# Focal length and aperture relationship problems

### Absolute and relative lens apertures

Also, remember that the focal length also affects the size of the opening. unless you're confused by the relationship between focal length, aperture, The main issue you'll face is that, in the day time, you're shooting into the. First things first, aperture does affect depth of field and in a normal shooting scenario One of the most unsettled facts in photography is if focal length affects depth of field. . highlighting the object/subject in relationship to the farthest background point of reference. Lance, that is exactly my issue here. The f-number of an optical system is the ratio of the system's focal length to the diameter of the is the diameter of the entrance pupil (effective aperture). .. there is no depth of field issue, and the brightness of stellar point sources in terms of The rapidity of a lens depends upon the relation or ratio of the aperture to the .

Why then, is the amount of light quadrupled if the opening is only double the size? The return of math and of the Inverse Square Law. Double the radius of the aperture means four times as much light entering the camera The formula for the area of a circle is: If you crunch some numbers, you will find out that by doubling or halving the radius of the aperture, you will quadruple or quarter the area just like when we were talking about the difference in the intensity of a given light based on distance.

When we bring this numeric data into a system for EVs, it is quite simple. A change in aperture that results in the light being either doubled or halved means you have changed your exposure by one EV, or stop. So, now that you know how aperture effects exposure, let us talk about those two "side effects" of aperture that we alluded to above.

The size of the aperture diaphragm not only affects the amount of light passing through the lens, it also affects image sharpness and is one of several factors that affect something called "depth of field. Without depth of field, the lens's razor-thin focal plane would cause problems for photography. Take a photo of a person and, for instance, the tip of their nose would be in focus but the rest of them would be completely blurry. Depth of field allows that focal plane to have a perceived depth.

Example of deep depth of field Depth of field is a function of lens aperture size, lens focal length, the distance between the subject and the camera, and something called the circle of confusion. For the purposes of this article, we will keep the depth-of-field discussion relevant to aperture. Depending on your camera and lens, by opening your aperture to its widest settings, you will narrow the range of the focal plane to a very small distance.

This can be used in photography for creative compositions with close-up photography and, most popularly, for making distant backgrounds blurry when taking portraits. Adjusting your aperture diaphragm the other way, to its most narrow setting, extends the depth of that focus plane and allows a large range of the image to be in sharp focus. Deep depth-of-field techniques are used commonly in landscape images.

For a varsity-level, three-part depth-of-field discussion, click here. Large depth of field small aperture Not only does the aperture control the amount of light passing through the lens, it affects the angle of the light rays as they transit the lens. To be clear, we are not talking about how the lenses are bending light, we are talking about how light, when it passes by an object, is slightly bent by that object-in this example, the blades of an aperture diaphragm.

This bending of the light is called "diffraction" and is a characteristic of light's wave properties. When you constrict a lens's aperture diaphragm, you are bringing that diffraction closer to the center of the image.

For our purposes, we will assume that the aperture is fully open blocking nothingso we can refer to the lens size and the aperture size interchangeably to mean the same thing. Focal lengths The focal length of a lens is a property of a lens itself unrelated to the scenedetermined by the curvature of its surfaces and the refractive index of its material.

## Understanding Exposure, Part 2: Aperture

It is defined as the distance where parallel rays from an object infinitely far away converge to a single point. Lenses with longer focal lengths produce a larger image for the same object at the same distance. For objects that are sufficiently far away, the size of the image is approximately proportional to the focal length.

For example, a mm lens will produce an image that is twice as large as compared to a 50 mm lens. However, the law of conservation of energy comes into play here.

## Understanding Depth of Field - It's Not All About Aperture

The amount of light gathered by a lens is proportional to its area. If the outgoing projected image is enlarged, then the image must be dimmer at each point because the total energy of the image must remain the same.

This situation is analogous to moving a lamp farther from a wall or moving a video projector farther from the screen, both of which make the lit surface dimmer. Therefore, if two lenses have the same aperture diameter but different focal lengths, then the longer focal length lens will produce a dimmer image.

This effect can be counterbalanced by making the aperture diameter bigger in proportion to the focal length, which brings us to the topic of relative aperture sizes.

### Understanding Exposure, Part 2: Aperture | B&H Explora

Notation for relative apertures If we divide the aperture diameter by the focal length, we get a dimensionless number. The lowercase italic f is a variable that denotes the physical quantity called focal length.

The slash indicates division — e. This also explains why the aperture number seems to get bigger as we make the physical aperture size smaller — because the aperture size is the focal length divided by this number.

### f-number - Wikipedia

In real-world products, Nikon gets the notation right whereas Canon gets it wrong. Properties of absolute and relative apertures We have seen that if we hold the absolute aperture size constant while increasing the focal length, the image becomes dimmer. In particular, due to the inverse square law for radiation in 3D space, the image brightness is inversely proportional to the square of the focal length.

For example, doubling the focal length will make each point a quarter as bright. At the same time, doubling the absolute aperture size will quadruple the area of the lens. Putting these two facts together, if we double the absolute aperture size and double the focal length then there will be no change in the image brightness.