Initially this was a part of a different post but I thought it's worth it's own topic.
Comparing the Fuji APS-C and Nikon full-frame, in real life handheld shoots of non-moving objects with so so light, requiring 1/FL and above base ISO shots, my Fuji X-T1 system is as good or better than the D750. This is valid when you need equivalent DOF or even when you want the best out of the system.
1/FL surely means 1/eFL (equivalent FL) as shaking is not a function of focal length, but field of view. I'm sure you meant this, but just to make it clear.
The starting point is that Nikon has 1-1.5 stops advantage compared to the Fuji APS-C based on sensor size only.
If one wants to have advantage with a larger format, one needs to feed the larger format with more light. You will either need to trade DOF for light, shutter speed for light or increase ambient light.
More on this very subject here.
There are no free lunches for any format size.
If you use similar aperture lenses for the Nikon they are pricier and heavier. After using both system in parallel now I get it when they say F1.4 is F1.4 regardless of the sensor.
f/1,4 means nothing more than that the entrance pupil of the lens has the diameter of the focal length divided by 1,4. It has nothing to do with the format used. However,
the effect of that focal length and that aperture are different with different formats. A camera with iPhone sensor sized imager with a f/2,8 lens would really have not much in common with a medium format camera wiith f/2,8 lens. Unless of course we stop the medium format down by the relevant crop factor
In order to use the sensor advantage you need to pay and carry significantly more.
I heard that the Sony A7 series is quite small, and the now old and obsolete original A7 isn't terribly expensive any more. Unfortunately the lens lineup is small (in quantity, not size) and not excatly cheap. Luckily one can use M-mount lenses is manual focus is all right. They're not exactly large and some are quite affordable with great image quality.
Regardless, if we are happy with the performance curve limit of a smaller format and anything beyond that would be a waste, then it is likely that the smaller format camera+lenses will be smaller in size and weigth as the larger formats often lack in lenses with equivalent apertures: an APS-C might have 200mm f/5,6 lens, but it's hard to find a full frame 300mm f/8,4 lens, so in practise the FF user would probably have to use 300/5,6 which is naturally much larger than 200/5,6 due to significantly larger aperture diameter. Though, the FF user might want to consider the same 200mm f/5,6 the APS-C user, even if it's an APS-C only lens, and use it with a teleconverter. Of course some systems don't have TCs and they're not necessarily practical outside of the longer focal lengths.
If you use equivalent DOF lenses you might be cheaper or more expensive (the more F1.8 lenses the cheaper), you might be heavier (most likely if you don't get telephotos) or lighter, but strictly image quality you will not be ahead, if anything you might be worse in some situations. You can still use the full frame for their focus advantages, better flash, superior movies and lens selection.
The image quality is (almost) all about light - the more you collect, the better image quality. If you want to have the same DOF, the lens will transmit the same amount of light to the image sensor regardless of it's format (disregarding transmission losses of the lens for the sake of simplicity). But if you can have longer shutter speed or use a flash (or other means to increase light levels), the larger formats tend to have signal-to-noise advantage as their maximum signal collecting capacity (i.e. how much signal can be collected before saturation) tends to be larger. If not, then it's a toss up regardless of format.
When it comes to lens sizes, the small format advantage is a bit of a myth. This may sound controverisal or flat out wrong, but if we consider lenses which do the same job it is largely the case. To do the same job you need to have smaller f-numbers on smaller formats, thus the lenses will get larger and more complex - in
some cases the smaller format may surprisingly need a larger lens to do the same job a larger format does with a smaller lens. A good example is Cosina Voigtländer 25mm f/0,95 for M4/3 format - from the same company comes 50mm f/1,5 for full fame which is half the size, a bit faster (f/1,5 vs. f/1,8 normalized to same format), and likely optically superior.
Many of you might have already reached the same conclusion. Of course if the light is good or you only use a tripod the point is moot. If the full frame will come with less mirror/shutter shock then it might come ahead by a little.
Big sensor doesn't have an inherit advantage over small one if the light is fixed to certain amount - the same DOF and shutter speed and ambient light. Well, there is one inherit advantage - the the amount of light is so large that the smaller sensor will saturate some parts of the image, this won't happen with the larger sensor.
Amount shutter shock - the Sony A7 and A7s are quite immune to it with the electronic first curtain. A7r unfortunately is not.
PS. I don't have an m43 system so I'm not sure how it will compare other than the weight advantage will be bigger.
The weight advantage is mainly cause of the body and the slower lenses - if we compare mirrorless to mirrorless - if the lenses are of the same speeds, then the FF lenses might surprisignly be lighter. But if the lenses have the same f-number, then the M4/3 will certainly have an advantage. I'm just not sure what the point of comparing different formats with the same f-stop is in this context as f-stop is tied to the (physical) focal length.
