APS-C vs Full Frame. Sensor size and what is promised.
I had been surfing around youtube recently and saw some interesting videos/comments on things like ISO, sensor size, equivalence and so on. Some companies claim APS-C will give a smaller lighter package, others argue that if we look at the the equivalence, there is no advantage and in-fact full frame is better. But is that true? Does a full frame truly have 2x more light? Does a 17-50 f2.8 really equal to just a 24-70 F4? Is it true that APS-C is pointless since it doesn’t save any weight?
As you can see in the above, a APS-C can be larger and heavier then a medium format. So weight saving is a very subjective thing. Below we dive deeper into the sensor, equivalence and weight.
First lets look at the argument
A image taken by a 50mm F2 on an APS-C will look similar to 75mm F2.8 (or F3) on a Full Frame. There is probably not much debate on the looks and output on the photo. Where the argument lies is when companies market a APS-C lens and claims it saves weight when compared to a Full Frame similar lens.
Case in point, Fuji 16-55mm F2.8 vs 24-70mm F2.8. Definitely the Fuji APS-C version is smaller and lighter by about 300+g. The Full Frame camp will argue that it is not equal and that a 24-70mm F4 is more comparable. The APS-C side will then counter that a 16-55mm F2.8 is longer (approx 24-83mm) and you still get F2.8 in terms of light. This is where things turn ugly, people will start arguing that Full Frame takes in twice as much light and thus produce better image quality and bokeh. We will not touch on Bokeh since a Full Frame F2.8 is definitely better in that aspect. For this write up, lets look at exposure, weight and image quality.
In exposure there are 3 parameters, Shutter Speed, ISO, Aperture. Aperture is usually based on the 35mm standard and talks about how much light is let in by the lens. Shutter on the other hand is how much light is let in capped by the set duration.
Returning back to the lens, a F2.8 will always result in a F2.8 shutter. The 16-55mm F2.8 can take an image at 70mm with a shutter of 1/100 while a full frame equivalent in looks, which is about F4, will need 1/50 of a shutter. The fact remains that a F2.8 lens is always faster then a F4, even if they look similar due to difference in sensor size. But where does the extra light and sensor area go to?
What this chart shows is the dynamic range: the amount of information captured and thus usable. This affects us greatly when we do stuff like pull shadows and recover highlights for those crazy demonstration of black image to full colored +5 exposure.
So if we look at the chart above using the same company (cause everyone defines their own ISO) camera, you will notice that the D500 Dynamic Range at ISO100 is the same as D850 at ISO 165. So a full frame with 1 stop of ISO boost is the same as an APS-C at base.
So with this in mind, revisiting the lens, a 16-55mm F2.8 gives you faster shutter speed then a F4 equivalent. All the full frame camera need to do is raise their ISO by 1 stop and it equals out. In-fact it will be close to totally equaling out including image quality and whatever advantage a full frame sensor is suppose to have. So yes mounting a F4 version of the 24-70mm on a full frame essentially makes it the same as an APS-C 16-55mm F2.8.
We have gotten quality out of the way, let us look at the weight. We take the Sony A7m3 with the 24-70mm F4 vs the Fuji XT3 + 16-55mm F2.8. It is not exactly equal since the Fuji version does zoom in almost 15% more (70mm vs 83mm)
Sony: 650g + 426g = 1076g
Fuji: 539g + 655g = 1194g
Now its quite clear the XT-3 combination is heavier. It weighs some 120g more but gives you 15% more zoom. But wait, the A7m3 is the lightest Full Frame around however the XT3 is probably some of the heaviest of its type. Let us use the XT30, which for most works and outputs the same as a XT3 except for a smaller body and less buttons.
Fuji: 380g + 655g = 1035g.
Now we can go even further with some of the other bodies in the X series but you should get my point. APS-C body can get smaller then a Full Frame. At least no maker has made a lighter full frame then a APS-C ever was. So as a set of camera and lens, APS-C will be lighter. Just for a comparison to the lightest setup you can make with a APS-C that has a full frame equivalent.
Fuji A5 with 18mm F2 : 361 + 118 = 479g
This is lighter then any full frame camera and you only give up 1 stop of dynamic range at the lowest ISO.
So the question now should changed: why is a good APS-C lens heavier even though it covers a smaller sensor and image wise just equates to a 1 stop slower Full Frame. We need to look at lens construction and the demands of APS-C.
A lens is built up of many parts. Depending on what you use to build and how rugged you built it, more then just glass, the items around it will carry that extra weight.
If we go back and view the 24-70mm F4 by Sony, it’s made out of plastic vs the 16-55mm which is made out of metal. Considering the extra zoom and just the material difference, it is where the extra weight goes. There are probably many other things inside such as AF motor, the type of element and electronics. Many of such things do not scale in weight from Full Frame to APS-C. There’s just no way to get around physics and if you make something with a heavier material, or what to get more (range in this case) it will cost something. There are probably ways to offset such as using more exotic glass like Leica but that will bring the price to stratospheric levels.
There is also one more parameter that will result in an APS-C lens to be heavier, and that is the quality needed of the lens to perform similarly as a full frame equivalent.
Firstly, I am no lens designer and if there is a mistake I am open to corrections. That said base on my understanding, a lens job is to focus the light. Many things affect this, from the quality of the glass to the lens tolerance and design. However, no lens design is perfect. That is why sharpness varies from lens to lens and even within the same lens there is some variation. Which brings up the simplified diagram above. At the same megapixel, a Full Frame has bigger sensor sites then a APS-C. Therefore, to design a lens that can work as good on a APS-C as it would look on a full frame is actually harder as the tolerance is lower. You may be using less material due to the smaller sensor, but you will need better material and overall better quality to work as well. This usually translates to some extra weight be it more/better lens elements or material with tighter tolerance. Which pretty much explains why good APS-C lens cost quite a sum of money and probably cost more with all the glass needed for correction. A good APS-C lens can easily perform to whatever full-frame has, it just cost the same as a full-frame lens and weighs as much.
So as you can see above, a top notch APS-C lens, the 200mm F2 by Fuji, really performs as good as anything out there and maybe a little more due to the advances in technology. Perfectly sharp, highly corrected, extreme fast AF with Linear Motor. All this can be done, at a cost that is similar to the Full Frame 200mm F2. It does come with a tele-converter and considering all the advances, it definitely is as good of a deal if not better then its equivalents.
So where does this all leave us in the end? Full-Frame and APS-C are really about the same. APS-C can be lighter by basis of smaller cameras and if you use a lens of similar built with specs comparable to its full frame counter part. APS-C performs similar to a Full Frame that has added 1 stop of ISO and using a 1 stop slower aperture lens. A Full Frame given the best condition, will always outperform a APS-C camera when it comes to image quality at the lowest ISO. Sharpness and image quality is more of a metrics of the lens then the sensor itself, unless you need the 1 extra dynamic range at the lowest ISO.
So if you want the lightest possible package, an APS-C is still the way to go. What you lose is 1 stop of dynamic range and probably 1 stop of noise compared to the best of Full Frame. That probably wont affect you as much unless you love to shoot in extremes: Extremely Dark or Extreme Bright. One needing the noise performance while other requiring the best dynamic range. Just remember that APS-C does not equate to a cheaper system or a lighter system if you want full equivalence to the full frame versions.
Of course there are still other things to consider. Very technical stuff such as read out speeds which is bias to smaller sensor size and that not all full frame performs as good as a D850 when it comes to dynamic range. If you were to browse around photons to photo, you will see modern APS-C is quite comparable to some Full Frame Sensors or almost no difference. If we are to look at the a Full Frame with similar frames per second as the XT3, that will be the A9, D5, 1DX. Non of them really performs better then the XT3 sensor in terms of image quality.
So what about Medium Format? Replace APS-C as Full Frame, Full Frame as Medium Format, and you get the same explanation. The biggest reason why Medium Format cost more and produce even higher image quality is because its a niche product, built for people who wants something better and willing to pay for it. If you get the best of everything for Full Frame, you can achieve 95-98% at maybe 80-90% of the cost. In the end, you pay for what you get and no one can beat physics for free
And a picture from the GFX50s, Medium Format Crop Sensor Camera