Extramedia - The Blog

Understanding image resolution

2009-07-02T18:53:00.034+01:00

Resolution is one of the most misunderstood terms used in digital imaging, among both enthusiasts and professionals alike.
To get the ball rolling on this topic, I’m gonna just come right out and say something that might get the odd raised eyebrow, but nevertheless true: “DPI (resolution) is practically irrelevant until you want to print your image”.

Surprised?! Well, some of you at least. What is important is the total number of pixels in the image. The resolution itself can be set to whatever you want (72, 150, 300 or 1000 dpi) and as long as the total pixel count in the file remains the same, the quality is maintained too.

So what is DPI after all?
DPI stands for Dots Per Inch, and since computer monitors don´t display in dots — but rather in pixels — this can only refer to the dots made by printers when reproducing a digital image... because that’s what they do... dots... millions of them! An important point to remember is that the DPI of a file is just a way to measure it. Maybe you’ve heard someone say that they need a file at 300 dpi... well, ANY image file can be measured at 300 dpi, whether it has been saved that way or not!

Here’s how it works:
Let’s say you have a 2848x4288 pixel image at 72 dpi.
At this resolution you could produce a 39.5 x 59.5 inch (100.3cm x 151 cm) printed image. So, if you divide the numbers of pixels by 72 dpi you will get the total number of inches that the image can be printed at:
2848 / 72 = 39.5 inch (100.3 cm)
4288 / 72 = 59.5 inch (151 cm)
Print size: 39.5 x 59.5 (100.3 x 151 cm)

to illustrate this, let’s take a look at the Image Size dialogue box in Photoshop CS3:

Several things are worth noting in this screenshot:

1 - at the bottom of the document window you can see the file size is 34.9 Mb (MegaBytes)

2 - the Pixel Dimensions in the upper section of the Image Size dialogue box: 2848x4288

3 - in the Document Size section, note the big physical dimensions if we wanted to print at this resolution of 72 dpi.

Now let’s change the resolution of the image without touching the number of pixels, therefor preserving it’s quality. Here’s how:

Step 1: In Photoshop (or Photoshop Elements) choose Image > Image Size

Step 2: At the bottom of the Image Size dialogue box, uncheck Resample Image (important). By doing this, the pixel dimensions are locked, therefor locking the quality. Now enter 300 into the resolution box and watch what happens.

What has changed? Now the image is 300 dpi and consequently the physical dimensions have decreased to 9.5 x 14.3 inch (24 x 36.3 cm). In other words, the pysical dimensions of the image have changed but not the pixel count. See how the size of the image didn´t change at all? Still those good 'al 34.9 MB. We haven´t altered the pixels (quality), only the measurement (dpi) which affects the physical output of the image.

IMPORTANT: As long as you uncheck the Resample Image box you can change the resolution as many times as you like, and you won´t be affecting image quality at all.

Ok, sounds great but, what resolution do I need?
Well, that depends. Assuming you want to have your image printed, first you must determine what kind of printer it will be reproduced on and at what size. If a comercial printer (offset) is your image’s final destination then 300 dpi is usually required, unless it’s for a newspaper for which anywhere between 150 and 200 dpi will suffice. High end giclée inkjet printers can achieve very good results from 150 dpi files, although even with 100 dpi files the outcome is above average most of the time. For best results a 200 dpi file is required though.

So, going back to our initial 2848x4288 pixel image example I could print the following on a high end giclée printer:

- 28.4 x 42.8 inch (72.3 x 109 cm) — For an above average quality print at 100 dpi:
- 19 x 28.5 inch (48.2 x 72.6 cm) — For a very good quality print at 150 dpi
- 14.2 x 21.4 inch (36 x 55 cm) — For an Excellent quality print at 200 dpi

To find out at what sizes you can print your images at different resolutions depending on the requirement of the output device, just remember the formula... ok, for those who’ve forgot, just divide:

Width in pixels by desired output resolution = print width

Height in pixels by desired output resolution = print height

Well, that's a wrap on resolution and I hope this ~~small~~ article has explained resolution well enough to save you the cold sweats whenever you need to print your images.

Stay tuned!

Getting rid of the "Purple Fringe"

2009-09-09T20:57:00.029+01:00

It's been a while since I last posted, due to the holiday season and my overall busy-ness (and business), so I'm glad to be back posting, hopefully on a more regular basis.

Ideally I would be writing about basic Photoshop post production tips first and slowly moving up to the more advanced techniques, but as the dreaded "purple fringe" problem is as widespread as the type-A you-know-what, I decided to write up a quick n' simple tutorial on how to get rid of it.

Now, as many of you may know, there's more than one way to skin a cat, so to speak, in Photoshop, but this is, by far, my favourite and most effective way to skin this pussy, IMO. I've used it dozens of times and has never let me down. I love this technique because it gives me pinpoint precision and control and I can eliminate it in the blink of an eye.

Please note that this is not an entry level technique (although beginners can benefit from it too), being more oriented to the intermediate-advanced user/photographer, mostly because they have the knowledge to identify it as being a problem. The reasons why purple fringe occurs is beyond the scope of this post, therefor will not be discussed. Maybe in a coming post.

To start off, lets take a look at what purple fringing looks like in the image below (click for larger view):

Please forgive the image quality but this is an old, out of focus image taken ages ago with a compact camera. Anyway, the purple fringe is visible running down the guy's shoulder and arm and around his pouch.

Step 1:

Open your image in Photoshop and create a new empty layer, that should be placed on top of your image layer.

Step 2:

Change the blending mode of the new layer to "Color".

Step 3:

Select the brush tool and use a soft edge (hardness at "0"). The size depends on the area you are working on, but I usually use a size that overlaps both the "good" and "problem" area slightly. Don´t forget to zoom in!

Step 4:

Use the Eyedropper tool to select some colour on the outside of the fringe (or Option + click on a Mac or Alt + click on a PC).

Step 5:

Paint over the fringe

Here is the result:

Most of the time purple fringing pops up in small areas, hence my preference for this technique. When this maladie takes on mammoth proportions though, more "overall" techniques might be in order.

Sometimes you might find that this particular CA (Chromatic Aberration) problem isn´t so purple afterall, but is rather green or some other lovely colour. No worries... this technique isn´t fussy when it comes the colour of your fringe!

Welcome to Extramedia's blog

2009-07-01T20:35:00.035+01:00

Initially I was planning on setting up a page on our main website dedicated to helping customers with photography related issues. Not a troubleshooting page, but rather a place where visitors can learn something new about photography, photoshop, the entrails of digital imaging and the likes. It became obvious why this idea wasn´t the best, for the simple reason that I wouldn´t be getting any feedback from the readers, which I like to use as my compass. So blogging seemed the way to go. And here I am. My name is Nick Kilford, I am a communication designer and an exclusive stock photographer at iStockphoto. Also co-owner and manager at Extramedia.pt, located in Lisbon, Portugal.

Why are you publishing in english?
Well, although I'm fluent in portuguese, english is, in fact, my native language. It's the language I feel most comfortable expressing myself in. Also, I believe I'll be able to reach more readers by using english and I know most tugas can handle it pretty well (yeah, we're a smart bunch!). Nevertheless, you might find a splash of portuguese here and there.

What can readers expect from this blog?
Photography talk in general, stock photography in particular, articles written by moi, others written by other professionals, photo editing tutorials, printing, links to interesting stuff on other blogs, interviews, creativity, book recommendations, videos and everything to do with digital imagery.
Language used will have an informal tone, as I want this blog to be a sideway step outside the main site, where the sharing of knowledge and ideas is paramount. I have been a designer for 10 years now, a stock photographer for almost 3, have been using Photoshop since version 5 on a regular basis and been running a printing business for 3 years. So, although I may know a trick or two in these areas, I believe that learning is an endless road and I'm certainly gonna pick up a few things along the way.
I will post as frequently as time allows and will try to answer all questions thrown my way ASAP and to the best of my knowledge, so before you start sending me hate mail, please gimme a couple 'a days.