Resolution can be a confusing subject so I hope to make it clearer for you.
Updates:
6th November ‘07 – Scanner software (with screenshot). Red Bubble products. General tidying and clarifying. Table of Contents.
10th November ‘07 – Added posters.
Contents:
Terms Used
DPI
Digital Camera Resolution
Aspect Ratios
Screen Resolution
Scanning & Resizing Images
...100% Reproduction
...100% Plus Reproduction – Enlarging
...Less Than 100% Reproduction – Reducing
Red Bubble Products
Terms Used
Resolution – the degree of sharpness of an image as measured by the number of dots per linear inch in a hard-copy printout or the number of pixels across and down on a display screen. Digital camera resolution is often referred to in mega pixels or MP. Mega means million, so 2mp = 2 million pixels.
Pixel – (Pix, short for pictures + el, short for element)
The basic unit of the composition of an image on a television screen, computer monitor, or similar display.
DPI – Dots Per Inch. This is a printing resolution term. Printers form dots on paper etc.
PPI – Pixels Per Inch. This is a screen resolution term. Screens display pixels.
Aspect Ratio – Usual form: 4:3, 3:3, 16:9, etc. E.g. a 4:3 image could be 4×3 inches, 8×6 inches or 12×9 inches, and would be in the 4:3 aspect ratio but different sizes.
DPI
Although DPI refers to printing resolution it has come to be interchangeable with PPI when used to describe the resolution of screen images.
The Pixels Per Inch refers to a linear inch not a square inch as you might think. So a 300PPI image is actually 300×300 pixels per square inch or 90,000 pixels.
Digital Camera Resolution
300PPI is photographic quality resolution, so a standard 6in x 4in photo at 300PPI is:
6×300 wide = 1800
4×300 high = 1200
1800×1200 pixels = 2,160,000 pixels or 2.16mp
If a 6in x 4in photo only needs 2 mega pixels why are camera sensor resolutions steadily increasing each year? Improvements in the technology and camera companies hoping their customers will think more mega pixels means better quality images. In fact the reverse is true. The more pixels you cram onto the same sized sensor chip the more ‘noise’ or electric charges effects images.
Most current compact camera sensor chips are around 1 square cm. So if you go from 4 or 5mp to 7 or 8mp on the same sized chip you are degrading the quality of images to some degree. This is especially noticeable in low light situations where the shutter is open for longer than usual. Most compact cameras become noisier as you get past the ISO 200 equivalent of film.
Aspect Ratios
Photographic prints generally have an aspect ratio of 3:2, e.g. 6×4 inches, whereas digital images are generally at the ratio of 4:3, e.g. 1600×1200 pixels, although some digital cameras have settings for the 3:2 ratio. This setting just ‘turns off’ the extra pixels to achieve the 3:2 ratio.
This means that you have to crop a 4:3 ratio digital image to the 3:2 ratio if you are intending to print at a photolab.
Screen Resolution
Computer monitors used to display at a resolution of 72PPI and often images intended for screens are at 72PPI, e.g. digital camera images are often at 72DPI when you open them in Photo Editing software. As monitor quality has improved, the resolution has increased to around 100DPI, but depending on the monitor size ie. 17”, 19” etc., the display size chosen ie. 1024×768, 1280×1024 etc. and the quality of the monitor, the resolution varies.
Standard monitors (as opposed to widescreen) use a 4:3 aspect ratio, eg. 1024×768 pixels, or a roughly 4:3 ratio, eg. 1280×1024 pixels is a 4:3.2 ratio.
Scanning & Resizing Images
100% Reproduction
If you want to scan an image and reprint it at the same size then you must match the photographic resolution (300DPI), with the scanned resolution of 300PPI.
E.g. you have a 6×4 inch photograph that you want to scan into your computer, increase the brightness and then burn to CD or copy to thumbdrive to be printed at a photolab.
You scan the photo at 300PPI because this matches the photolab’s print resolution of 300DPI. The scanned image has not increased or decreased in resolution, therefore it will print at the same size as the original photo.
100% Plus Reproduction – Enlarging
If you want to scan an image and print it at a larger size, then you have to increase the scan resolution from 300DPI to something higher.
E.g. you have a 6×4 inch photograph (must be a film photo as these are printed at very high resolution and therefore enable enlargements without loss of quality) and you want to enlarge it to 8×12 inch and get it printed at a photolab.
First, let’s take a look at an example of scanning software:
Starting at the top and working downwards:
Colour Mode: Black and White, Grayscale, Colour
Black and White – reduces the image to two tones: black and white.
Grayscale – what most people call black and white, ie. an image that goes from black through shades of grey, to white.
Colour – colour.
Output Resolution: DPI (notice even scanner software refers to DPI…)
300DPI – if you want photographic quality
Selection: – for this example a 6×4 inch photograph (film) is being scanned
Width: 4
Height: 6
Units: inches
Note: the icon to the right of the Height & Width options is to lock/unlock the aspect ratio of the selection. When selecting the image to be scanned, ie. with the mouse, the selection maintains the aspect ration entered.
Print size: for this example an 8×12 enlargement is required. Conveniently the dimensions are at the same aspect ratio so no cropping is required.
Width: 8
Height: 12
The image is then scanned and the software scans the image at 200% or 600PPI.
Doubling the scanned resolution quadruples the size of the final print size:
Less Than 100% Reproduction – Reducing
If you want to scan an image for use on screens only, e.g. the WWW: then you wouldn’t need to scan at 300PPI for many images. Scanning at 100DPI would probably be enough. It depends on the size of the image to be scanned and the display size.
6×4 inch photos are 15cm x 10cm, so scanning at 100PPI would make a good sized screen image at 600×400 pixels. If you scanned the photo at 300PPI it would be 1800×1200 pixels making it larger than most people’s monitor display size.
The file size for a 6×4 inch photo at 300PPI would be approximately 500 kb to 2000 kb, whereas a 6×4 inch photo at 100PPI would be round 100 kb to 300 kb.
Red Bubble Products
How do I get my digital camera image to the right size for a Red Bubble card, print or T-shirt?
Firstly Red Bubble has minimum pixel dimensions:
Cards – 1300×900 pixels
Small print – 2400×1600 pixels
Medium print – 3240×2160 pixels
Large print – 3840×2560 pixels
Small poster – 2500×3500 pixels
Medium poster – 3500×5000 pixels
Large poster – 5000×7100 pixels
T-Shirts – 2400×3200 pixels
Secondly, Red Bubble uses the usual photographic aspect ratio of 3:2 (excluding posters and T-shirts), and digital cameras generally use the 4:3 ratio, unless there’s a setting for 3:2. Check if your camera has this setting if you would like to save having to crop your photos. The advantage of keeping the 4:3 ratio setting is you can choose where to crop if needed later.
If you have 4:3 ratio images there’s going to be a mismatch:
The 4:3 image must be cropped to 3:2 using image editing software; such as Gimp or Photoshop, that lets you choose an aspect ratio when using the selection tool.
After cropping you may have to resample the image to increase the pixels. Resampling is increasing an image’s pixels by using a mathematical algorithm to ‘guess’ where to put the new pixels and not degrade the image quality. There’s a limit to how much a given image can be resampled, and depends on the quality of the resampling algorithm used.
Photoshop’s Bicubic resampler is considered the best and can achieve 200% plus increases without noticeable degradation.
Cards: 1300×900 pixels
This equals 1.2mp, so as long as you set your camera to at least 2mp you will have enough pixels.
Small prints: 2400×1600
This equals 3.8mp, but due to the aspect ratio mismatch problem and the resulting cropping of the camera images, 4mp is not enough, so set your camera to 5mp if available, or 4mp as a second best option.
Medium prints: 3240×2160 pixels
This equals 7mp, set your camera to 8mp if available, or 7mp as a second best option.
Large prints: 3840×2560 pixels
This equals 10mp, set your camera to 11mp if available, or 10mp as a second best option.
Small poster: 2500×3500 pixels
This equals 9mp, set your camera to 10mp if available, or 9mp as a second best option.
Medium poster: 3500×5000 pixels
This equals 18mp which isn’t widely available as a camera resolution so some sort of interpolation (resampling) will be necessary. DSLRs have larger sensor chips which should allow greater enlargements than a compact camera’s small (typically 1cm square) sensor chip.
Large poster: 5000×7100 pixels
This equals 36mp. See above.
T-shirts: 2400×3200 pixels
This equals 7.7mp, set your camera to 8mp if available, or 7mp as a second best option.
If you only have a 4mp camera but want to upload images for larger than the cards and small prints don’t fret, as mentioned above, resampling with a good algorithm could get an acceptable image for the medium print size at least.
Other options:
Prints: photo collage made up of 4mp images
T-shirts/Prints: create art from scratch on computer at desired size
T-shirts/Prints: Using filters to transform photos into graphics allows for more resampling increase without as much degradation. Eg. posterize, half tone screen, and mosaic to name a few.
peter
,
8 months ago
Great post davoid! Cheers.
Lisadee, 8 months ago
Amazing Davoid – you could charge for this info! Thanks.
davoid, 8 months ago
Thanks Peter & Lisa. Hopefully it will help fellow bubblers understand a complex subject.
I’m adding things as I think more about it, so check back if you are interested.
Patricia L. Ba..., 8 months ago
Great info. Thanks.
mick8585, 8 months ago
THanks Davoid. Straight to my faves.
davoid, 8 months ago
You’re welcome. Hope it helps.
John Hurle, 8 months ago
Thanks Davoid
Andy2302, 7 months ago
Thanks, Davoid. This is exactly what I needed to know. Especially the aspect ratio.
How good is PaintShop Pro X at Re-sampling? Is 125% too much?
Andy P
davoid, 7 months ago
No worries Andy.
re: Paint Shop Pro X’s resampling algorithm
I had a look online but couldn’t find the algorithm used. Check the image size menu options. Bicubic is the best.
re: 125% too much?
Percentages can be a little confusing with resampling.
Eg. an image scanned at 100% is the same size as the original
An image resized via image transform at 100% is also the same size, ie. no change.
So 125% is really a 25% increase which is a relatively small increase and should mean little degradation of the increased image.
If you resample an image via the resize menu and change the pixels per inch from 100 to 200 this actually quadruples the size of the image: double the width and double the height. Most people would think: ‘I’ve increase the pixels per inch by 100% so the image will be 100% larger. ’ They have increased the pixels per inch by 100% but as i said it quadruples the image ie. 400% increase.
So getting back to your 125%. If you mean increase your image by 225% ie. a 10cm by 10cm image becomes a 22.5×22.5 cm image, then this still should be OK, re: image degradation due to resampling. It depends on each image, the fine detail present, and the algorithm used, as to how much resampling is possible.
The more the image is increased the softer it tends to get, so often sharpening (ie Unsharp Mask in Photoshop) is used to tighten the image back a lttle.
I hope this makes things clearer.
Andy2302, 7 months ago
Thanks, Davoid. I’ll have to check “Bicubic” out.
For the re-sampling % I was looking at the original size, say 2800, and making it 3500. to come up with 125%. I’m a rookie at this, lol. I’m only used to re-sizing down for web posting ~ not up.
Yes, I unsharp mask a bit on re-sized pics.
davoid, 7 months ago
No worries Andy.
Sounds like you have a fair idea of what your doing. I probably gave you too much info…
Going from 2800 pixels to 3500 pixels should produce good results as its not much of an increase.
re: Bicubic
I noticed that Photoshop CS2 has added:
Bicubic softer
Bicubic sharper
to the options…
Phillip Moore, 7 months ago
I’m pretty new to this as well and I use CS3’s bicubic resampler, and I have started to uncheck the resample box when adding ppi. Thanks for your advice. I still resample up some, but am more careful about it than I was.
Steve Berry, 7 months ago
Hi Daviod,
Do you know how i can resize for a t-shirt in photoshop 7 that will be big enough in redbubble( I have tried but can’t get it to work) Any suggestions? Thanks. Johanna
Nancy Fischer, 7 months ago
Bless your heart!
davoid, 7 months ago
Thanks Nanmarie.
Stephen Mitchell, 6 months ago
Great piece. Easy read.
Interesting question: ‘If a 6in x 4in photo only needs 2 mega pixels why are camera sensor resolutions steadily increasing each year?’ While your answer is valid about camera-manufactures fooling the general public about the importance of megapixels, more photographic-artists want to print at 12×8, or even banner-size , without losing resolution. That’s where the 10.1megapixel Canon 40d is beneficial. But still too damn expensive!
Another option for cropping without losing any quality: IRFANVIEW , its a freeware program, and very easy to learn.
grouchyolhippie, 6 months ago
One more element needs to be considered in the quality of finished images that is not mentioned here at all, and that is the degradation that occurs when using the .jpeg format for saving images.
The first problem with using the jpeg format to save images, is that it is not lossless.
In other words, each time a jpeg is saved from another jpeg it compresses it again,
and in the process literally loses some of the quality of the original image that it is being saved from.
The loss of quality directly corrosponds to:
1. The amount, or percentage, of compression being applied to the image being saved. The higher the compression, or percentage, the more of the quality of the original image that will be lost. Over-compression will seriously affect anti-aliasing and gradient characteristics of the original image.
2. The number of times that the image has been previously saved in the jpeg format. As an example, When the image to be saved was already in the jpeg format to begin with, saving it again as another jpeg formatted image increases the loss of image quality yet another time, as it is being compressed from an image that has already been previously compressed from the original image. This is often reffered to as the number of ‘generations’, and in the example just given, the resulting image file saved would be a second generation jpeg. The higher the number of generations from the original image, the lower the image quality becomes.
To get around this problem with the jpeg format, I recommend the following tips…
First, always save your original photo or artwork as a png (Portable Network Graphics)
formatted file from the start. Do all your editing, such as color corrections and cropping, on this png file till you get the final image you wish to submit. The reason for this is that the png type format is a completely lossless format, and will remain the same quality irregardless of the number of times it is being saved.
Second, if your final image must be in the jpeg format, then be sure to use the manual compression settings available in both Photoshop and PaintShopPro (all versions of either image editing product offer this). Manually adjust the amount of compression for the jpeg starting with the smallest amount of compression, or percentage, and increase it slowly upwards until the finished file size meets the file size requirements asked for in your submission.
I really hate to see people working so painstakingly to preserve the image quality of their work, only to have all that work tossed out the window by improperly saving their image using the jpeg format.
I hope this will help all of you in getting the best image possible submitted for both display and printing of your artwork.
Peace and Love,
Grouchy ;)
Wulfrunnut in reply to grouchyolhippie’s comment, 5 months ago
Interesting Grouchy,
What is the trade-off between gradually increasing the amount of compression in a jpeg save as you suggest or, alternatively, gradually reducing the resolution (ie pixel count) but saving with minimum compression to achieve the same file size.
Always been worried about degrading my pics!
Also, thanks Davoid. Hadn’t realised RedBubble use 3:2 ratio rather than 4:3. Alarmed to realise my carefully prepared artwork is going to be randomly cropped for any potential purchasers.
cheers Steve
grouchyolhippie, 5 months ago
Hey Steve,
I have found that it is best to work on your artwork in an completely uncompressed format until you have your final image. Resizing your work to a desired pixel count should also be done on your uncompressed work file, as usually blurring and/or sharpening of the image after resizing has much better results in the process quality.
Once you have your work finalized, I would recommend using the PNG format to produced your compressed version of your work. Use the optimized palette option when saving your png file, as this will result in an incredibly reduced file size without any noticeable reduction in the quality of the image. After much experimentation over the years, I find that using this method will give you the best image quality to file size ratio.
Using PNG format also gives you the added benefit of background and alpha channel transparency that other formats do not support. You may have noticed that this is also
a required format for the t-shirts here at RedBubble.
If you must save the finished work in a .jpg format, then be sure to use the option on your image editing software that allows you to view the selected compression in a ‘before and after’ preview window. This will let you see the effect of your chosen amount of compression in real-time before you save the file.
Last, but surely not least… NEVER, and I mean NEVER!... save a jpg file from another jpg file, as each resulting compression will extremely degrade the quality of your image.
The problem with standard jpg compression is that it is ‘lossy’, meaning that it literally throws away a great amount of the image data when the image is compressed. The higher the compression, the more of the image data that is ‘thrown away’. So when you save from a previously compressed jpg, you are in throwing away more of the image data that has already had image data thrown away on the previous compression. You can see the result of this all over the web, as folks pass around jpg files, resaving them as jpg again, pass them along again… vicious cycle… and each time the image quality gets shredded further.
There is a ‘lossless’ jpg format, but you will find that it is very rarely supported by anybody, and I would just stay clear of it all together, as I personally believe it is
already becoming an artifact of progress… kinda like the old Edsel car.
Hope this will help you in finding the best balance between file size and quality!
Peace,
Grouchy
William Hallatt, 5 months ago
Thank you, thank you, thank you….thankyouthankyouthankyou :D
davoid, 5 months ago
Glad I could help William.
bev langby, 4 months ago
Hi David i havent read all of this yet but know it will help me understand this area that alludes me , im just looking for a program to increase the size of my pic seems i havent been doing them at the right size for printing , i hate the techie side of the puter , lol just want to paint ,but its all caught up with me , it does in the end so they say lol
cheers and thanks Bev
bari titen, 3 months ago
thank you for your info on this…i tend not to have any patience when someone is standing over my shoulder trying to explain how all this works…this is easier…thanx!
davoid in reply to bev langby’s comment, 2 months ago
Hopefully it’s understandable. I tried to make it clear and simple for someone not familiar with this confusing subject.
If you are looking for software to increase the amount of pixels in an image, Photoshop would be my recommendation. I use the Bicubic option.
davoid in reply to bari titen’s comment, 2 months ago
Glad to help. I hope it sheds some light.
silencieux, 2 months ago
MERCI !
bchrisdesigns, 2 months ago
Wow, such great information here!
davoid in reply to silencieux’s comment, 2 months ago
vous êtes bienvenu.
davoid in reply to bchrisdesigns’s comment, 2 months ago
Glad you like it.
Flegge, 2 months ago
do you have any idea why som of my t-shirts end up a little grey when i upload them?
but only the ones with many different colours
davoid in reply to Flegge’s comment, 2 months ago
No idea, sorry.