We often confuse the notions of pixels, photosites, sensor size, resolution and image definition … Here is a simple guide (I hope) to understand these concepts. Do not hesitate to send me feedback.
Sensors and Photosites
The sensor of a camera is composed of a very large number of photosites. To simplify, let’s say that for each pixel (px) of your camera, you need a photosite. These photosites are a kind of photoelectric cells, sensitive to light. A bit like the photovoltaic cells of a solar sensor.
The bigger the photosites are, the more they react to light and convert it easily into electric current. Exactly like a big solar collector that produces more than a small one.
The size of a sensor varies from one camera to another. There are medium format, Full Frame, APS-C, Micro 4/3, etc.
Let’s take for example 2 cameras with 24 million pixels each. The first one has a full format sensor that measures 24×36 mm and the second one has an APS-C sensor that measures 23.5 x 15.6 mm. Since the number of pixels is the same, the number of photosites is the same too. The 24 million photosites will be necessarily smaller on the APS-C camera since the sensor measures only 23.5×15.6 mm against 24×36 for the full format. The 2 cameras will deliver images of 24 million pixels (mpx) each or 6000×4000 pixels.
Definition of the image
What we call image definition is simply the number of pixels that compose it. It can be 12, or 24 million pixels (mpx). As we have just seen, the size of the sensor (FF, PAS-C, 4/3, etc.) does not influence the image definition. There are Micro 4/3, APS-C and Full format cameras with 24 million pixels. They have the same number of pixels, so they have exactly the same definition.
As a reminder, the definition of the Full Frame (FF) sensor of a CANON 5DSR is 50.6 MPX. The size of the images it delivers is 8688 × 5792 pixels, that is to say a definition of 50.6 mpx. The definition of the sensor of a Canon 6D MarkII is 26.2 mpx. We will have an image of 6240 X 4160 pixels. An image definition of 26.2 mpx, so 2 times less.
Difference between a full frame sensor and an APS-C sensor
If I go back to my example of my 2 cameras of 24 million pixels (mpx) each. One with a Full Format sensor and the other with an APS-C sensor. The 2 deliver images of the same definition, but the photosites are larger on the full format sensor which is 1.5 larger. But the size of the photosites has an influence on the ISO sensitivity. The tighter the photosites are, the more difficult it is to manage ISO noise. The other advantage of large photosites is to provide a better dynamic range. This allows to have more details in the darkest and lightest parts of the photo.
This is essentially the advantage of large sensors over small ones. For an equivalent number of pixels. In the case of a full format sensor with 50 mpx, the photosites will be tighter than on an APS-C camera of 24 mpx for example. In this case, it will lose all or part of its advantages.
What are the pixels for?
A high definition image allows for large quality enlargements as we will see later with the notion of “Image Resolution”. Point. You can also use it to crop, an argument often heard, but not really decisive.
Resolution and image definition
The confusion between resolution and image definition is sometimes maintained by some vendors because it facilitates the sale of large machines. The definition of the image corresponds to its dimensions in pixels.
The resolution of an image
The resolution of an image is the way its pixels are distributed when you want to print this image. A good image resolution will concentrate enough pixels so that the image observed at a comfortable distance appears sharp, as in the view on the right below. In other words, the higher the resolution and the more pixels per centimeter/inch, the sharper the printed image will be.
The right resolution – the right pixel density – depends on the printing technique, the size of the print and the viewing distance of the photo. It varies only for printing. We generally recommend a value of 300 PPP for a high quality print. This value can go down to 150 PPP for very large formats or depending on the type of printer.
Note: the resolution is expressed in “PPP” (pixels per inch) or “DPI” (Dot per inch) which is exactly the same.
So when you are told to choose a resolution of 300 dpi for images to be printed, it means that 300 pixels per inch (300 dots or “dot”) per inch is required for a good print.
Resolution and printing
The notion of resolution is only important when you want to print. Let’s take as an example an image from a 24 mpx camera. Its definition is 24 mpx or 6000×4000 pixels. In Photoshop, I display the dialog box “Image Size”. The original dimensions appear at the top of the screenshot below: 6000×4000 px. With such a definition, the software tells me that I can print with a resolution of 300 PPP up to the size of 50.8 cm (in blue).
I can also print smaller. The resolution will be higher, but this will not have any influence because beyond 300 PPP, our eye does not see any difference.
Now let’s try to print the same image in 90×60 cm format. The print resolution this time has dropped to 169.33 pixels per inch (PPP). Not enough in theory for a high quality print. We will see later that other notions come into play.
Resolution and viewing distance
Studies have considered that 300 PPP is the maximum resolution that the human eye can discern. This has therefore become the standard for high-end printing in a specialized laboratory, etc. In practice, the resolution required for printing on an inkjet printer can be lower, up to 150 dpi, depending on the model.
Then comes the distance at which you look at the picture. The larger the print, the further back you stand to look comfortably. And the lower the resolution can be. The same is true for an HD TV screen. Seen from up close it always looks less sharp.
Finally, it is possible to artificially increase the size of an image by adding pixels. This is called interpolating or resampling an image. When these techniques are well mastered, there is no need for a 50 MPX file to make very large enlargements. For example, during a recent exhibition, I shot images from 12 MPX in 150×100 cm format. The final resolution chosen was 200 PPP (Ok because you don’t look at these kind of large prints too closely) and the photo was then interpolated (adding pixels by software). The result was excellent.
Definition and screen display
Again, it’s very simple: the higher the definition of an image, the larger it will be displayed on your screen. An image of 2000 pixels wide at 100% (maximum display) will be displayed full screen on a 1920×1200 pixels monitor, but will not fill a 4096 x 2304 px monitor (4K standard).
Resolution and screen display
In the screenshot below the image has a resolution of 72 PPP, against 300 PPP for the photo on the following screenshot. And the 2 images look exactly the same on my screen (and on yours).
The second image has a resolution of 300 dpi, but it displays exactly the same on the screen. On the other hand, its print size has been reduced from 70.56 cm to 16.93 cm
The resolution of an image does not influence at all the way it is displayed. Neither in size nor in quality.
A very old misconception … the 72 PPPs
We’ve been telling you this for as long as digital photography has existed: photos on the Internet must have a resolution of 72 dpi. Sometimes you can also read 96 dpi. In both cases it is true and false. In fact it doesn’t matter. Whatever the resolution of an image, its display will be identical on the same screen.
Screen Resolution and Definition
And yes, there is resolution and definition of the image and resolution and definition of the screen.
Think of your screen as a camera. The definition varies from one camera to another, 16, 24 or 50 MPX. The same goes for screens: the latest ones display 1920×1200 pixels (px) or just over 2 MPX. But some go up to 3840×2160 pixels or more than 8 MP (ultra-HD). So much for the definition of your screen.
For the screen resolution, it is the same as for the resolution of an image. We take the number of pixels displayed by your screen and divide by its surface. So if your screen displays 1920×1200 pixels for a diagonal of 22 inches, its resolution will be higher than for a screen of 24 inches.
Full Sensor Advantage?
One could easily draw this conclusion. If we take a 24 MPX APS-C sensor, compared to a 50 MPX full frame, we will have photosites of relatively similar size. In fact, the very high definition of some cameras (36 to 50 MP) are not really useful. I have shot images from a 12 MP sensor in 150×100 cm format. This requires care (and a good image to begin with) but the result is very good. So what is the purpose of these maxi definitions? To crop some will say, to sell too, it is an argument often unstoppable. In fact, the Nikon D5 or Canon Mark professional cameras have few pixels (a little more than 20 MP). This allows them to go up in ISO sensitivity and to display a very good dynamic range.
Notes
Pixels (px): basic unit of a digital image4K: definition of 4096 pixels for image widthUltra HD: standard for television sets – definition of 3840×2160 pixels
Conclusion
What is the definition of the image? Its dimension in pixels. It is fixed and depends on the sensor of the camera
What is the resolution of an image? It is the density of pixels per inch or square centimeter. The more pixels there are (up to 300 dpi), the sharper the image appears.
What is the right resolution for an image to be printed? 300 dpi is suitable for most cases. There is no need to exceed this resolution. 240 / 200 dpi can be used for larger prints
What is the right resolution for an image to be displayed on a screen? 70 to 300 dpi, the resolution of an image for the web does not matter.
What to do if you don’t have enough resolution to print a large print? The software, Ligthroom, Photoshop, etc. propose to increase the resolution of the images. It is possible up to 100 or 200%.
Is it better to use a big or a small sensor ? For the same definition (e.g. 24 mpx) an image from a large sensor will have a better ISO noise management and a better dynamic range, but the resolution and definition of the image are not the only criteria to take into account far from there.
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