Understanding Depth Of Field

Depth of field is one of the most important concepts of creative photography but often the least understood. In order to take compelling images, it is very important to understand this concept clearly. Here I will try to explain this concept in simple terms. When I started to understand this concept, following questions came to my mind and getting answers to these will help understand this concept.

Question 1: What is depth of field(DOF)?
Ans: Depth of Field(DOF) is the distance between the nearest and the farthest objects that appear to be in sharp focus in a photograph.

Question 2: How can you control DOF?
Ans: DOF can be controlled by opening or closing the apperture of the lens. Big apperture(small f number, e.g F2.8) will lead to shallow DOF. Small apperture(High f number e.g. F16 )will lead to deeper DOF. You can use shallow DOF to only focus on the subject and haze out the foreground and background.This is often used in portraits or closeups to blur distracting fore and back grounds. You might use deeper DOF to have all of the photograph in sharp focus e.g. in a landscape photograph.

Question 3: Does DOF depend on the camera format?
Ans: Not really, but it does seem to effect in the final image. E.g. 35mm film cameras or full format DSLRs seem to have shallower DOF than the Digital point and shoot or DSLRs that have smaller sensor than the 35mm film frame. Here is the reason. DOF depends on the distance of focus. Lesser the distance between camera and the subject, shallower the DOF and vice versa.

Let us understand this with an example. Let us say that we have two cameras with different sensor size but mount the same focal length lens on both. Now if you take a picture of some subject from the same distance, the image on the camera with a smaller sensor will appear to be larger. This is because some part of the image formed by the lens will be cropped(There is no sensor to capture the outer part of the image!!).So to make the image appear the same size on the final print, you have to move little bit back. By doing so, you increase the DOF.

Let us take another example. The point and shoot Digital cameras seem to have infinite DOF. The reason is that these cameras have tiny sensor even though they have the same 12 Mega Pixels as many of DSLRs. To get same size image as with DSLR, these cameras have to use lens with smaller focul length. Smaller the focal length, greater the DOF. A 50mm lens on these will appear to be a big zoom as only small middle part of the image will be captured on those 12 MP!! Notice that a X10 optical zoom point and shoot Digital camera will have a lens with 6mm-60mm focal length!!

The depth of field calculator below allows a better understanding of the various factors that affect depth of field. It is important that for digital cameras the actual focal lengthh must be used and NOT the 35mm equivalent focal length (as determined by the focal length multiplier).

Depth of Field Calculator
Negative format
Lens focal length
Selected aperture
Subject distance (Meters)
Hyperfocal distance for this lens/aperture combination.
Near limit of acceptable sharpness.
Far limit of acceptable sharpness.
Total depth of field.

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