I'm very (very) new to photography world.. I've been trying in the past few days to explore very deeply to this world.. But the more I dived in the more confused I became.    For example most of the videos that I've seen in YouTube and a lot of articles said that the focal length is the measurement between the front lens and to the convergence point, but some said (and it sounds more sense to me) that it actually from the convergence point to the sensor, so what is the truth?
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?
Trigonometric Uncertainty Propagation Can't preview smoke simulation on Blender 2.82 Whenever I add a Quick Smoke effect, I can't preview it on the viewport unless I bake it.   And Yes, I added the Smoke Domain on the object.   I've seen tutorials like this  and this  that clearly shows the possibility of previewing the smoke before baking it.   I'm using Blender on a Mac with AMD GPU. I would guess that this is something related to OpenGL, as thoroughly explained here -  but that's for Windows.   Does anyone have any idea how to fix this? Why didn't all the Jedi at the Jedi Temple sense Anakin and his legion of Clones’ thoughts? If you go back to Attack of the Clones, Anakin and Obi-Wan sensed that poisonous alien worms were about to kill Padme. Back in Revenge of the Sith, the Jedi could take on the legion or if there were too many, they could at least escape. What does the following for loop syntax mean in C? Snap entire selection to the 3d cursor in edit mode Is there any way to snap an entire selection to the 3d cursor in edit mode?  Shift+S > Selection to Cursor snaps each individual vertex to the cursor, so the shape is lost: Storing Images in DB - Yea or Nay? So I'm using an app that stores images heavily in the DB. What's your outlook on this? I'm more of a type to store the location in the filesystem, than store it directly in the DB.  What do you think are the pros/cons? How do I run a command before or after every command written in the console? stackoverflow.com without OpenID is it possible to login/create user in stackoverflow.com without creating/login using OpenID? i hate using OpenID, it's really confusing if you create different user/account on different websites :(     Note: the following text was submitted as an edit suggestion. I approved it, giving the user the benefit of the doubt and assuming that it was the OP's other account. -   even when i want to post this question under my new openid, i still logged as the old user, so i can't view my own question when i logged under my userid Is it possible to link multiple leaders lines to one piece of annotation?  I'm working in ArcGIS 10.2 Desktop Advanced with non-feature linked geodatabase annotation. I have hundreds of numbers labeling various polygons, but some polygons share a number. Instead of creating multiple numbers/annotations for each polygon I'd like to have one number with multiple lines pointing to each polygon.   So, in the image below, the number 5 corresponds to multiple polygons. Here I've moved 5 from it's ideal location to show that the second leader line is not connected to the number annotation. My workaround has been to create another piece of geodatabase annotation with a blank text string to serve as a placeholder for a leader line. The other option was to use map/graphic annotation to create the second leader line, but with the multiple disadvantages of not being in the same annotation feature class as the number annotation.    My goal would be for each piece of number annotation to be connected to multiple leader lines like appears below. Is it appropriate to buy a "thank you" gift for a PhD supervisor? I am nearing the end of my PhD and I would quite like to buy a "thank you" gift for my supervisor (although, she's the socially awkward type who I'm sure will be greatly embarrassed to receive one).  I'm just wondering whether a gift is the done thing? And if so, what sort of gift seems appropriate? I suppose a bottle of wine would be a safe choice? Can't save my render as an image I seem to be having issues when I try and save the output file as a .png, or .jpeg for that matter. I've set up my user preferences, File, Render Output to my desktop as well as render output in the editor window. What am I missing here? I'm obviously very new to this program and your help would be appreciated. Thanks for your time How do I set the page title? I know I could change the page title with drupal_set_title(t('Amy page title')), but when I try it in Drupal 8 I get the There is no such function error.  How can I change page title in Drupal 8? How do I identify which CPU a MacBook uses? Get table names using SELECT statement in MySQL What are independent parameters in Hellmann–Feynman theorem? A typical example in textbooks about the application of Hellmann–Feynman theorem is calculating $\left\langle\frac{1}{r^2}\right\rangle$ in hydrogen-like atoms.  has a nice demonstration of this. At some point in the Wikipedia derivation is used that $$\frac{\partial n}{\partial \ell}~=~1. \tag{1}$$   But why is eq. (1) true? I know that  $$n=n_r+\ell+1,$$   but $n_r$ is just another variable with different physical meaning, so why is $n_r$ independent from $\ell$, whereas $n$ is not? The Wikipedia proof for Hellmann–Feynman theorem does not address the problem of independence of different parameters. What variables are kept fixed during the differentiation $(1)$ and why?  The Wikipedia page seems to have only a vague notion of $\frac{\partial\hat{H}}{\partial\lambda}$ and $\frac{\partial E}{\partial\lambda}$, unlike in, e.g., thermodynamics, where all partial derivatives are typically written like  $$\left( \frac{\partial U}{\partial V} \right)_S \qquad ,\qquad  \left( \frac{\partial U}{\partial V} \right)_p \qquad ,\qquad  \ldots $$ so that it is clear which variables are kept fixed during the differentiations. What is the difference between Θ(n) and O(n)? Sometimes I see Θ(n) with the strange Θ symbol with something in the middle of it, and sometimes just O(n). Is it just laziness of typing because nobody knows how to type this symbol, or does it mean something different? I just made a minor , and  the grammar correction was unnecessary.  The original text was     Another factor--sometimes life happens and you can't fly. In the old days you could simply sell your ticket to someone else, now you either have to eat a hefty change fee or lose it outright. That's money in their pockets that they didn't used to get.   It was corrected to:     Another factor--sometimes life happens and you can't fly. In the old days you could simply sell your ticket to someone else, now you either have to eat a hefty change fee or lose it outright. That's money in their pockets that they didn't use to get.    Was this grammatical correction an improvement? Was the original text grammatically correct? Is the edited text grammatically correct? Express js - can't redirect What does it mean when Git says 'rewrite' or 'rename' in a commit message? meaning of powers on trig functions I always forget this, when a trig function has an exponent does that mean multiply itself or apply itself to the result recursivly?  e.g. does $\sin(x)^2=\sin(x)\sin(x)$ or $=\sin(\sin(x))$? What about $\sin^2x$?