Does focal length change when I adjust the focus? When I adjust the focus of my camera, in general, does this affect the focal length? I have heard that it does.
Is it normal for focal length to change while focusing? When I change the focus distance on my Pentax 55-300mm lens, the focal length appears to change.  More specifically, the lens appears to zoom in as I focus towards the minimum focusing distance.  This is most noticeable at the 55mm end of the lens.  Is this normal?
What is the value of $\arctan(1/2)+\arctan(1/5)+\arctan(1/8)$? How do I get and modify the source code of packages installed through apt-get? How do you concatenate matrices? And how do you make block matrices? How do I get a platform-dependent new line character? How do I get a platform-dependent newline in Java? I can’t use "\n" everywhere. How do I update Custom Label's value using apex? Which one would be a better option metadata api or tooling api? Please provide example I'am working on post deployment automation process in salesforce. I am stuck on how should I update values in custom labels which needs tobe updated manually after a sandbox refresh. Floquet and Bloch's theorems : connection? It is often stated that  and  are equivalent, even the Bloch's theorem is often referred as Floquet-Bloch theorem.  However, it seems quite confusing to me since the former involves a second order differential equation (Schroedinger equation with a periodic potential) while the latter is defined for a first order one.  Can someone clarify this to me? Also it would be nice if I can get references that connect the two. A google search gives number of methods to hide ALL tabs in chrome , but I want to hide 1 particular tab / change the display name of that tab.   Reason: I have an online music playing website open, which keeps showing tracks in the tab name and I just want to remove it from view of my work. In R, what does qqnorm actually do? qqnorm(x) plot(qnorm(seq(80)/81),sort(x))   Finding that the plots produced by the commands above are slightly different from each other, I tried this:  qqnorm(qnorm(seq(80)/81))   I got a slightly less than perfect line.  I'd have tried regressing the result of qnorm(seq(80)/81) on the variable that was plotted on the $x$-axis and plotting residuals against the predictor, expecting to see some graceful curve, but for the fact that I don't know what to use as the predictor.  Possibly such a residual plot would reveal more than just the graceful S-shaped thing I'd anticipate.  So my question is this: if the thing on the $x$-axis in the plot produced by qqnorm is not what I get from qnorm(seq(80)/81) (and what I did shows that indeed it is not), then what is it? I have a problem with derivation of some function. This is related to computing the Poincare Map of Logistic Population model with Periodic harvesting. This function is:  $\phi(t,x_0) = x_0 + \int_{0}^{t} f(s,\phi(s,x_0)) ds $,  where $\phi(t,x_0)$ is a function: $\mathbb{R}  \times \mathbb{R} \rightarrow \mathbb{R}$.  I'm trying to differentiate  equation of this function both sides  with respect to $x_{0}$. In book from this equation comes from the result of this differentiation is:  $\frac{\partial \phi}{\partial x_0} (t,x_0) = 1 + \int_0^{t} \frac{\partial f}{\partial x_0} (s, \phi(s,x_0)) \cdot \frac{\partial \phi}{\partial x_0} (s, x_0)ds$.  I can't understand expression under the integral. In this book it is explained that it is clear from chain rule.Could someone explain to me this transformation in easy way? I will be grateful for your help Best regards “Have you seen...” or “Did you see...?” In the US, when people work together, they may look for a colleague or any person to say something or for any other reason, asking colleagues or other people this type of question:         Have you seen Michael?   (OR)        Did you see Michael?          Which is considered right if the question refers to an activity (in our case "see") during the day and not in the past in general? Feed randomly gone on SE Android app SE Android App Version: 1.0.84  There is a display issue in the Feed section. The Earlier today, the below sections and the post title are not visible.  There is no exact steps to reproduce. It doesn't happen when the app is initializing. But if you open the Stack Exchange app, navigate to some other apps, then come back after few minutes, I can see this issue (but not all the time).  When the issue is there, if I manually click the Feed option in the navigation sidebar, the display issue is fixed.  I have recorded the issue: Is cross-posting wrong (to an external site)? I made a question at Stack Overflow long ago and also posted the same question in a non-Stack Exchange website (Xtreme VB Talk). The question .  According to the answer, I shouldn't have cross-posted between Stack&nbsp;Overflow and XtremeVBTalk, because     it increases the probability that wires will get crossed and everyone will waste time.   But I'm not sure why is that wrong. You see, I simply see Stack&nbsp;Overflow and XtremeVBTalk as two different communities with different users. So posting the question in both sites would just give me more answers. And if I got the answer in one of them, I would report back to the other site and let everyone know (and perhaps someone else would be benefited as well). I really doubt it is a waste of time - the more input, the better. The fact that someone was a user in both websites is simply a coincidence.  I realize that Meta&nbsp;Stack&nbsp;Exchange question  exists, but apparently it involves cross-posting within Stack Exchange sites, but this is about external websites instead.  I also realize that I was told that as "advice" (since my question was not flagged or anything), but well, I am now curious: Why would anyone consider this a bad practice? How to get a list of the filenames of the figures embedded in a document I have a TeX document that is embedding a huge amount of figures from a fig folder that is full of another humongous amount of other figures.  I would like to move the document along with its figures, but I don't want to take the whole fig folder, just the figures in there that are embedded by the document.  Is there a package (or any other way) that allows me to, for example, print out the filenames of all the figures embedded by the document? Something like:  fig/figurename1.eps fig/figurename2.eps ... etc   I could then just copy-paste that list and add some cp's in a plain text file and automate the copy of those files to my desired location by running the file in my shell:  $ cp fig/figurename1.eps wherever/fig/ $ cp fig/figurename2.eps wherever/fig/ ... etc   Of course this is just an idea, if somebody has an alternative to the "file listing and copy-pasting in a shell file" I would be happy to listen =) Why does sudo change the PATH? How to approximate the student-t CDF at a point without the hypergeometric function? Is there a way to closely approximate the CDF of a student-t distribution at a point $x$ without involving the hypergeometric function? For example, by using a series expansion, or expressing the CDF in terms of simpler functions?  The t-distribution can   $$t = \frac{X\sqrt{n}}{Y}$$  where $X$ is normally distributed with mean 0 and variance $\sigma^2$; $\frac{Y^2}{\sigma^2}$ has a chi-squared distribution. Maybe this expression can help simplify the t CDF?  I am trying to write an algorithm to compute the student-t CDF for any real number, so using the hypergeometric function will be extremely difficult. I am also trying to avoid integrating the PDF to get the CDF. How do I detach a process from Terminal, entirely? I use Tilda (drop-down terminal) on Ubuntu as my "command central" - pretty much the way others might use GNOME Do, Quicksilver or Launchy.  However, I'm struggling with how to completely detach a process (e.g. Firefox) from the terminal it's been launched from - i.e. prevent that such a (non-)child process   is terminated when closing the originating terminal  "pollutes" the originating terminal via STDOUT/STDERR   For example, in order to start Vim in a "proper" terminal window, I have tried a simple script like the following:  exec gnome-terminal -e "vim $@" &amp;&gt; /dev/null &amp;   However, that still causes pollution (also, passing a file name doesn't seem to work). Aspect (simple, perfect and progressive): What are the differences? Could you please explain to me the differences between the simple, progressive and perfect aspects.  "Simple aspect" means completed action (action starts and finishes) but I don't really understand the fine details.  For example, with these sentences:     It usually rains in the fall.   He smokes 15 cigarettes a day.   I slept for a long time yesterday.     Here's what I understand: if I didn't complete sleeping I should use "I was sleeping". If it doesn't stop raining we can't say "it is finished". Is that correct?  For continuous aspect, duration is an important factor – but how?  For example:     She is wearing a jacket.   Normally "to wear" action is completed but we can use here present continuous aspect.  Another example:     She has been wearing the same jacket for 3 days.   and why can't I use continuous aspect here?     I have cleaned my room.   It means that my room is clean now. Action is finished but why can't I say "I have been cleaning room" like "They have been playing in the mud"? Action is completed but we don't care here about completed, we are caring about duration, why? How to change the language for the front-end only? Baffling Elevation Profile out of Profile Tool Plugin When using the Profile Tool plugin I get a graph that looks like the 2nd derivative of what I want. That is, it looks like the rate of change of the slope of the elevation profile. I just want the elevation profile! The data is present for good 1' contour lines. Why is the Profile Tool interpreting the data in such a bizarre way? What am I missing? And what are the Min and Max units? I'd like to work on a bbcode filter for a php website. (I'm using cakephp, it would be a bbcode helper) I have some requirement.    Bbcodes can be nested. So something like that is valid.  [block]       [block]       [/block]       [block]           [block]           [/block]       [/block]   [/block]     Bbcodes can have 0 or more parameters.     Exemple:     [video: url="url", width="500", height="500"]Title[/video]   Bbcodes might have mutliple behaviours.     Let say, [url]text[/url] would be transformed to [url:url="text"]text[/url] or the video bbcode would be able to choose between youtube, dailymotion....  I think it cover most of my needs. I alreay done something with regex. But my biggest problem was to match parameters. In fact, I got nested bbcode to work and bbcode with 0 parameters. But when I added a regex match for parameters it didn't match nested bbcode correctly.   "\[($tag)(=.*)\"\](.*)\[\/\1\]"  // It wasn't .* but the non-gready matcher  I don't have the complete regex with me right now, But I had something that looked like that(above).  So is there a way to match bbcode efficiently with regex or something else.  The only thing I can think of is to use the visitor pattern and to split my text with each possible tags this way, I can have a bit more of control over my text parsing and I could probably validate my document so if the input text doesn't have valid bbcode. I could Notify the user with a error before saving anything.  I would use sablecc to create my text parser.   Any better idea? or anything that could lead to a efficient flexible bbcode parser?  Thank you and sorry for my bad english...