Why is depth of field affected by focal length? - Photography Stack Exchange
Three main factors that will affect how you control the depth of field of your images are: aperture (f-stop), distance from the subject to the camera, and focal length. Understanding depth of field is one of the first big hurdles in photography. Here's How aperture, focal length and focus control sharpness. The equations for the DOF limits can be combined to large in comparison with the lens focal length, this simplifies to.
The limit of tolerable error was traditionally set at 0.
Depth of field - Wikipedia
Similarly, for subminiature photography for example the Tessina with a frame format of 14x21mm, 8x12 inches corresponds to Some authors, such as Merklinger have suggested that distant objects often need to be much sharper to be clearly recognizable, whereas closer objects, being larger on the film, do not need to be so sharp.
The loss of detail in distant objects may be particularly noticeable with extreme enlargements. Achieving this additional sharpness in distant objects usually requires focusing beyond the hyperfocal distancesometimes almost at infinity.
For example, if photographing a cityscape with a traffic bollard in the foreground, this approach, termed the object field method by Merklinger, would recommend focusing very close to infinity, and stopping down to make the bollard sharp enough.
With this approach, foreground objects cannot always be made perfectly sharp, but the loss of sharpness in near objects may be acceptable if recognizability of distant objects is paramount. Other authors Adams51 have taken the opposite position, maintaining that slight unsharpness in foreground objects is usually more disturbing than slight unsharpness in distant parts of a scene.
Moritz von Rohr also used an object field method, but unlike Merklinger, he used the conventional criterion of a maximum circle of confusion diameter in the image plane, leading to unequal front and rear depths of field. The depth-of-field scale top indicates that a subject which is anywhere between 1 and 2 meters in front of the camera will be rendered acceptably sharp. Out-of-focus highlights have the shape of the lens aperture.
Several other factors, such as subject matter, movement, camera-to-subject distance, lens focal lengthselected lens f-numberformat sizeand circle of confusion criteria also influence when a given defocus becomes noticeable.
For a given f-number, increasing the magnification, either by moving closer to the subject or using a lens of greater focal length, decreases the DOF; decreasing magnification increases DOF. For a given subject magnification, increasing the f-number decreasing the aperture diameter increases the DOF; decreasing f-number decreases DOF.
If the original image is enlarged to make the final image, the circle of confusion in the original image must be smaller than that in the final image by the ratio of enlargement.
Depth of field
Cropping an image and enlarging to the same size final image as an uncropped image taken under the same conditions is equivalent to using a smaller format under the same conditions, so the cropped image has less DOF. Stroebel, — When focus is set to the hyperfocal distancethe DOF extends from half the hyperfocal distance to infinity, and the DOF is the largest possible for a given f-number.
Relationship of DOF to format size[ edit ] The comparative DOFs of two different image sensor format sizes depend on the conditions of the comparison. The DOF for the smaller format can be either more than or less than that for the larger format. In the discussion that follows, it is assumed that the final images from both formats are the same size, are viewed from the same distance, and are judged with the same circle of confusion criterion. Derivations of the effects of format size are given under Derivation of the DOF formulae.
Though commonly used when comparing formats, the approximation is valid only when the subject distance is large in comparison with the focal length of the larger format and small in comparison with the hyperfocal distance of the smaller format. Moreover, the larger the format size, the longer a lens will need to be to capture the same framing as a smaller format.
Conversely, using the same focal length lens with each of these formats will yield a progressively wider image as the film format gets larger: Therefore, because the larger formats require longer lenses than the smaller ones, they will accordingly have a smaller depth of field. However, changing the focusing distance is often the least convenient way to control depth of field - it's much easier to simply select an alternative aperture setting.
The only thing you need to be aware of is that shifting from a large aperture to a small one can lead to blurred photos.
They can do, but the choice of aperture has to be balanced with the shutter speed and ISO in order to maintain a consistent exposure. Check out our guide to the Exposure Triangle for a more detailed explanation, but here's a brief overview.
Larger apertures let in more light, so faster shutter speeds can be used to freeze movement. Switch to a smaller aperture, and the amount of light passing through the lens is reduced. Consequently, the shutter speed has to become slower, increasing the risk of camera shake and subject movement. To get round this, you could increase the ISO. This allows you to use smaller apertures to increase the depth of field and use faster shutter speeds.
Okay, so how does the type of camera affect depth of field? It's the size of the imaging sensor inside the camera that makes the difference. The larger the sensor, the shallower the depth of field will be at a given aperture. This is because you'll need to use a longer focal length or be physically closer to a subject in order to achieve the same image size as you get using a camera with a smaller sensor - and remember the effect that focusing closer has on depth of field. Is it true that longer lenses produce a shallower depth of field?
The focal length of the lens does appear to have a significant impact on depth of field, with longer lenses producing much more blur. A mm lens focused at 12ft will have a wafer-thin depth of field compared to a 20mm lens focused at 12ft. As the object moves farther from the lens, the image moves closer to the lens and decreases in size. As a result, the angular size of the bent light cone increases.
Even though the first and second object shifts are the same size, the second image shift is smaller than the first image shift.
Understanding Depth of Field for Beginners
In other words, the image position is less sensitive to the object position when the object is farther from the lens. The answer to this question is not very straightforward because we have two competing forces. Depending on the relative strengths of the angular and sensitivity effects, the DOF might decrease or increase!
To proceed any further, we will need to quantitatively determine the depth of field for different object distances.
- How Do Object Distance and Focal Length Affect Depth of Field?
For a lens with a given focal length, the general procedure would be as follows: Choose a central object position. Determine the corresponding image position. Assume that the camera sensor is located at this position. Choose another object position, locate its corresponding image, and determine the size of the circle of confusion it produces. Repeat step 4 many times to find the object positions at the extreme edges of the approximate focus range.
This will determine the depth of field at this central object position. Repeat steps many times for different central object positions.
It is possible to do this graphically by drawing bazillions of ray diagrams. However, it is much easier to calculate the depth of field numerically using equations that describe a simple model of a lens. The Thin Lens Equation The thin lens equation is the key to numerically determining the depth of field. For a point-sized object that is radiating light towards a lens, the equation relates the object-to-lens distance dothe image-to-lens distance called diand the lens focal length called f in the following way: Strictly speaking, this equation only applies to idealized lenses that have zero thickness.
Of course, such lenses do not exist in real life. However, camera lenses are carefully designed so that they function almost exactly like perfect thin lenses. Note that the thin lens equation contains all of the information we learned from the ray diagrams I drew above.
Specifically, we can use the thin lens equation to discover the angular and sensitivity effects. To do this, we can solve the equation for the image distance and then plot di vs do for a particular focal length, say 50 mm: