Why does the angle of view narrow as you zoom in? Why does a 4mm lens have a wider angle of view than an 8mm lens ?
What is "angle of view" in photography? I read on the :     Focal-length determines the angle-of-view seen through a lens for a   given sensor-size. With a full-frame sensor a lens gives the same   angle-of-view as it would on a 35mm film camera. With a smaller   sensor, the angle-of-view becomes smaller. The crop-factor, also   called FLM, is the ratio representing the difference in equivalent   focal-lengths. So a 150mm on a full-frame DSLR such as the Nikon D700   gives the same angle-of-view as a 100mm on a D7000 since its FLM is   1.5X.      Short focal-lengths show a greater angle-of-view compared to longer   ones. [...]   In layman's terms, what is angle-of-view? Is it the same as focal length? If not, how is it different? How is it used? Why do I need to know about it?
/boot full of kernels – what to delete? We have some Ubuntu 16.04 servers. unattended-upgrades are automatically enabled since 16.04 and /boot is a separate partition. Due to the automatic security updates, the boot partition is running out of space with new kernels. We can't reboot the systems (for availability reasons), so the machine is still using the penultimate kernel. Which kernels should I remove? All but the current, the oldest and the newest? Do you guys have some recommendations? I have also noticed that the newest kernel has the status &quot;Half Configured&quot;. This kernel would probably not work, so should I remove this one and use an older kernel? Output of dpkg -l | grep linux-image: ii  linux-image-4.4.0-21-generic --&gt; old kernel ii  linux-image-4.4.0-34-generic --&gt; current kernel ii  linux-image-4.4.0-36-generic --&gt; new kernel ii  linux-image-4.4.0-38-generic --&gt; new kernel ii  linux-image-4.4.0-42-generic --&gt; new kernel ii  linux-image-4.4.0-45-generic --&gt; new kernel ii  linux-image-4.4.0-47-generic --&gt; new kernel ii  linux-image-4.4.0-51-generic --&gt; new kernel ii  linux-image-4.4.0-53-generic --&gt; new kernel iF  linux-image-4.4.0-57-generic --&gt; new kernel I wonder whether it is possible to perform within R a clustering of data having mixed data variables. In other words I have a data set containing both numerical and categorical variables within and I'm finding the best way to cluster them. In SPSS I would use two - step cluster. I wonder whether in R can I find a similar techniques. I was told about poLCA package, but I'm not sure ... Is there a way to add a timer to a Beamer presentation? I have two directories that should contain the same files and have the same directory structure.  I think that something is missing in one of these directories.  Using the bash shell, is there a way to compare my directories and see if one of them is missing files that are present in the other? Keep values selected after form submission What is the difference between 'can', 'could', 'may' and 'might'? I'm a native English speaker and I've been doing some research into English grammar for a programme I'm working on. However, on looking into modal verbs, I've only just come to appreciate how subtle and complicated they are.  What is the difference between these sentences? What possible different response might one have to each?        I can go to the cinema tonight.   I could go to the cinema tonight.   I may go to the cinema tonight.   I might go to the cinema tonight.      Granted, the first of these looks more like a statement of ability or possibility than the three that follow. But in what ways does it differ from the others, since all suggest ability and possibility? And crucially, what distinguishes the other three.  Perhaps it's a matter of tone, of context, of the speaker... The question may appear quite simple but I haven't found a satisfactory answer elsewhere.    looked quite useful in that it covered levels of confidence and certainty but it was talking about the potential state of something rather than the possibility for the person who is speaking. Are there any differences in that sense?   looked at these verbs used in question form (e.g., differences between 'Can I...?' and 'May I...?') but I'm not so interested in that for now.  Any thoughts appreciated. How do I easily switch between windows rather than applications in GNOME Shell? "Has/Have/Had" as a main verb and auxiliary verb I need help regarding the use of has/have/had both as a main verb and as an auxiliary verb.  In the sentence below, the word has acts as the main verb. I would normally follow it with the phrase "so do I."        Kenny has a car, and so do I.      On the other hand, the word has in the following sentence acts as an auxiliary verb. I would normally follow it with the phrase "so have I."        Kenny has bought a car, and so have I.      a) Are sentences 1 &amp; 2 above correct? I'm under the impression that so do I is used when has/have/had is the main verb of a sentence, and so have/had I if it's an auxiliary verb in a sentence.  b) Does the verb has/have always need a supporting "auxiliary verb" (either implied or mentioned)? (e.g., Kenny has a car = Kenny does have a car.) How to reduce Eclipse's memory usage? I was curious about how other people use the this keyword. I tend to use it in constructors, but I may also use it throughout the class in other methods. Some examples:  In a constructor:  public Light(Vector v) {     this.dir = new Vector(v); }   Elsewhere  public void SomeMethod() {     Vector vec = new Vector();     double d = (vec * vec) - (this.radius * this.radius); } Distance between surfaces From a previous question, we have that $\sqrt[3]{3} \notin \mathbb{Q}(\sqrt[4]{3})$, and I am assuming that we would need to use this to justify the answer to the question.   Would it be right to use some idea of a splitting field here? Or is it something to do with $\mathbb{Q}(\sqrt[3]{3}, \sqrt[4]{3})$ not containing a primitive 3rd root of unity?   This is a past exam paper question worth 8 marks, so it doesn't seem like it's a few lined answer, but I am a little stuck on where to go. Will I miss anything by watching Dragon Ball Z Kai instead of the other series? I've read that Dragon Ball Z Kai is an HD remaster and recut of Dragon Ball and Dragon Ball Z. However, Kai has  while Dragon Ball has  and Z has .  If I watch only Dragon Ball Z Kai, do I miss anything I would get by watching the other two series? And is anything censored in Kai that is not censored in the others? The repository 'http://dl.google.com/linux/chrome/deb stable Release' does not have a Release file After executing this command  sudo apt-get update &amp;&amp; sudo apt-get -y upgrade   I get this error  W: The repository 'http://dl.google.com/linux/chrome/deb stable Release' does not have a Release file. N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use. N: See apt-secure(8) manpage for repository creation and user configuration details.   I've seen past inquiries on same issue but the answers given are specific to other repositories.  Any help will be appreciated. I've setup a HTTPS and FTP site in IIS but how can I add users to use the FTP site?  Does they necessarily need an account with password in the computer to be able to use it? Where does the extra kinetic energy of the rocket come from? How, on a physical level, does ATP confer energy? When ATP is used as the energy currency to make, say, reaction X + Y → Z happen, is what happens on a physical level down at the molecular scale that during the reaction    ATP + H2O → ADP + Pi &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ΔG˚ = −30.5 kJ/mol (−7.3 kcal/mol)    that 30.5 kJ/ mol is conferred by ATP molecules physically bumping around the reactants X and Y, the kinetic energy of the above reaction being what does it?  I mean, is the energy coin of ATP conferred to reactions by molecular collisions, or is it an electric field effect in the spatial geometry the way the ATP molecule tends to break apart? The Half-Elven and their choices When I format a drive to NTFS under Windows and plug it into the Ubuntu machine it is perfectly usable, but when I format it under Ubuntu using Disks (delete all partitions and create one single partition with NTFS) it does not show up when plugged into the Windows machine. Please note that in both cases the USB drive contains only one, primary partition formatted to NTFS, for example: /dev/sdb - the actual device /dev/sdb1 - the only, primary partition I've observed this behaviour both with a solid state USB pen-drive as well as an external USB magnetic HDD. How to (re)format a USB HDD or a stick to NTFS under Ubuntu so it is usable under Windows? Any way to embed Pygame in webpages?