nine ! it's the number of pages for discuting again and again and again and again about ff vs apsc ... we all know the answers since times ... and ... no picture ...
As a professional photographer here in France, i used to work "FF" ... I started with film olympus OMD ... had canon film ... started the "digital" adventure with a pentax k200d for personal use ... then i bought a k20d and a k7 ... and after a first k5 that was top (except af) ! ... and i was sundenly tempted with FF, just because it could give me again the "view" i was used to with film camera ... So i went canon gear with a 5D, then a 5d II but i didn't like canon bodies ...
i went to Nikon and it was superb ... with a d700, a d7000 then a D3 followed with a d3s and a d4 ... and pro optics ...
During all of these changes ... none of my client have told me about my gear or an upgrade or downgrade of quality in my pictures ... non one saw the differences !
Then one day i realised that ... like Zack Arias said ... in my pictures there wasn't as much unbelievable difference to be seen for my clients or myself (as i m not a pixel peeper ) in apsc vs FF ( but it was case with superior format ! ) ... i realised that only the confort of the viewfinder was real and significant advantage ... that having pro bodies at these years : af was effectively better 2 ...
And i realised that i was carring a lot ... and i realised that finaly ... Body was just like film ... like ink cartridge in the printers ...
And i saw the evolution in apsc body with same quality than "Pro FF" where giving : Excelent AF, all options, Burst, continuous modes, good wb ... etc ...
as i was in love with pentax design and usability since years ... i then check the k3 spec ...
As a pro photographer, i' m used to work in studio, and shoot artists, on-stage, backstage, day, night ... i realised that apsc had never limitated me and my picture quality was as i expected... i checked my Lightroom database ... and had a more precise look at my exif ...
I can asure you that if i show you my database without exif for scenic pictures : you won't be able to tell from wich body it come.
With k3 body i have a compact end versatile system that deliver unbelievable pictures in all stages in a compact bag ! ... I sold all my nikon gear and go back ricoh for my pro equipment and i'm more than happy ! pictures just pop out ! and it's a real pleasure to work with ... it's a very well balanced camera ! as good in use as a pro body like my d4 was ... but without beeing big as a fridge !
At that point ... i also have some tests with mirorless ... and it's just great and amazing ! just like i was in love working with film body ! (25 years ago OM10 ) and i would prefer ricoh giving me a real top high end mirrorless with a top apsc sensor than a FF ! ...
beetween apsc vs ff ? i totaly agree with zack : differences for working are
NEGLIGABLE ... quality in viewfinder, perf of the body, WR, Burst, Frame rate, functionality and handling are THE IMPORTANT diferences ... Lens are the real invest, not body ... with a good lens today i can perform the same picture for my clients with a k3, a D3s, a K50, a XE-1 or other camera ... and the result will shine.
In real world : in all complicated spots ( scenic, hard light ... ) Apsc iso 3200 is more than enough with a good lens and deliver as good quality than FF, iso 6400 is good for hardest situation and quality is great ...
Night photography ? No one shoot the quality at 100 000 iso but use tripods ... in sports ? a K3 6400 with a good 2.8 or 4 prime lens is more than enough to get the shoot ... Not so long ago, my d3s didn't do as much than the k3 : same quality but 24 Mpix ! ... in studio ? i managed enough light and primes lenses : on those apsc bodies i have an awsome quality ... for extra or very very beauty i will use big film format or 645 like ...
So comparing what ? a body that is like an ink cardrige and will evoluate every 2 years but coast more (have a look at a d4 price and what gives real more than a k3 ??? ) ? a camera that doesn't realy do so much ? a camera that will hurt you to cary ? ...
Just put your apsc in it's real limits and you won't need something else than your "photo touch" to be in National Geographic or some good newspapers ...
Talking about sports ... great ! ... i was shooting sport for press one day ... than the day after i saw one of my picture in a local magasine ... great ! ... when i saw the printing quality ... i was crying ... : why am i so disturbing holding such heavy systems when pictures will be printed just like a stikker ? ...
Zack is right and i agree with him : there's no job i can't do well with last generation apsc sensor ... FF doesn't gave me very much for the coast, Kg and less compacity ... what was realy true 6 or 8 years ago, when apsc were bad at more than 1200 iso and only FF where delivering good at 3200 isn't true today ... APsc (some of them ) shines <= 6400, do great job at 12800 ... so why needing more to cary ? ... taking without flash a marathonian in the night without light at 240 000 iso for a press picture of 5x6 cm printed in black and white on a recycled paper ?????????
best for me at the moment : Pentax K3 and some Mirrorless i won't do the promotional here... then YOU ... just do the job !
Here is my point of view and experiences as a pro photographer ... and i won't return Pro FF body ... but apsc for the job or medium format or more (film) for the extra quality ! : Pentax understood that ... everything else is marketing based on frustration ... and i m sory for my poor english ;-)
So tell us : in the way you picture, What are those things you can't do with a k3 and good prime you could do with a d4 and equivalent good prime ????
@ Pentax : i don't need FF anymore, give us a great great great mirrorless APSC with a demoniac sensor and a pure big EVF and all functionalities of the k3 ! and it will be dream ! and give us opened lenses !!!!!
And for those who doesn't believe and want FF and couldn't wait : do you know that you're not married with an industrial ?
Just take what you think you need and you can buy if it makes you happy ;-) but take pictures that speak ! and think about it : what you will carrie for hours and hours and hours all around the world ! ;-) ( i not so long ago do a report of 2 days long non stop ! i was proud to carry not much and comming back with 2800 pictures with only an apsc and 2 lens that were fiting in my pockets viewing some friends with their top gear incredible bag ( show room bags !!!! ) such suffering caring them ! )
And i also agree with dave : what was good in FF was viwfinder with a 50 and "all eyes opened" ;-) you should try some of the last mirroless apsc with big evf and a 35 attached and you 'll be surprised ...