Sensor Sizes, Noise and Dynamic Range

Much ado is often made about sensor sizes. With regards to Fuji, we know that the 2/3” sensor in the S100fs is the largest consumer sensor on the market. Or was. Fuji have discontinued that model in favor of the S200EXR, which has the EXR sensor at 1/1.6” size. Is that a lot smaller? Can yo upicture the relative difference in size? Can you picture the actual sizes superimposed on one another?

And then we have the inevitable comparison made between the various sensor sizes. This comparison works on crop factors, the simplified definition of which is the relative size of the diagonal. Which itself more or less defines the required image circle size. Which defines the required glass size. And so on.

Now, we often get these fun debates about relative image quality. The best bridge cam has a 1/1.6” sensor today. How does this really compare in size to the Full Frame sensor, or the APS-C sensor?

Well, I’ve created a rather large chart to illustrate the differences. First, I show the relative difference between the four more popular sized sensors, all used in modern Fuji cams. The sizes are:

• 2/3” – S100fs
• 1/1.6” – S200EXR, F200EXR
• 1/2” – F70EXR, F80EXR
• 1/2.3” – HS10

Below the extremely enlarged relative sizes, I show the actual sizes (sized for the typical LCD monitor resolution of 100ppi) and you can easily see that the actual differences in small sensor sizes is vanishingly small. In fact, so are the sensors themselves. The largest of them still fits pretty much inside your pinky finger nail.

Note: you *must* click the image to see the large version. That gives you a true reading on the actual sizes under discussion. The actual sizes are, of course, relative :-)

Let’s examine this another way, just to put some concrete numbers to the visual impact of these differences. The difference in *area* from the largest sensor in use in what we must call “small sensor cameras” to the smallest sensor in common use today is a factor of 2.04 … the HS10’s 1/2.3” sensor versus the S100fs 2/3” sensor. (Note that the 1/2.5” sensor was popular over the last few years and is even smaller than the HS10 sensor, but the 1/2.3” Sony BiCMOS sensor is sweeping those older sensors off the table for performance reasons.)

These area size differences compared to the S100fs and the to a Full frame sensor are captured in a table:

So what does this all mean?

Well, it is fairly obvious that the two most popular Nikon sensor sizes are rather large when compared with even the largest small sensor. In fact, the table shows that the area difference ranges from 15 times to 30 times. So, where the S100fs can capture twice the light that the HS10 can (one doubling of size) … the D700 can capture 5 stops more light than the HS10 and 4 stops more light than the S100fs … making for a truly massive difference in image quality, as defined by noise and dynamic range, between the entire small sensor cluster and the large sensor cluster. And the differences pile up as sensitivity rises.

This is corroborated by comparing cameras on the DXOMark site. There, you can get a direct read on small sensors versus large sensors in RAW mode (i.e. ignoring jpeg engine differences.)

Here is an example … a D700 (FF sensor) against a D90 (APS-C sensor) against a Canon G11 (1/1.6” sensor) against a Panasonic with a 1/2.3” sensor against an S100fs (2/3” sxensor.)

First, noise.

So we see that the sensor sizes are tracking very well here. In fact, if you look at crop factors versus stops of noise difference, you can see a strong correlation. The laws of physics are speaking here.

The graph is straight forward. The middle two lines are the 2/3 and 1/1.6 sensors. And the differences are negligible. No reason to get twisted knickers over it *when shooting RAW.* The 1/2.3 sensor is about 1 stop down from that. The APS-C sensor is 2.5 stops above the best small sensor. and the FF sensor is 3.5 stops. Tracks the 4x crop factor nicely.

In plain English now … you are shooting a birthday party for your kids. You want to get a great shot of the blowing face lit by the candles on the birthday cake. This is going to require 1600 or 3200 ISO.

Well … the smaller sensors are well below the cur off for excellent image quality of 30dB. Thus, you will be hammering the image with noise reduction. On the other hand, the D700 is still at that magic level at 3200 ISO. The D90 at 1600 ISO. So if you nail the exposure with either, you get a frameable karge print. With the smaller sensor, you will be lucky to score a decent WEB image.

Now … dynamic range.

This one is not quite as predictable. In a straight up shooting war, the two sensor size groups cluster quite tightly. And the clusters themselves are about 2 to 3 stops apart. Tracking a little less closely than you expect from the crop factors.

But … this is only part of the story. When you actually shoot, say, the D300 against the D700 (I do that), you find that the D700 has significantly cleaner shadow detail. Sometimes the difference is astounding. This is reflected in the latitude for improper exposure. Underexpose with the D300 and you will suffer somewhat. Underexpose the D700 and you will not really notice a drop in quality (even though it is there.) And underexpose a small sensor … well, suffer baby.

So these sensor size clusters … the big ones against the little ones … they matter. The image quality is indisputably better when the sensor is bigger, all other things being equal (and they generally are, despite the hopes and dreams of the staunch defenders of the small sensor.) Scotty said it best, when he shouted “Ye canna change the laws of physics!”

P.S. What makes the EXR sensors special is that they will combine adjacent “like colored” pixels to form a sensor of half the resolution, which drops the pixel density drastically. This, in turn, changes the nature of the noise and generally allows for much less noise reduction, with the attendant dramatic improvement in shadow details and low contrast details. The chroma noise also improves, although it is tough to say why. But the F70EXR specifically is almost chroma noise free in EXR modes … and that’s pretty amazing …