Andrew Cooke | Contents | Latest | RSS | Twitter | Previous | Next

C[omp]ute

Welcome to my blog, which was once a mailing list of the same name and is still generated by mail. Please reply via the "comment" links.

Always interested in offers/projects/new ideas. Eclectic experience in fields like: numerical computing; Python web; Java enterprise; functional languages; GPGPU; SQL databases; etc. Based in Santiago, Chile; telecommute worldwide. CV; email.

Personal Projects

Lepl parser for Python.

Colorless Green.

Photography around Santiago.

SVG experiment.

Professional Portfolio

Calibration of seismometers.

Data access via web services.

Cache rewrite.

Extending OpenSSH.

C-ORM: docs, API.

Last 100 entries

[Link, Politics] Cubans + Ultrasonics; [Book, Link] Laurent Binet; VirtualBox; [Book, Link] No One's Ways; [Link] The Biggest Problem For Cyclists Is Bad Driving; [Computing] Doxygen, Sphinx, Breathe; [Admin] Brokw Recent Permalinks; [Bike, Chile] Buying Bearings in Santiago; [Computing, Opensuse] Upgrading to 42.3; [Link, Physics] First Support for a Physics Theory of Life; [Link, Bike] Peruvian Frame Maker; [Link] Awesome Game Theory Tit-For-Tat Thing; [Food, Review] La Fabbrica - Good Italian Food In Santiago; [Link, Programming] MySQL UTF8 Broken; [Link, Books] Latin American Authors; [Link, Computing] Optimizatin Puzzle; [Link, Books, Politics] Orwell Prize; [Link] What the Hell Is Happening With Qatar?; [Link] Deep Learning + Virtual Tensor Machines; [Link] Scaled Composites: Largest Wingspan Ever; [Link] SCP Foundation; [Bike] Lessons From 2 Leading 2 Trailing; [Link] Veg Restaurants in Santiago; [Link] List of Contemporary Latin American Authors; [Bike] FTHR; [Link] Whoa - NSA Reduces Collection (of US Residents); [Link] Red Bull's Breitbart; [Link] Linux Threads; [Link] Punycode; [Link] Bull / Girl Statues on Wall Street; [Link] Beautiful Chair Video; Update: Lower Pressures; [Link] Neat Python Exceptions; [Link] Fix for Windows 10 to Avoid Ads; [Link] Attacks on ZRTP; [Link] UK Jazz Invasion; [Review] Cuba; [Link] Aricle on Gender Reversal of US Presidential Debate; {OpenSuse] Fix for Network Offline in Updater Applet; [Link] Parkinson's Related to Gut Flora; Farellones Bike Park; [Meta] Tags; Update: Second Ride; Schwalbe Thunder Burt 2.1 v Continental X-King 2.4; Mountain Biking in Santiago; Books on Ethics; Security Fail from Command Driven Interface; Everything Old is New Again; Interesting Take on Trump's Lies; Chutney v6; References on Entropy; Amusing "Alexa.." broadcast; The Shame of Chile's Education System; Playing mp4 gifs in Firefox on Opensuses Leap 42.2; Concurrency at Microsoft; Globalisation: Uk -> Chile; OpenSuse 42.2 and Synaptics Touch-Pads; Even; Cherry Jam; Lebanese Writer Amin Maalouf; C++ - it's the language of the future; Learning From Trump; Chinese Writer Hu Fayun; And; Apricot Jam; Also; Excellent Article on USA Politics; Oh Metafilter; Prejudice Against The Rurals; Also, Zizek; Trump; Why Trump Won; Doxygen + Latex on CentOS 6; SMASH - Solve 5 Biggest Problems in Physics; Good article on racism, brexit, and social divides; Grandaddy are back!; Consciousness From Max Entropy; Democrats; Harvard Will Fix Black Poverty; Modelling Bicycle Wheels; Amusing Polling Outlier; If Labour keeps telling working class people...; Populism and Choice; Books on Defeat; Enrique Ferrari - Argentine Author; Transcript of German Scientists on Learning of Hiroshima; Calvert Journal; Owen Jones on Twitter; Possible Japanese Authors; Complex American Literature; Chutney v5; Weird Componentized Virus; Interesting Argentinian Author - Antonio Di Benedetto; Useful Thread on MetaPhysics; RAND on fighting online anarchy (2001); NSA Hacked; Very Good LRB Article on Brexit; Nussbaum on Anger; Tasting; Apple + Kiwi Jam; Hit Me

© 2006-2017 Andrew Cooke (site) / post authors (content).

Depth of Field

From: "andrew cooke" <andrew@...>

Date: Sun, 23 Dec 2007 16:58:36 -0300 (CLST)

From
http://www.flickr.com/groups/panasonicdmc-lx1/discuss/72157603526533457/#comment72157603525884094

in
http://www.flickr.com/groups/panasonicdmc-lx1/discuss/72157602866039023/#comment72157602895143765
the depth of field in compact p&s cameras was shown to be much greater
than in a 35mm lens at the same aperture (f number).

why is this?

naively i would expect that if you take a lens and put it in a magic
"shrinking machine" that scales it to (say) half size, then you 
would get much dimmer pictures (smaller absolute aperture - 1/4 area of
lens) that cover a smaller area of the focal plane (1/4 area of sensor),
but that  he f number would stay the same (it does, right?).

furthermore, since everything seems to be "the same but smaller"
i would expect the depth of field to be the same (when you make a photo 
smaller or bigger in photoshop the depth of field in the image doesn't 
change).

yet clearly this isn't the case, and it's the thing i miss most in my
camera.  does anyone have a good intuitive explanation why?

[...]

ok, so it's quite simple really :)

in my argument above (which is a "scaling argument") i imagined
taking a lens (well, an entire optical system) and shrinking it.

what i forgot is that when i shrink the camera i also have to shrink the
world it is inside!

less cryptically, there's an extra distance, in the "real world"
that i had forgotten to take into account - the distance from the camera
to the subject.  this may seem unusual, because typical, simple geometric
optics considers subjects that are at an "infinite" distance -
this simplifies things (shrinking infinity is still infinity) and is often
a reasonable assumption.  but depth of field doesn't work "at
infinity", it only works for relatively close objects, so this is not
a good approximation to make.  this is why, incidentally, the first link
above keeps talking about "intermediate distances".

so, once you realise that the distance from the lens to the subject is
important things become a lot clearer.  the shrinking argument is correct,
but you have to shrink the distance to the subject too.

so, for example, the depth of field for a 50mm lens with a 35mm sensor at
10m is the same (in relative terms) as a 25mm lens with a 17.5mm sensor at
5m (we have simply shrunk everything by a factor of 2, geometry - angles
and relative resolutions - stay unchanged).

almost there...

the problem then, of course, is that when we compare depth of field for
different sized sensors we are implicitly talking about taking pictures of
things <em>at the same distance</em>.  and we all know that as you get
closer to the lens depth of field becomes more important (ie smaller). 
conversely, moving further away makes depth of field effects less
pronounced.

so if we want to compare depth of field for two different sensor sizes
<em>with the subject at the same distance</em> then in the smaller sensor
case it is "as if" the subject was futher away (compared to
where it would be if we simply scaled everything).  and, as a consequence,
depth of field for the smaller sensor is larger (less pronounced).

i don't know if that helps anyone except me, but i find that much more
intuitive than looking at formulae...

Comment on this post