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Understanding resolution

Film vs. digital resolution.

Camera, screen, and print resolution can be confusing and I think there are some misconceptions out there and misused terminology.                                                                                                                                   Many photographers may not even be on the same page with the different factors that determine resolution.

Hopefully I can clear that up a bit and possibly shed some light on the relationship between camera resolution in pixels and how that relates to print size and resulting detail.

Not really until digital photography was resolution talked much about in relation to the camera. When shooting film, the film size used was generally determined by what you intended to do with it. If you only needed magazine sized prints for publication, 35mm would do; if you needed larger prints, medium or large format film was used. Today resolution is often discussed independently of prints in digital form. I think that is where a disconnect has formed.

Since most people today have very easy access to a digital camera in some form, most images taken never see print and stay in digital form via their computer or on social media. The vast majority of casual digital camera users have no concept of resolution since they just post whatever size comes out of their camera. I think the lack of understanding about resolution has also given some rise to the megapixel race that we see from many camera makers today. More pixels or higher resolution is not necessarily always better     (I talk about that more here) or necessary. In my opinion, about the only reason for any casual camera user to own a camera with a resolution over 12 megapixels would be if they were producing large prints.

resolution-data_1

Megapixels.

Since I mentioned megapixels, let’s start with that. First off, for what we are discussing, a pixel is to a digital camera sensor as inches are to a print, tiny squares of light catching technology that as a whole, make up a camera sensor. What is a megapixel? It sounds like one giant pixel! Actually, a megapixel is one million pixels and describes the camera’s sensor resolution capability in area. The pixels are small enough that we don’t see them (hopefully), but if we zoom in enough on an image on our screen, each pixel on screen represents a pixel on the sensor (see below). For a while, 12 megapixels was pretty standard. Why is that you may ask? 12 megapixels is the perfect size to cover a typical magazine spread across two pages or the largest advertisement that would fit in a normal magazine, much like 35mm film.

So how are megapixels figured? Just like when determining the area of any space, we would multiply the length by the height. In this case we might have 4000 pixels long x 3000 pixels tall, which gives us 12 million pixels or 12 megapixels.

Individual pixels.

Pixel density (pixels per inch).

We have to get a bit more complicated and add another variable to the mix to talk about how megapixels relate to print size. We have to talk about pixels per inch. A minute ago I said that 12 megapixels would cover an entire magazine spread. More specifically the image would be around 12”x18”; however, that is only true if the ppi or pixels per inch is at 240, which describes the pixel density.

SIDE NOTE: There is a large misconception when talking about pixel density. Many people confuse dpi or dots per inch with pixels per inch but these terms are not interchangeable. Dpi relates to printing and print resolution so be careful not to get the two confused.

Pixel density (i.e. our 240ppi number) can be changed to increase or decrease the size of the image. If I change the number of pixels per inch, the size of the print will change because the total number of pixels remains the same. I am just spreading the existing pixels out, causing fewer per inch and therefore taking up more overall space. To better understand what’s happening here, we just have to look at a bit of simple math. Sticking with our 12 megapixel image, if we are at 4000 pixels in length with 240 pixels per inch that’s 4000 ÷ 240 = 16.7 or 16.7 inches in length. So if we change the ppi number we can manipulate how large the image will be.

There are a couple of reasons this may be desirable. You may want to get a larger print out of a lower megapixel camera, or you may want to produce a smaller print with greater detail. Without relying on software to make our image larger or smaller, we are dealing with a finite amount of pixels.

 

Different pixel densities and the resulting print size.

12 megapixel image long side = 4000 pixels.

@300 ppi = 13.3”

@240 ppi = 16.7”

@200 ppi = 20”

@150 ppi = 26.7”

@72   ppi = 55.6”

resolution-blog_2resolution-blog_3

 

Keep in mind, there is generally a minimum standard resolution for printing where the print begins to lose detail and will not look its best. For most printers, that is around 150ppi; although for the absolute best detail, I always try to stay around 240 to 300ppi. Each photographer may have a different comfort level as to how much they feel they can stretch an image size. In stark contrast to a professionally printed image, the Internet standard is 72ppi! This means if you are only posting images online, the resolution needed is minimal. Did you know that the two most common megapixel sizes in a point and shoot camera are 16 and 20 MP? If you are never printing your images that is massive, and large images and file sizes will fill up the space on your computer never to be seen again.

If you upload a 12 MP image to Facebook straight out of the camera, Facebook has to knock that down to a manageable size, and in doing so, you will most likely lose a lot of quality.

Print resolution and print sizes.

Let’s talk more specifically about prints and print sizes and since I have spoken out of one side of my mouth about how megapixels are overrated, let me speak out of the other. If you are intending to produce large prints (12”x18” or larger), you need megapixels! There is a big difference between using images for screen and using them for print. As high as 36 megapixels sounds, which is the resolution of the Nikon D800 camera that I use, that digital pixel real estate gets used up very quickly when you want to produce large prints. In the past, I have encountered clients that want to be able to print an image incredibly large, but they do not understand what that entails.

Again, instead of thinking of print sizes in length and height, we have to think about them in area. An image comes out of my 36 MP Nikon natively at just over     20” x 30” @ 240ppi. I can stretch that to 30” x 40” @ 163ppi, but I don’t really feel comfortable going larger than that without a really good enlargement scaling software. However, even with great software, it’s never as good as the original in my opinion. To get to the next size up in a large print, such as 30” x 40” @ 240ppi straight out of the camera, my options are limited. I would have to own a camera that shoots nearly 70 megapixels! Why the huge jump? 36 MP to 70 MP.

Common megapixel camera sensor sizes and their native print resolution.

12 MP  =  16.7″ x 12.5″

16 MP  =  19.2″ x 14.4″

20 MP  =  22.8″ x 15.2″

24 MP  =  25″ x 16.7″

Even though it looks like only one size up and a 10 inch increase in height and length, it’s far more than that in area. The jump from 20” x 30” to 30” x 40” is huge and it gets even larger as we go up. The area for a 20” x 30” is 600 square inches whereas the 30” x 40” is double at 1200 square inches. Then we see the correlation with megapixels, which is also nearly double. To shoot a single 70 MP image natively we really only have two options; a medium format digital camera or large format film. A medium format digital camera, such as the Phase One digital back costs tens of thousands of dollars, so it’s rather cost prohibitive. A Phase One XF 100 MP camera system is capable of producing a native sized image of 48”x 36”. That’s pretty great, but in some cases film still wins!

Hopefully this helps you understand how resolution works from image in camera to print sizes and always feel free to email me any questions you may have.

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