AG Photo luxury
Resolution
The image resolution and the perceived resolution
The resolution of an image is a very important characteristic that determines its quality. Before we talk about image resolution, however, it is necessary to speak of the human eye resolution. The human eye is able to distinguish two points as separate entities only when between the two there is a certain distance. When two points are too close, the eye does not perceive more distinct objects, but just one.
In order to obtain a sharp image, it is necessary that its resolution is greater than that perceived by the human eye.
The human eye, at a distance of 20-30cm (at greater distances the situation gets worse), has a maximum resolution of 0.1mm. It means you can see up to 10 distinct points in a single mm. If the points increase, the human eye is no longer able to see them as separate.
Definition of resolution of an image: the amount of the smallest fundamental points per unit length.
On a screen, the smaller fundamental point is the pixel while on printed paper is the point of ink. So, when it has to do with the resolution of a screen, with any technology it is built, you have to use the unit of measure of the pixels/cm or pixels/inch. However, it is more frequently used the second unit of measurement also called ppi (pixels per inch).
But when it has to do with the resolution of an image printed on paper, you have to use the unit of measurement known as dpi (dots per inch).
The optimal resolution
As mentioned earlier, to have a resolution so as to generate a sharp image, defined and with soft outline, it is necessary to exceed the maximum resolution the human eye. The human eye has a maximum resolution (with a great view and no defects) of 0.1mm, and allows us to distinguish 10 points in a single mm. So in a cm, it is possible to distinguish up to 100 points. The inch is Anglo-Saxon unit of measurement, not valid in the metric system, but commonly accepted and has a value of 2.54 cm. If in a cm the human eye distinguishes up to 100 points, in a inch it distinguishes 254.
Therefore, the resolution of the human eye is equal to 254pp. In order to have an adequate resolution, the image has to have a ppi value that is greater than 254. The standard has been conveniently set at 300ppi.
At 300ppi, our eye is not able to distinguish single pixels but sees a well-defined image with soft contours.
In figure 01, we see three slanted lines.
The uppermost line has a low resolution, and we can distinguish the single pixels that make up the image. It looks more like a flight of steps than an inclined line. The second line also has a low resolution but better than the first line. We are still able to see the pixels that the line is made up of. In the third line, the resolution is the highest, and our eye is not able to see the individual pixels, and, therefore, we see a continuous line. Two factors should be taken into account: the size of the pixels and their distance that is only a different way of giving a definition to the word resolution.
Resolution of images on the screen and on paper
Whether we talk about points or pixels, the discourse is always the same because the optimal resolution has to be greater than 254ppi or 254dpi. However, at times ink dot does not correspond to a pixel.
In fact, this happens on some printers, whether they use more points to represent a single pixel. When the ink dot corresponds to the pixel, the optimal resolution is 300dpi.
01 Three examples of slanted lines with different resolutions.
Often in printer technical specifications, we find resolutions equal to, for example, 1200 dpi. This means that every pixel is represented on paper as four dots, one for each cartridge. In this case, the true resolutions will be 1200/4 and therefore it is still 300dpi because 1200 dots represent 300 pixels. The resolutions of one screen, monitor, display, or TV set (with cathode ray technology, plasma, LCD, LED and so on) are expressed in ppi and normally it is 72 ppi. There are monitors, like the ones I use while I am writing, have a resolution of 109ppi and are always inferior to 300ppi. In spite of this, the resolution of screens is good because often we observe screens from a distance that is greater than 20cm and not everyone has the best possible vision and 255 ppi would also be enough.
The resolution and the number of total pixels
According to the definition of resolution that we have given, all surfaces being equal, an image is more resolute if it is divided in a greater number of points and, in effect, this is so.
But often the term “resolution” is commonly used in the wrong way in order to describe the total number of pixels of the digital camera.
In digital cameras, the total pixels are measured in Mp (megapixel) that is in millions of pixels. Therefore a rectangular sensor in 3/2 format that has the long side of 3000 pixels and a short side of 2000 pixels will have a surface area (total number of pixels) of 6000000 or 6 Mp. As I was saying, this is not the resolution but the total number of pixels. According to the defintion, in fact, we often have different resolutions of 6 Mp images. This depends solely on how much space contains the 6 Mp. It is easy to understand, at this point, how the resolution is a characteristic of the printed image and of the screen. It doesn’t make sense, therefore, to talk about the resolution of an image file or of a digital camera that made it. For both situations, it is best to talk about the number of total pixels because the electronic image will have a different resolution if it is seen in its actual dimensions and without enlargement on a screen with a high or low resolution. It depends, in fact, on the surface area on which the pixels will be visualized. On the contrary, the number of total pixels of a file are a characteristic that does not change and that determines the size of it. If the file of the digital photo is 6 Mp with a 3/2 format, it means that the photo has a 3000 pixel side and another with 2000. If the surface area is doubled (12 Mp, 4300p x 2800p), we would have a much bigger photo. But if both were seen on a monitor with a resolution of 72ppi, their resolution, if seen in real dimensions (in other words, without enlarging or shrinking), it will be the same and therefore equal to the 72ppi screen. The second image, though, would occupy on the screen twice the surface area. If instead the photos were printed, their resolution would depend on the size of the printed format chosen. If the format is less than or equal to 30x20 cm (see table 11), the photos would have a sufficient resolution. For a larger format, the surface area of paper is too much to distribute the pixels of the 6 Mp photo with a resolution of 300dpi and therefore would appear grainy.
Therefore, the resolution is really a characteristic of the screen, monitor, display, TV (Plasma, LCD or LED) or of the printed photograph while the number of total pixels is a characteristic of the sensor in the digital camera, videocamera and the image file: these two things should not be confused.
When we look at a photo on the monitor, if the number of total pixels of the screen is equal to 1 Mp, as are many screens commercially available, the photo seen has a total of 1 Mp or 14 Mp , it will not change much of anything for the person looking at it. If the screen that you are using has 4 total Mp you will see the difference, but you will not see it anymore if the first of the two photos will have 4 total Mp and so on. Having said this, it is however good to have more pixels so that we will have the possibility to enlarge more and thereby maintaining a resolution in ppi that is equal to that of the screen we are using.
An interesting curiosity is that even though the progressive increase in pixels in the digital camera has made digital photography more advanced, it does still not rival the human eye. In fact, in 2014, we arrived at having reflex cameras with 36 total megapixels while the human eye possesses 125,000,000 rod receptors and 6,500,000 cone receptors (rods & cones). The first serves to distinguish weak light, shapes and movement while the second one distinguishes between strong light and colors.
Introduction
The camera
How it is made a Reflex camera
Photographic lenses
Equipment and accessories
Take a picture:
Buyer’s guide
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02
Image ratio: 3/2
A=1,8 inch
Pixel: 30x20
Surface: 600 total pixels
Resolution: 16,7 DPI
03
Image ratio: 3/2
A=0,9 inch
Pixel: 30x20
Surface: 600 total pixels
Resolution: 33,3 DPI
04
Image ratio: 3/2
A=1,8 inch
Pixel: 60x40
Surface: 2400 total pixels
Resolution: 33,3 DPI
05
Image ratio: 3/2
A=1,8 inch
Pixel: 60x40
Surface: 2400 total pixels
Resolution: 66,7 DPI
In figures 02, 03, 04, 05, we can see four images that show different examples of resolution and size. The examples were chosen ad hoc in order to be able to understand the factors that influence the resolution of an image.
In the captions of each image, we find the figures information. The pixels are represented by small red and black squares; the fact that there are two different colors is not important so for the purposes of not confusing them and therefore the black squares and the red ones are absolutely identical. All four images are in 3/2 format and therefore the same as the current reflex. In figure 02, we have an image of a certain size with the number of pixels equal to 600. In figure 03, on the other hand, maintaining the same number of pixels, the size has been reduced by half (half of the side) and consequently the resolution is doubled with respect to figure 02.
In figure 04, the size of the photo is the same as figure 02, but the total pixels are four times greater, obtaining a double resolution but equal to that in figure 03. Therefore, in figure 04, we have a total number of pixels that are 4 times greater than those of figure 03 and the resolution is the same.
In figure 05, the dimensions of the image had been again reduced by half (half the side) and the number of pixels brought to four times the value of the first image (02). In this case, the resolution is four times greater than the resolution in figure 02 and two times greater than the resolution in figures 03 and 04.
It is worth noting that increasing the resolution, our eye always distinguishes fewer pixels and, as such, tends to see instead a more homogenous “fabric” that together with the two colours, red and black, at high resolution, appear more as a bordeaux red.
In figure 06, we can see how redoubling the side of a rectangle makes the surface area increase four times.
But the resolution is defined as a point of unity of length and not of surface area and therefore varies in a linear manner with the side of the rectangle and not with its surface area exponentially.
06 Side x2 = area x4
Image formats
Below, in figures 07, 08, 09, 10, we can see the most common image formats, whether they are from a photographic sensor, a videocamera sensor, a television set, a printed or displayed photograph or any kind of screen of monitor.
The first type is the 4/3, used mostly in old cathode ray TV sets, in compact digital cameras and old computer monitors. The 3/2 format is more panoramic and is used in digital reflex cameras. The 16/9 format is mostly used on LED, LCD and Plasma screen television sets and on many computer monitors. The last image type, the 21/9 format, is the most extreme of the four types and is used mainly on cinema screens and some new TV’s.
07 Image in 4/3 format.
09 Image in 16/9 format.
08 Image in 3/2 format.
10 Image in 21/9 format.
Total number of pixels and printed formats
The table in figure 11 helps us understand quickly that if we want to print our 10 x 15cm photos, it sufficient that our digital camera is 1,5 megapixel. If one day, we wanted to print larger photos, for example 30 x 20cm, a 6 Mp camera would do the job. From this table it is easy to understand that once you get beyond a certain value, the number of Mp for printing no longer has an impact.
If we consider the monitor, we have to divide the values that we have obtain by 4 (roughly) by reducing still the number of pixels necessary to see the image correctly. On the monitor, though, it is useful to be able to make an image larger to see a particular detail and therefore having a greater number of pixels is important.
In conclusion, I have to add that it would be enough to surpass the threshold of 254dpi ever so slightly to get the perfect image for our eyes. The table takes into consideration the 300dpi standard. This means that the necessary megapixels are still less. For a 45x30cm photo, 12mp are enough. If we then consider that at such a size the photo will probably be hung on the wall and seen from a greater distance that if we were to hold it in our hand, the necessary resolution is still less due to the greater distance. The perceived resolution by the human eye is no greater than 100 points per centimeter but inferior in this specific instance.
11 In the table, I list all the print formats in centimeters that are available on the market for the 3/2 format. From 50x33 and on up, there are formats that are not as common and not easily found even though not all laboratories allow prints up to 45 x 30 cm. In the pixel column, I assigned to each format the number of pixels necessary (side by side in the rectangle) to obtain a print with the best optimal resolution (300 dpi). In the last column (Mp), we have the number of necessary Mp that will allow us to have a resolution of 300 dpi in various formats.