Two Major Meanings of Full Frame - Choice of Shallower DoF and Most Optical Quality from (FF) Lenses
A recent discussion at my last Blog entry inspires me to write something more about the topic..
As you can see, a 55mm/1.4 lens on APS-C will give the user a 84.4mm equivalent AoV (Angle of View) to Full Frame but only at f/2.15 DoF wise, even when it is wide opened at f/1.4. So, in comparison, if you have one of those excellent previous Pentax 85/1.4 film lenses which is closed down to f/2.2, without any doubt, the optical quality of the image must be higher than that of the 55mm lens that must be wide opened in order to get the *same* shallower DoF. And, if the user just wants even shallower DoF with an APS-C system, he is just helpless!
Furthermore, to use lenses that have a mis-matched image circle is just a waste of the lens and its designed resolution. Unfortunately, this is just the case for many Pentax "DA" lenses that actually have a FF image circle, e.g., the DA*55, DA 40 and 70 Limited etc. With cropped centre of the lens, there is always a "strong" argument that the "sweet spot" of the lens is used. That is true to a certain extent but those people do have forgot another even more important and real point, i.e., the effective resolution of the cropped centre of the lens is decreased considerably!
Just say even the best Leica lens can have a centre resolution of 80 lp/mm at a MTF of 50%, that lens is NOT going to beat a cheap P&S DC which has a really small sensor at 1/2.3" etc. in terms of lp/mm figures! So, you can see how a mis-match image circle will have a real adverse impact on the optical performance and final effective resolution that can be delivered by a lens!
Grass is not always greener at the other sides neither, though. Whilst in Pentaxland there is always the big chaos of lenses with different image circles in the same lineup with the same nomenclature, i.e., the DA "family", quite some users of Canon and Nikon do also have committed the same mistake in matching their lenses with their cropped APS-C bodies. Yes, compatibility and future upgradability is ensured for buying FF lenses to use with their APS-C body but that is not going to help them to have better IQ and more effective resolution than to use lenses with matching image circle than to mis-match, i.e., to put real APS-C lenses on their APS-C bodies rather than to use FF lenses! Say for two real examples according to my real experience: my Canon cheapo EF 50/1.8II performs obviously better on my old 12M 5D resolution wise than it is put on a 550D with 18M of which the 50% higher resolution sensor just won't help. The other example is that when a EF-S 55-250 lens is put on the 550D, the results are not noticeably worse than when I put my EF 70-200L on the same 550D, despite that the L tele-zoom is just four to five times more expensive! It is JUST because it is a FF lens! (No matter how high grade it is..)
So, the above are just the two major points of why Full Frame! Those are actually have being overlooked by many DSLR users. Even now noise has been somehow improved for the late APS-C sensors with new manufacturing and image processing technologies. But the physics is still here and it is always in action - with the same technology, one can get either more resolution and/or lower noise with a Full Frame sensor!
Nevertheless, there are always two sides of anything. If you're shooting macro photos more often for which the shooter always desired more DoF (but not less) and that in particular if you're shooting insects or butterflies etc. which are really small and/or you couldn't come closer and always want longer reach, the APS-C cropped machines are for you! Ditto for bird shooting, which the longer reach is always desired!
So, someone would put case to extremes then. For example, they would say that the Medium Format is better than the FF and that the micro-4/3 does have tighter crop and longer reach than the APS-C. Well, the simple answer to both proposition is Yes but do think more about the other issues and compromises also! And, old wisdom told us that put things to extremes always do no good and generality is hurt! :-o
In fact, there are a lot of "calculations" on the web in comparing APS-C Vs FF, e.g., diffraction limit for maximum resolution, DoF and so on, which I think whilst most of them are correct for the maths, they are not much meaningful in practical sense. These are not going to help one to know about if FF or APS-C will suit him/her better - Only understanding the basic but major differences I've iterated above and then think/check with one's own need(s) should be more helpful and useful!
If Size Really Matters, What's the Meaning of Micro-4/3 Now?
FA*85 on FF Vs 43 Limited on 2X (m4/3)
Compatibility of DA Lenses on Full Frame
Some Macro Photos to Share (DFA 100 on 5D)
Sample Photos of Full Frame Fisheye and 43 Limited on 5D