the attention of the shot away from the background and onto the person. It seems kind of weird, but large numbers, say f/ 22, means a small aperture, but a corresponding deep depth of field. The opposite is also true; small numbers like f/ 2.8 means a large aperture and a short depth of field. It may help you to think of it like this:

Large numbers = large depth of field, small numbers = small depth of field.

So... onto ISO/ASA. These numbers are from the days of film cameras, and refer to the film's sensitivity to light. The lower the number, the less sensitive it is to being exposed to light. It is the same for digital cameras, except with digital, we tell the camera how sensitive to light we want it to be by the way we select the ISO or ASA.

What really matters here is understanding why we select one ISO/ASA over another. The general rule of thumb is to select the lowest ISO/ASA that will give you the results you want. The reason? The higher the ISO/ASA number, the more noise you get in the shot. What is noise? Try zooming in on a picture to the point where you can see the individual pixels... and do it in one of the darker areas of the shot. You will notice that several pixels really aren't dark, but are a bright pink, purple or red. That is noise. If there aren't very many of these off-color pixels, you tend not to notice them. But as the ISO number increases, so does the noise and so does the chance you will notice it on the finished product. (In the days of film, the same thing happened, only the shot was said to be "grainy").

Under brightly-lit conditions, even when there is quite a bit of motion and you need a fast shutter speed to avoid blurring, you can usually get away with a low ISO/ASA of 100 (or 80, if your camera goes that low). As you move to less well-lit conditions, you may have to compensate by increasing the camera's sensitivity to light and going to ISO/ASA 200. If you are at a sporting event with lots of action but not much light, you might go to ISO/ASA 400 to get the shutter speeds you need which allow you to freeze the action. Beyond that, most folks don't use the ISO settings above 400 unless they're doing night photography without flash or purposely going after a grainy effect.

I hope that helps... good luck and have fun with it.

Attachments:

• DSC_0010.JPG, 2.5 MB

Permalink Reply by Maeda on June 9, 2008 at 5:45am

Thank you very much, David! Now I understand. I guess I have to play with my camera regarding ISO and DOF.. Maybe the shutter speed too so I can explore the most of it! Thanks again! You helped a lot!