DOF

This is the acronym of “Depth Of Field“. I have been usually using this word in my blog, giving sometimes a brief description about it. But in today post, I am going to talk a little more about this topic in relation with my experience using different digital cameras.

I bought my 1st camera on April , 2004. It was a Canon Ixus 400. The Ixus belonged to a Canon compact series with a slim body, with great quality for a point-and-shot camera, but with so few manual settings. After breaking the LCD of this camera, I changed to a Panasonic DMC-FX9. I was less happy with this camera ‘cos it had 6 MPixels vs. 4 MP of the old Canon, but it provided less details and more noise. In 2007, I began to take photography more seriously and changed to a ‘bridge camera‘, the Sony DSC-H5. This camera included manual and semi-automatic modes and provided a bigger zoom. In 2008, I went on holidays to Miami (USA), with the idea of buying a Canon EOS 450D due to the high euro/dollar ratio, but I bought it finally in Spain through a website due to the limited warranty to USA. The change to a DSRL with an APS-C sensor was the way to a new world of sharpness and digital edition in RAW, and more flexibility with lenses and creativity using my dear Sigma 10-20mm wide angle lens for architectural and landscape photography. Finally, this year, I have recently bought a full frame camera, the Canon EOS 5D Mark III, and a new era for control of focus, more sharpness, and high ISO settings has begun for me ….

Now, you can be asking why I have described you my old cameras and some of the decisions that I took for buying them, if I am talking about DOF. So simple, you are going to discover it in the following paragraphs.

Depth of field (DOF) , as a definition, is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image.

The DOF depends on several factors:
– Aperture
– Focal Length
– Sensor size or ‘cropped factor’ (in relation with full frame camera sensors, 24 x 36 mm)
– Subject distance

The aperture is usually specified with the f-number, the ratio of lens focal length to aperture diameter. Reducing the aperture diameter (increasing the f-number) increases the DOF. So you are wondering, why you shoot a lot of your pictures with f/11 , if the maximum f-number in your lenses is usually f/22 ? Due to a problem called diffraction, that occurs in camera when you increase you f-number, reducing the amount of light transmitted and the the sharpness in the image borders. f/11 is an usual value for what we can call ‘critical aperture’. The critical aperture is the optimal f-number value or values for which you obtain overall sharpness (including the extremes and center of the picture), avoiding defocus and diffraction. So as you are seeing here the practical use of the aperture depends on the amount of sharpness or blur that you need or want for creative effects in your composition, but taking into account the optics defects. If you want so little DOF, just only to get the subject in focus, for example for portrait photography, you should use your lens ‘wide open’ with f-number bellow f/5.6, usually f/2.8 or less. For landscape photography as you want to get all in focus, sharpness from the foreground to infinity, you should use f/13, f/16 … but if you can , don’t go to more ‘cos you will have blur in the extremes due to diffraction.

The focal length also controls the DOF. With telephoto lenses (long focal lengths) , you reduce the DOF. This is the reason to use them in portraits, to get only in focus your subject or part. And with wide angle lenses (short focal lengths), you increase DOF, being the most useful lenses for landscape and architectural photography.

DOF is, to a first approximation, inversely proportional to format size. I am going to explain this firstly with cropped format DSRL ( e.g APS-C cameras with a 1.5 for Nikon or 1.6 for Canon sensor size ratio). These APS.-C camera systems allow using many of the same lenses on both full-frame and “cropped format” cameras.. If pictures are taken from the same distance using the same f-number, same focal length, and the final images are the same size, the smaller format has less DOF. If pictures taken from the same subject distance using the same focal length, are given the same enlargement, both final images will have the same DOF. The pictures from the two formats will differ because of the different angles of view. If the larger format is cropped to the captured area of the smaller format, the final images will have the same angle of view. So as consequence, if you compare with compact camera where sensor size is still smaller, you will discover why you usually have overall image in focus with your compact camera and your are so happy with your landscape photography, although your camera exif data said that the picture was taken with f/5.6 or less, but you are angry cos you can’t get the bokeh ( blur in background) that get the photographers that shoot your favorite model.

The subject distance is very critical when you are very close to your subject, reducing the DOF. I have discovered after using several cameras , that it is specially critical with Full-Frame cameras and you should consider specially where you are focusing with your camera in interiors with low f-number although you were using wide angle lenses. So in order to solve this problems, it is useful to consider the use of hyper focal distance.The hyperfocal distance is the nearest focus distance at which the DOF extends to infinity, focusing the camera at the hyperfocal distance results in the largest possible depth of field for a given f-number.

The image posted shows how the DOF is extended from a few meters to infinity, but some part of the image is defocus, the tiles on the right side, due to the short f-number, f/4.