“How big does my file need to be?”:
You might think this seems like a simple question, but this one little query prompts so many other questions it can get a bit overwhelming. My go-to analogy for file dimension is vehicle speed - so the above question is kind of like asking “How fast do I need to drive?”. Without taking speed limits into consideration of course, you would first need to figure out how far you need to go and how long you have to get there.
A lot of the terms we use in photography and digital imaging have been co-opted and corrupted from the world of computing, a quick web search of the word “resolution” will give you a large number of definitions and most of them have nothing to do with photography or imaging. For the sake of our collective sanity, I’m going to define some terms as we use them here at Old School Photo Lab.
PIXEL DIMENSIONS or FILE DIMENSIONS: A set of two numbers referring to the number of pixels on an image’s horizontal axis and vertical axis. Example: 1200x1800px (pixels).
PRINT SIZE: A set of two numbers referring to the final width and height of a print. Example: 4x6” (inches).
PRINT RESOLUTION (Commonly just “RESOLUTION”): The number of dots per inch (dpi) at a given physical print size (each “dot” represents a “pixel” but “pixels per inch (ppi)” is used when referring to monitor display). Example: 300dpi.
FILE SIZE: A measure of how much memory your file takes up, usually measured in KB or MB. (“File Size” and “File Dimensions” do not always directly correlate and will vary depending on the file format and compression.)
Opening the “Image Size” menu in Adobe Photoshop can give you an idea of what we’re talking about:
Take note that in the above image, there are boxes dividing the dialogue box into two sections. When working with image files, this is how you need to start thinking when it comes to “how big” your files should be. The numbers in the top box are your “Pixel Dimensions” and the numbers in the bottom are what Adobe calls your “Document Size” which contains width and height for the “Print Size” and “Resolution”. The numbers in the top box relate directly to the numbers in the bottom box… this is where we go back to my vehicle speed analogy:
“drive time” x “how fast you drive” = “distance”...2hrs at 60mph = 120mi
…is the same thing as…
“print size” x “print resolution” = “pixel dimensions”…6in x 300dpi = 1800px
Using this equation we can figure out what kind of pixel size we need for a given print size in just a couple of steps:
To yield a 4x6in print at 300dpi…
4in at 300dpi = 1200px
6in at 300dpi = 1800px
…we need a 1200x1800px file.
Using this same method but by dividing instead of multiplying, we can work backwards from a set of pixel dimensions to see what an acceptable print size would be:
A high quality 300dpi print from a 2400x3000px image would be…
2400px at 300dpi = 8in
3000px at 300dpi = 10in
…an 8x10in high quality print!
RECOMMENDED PRINT RESOLUTION (dpi):
“High Quality Print” is a little hard to define, and the definition is extremely subjective. The generally accepted standard in the photo industry for print resolution is 300dpi, but as one starts moving into larger prints that number will drop. The reason for a drop in dpi requirement as prints get larger has to do with the fact that larger prints are viewed from farther away (“viewing distance”). As the viewing distance increases the print resolution can drop since your eyes can’t see the individual dots that make up an image from far away. For example, billboards are printed from files that are fairly low dpi since they are viewed from hundreds of feet away. Some benchmarks we use here at the lab when talking to customers and would be a good guide to keep in mind are:
Print up to 11x16in… at least 300dpi
11x16” up to 16x24in … at least 220dpi
16x24” up to 24x36in … at least 150dpi
Working from what we learned earlier, what size print can we get from a file that is 3000x4500px?
High quality print @ 300dpi ... 10x15in
Larger viewing distance print @ 150dpi = 20x30in
There are of course, exceptions to every rule! Sometimes small prints made from a file at 150dpi will look just as good as ones from a file at 300dpi – it can depend greatly on a large number of factors including the quality of the original image, the scanner used, and things like file compression. Scans from the Frontier scanner here at Old School Photo Lab can generally print a couple of sizes larger than would be expected if we’re talking just about pure math. We print 5x7's from 1200x1800px (a “basic scan” from 35mm) files and 24x36's from 3000x4500px (a “high resolution scan” from 35mm) files on a regular basis without any noticeable quality issues.
SCAN SIZE AND END USE:
The end use of your images should always have something to do with how large of a scan you make or order.
For sharing on the web, projecting on a screen, or sending through email a smaller scan will work just fine. Scanning from a 35mm film frame at 1200x1800px (4x6in at 300dpi) will more than fill most computer monitors.
If your images require heavy editing/retouching, or will be printed large, a high resolution scan is preferable. Keep viewing distance in mind when thinking about large prints – a file for a 24x36in print at 300dpi will yield a slightly higher quality print, but the size of the file will make image editing very slow on most consumer computers.
FILE SIZE, FORMAT, AND COMPRESSION:
The most common file format in use today is the JPEG (.jpg) format, which uses a compression standard to make image files smaller and more manageable. JPEGs are very useful and as long as they are handled correctly will yield a high quality image when viewed or printed. People get in trouble with JPEGs when they set the “quality” too low when saving or if they edit and save the same image repeatedly as a JPEG (which will decrease the quality). “Quality” settings reflect the compression rate for an image: low quality = higher compression and smaller files, but the image will suffer. Using a “High Quality” setting when saving JPEGs will result in a slightly larger file, but will give you a much higher quality image. Both of the images below have the same pixel count (360x360px), but the top image is a High Quality JPEG and the bottom is a Low Quality JPEG.
|High Quality (low compression) JPEG 180KB|
|Low Quality JPEG (high compression) JPEG 39KB|
Another common file format is TIFF (.tif), an uncompressed file format. TIFF files are MUCH larger than JPEG files and it isn’t really practical to use them for display on the web or sending through email. The difference between a High Quality JPEG and a TIFF is barely noticeable unless viewed at high magnifications.
Some labs offer the choice between JPEGs and TIFFs - at our lab, we generally work with JPEG files, but we will soon be offering the option to have an Archive CD made with both TIFF and JPEG files. Personally, I work with JPEGs 99% of the time and rarely use TIFFs.
OTHER SCAN TERMINOLOGY:
“BASE SCANS”: If you are ordering scans from a lab and they offer “low resolution” and “high resolution” scans, find out what the actual pixel dimensions of the files will be. Sometimes “low resolution” refers to what used to be called a “Base Scan” and is really only suitable for screen viewing. “4 Base” is a more acceptable low res scan these days, but the “Base” scan terminology was a Kodak standard is not commonly in use today. For sharing on the web, small prints, or scrapbooking a scan of around 1200x1800px should suffice.
“SCAN AT XXXdpi” or “SCAN AT XXXX%”: You may have seen people or labs say something like “We scan 35mm slides at 4000dpi.” From this post you know that this is only part of the equation and it may seem like an incomplete statement. What they are saying is that they are scanning the slide or negative at its original physical size at the given dpi. A 35mm slide is 1x1.5in, so a “4000dpi” would yield a 4000x6000px image.
“UP-RES”: If we have a file that is only 400x600px, we can’t just add more pixels to make a larger image. Occasionally you may come across programs or apps that say they can “UP-RES” your image, which is technically true in the fact that they can add pixels but false in that they cannot add details.
|16x16px image at 3200%|
|16x16px UP-RESed to 256x256px|
|True 256x256px image|
Contributor Bio: Jake Bouchard got his start in photography as a teenager and attended the Hallmark Institute of Photography to pursue a career as a photographer. While attending school, he realized that working in the darkroom and making prints was where his talent really lay. After graduating in 1999, Jake worked as an occasional photographer and photo assistant, but always went back to lab work. He has been employed at a number of different photo labs in ME and NH, ranging from professional commercial labs to small neighborhood photo processors. He finally settled at Photosmith in Dover, NH where he brings his photography and lab experience to the 31 year tradition of quality photo finishing. Jake's career in the photo industry has coincided with the decline of film photography and the rise of digital. Film photography is his passion and through his work with OldSchoolPhotoLab.com and 120processing.com, he hopes to help keep the world of film photography alive.
Also, read Jake's other guest post: Tips For Getting the Best From Your Photo Lab
Old School Photo Lab: http://www.oldschoolphotolab.com/
120 Processing: http://www.120processing.com/