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IMAGING TIP: How Big Can I Print From My Digital Camera File?
by Rich Seiling

Many photographers are confused by resolution, and the way it affects image quality. This is particularly true when digital cameras are involved, because large prints have to be made at lower dots per inch than the "standard" of 300 dpi.

A common question we hear at West Coast Imaging helps illustrate the confusion: “I have a 6 megapixel camera. How big of a print can I make from a 3008 by 2000 pixel file at 300 dpi?”

The simple answer
If you have 3008 by 2000 pixels and choose to lay them down at a resolution of 300 pixels per inch, it’s simply a matter of dividing the number of pixels on each side by 300. In this case, dividing 3008 by 300 gives you the width of 10.027", and dividing 2000 by 300 delivers a height of 6.667", rendering a final file that is 10.027" by 6.667". 

Yet, this is not the answer most photographers are seeking. In virtually every case, what the photographer wants to know is, “How large can I print my digital camera file, and still have it look good?"

The challenge is to learn the difference between absolute resolution and relative resolution. Until you learn the difference between the two, you won't have the answer you are looking for.

Do you want to know absolute resolution, or relative resolution?
So what’s the difference between absolute resolution and relative resolution, and how does it apply when you're trying to decide how big to print your digital camera files?

We can get to the heart of the matter by opening a digital camera file in Photoshop, then bringing up the image size dialog. Here we see the “size” of the file expressed in two different ways. 

Absolute Resolution
The first way is to look at the Pixel Dimensions. In this box, we see that the file is 3008 pixels wide by 2000 pixels tall. These are absolute pixels. Each and every pixel on this 3008x2000 pixel grid corresponds to a pixel generated by the digital camera from the CCD or CMOS sensor, then stored as digital data in the file. In the case of a digital camera file, the number of pixels we have is limited by the size of the sensor.

Therefore, if we only have a 6-megapixel sensor and set it to capture at full resolution, we only have six million pixels in our file. This gives us the absolute resolution--the absolute number of pixels contained in the file. 


Relative Resolution
Now that we know that the number of pixels we have is fixed, we can look at how tightly they are packed into a print, which is the relative resolution. This is displayed within the Document Size box inside the Image Size dialog. 

Let’s begin by un-checking the Resample Image checkbox. This tells Photoshop that we don’t want to make up any new pixels, but instead want to either pack the pixels closer together, or spread them further apart. 

The Document Size box gives us options for Width, Height, and Resolution. These are relative to our pixel dimensions. The easiest way to understand this is by typing in different resolutions and watching what happens to the width and height in the Document Size box.

If you type in 300 dpi, you’ll see it creates a 10.027" by 6.667" image. If you type in 200 dpi, it creates a 15.04" by 10" image (see illustration below).

If you type in 400 dpi, it makes a 7.52" by 5" image (see illustration below).

 

Now, as you type in different resolutions, watch the Pixel Dimensions and notice that these do not change.  This is the key to understanding absolute versus relative resolution.

We only have so many pixels in our digital camera file or scan. The larger we print an image, the further we have to spread out these pixels. It’s like having a room to paint and trying to use a pint of paint when a gallon would be better. We may be able to get some color on the wall from the pint-size can, but it will be a thin coat, and probably won’t look very good. Instead, if we choose the gallon size, we can get good coverage and the color will look thick and strong.

That’s the question the photographer really wanted to know: How far can I spread the pixels out, and still make a good looking print?

There is no simple answer to this question because it depends on what device you are printing on, the subject matter, and what you think is acceptable quality. However, there are some benchmarks that will help you recognize when you'll be degrading the quality of the image. Let’s look at the benchmarks for two common printers:

Chromira and LightJet printers:
These printers make excellent quality prints from 200 dpi files. Even though their native resolution is 300 dpi, through sophisticated built-in interpolation and the use of photo paper, 200 dpi is more-than-enough to make prints 16x20 and larger. Whenever you drop below 200 dpi, you will start to see a degredation in quality. How much depends on the subject matter and the size you are printing.

I recently made some 24x32 prints from the Canon 1Ds MK II 16-megapixel camera, and the original 1Ds 11-megapixel camera. The prints were made at 128 dpi and 105 dpi, respectively, and they looked very good. When compared to prints from film, there was a noticable loss of resolution, yet they still held up surprisingly well. This means prints made at 100 dpi can still look good--but the quality isn’t the same as printing from 200 dpi files. When making prints at less than 200dpi, you'll need to test the different resolutions yourself to see what meets your expectations. 

Epson 9600 printers:
My experience is that any time you give the Epson 9600 less than 360 dpi, you will see noticeable jaggies in the image. This is particularly evident in text. 360 dpi is a serious amount of resolution. Fortunately, there is a trick to help us use smaller-resolution files.

My testing shows that if you have a file smaller than 360 dpi, you can make a better print by interpolating the file up to 360 dpi using Photoshop. So, let's say you have a 240 dpi file. If you printed it as-is, you would see some jaggies in the print. But if you take that same file and use Photoshop to size it up to 360 dpi, it will not have jaggies, and the print will look sharper than the original 240 dpi file.

Using this trick, I have found that all I need for the Epson 9600 is a 240-dpi file. Having more resolution doesn’t increase quality, but if you use less than 240 dpi, you will start see a decrease in quality. For critical applications, you'll need to test the resolutions yourself to see how it meets your expectations.

Is that your final answer?
Now that we have all of this knowledge, let’s ask the question again: How far can I spread the pixels out, and still make a good looking print?

A 3008 by 2000 pixel file will create prints in the following sizes:

  • 20x30 @ 100 dpi

  • 16x24 @ 125 dpi

  • 20x13.333  @ 150 dpi

  • 11.5x17 @ 175 dpi

  • 10x15  @ 200 dpi

  • 8.9x13.4 @ 225 dpi

  • 8x12 @ 250 dpi

  • 7.2x11 @ 275 dpi

  • 6.7x10 and smaller at 300 dpi

My experience is that most photographers who haven’t used medium format film or larger would find the 20" by 30" acceptable. For photographers who have used medium format, anything larger than 10" by 15" will begin to fall short of your expectations.

If you need more of an answer than what I've spelled out here, your best option is to make test prints of your files, using different resolutions. Use our print lab to test your prepared files on both the Chromira and the Epson 9600. Seeing your own images printed at different resolutions is the best way to answer your resolution questions for good.

Text and photos ©2005 Rich Seiling, All Rights Reserved. This page may not be reproduced without the permission of the copyright holder.

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