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How & Why Sensor Size Affects Image Quality (APS-C vs FF vs compact)
Posted By: Adam, 06-20-2013, 04:08 PM

Today I wanted to write an article that shows how sensor size affects image quality at a very high level. I'll start by saying that while my results are informal and not perfectly-controlled in every aspect (the ideal test would include 3 cameras with the same number of megapixels), I've tried to eliminate as many inconsistencies as possible in hope that my conclusions will generally be valid. In the remarks to follow, image quality from a compact camera (represented by the Pentax Q7), an APS-C camera (represented by the Pentax K-50), and a full-frame camera (represented by the Nikon D800) will be compared.

I've done the following to reduce the number of confounding factors:
  • All 3 cameras are of the same generation (current models as of the time this post was written)
  • All 3 cameras were using consumer-grade zoom lenses (02 zoom, Sigma 17-70mm F2.8-4 "C", Nikon 24-120mm F4)
  • All photos were taken in aperture priority mode near the sharpest aperture setting
  • All photos were taken at focal lengths delivering virtually the same horizontal field of view (Q7 = 10mm, K-50 = 34mm, D800 = 50mm)
  • All photos were shot at default settings in JPEG mode
  • No post-processing was applied except for monochromatic auto-levels in order to suppress differences in color
    to allow you to focus on differences in detail and dynamic range
  • All images are losslessly compressed (.PNG format)
  • All images were shot at the same sensitivity (ISO)
So, without further ado, here we go.




Everybody says that the bigger the sensor a camera has, the better the image quality. But why is this? Clearly, larger sensors have an advantage in terms of overall surface area. The additional area lets sensor manufacturers improve the sensor in two respects: the size of individual pixels (pixel pitch or pixel area), and the overall number of pixels (resolution).
  1. The bigger the pixel pitch of a sensor, the higher the signal-to-noise ratio. This leads to better color reproduction, detail, and dynamic range as the color values that pixels record will be closer to the true color values.
  2. The higher the resolution, the better the overall clarity. Because most photos are scaled to a smaller size before being printed or displayed, the extra pixels can be used to suppress much of the noise that was found in the original image that was recorded.
Let's look at a few different sensors to compare pixel pitch and resolution:

Camera FormatSensor Area (sq. mm) Resolution (megapixels) Pixel Pitch (micrometers) Pixel Density (megapixels / sq. cm)
Pentax 645DMedium145240.06.02.8
Nikon D800FF86136.34.84.9
Nikon D600FF86124.35.92.8
Nikon D4FF86016.27.31.9
Nikon D7100APS-C36624.03.96.6
Pentax K-50APS-C37116.34.84.4
Pentax Q71/1.7"4312.41.929.3
Pentax Q1/2.3"28.512.41.543.4
Manufacturer-specific/generational hardware differences aside, if we shoot with take the Pentax K-50 and Nikon D800 side-by-side, we should observe very similar levels of overall noise. However, since the D800 has twice the resolution of the K-50, once its files are scaled to a 16-megapixel size, they will appear to have significantly less noise and therefore more clarity. On the other hand, if we took the same photo with the K-50 and the D4, the original file from the D4 should have much less noise due to the fact that its pixels are physically much larger. You can make further comparisons on your own using the data in the table above. Notice how big of a jump 1/1.7" to APS-C is compared to APS-C to FF in terms of sensor area. This jump is of course correlated to the respective difference in image quality.

So why is pixel pitch, resolution, and signal-to-noise ratio such a big deal? The short answer is that if we have low noise, more of the data recorded by the sensor is accurate, and so the overall image is clearer. Similarly, if we have more resolution, we have more room for error in the original image because we can make up for it when scaling photos.

But I want to take my explanation a little bit further than that. Having a background in computing, I have personally implemented and worked with a host of image processing algorithms, including the algorithms that convert raw sensor data to generate full color images. This has given me a great deal of insight into a number of things concerning how images are generated and enhanced. The big issue at hand is that color digital image sensors only record a fraction of the color data (light) that they are trying to portray. Each pixel on a color sensor is only sensitive to one of three color channels: red, green, or blue. Most sensors today are designed using a Bayer pattern, which uniformly lays out the pixels in a checker pattern consisting of 50% green pixels, 25% red pixels, and 25% blue pixel (because green light is the most abundant). Thus, after the sensor captures an image, at each individual pixel location, a computer algorithm has to try to calculate (a.k.a. guess with high accuracy) the intensity of the two missing color channels (this is known as demosaicing). Demosaicing techniques generally work by analyzing the neighbors of each pixel and calculating missing color intensity values based on the intensities found at neighboring locations. But what if the neighboring intensities are not accurate due to noise? The short answer is that the effectiveness of demosaicing drops dramatically, as the guesses about the missing two color channels could be bad or even flat-out wrong. This is what leads to significant loss of edge detail and color noise. If we have a lot of pixels in an image, the probability of having a good guess within any given area is high, and that's why scaling (downsampling) reduces the apparent noise. Similarly, if we have low overall noise to begin with, the original representation of detail and color will be accurate, so we will not need to do any (or as much) downsampling to get a good image.

And this brings us back to sensor sizes. If we have a very large sensor, its tolerance to noise will be higher, either due to higher resolution or due to increased pixel pitch. This, together with the precision of the optics that deliver light to the sensor, is what ultimately dictates image quality. So, when you go to choose a camera & sensor format, the main question you have to ask yourself, as least as far as image quality goes, is when is the difference big enough to warrant a larger sensor size and is my lens good enough.

To help answer this question, I have prepared a number of test photos. Here is the same scene shot with the D800 (FF), K-50 (APS-C), and Q7 (1/1.7" compact size):

1.
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2.
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3.
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Size:  1.29 MB

Can you guess which is which? It's probably going to be very difficult to tell a difference between the D800 and the K-50 shots, though you might be able to spot a little less detail in the Q7 shot, even at this web resolution. The answer is found in gray below.
#1 is from the K-50, #2 is from the Q7, and #3 is from the D800.


Now, let's zoom by about 2x.
1.
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2.
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3.
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Is the difference more discernible spot now? It should be, but try to guess again!
#1 is from the K-50, #2 is from the D800, and #3 is from the Q7.

Finally, let's zoom in to the limit of the Q7's resolution

1.
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2.
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3.
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Here, it should be obvious that the first photo is from the D800 while the last is from the Q7. The larger the sensor, the better the colors and the more detail you see.

All the photos above were taken at each camera's lowest sensitivity setting, meaning that their respective image quality was at its absolute best. But what happens as you crank up the ISO? Read on for some examples.

Views: 14,016
06-22-2013, 05:33 PM   #16
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QuoteOriginally posted by Big Dave Quote
been attached
You might want to fix this........... I truly could not understand what you meant. But that's just me having a slow day.

06-22-2013, 06:30 PM   #17
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Adam; A great right up. Thank you for putting in the time and effort to cover everything you did. I have my academics at work talking about imaging algorithms and so forth all the time and it all goes screaming over my head
I found this article particularly interesting just having moved from a K-5 to a D800, but still have my Q (obviously the older, original model. Love that camera). I love my D800, but it is a bit of a tank, especially with the grip and a decent bit of glass on the front.

QuoteQuote:
Using cheap lenses with good sensors is worse than using good lenses on cheap sensors.
Complete agree, with one note. Especially in Nikon world, there is this constant thought that if its not Nikon, its not good. When having to replace as much of my Pentax kit with Nikon and having just dropped stupid dollars on the D800 body, I had to choose wisely on the best lenses my budget would allow. Having had wonderful results with Sigma lenses previously (I still miss my 20-40 2.8) the only Nikon brand lens I ended up with was the 50mm 1.8D. After careful researching, I happy with my Tamron and Sigma collection.
Nutshell version; If you look carefully enough you can get 99.9% of the clarity and performance (or in some cases, even better clarity and performance) for half or two thirds the price. Cheap(er) doesnt always equal Bad.
06-22-2013, 06:54 PM   #18
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thanks for this useful article..
06-22-2013, 08:28 PM   #19
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QuoteOriginally posted by Adam Quote
The Canon 5D is 4 years older than the K-x and E-P1. The sensor tech has to be roughly of same generation, otherwise you'd be comparing apples to oranges.
If you're comparing sensor size, then having a similar number of MP should be more important than the generation of the sensor. After all, MP has more effect on lw/ph (resolution) than Sensor size, when looking at APS-c and FF. The ridiculous pixel density of the the Q7 puts it in a different class. By the way, having seriously looked into a 5D, it would seem to be a pretty remarkable camera. If anything , based on it's following, I'd say the generation of the sensor might be the least important stat. The canon 5D and Nikon D700 are both very serviceable cameras if you listen to the folks who use them.

06-23-2013, 02:05 AM   #20
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QuoteOriginally posted by Adam Quote
The Canon 5D is 4 years older than the K-x and E-P1. The sensor tech has to be roughly of same generation, otherwise you'd be comparing apples to oranges.
Pixel pitch and density are the only important factors though. So you're incorrect.
06-23-2013, 02:43 AM   #21
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I don't want get involved in the ongoing discussion -- too many incorrect statements already -- but thought it may be useful to point out three good resources on the topic of sensor size:

1. Digital Camera Sensor Sizes: How Do These Influence Photography?
2. The true reasons for a full frame camera.
3. Equivalence.

P.S.: Everyone should know that Ken Rockwell is the Chuck Norris of Photography.
06-23-2013, 03:11 AM   #22
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Nikon D800 is using New sensor,Pentax K50 is old sensor

Nikon D800 is using New sensor,Pentax K50 is old sensor..Lets wait for the next replacement for K5..and compare it again to K?? aps-c
06-23-2013, 10:39 AM   #23
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Thanks Adam. Really helpful. Am I right in thinking that this (limitations of demosaicing) may be a major factor in explaining the purple fringes that plague my pictures (K5 - kit lenses and one tamron 10-24)? Sometimes blatant, sometimes only noticeable upon closer inspection, but pretty much always lurking. They love to deface the detailed high contrast areas, such as the branches of bare trees against bright skies. I was blaming the CA of the lenses, but if I'm understanding this correctly the effectiveness of any demosaicing algorithm could be more important in terms of how many fringes appear on the RAW? Do manufacturers such as Pentax develop and tweak their own algorithms, or are these determined by the sensor manufacturer? (Wondering whether I would get the same results with any APS-C). I've attached an example. Please feel free to comment on other possible causes etc. (anyone interested!)




could there be differences in the algorithms employed by different manufacturers? Not that I want to move from my K5 - fits like a glove, and I can live with correcting the aberrations in

Attached Images
 
06-23-2013, 11:45 AM - 1 Like   #24
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QuoteOriginally posted by Calmsea Quote
Thanks Adam. Really helpful. Am I right in thinking that this (limitations of demosaicing) may be a major factor in explaining the purple fringes that plague my pictures (K5 - kit lenses and one tamron 10-24)? Sometimes blatant, sometimes only noticeable upon closer inspection, but pretty much always lurking. They love to deface the detailed high contrast areas, such as the branches of bare trees against bright skies. I was blaming the CA of the lenses, but if I'm understanding this correctly the effectiveness of any demosaicing algorithm could be more important in terms of how many fringes appear on the RAW? Do manufacturers such as Pentax develop and tweak their own algorithms, or are these determined by the sensor manufacturer? (Wondering whether I would get the same results with any APS-C). I've attached an example. Please feel free to comment on other possible causes etc. (anyone interested!) could there be differences in the algorithms employed by different manufacturers? Not that I want to move from my K5 - fits like a glove, and I can live with correcting the aberrations in
Aberrations like that are usually caused by the lens, not the sensor. Bad demosaicing can lead to two types of artifacts: zipper artifacts or false color/moire. In the case of zipper artifacts, they will usually be orange, blue, red, or green, but you'll be able to see a distinctive checker pattern when the demosaicing is in fact to blame. Really basic demosaicing techniques may also make fine details vanish. Here's an example of a 200% blowup of a photo from the Ricoh GR. It's got zipper artifacts as well as some fringing from the lens.

Demosaicing algorithms differ from manufacturer to manufacturer- that's why some camera produce JPEGs better than others, even if the hardware is the same. The same goes for raw converters.

Because of the simple fact that sensors only record only a fraction of the color information they're trying to portray, there never can be a perfect demosaicing algorithm. Some work better than others based on the circumstances.
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06-24-2013, 06:32 AM   #25
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Thank you for nice post
07-02-2013, 03:34 PM   #26
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QuoteOriginally posted by jct us101 Quote
Pixel pitch and density are the only important factors though. So you're incorrect.
By "density" do you mean fill factor? The 5D has a low fill factor by today's standards as it does not use micro-lenses. While a FF should have a 1.2 stops shot noise SNR advantage over an APS-C with the same type of sensor tech (quantum efficiency), the 5D is only 0.1-0.3 stops better than the K-5 as regards shot noise performance.The QE is 25% (5D) vs 46% (K-5).

See Sensorgen - digital camera sensor data and https://www.pentaxforums.com/forums/photography-articles/129754-comparison-sn...s-formats.html

The 5D's pattern noise and read noise performance is also inferior to Exmor sensors.

Dan.

Last edited by dosdan; 07-02-2013 at 03:44 PM.
01-02-2015, 08:32 AM   #27
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HI Adam very nice explanation. Just a question as to why you would use the K50 when the K5II was out. Was this just to better help show the difference between sensor sizes. Although it is a given that when pushed the D800 will out perform the K5II I would think the K5II would hold up better in this test. For curiosity I would have liked to see the K5II used or better still the D7100 since the K3 was not out at the time of this writing.
01-02-2015, 05:55 PM   #28
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QuoteOriginally posted by MikeD Quote
HI Adam very nice explanation. Just a question as to why you would use the K50 when the K5II was out. Was this just to better help show the difference between sensor sizes. Although it is a given that when pushed the D800 will out perform the K5II I would think the K5II would hold up better in this test. For curiosity I would have liked to see the K5II used or better still the D7100 since the K3 was not out at the time of this writing.
Glad you liked it! Those are just the cameras I had on me at the time I wrote the post. It was more of a spontaneous piece that I thought of writing while researching image processing algorithms, hence it's posted here rather than on the homepage.

In daylight I don't think there would be much of a practical difference between the K-50 and the K-5II. But I agree that it would be fun to redo these tests with identical resolutions such as a 16MP FF, 16MP APS-C, and 16MP compact, or something along those lines.

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01-02-2015, 09:38 PM   #29
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QuoteOriginally posted by buds_vai Quote
Nikon D800 is using New sensor,Pentax K50 is old sensor..Lets wait for the next replacement for K5..and compare it again to K?? aps-c
I get where you're going, but they use the same level of technology, and heck, the K50 was introduced a year and a half - a full generation - later than the D800. The K-50 shouldn't have any excuses.

Sensor performance is roughly the same now as it was 4 years ago, which is unprecedented. If you want better performance right now you have to increase the sensor size.
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