Put simply, focal length as denoted by the numbers on your lens (35mm, 50mm etc.) is the distance between your sensor or film and your camera lens when it’s focused to infinity.

Focus distance

Important Optics Formula: Sin i / Sin r = μ [refractive index of the second medium with respect to the first medium] Also μ = c/v or v 1 /v 2 μ = Real depth

One reason the focal length doesn’t matter when shooting street is that nobody knows, or cares, how near or far things really are in your images, so long as they look good.

Focallength

In this lab, you will take measurements to determine how the amplitude and the period of waves are affected by the transfer of energy from a cork dropped into the water. The cork initially has some potential energy when it is held above the water—the greater the height, the higher the potential energy. When it is dropped, such potential energy is converted to kinetic energy as the cork falls. When the cork hits the water, that energy travels through the water in waves.

When I’m out shooting on the street, I can adapt my images to suit the focal length. However, if you’re shooting something that needs a particular look, such as portraits choosing the right focal length is vital.

We could have also used the equation v w = λ T v w = λ T to solve for the wave velocity since we already know the value of the period ( T=0.5s) ( T=0.5s) from our calculation in part (a), and we would come up with the same answer.

In addition to amplitude, frequency, and period, their wavelength and wave velocity also characterize waves. The wavelength λ λ is the distance between adjacent identical parts of a wave, parallel to the direction of propagation. The wave velocity v w v w is the speed at which the disturbance moves.

When taking these pictures, the three objects did not move. I did have to move the camera further away from the pickle as I zoomed in to the higher focal lengths, to have the bottle the same size in each picture, but that was the only change I made.

This video is a continuation of the video “Introduction to Waves” from the "Types of Waves" section. It discusses the properties of a periodic wave: amplitude, period, frequency, wavelength, and wave velocity.

Because when you’re getting started in photography, all those numbers and terms can be pretty intimidating. They lead to a lot of questions for beginners around the subject of focal length.

For these three pictures, I had to stand further away from the lamppost to make it appear the same size in each picture as my focal length increased (as I zoomed in).

Consider the periodic water wave in Figure 13.7. Its wavelength is the distance from crest to crest or from trough to trough. The wavelength can also be thought of as the distance a wave has traveled after one complete cycle—or one period. The time for one complete up-and-down motion is the simple water wave’s period T. In the figure, the wave itself moves to the right with a wave velocity vw. Its amplitude X is the distance between the resting position and the maximum displacement—either the crest or the trough—of the wave. It is important to note that this movement of the wave is actually the disturbance moving to the right, not the water itself; otherwise, the bird would move to the right. Instead, the seagull bobs up and down in place as waves pass underneath, traveling a total distance of 2X in one cycle. However, as mentioned in the text feature on surfing, actual ocean waves are more complex than this simplified example.

Had I been in the same park to take actual photos with a 55mm lens, and not just getting shots to demonstrate focal length, I wouldn’t have taken that third shot. I would have found other things to take pictures of. Things more suited to the focal length available to me.

You might think the images in that post are rubbish, or you might think they aren’t bad. I would be surprised to hear you thought they’re rubbish because of the focal length though.

With the help of some Branston Pickle, Aesop’s Fables, and two men punching each other in the face, I’ll give you another visual.

All waves carry energy, and the energy of earthquake waves is easy to observe based on the amount of damage left behind after the ground has stopped moving. Earthquakes can shake whole cities to the ground, performing the work of thousands of wrecking balls. The amount of energy in a wave is related to its amplitude. Large-amplitude earthquakes produce large ground displacements and greater damage. As earthquake waves spread out, their amplitude decreases, so there is less damage the farther they get from the source.

The speed of propagation vw is the distance the wave travels in a given time, which is one wavelength in a time of one period. In equation form, it is written as

The main options for cameras with interchangeable lenses are FX and DX in DSLRs, APS-C in Sony (and other) mirrorless, and micro four thirds or m43 in Olympus (and other) mirrorless.

Wave velocity is sometimes also called the propagation velocity or propagation speed because the disturbance propagates from one location to another.

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For example, a 35mm lens on a DX or APS-C sensor will give something close enough to 50mm when x1.5, and a 50mm lens will give something close to 85mm, which is handily a great length for portraits.

I’ve already chosen my lens, which is always a fixed length, and am far more concerned with what I can capture with it, and how, than I am with why.

As well as allowing for a shallower depth of field to blur the backgrounds, which is a whole other article, longer lenses can help make people look better.

I’m happy to hear you’re taking a photography class. I took some a few years ago and they’re the best money I ever spent on photography. Far more so than any gear. I hope you get a lot out of your classes too. 🙂

The first, which is perhaps the single most noticeable, is how close to the camera your subjects appear. It’s the basic action of zooming in and out.

Focallengthcamera

For waves, these variables have the same basic meaning. However, it is helpful to word the definitions in a more specific way that applies directly to waves:

The first step to understanding this is to ask, what kind of camera do you have? And more importantly, what size sensor does it have?

Since one definition of wavelength is the distance a wave has traveled after one complete cycle—or one period—the values for the wavelength (λ=0.9 m) (λ=0.9 m) as well as the frequency are given. Therefore, we can use v w =fλ v w =fλ to find the wave velocity.

focal length是什么

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These fundamental relationships hold true for all types of waves. As an example, for water waves, vw is the speed of a surface wave; for sound, vw is the speed of sound; and for visible light, vw is the speed of light. The amplitude X is completely independent of the speed of propagation vw and depends only on the amount of energy in the wave.

Crop factor can be an annoyance when it negates truly wide angle lenses on non-full frame cameras, but it can usually be worked around for more common focal lengths.

So whilst out with whatever lens you might have, you should be adapting your mind to suit what you see through the viewfinder, adapting your ideas, adapting your subjects, adapting your composition, and taking your photos accordingly.

From this relationship, we see that in a medium where vw is constant, the higher the frequency, the smaller the wavelength. See Figure 13.8.

The first picture, shot at 18mm, appears to show a greater distance between the objects while the one shot at 55mm is somewhat flatter, with the images appearing closer together.

The pictures taken at 33mm and 55mm were taken from this position only to show you the lessening distance between the lamppost and the tree.

Notice the distance from the lamppost to the tree, and the bin, and also how the blue cafe sign comes closer as the focal length increases.

I’m taking a photography class at a local School of Photography and always refer to your articles to flesh out my understanding of what I’m learning. You have such a gift at simplifying what can be a confusing subject. I just wish I had a book to write in a tag! For a visual learner especially, I can see the illustrations in my mind’s eye and they stay with me! Consider a book! Thanks 🙂

Calculate the wave velocity of the ocean wave in the previous figure if the distance between wave crests is 10.0 m and the time for a seagull to bob up and down is 5.00 s.

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And if you think others will get something from this focal length explanation too, help them find it by sharing or pinning.  😀

Geologists rely heavily on physics to study earthquakes since earthquakes involve several types of wave disturbances, including disturbance of Earth’s surface and pressure disturbances under the surface. Surface earthquake waves are similar to surface waves on water. The waves under Earth’s surface have both longitudinal and transverse components. The longitudinal waves in an earthquake are called pressure waves (P-waves) and the transverse waves are called shear waves (S-waves). These two types of waves propagate at different speeds, and the speed at which they travel depends on the rigidity of the medium through which they are traveling. During earthquakes, the speed of P-waves in granite is significantly higher than the speed of S-waves. Both components of earthquakes travel more slowly in less rigid materials, such as sediments. P-waves have speeds of 4 to 7 km/s, and S-waves have speeds of 2 to 5 km/s, but both are faster in more rigid materials. The P-wave gets progressively farther ahead of the S-wave as they travel through Earth’s crust. For that reason, the time difference between the P- and S-waves is used to determine the distance to their source, the epicenter of the earthquake.

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Since wave frequency is the number of waves per second, and the period is essentially the number of seconds per wave, the relationship between frequency and period is

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I adapt to what I have and if this is a prime lens then we’re essentially trying to make the best of the restrictions we have given ourselves.

However, there are further, less conspicuous ways in which focal length changes how your images look. While they may not be quite so obvious to the beginner, they can be just as important depending on what you’re shooting.

Use these questions to assess students’ achievement of the section’s Learning Objectives. If students are struggling with a specific objective, these questions will help identify such objective and direct them to the relevant content.

just as in the case of harmonic motion of an object. We can see from this relationship that a higher frequency means a shorter period. Recall that the unit for frequency is hertz (Hz), and that 1 Hz is one cycle—or one wave—per second.

I said earlier that I don’t believe focal length is something to worry about once you’re out shooting, and this is where I can explain that further.

Personally, I take pictures to suit the focal length I have at my disposal, without even having the words ‘focal length’ enter my head.

In this animation, watch how a string vibrates in slow motion by choosing the Slow Motion setting. Select the No End and Manual options, and wiggle the end of the string to make waves yourself. Then switch to the Oscillate setting to generate waves automatically. Adjust the frequency and the amplitude of the oscillations to see what happens. Then experiment with adjusting the damping and the tension.

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As we saw above, shorter, wider angle lenses will add depth. Longer, narrower lenses will flatten the image. So what lens do we use for portraits?

Photography should be fun. Forget the numbers once you’re out there. Go enjoy it, and don’t come back without some great photos – whatever focal length it is you’re using. 😀

And that leads us down the next rabbit hole. Because if you’re going to understand exactly how the focal length of your lenses will make your images look, you’re going to need to learn a bit about crop factor.

This slow speed seems reasonable for an ocean wave. Note that in the figure, the wave moves to the right at this speed, which is different from the varying speed at which the seagull bobs up and down.

The values for the wavelength (λ=10.0 m) (λ=10.0 m) and the period ( T=5.00s) ( T=5.00s) are given and we are asked to find v w v w Therefore, we can use v w = λ T v w = λ T to find the wave velocity.

If you have a zoom lens, say an 18-55mm, you can use it any focal length between 18 and 55mm. Prime lenses are fixed at one value.

Use it to pick the right lens for what you’re going to shoot, then concentrate fully on the creative side of things. Not the maths.

So to repeat that earlier statement, on a full frame or FX camera, a 50mm lens will give you photos that look pretty much as the human eye sees the world.

Aperture

We know from seismic waves produced by earthquakes that parts of the interior of Earth are liquid. Shear or transverse waves cannot travel through a liquid and are not transmitted through Earth’s core. In contrast, compression or longitudinal waves can pass through a liquid and they do go through the core.

Understand a bit about focal length will help you choose the best lens for getting the best results you can. That’s important.

If a wider lens will make their features more prominent, longer lenses can, and often are, used to make them look more petite.

Depending on what you shoot – landscape, portrait, street, travel, anything else – understanding the best focal length for the photographs you’re going to want to get is important.

Not when you can always find a different angle or composition or, if it’s really not working, to just forget it and find a different subject.

Even though everyone is different, we do have an idea of the boundaries of normality when it comes to the human form. And no model wants to have their nose or chin enlarged by the camera.

Crop factor relates directly to the type of sensor you have in your camera, and will alter the effective focal length of the lens you’re shooting with.

In short, the 50mm lens you buy might not be giving you a 50mm focal length. The 35mm lens you buy might not be giving you a 35mm focal length either.

Lenscalculator

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The woman in Figure 13.3 creates two waves every second by shaking the toy spring up and down. (a)What is the period of each wave? (b) If each wave travels 0.9 meters after one complete wave cycle, what is the velocity of wave propagation?

This all means if you want to know you’re shooting the focal length as stated on your lens, you’re going to need a full frame or FX camera.

If you have a DSLR and aren’t sure, it will probably be DX. These are the more common type, and if you never asked when you were buying it, you probably got a DX.

Which of the settings—amplitude, frequency, damping, or tension—changes the amplitude of the wave as it propagates? What does it do to the amplitude?

focal length中文

[Figure Support] The same speaker is capable of reproducing both high- and low-frequency sounds. However, high frequencies have shorter wavelengths and are hence best reproduced by a speaker with a small, hard, and tight cone (tweeter), whereas lower frequencies are best reproduced by a large and soft cone (woofer).

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If you found this post on understanding focal length useful and want more practical photography guides, here are three more that can help you:

To find the period, we solve for T= 1 f T= 1 f , given the value of the frequency ( f=2 s −1 ). ( f=2 s −1 ).

I’m not thinking about what I could be capturing with a different focal length. I’m concentrating on what I can shoot and not worrying about what I can’t.

[BL] For sound, a higher frequency corresponds to a higher pitch while a lower frequency corresponds to a lower pitch. Amplitude corresponds to the loudness of the sound.

If it looks bad, I highly doubt thinking about the focal length would have been the best way for the photographer to make it a better picture.

Subjects appearing closer and fields of view becoming narrower are both rather obvious effects of different focal lengths.

For me, focal length is something you should probably know about as a photographer. And know about it well enough to explain it to someone else.

While that mini shoot was fun to do, we’ll always learn more from real world examples, such as these shots from the park.