Why does depth of field change with aperture size

So what really happens with depth of field is that objects are not really in focus off the focused plane, but rather the blur is considered negligible. Think of it this way: a thumbnail photo might look clear, but if expanded to be an 8x10" photo, it may be unacceptably fuzzy. In practical application, a so-called hyper-focal setting on a lens, may give an acceptable image of a scene when viewed on a small format display or print, but when expended or enlarged, will yield a more fuzzy appearance as it is in reality not completely in focus through the "depth of field.

As a photographer and physicist myself, I am trying to explain this phenomenon to my musician friend who has just bought an expensive camera. The best I can come up with is to take, at the extreme, a pinhole camera with infinite depth of field. As we stick a lens in front and widen the aperture it becomes closer to the familiar situation with object and image distance determined by focal length. Sign up to join this community.

The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more. Why does aperture size affect depth of field in photography? Ask Question. Asked 8 years ago. Active 2 years, 6 months ago. Originally Posted by jcgwakefield. In theory, would it be possible to take an image taken with a wide apperture and process it such that it appears as if it were taken with a small apperture?

With sound, I believe its possible to isolate the different wave forms which have been added together to form a rich timbre. So what I'm getting at is being able to take an image and remove the exposure caused by light waves which result in the image being out of focus or in other words remove from the image the exposure caused by light waves which arrived via a high divergence.

Surely, for every out of focus object, the light waves existed at the time of the photograph for the image to be in perfect focus? The answer is a qualified, "yes" when you shoot digital. See the "Composite Focus" section in this book.

I don't know about earlier editions of Photoshop nor do I know about the "focus stacking" ability of other photo editing programs. Here is a tutorial on "focus stacking" using Photoshop CS3. I have not worked with one of these supplemental programs. Last edited by rpcrowe; 14th September at PM. Setting aperture for depth-of-field With the aperture wide open, the only details that will be in sharp focus are those at the specific distance the lens is focused on.

Using a depth-of-field preview If your camera has it, a depth-of-field preview will allow you to see this change in sharpness, but you need plenty of light to see the image clearly in the viewfinder: as the size of the aperture decreases, so the viewfinder image will darken. The shorter the focal length, the greater the depth of field from near to far at any given aperture. Depth of field is specific to focal length. For example, a 20mm focal length delivers the same depth of field at a given aperture when used on a point-and-shoot camera, a camera with an APS-C sensor, or a camera with a full-frame sensor.

The only way to see the depth of field through the viewfinder is to press the depth-of-field preview button, which adjusts the aperture to the current setting. This is not a technical term! I made it up. So what does small enough mean? Well, there is are two versions: the physical small enough , and the physiological small enough. The physical small enough refers to the fact that any real-life camera sensor has finite resolution.

For example, as mentioned before, a digital camera sensor consists of an array of pixels. These pixels are little squares that have finite size. Say one square micrometer.

The physiological small enough is a bit more complicated and has to do with the angular resolution of your eye and how your brain processes visual information. If you make it small enough, it will eventually stop looking blurry. You can also try looking at a picture from very close up and from farther away.

It will look better from far away! I have chosen D2 and D3 in the previous picture so that P2 and P3 are at the limits of approximate focus.

So the range of approximate focus extends from D2 to D3. I call this range the focus range. This is not standard terminology either. We can now define depth of field : depth of field is just the size of the focus range!

Depth of field has huge impacts on photography. When a photographer chooses to use a small depth of field, he or she can draw attention to certain aspects of a scene. The photographer can choose a small range where things are in focus, while the rest of the image will be blurry. This actually has a name: bokeh. Alternatively, the photographer can choose a large depth of field to emphasize all aspects of a scene. I think this technique is particularly evident in landscape photography.

So that was just the prelude to the main point of this article. Before we continue any further, I should explain what an aperture is. An aperture is basically a hole that you place in front of your lens. You can adjust its size, which essentially shrinks or increases the size of your lens. It turns out that there are actually two reasons for why a small aperture increases depth of field. The first is entirely based on geometry. This allows for a wider focus range and hence a larger depth of field.

The picture below will help you visualize this. The top shows the lens with the widest possible aperture, while the bottom shows the lens with a much smaller aperture.

The other reason has to do with the fact that real-world lenses are not perfect. Instead, most cameras use spherical lenses. This effect is shown in the picture below. The top shows a perfect lens, while the bottom shows a more realistic one.

The net effect is that spherical aberration increases the size of the circle of confusion. In fact, it makes it impossible for the sensor to ever measure a single point. By blocking the outside light with an aperture, you can consequently shrink the circle and increase the depth of field. For example:. As an aside, if you want to know how I determined the location of the light ray intersection in the my pictures, check out the thin lens equation and geometric optics.

What a great description! The methodical way you presented this helped me understand things about optics that have always been a little fuzzy to me intentional play on words! Well done. Much appreciated. Easy to understand. Now I know! Thanks a lot for your explanation. I recently began studying photography and this really helps me understand. I will post a link to this post. It helped me a lot to understand why this […]. My son is teaching photography to year 10 students in Melbourne in July-August.

He is an art teacher but knows very little about photography. As a fast learning photographer, in my recent retirement, I am writing a course for him.

Your explanation of Depth of Field is the best I have seen.