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07-06-2010, 03:40 AM   #1
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Getting High Resolution for Large Canvas Art

I've got the opportunity to post images to a gallery that produces large canvas wall art prints and I'm really keen to do it. They have asked for high res images. When I use my K20 even at the highest setting I get image sizes of 4672 x 3104 pixels at 72 dpi. Is 72 dpi going to be a high enough res for printing large canvases? How can I get a higher res than that without losing quality?
Thanks in advance for much appreciated help

07-06-2010, 04:24 AM   #2
Ira
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It's a math thing, and it depends on the system they're using to print it.

Normally, they will ask you for a file of a particular size--8" by 10" for example, at a specific resolution, like 72 dpi.

So, for example, if you open the file in an image editing program, you may see that it's 20" by 16" at 72dpi, but when you change the dpi to 300, and you DON'T have resample image checked, the image becomes around 5" by 4" but it's now at 300dpi.

You've quadrupled the resolution, and the only way to do this without affecting quality (because you can't "invent" better resolution after the fact), is to reduce the size.

At the very least, the company who prints these should tell you how many pixels they need for width and height, and they can properly convert your file for their software. This kind of large format printing, even though it can show incredible detail, is often done at only 72dpi.

Do you have a link for this place with any specs mentioned?

Last edited by Ira; 07-06-2010 at 08:37 AM.
07-06-2010, 07:25 AM   #3
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QuoteOriginally posted by Ira Quote
It's a math thing, and it depends on the system they're using to print it.

Normally, they will ask you for a file of a particular size--8" by 10" for example, at a specific resolution, like 72 dpi.

So, for example, if you open the file in an image editing program, you may see that it's 20" by 16" at 72dpi, but when you change the dpi to 300, and you DON'T have resample image checked, the image becomes around 5" by 4" but now it's now at 300dpi.

You've quadrupled the resolution, and the only way to do this without affecting quality (because you can't "invent" better resolution after the fact), is to reduce the size.

At the very least, the company who prints these should tell you how many pixels they need for width and height, and they can properly convert your file for their software. This kind of large format printing, even though it can show incredible detail, is often done at only 72dpi.

Do you have a link for this place with any specs mentioned?
Right, I gotcha, thanks. So by making the size larger would be if I checked resample and then it would make the pixels bigger too and go blocky. They don't have any specs on the site, so I'll just have to go back to them and ask but I was trying not to appear too dumb!! Even if I am dumb
07-06-2010, 08:11 AM   #4
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QuoteOriginally posted by menessie Quote
Right, I gotcha, thanks. So by making the size larger would be if I checked resample and then it would make the pixels bigger too and go blocky. They don't have any specs on the site, so I'll just have to go back to them and ask but I was trying not to appear too dumb!! Even if I am dumb
Actually, when you have resample checked, what it does is "guess" at the pixels and add ADDITIONAL ones around each one to make it your new size. A pixel size is always a pixel.

By resampling, you're adding more pixels to the image, and since your image editing program doesn't really know what it's re-rendering, resampling up in size always results in a downgrading of quality, although 10%, even 20% up resizing can be acceptable. However, on your monitor, always view at 100% of image size to see the actual quality. It can look fine viewing at 50%, but once you view at 100%, you'll see the boxiness--and that boxiness will PRINT at any size. Downsize resampling is fine.

The whole business is complicated and not something you should concern yourself with too much just yet. The main thing is when someone asks you to supply a file, ask for their specs, and if it seems suspicious, ask here.

For example, your JPEG images are RGB, put printers (the machines) use CMYK inks. So what happens is depending on both the application the file is printed from (Photoshop, a layout program such as InDesign, etc.), and depending on the printer driver, this software makes the RGB to CMYK conversion for you. However, when I'm printing for myself directly from Photoshop, I always first convert my RGB images to CMYK, and save as a TIFF.

In a similar vein, let's say you have a 20" by 20" image at 72dpi, but I place it/use it in a layout program at only 5" by 5". Normally, I would go to PS and resize it to 5 by 5 without resampling, thus upping my resolution up to 288. Nowadays, most layout programs "fix" these kind of resolution problems on the fly, but it's nice to know the "right" way to prepare your files if they're going to be used for actual, real offset printing, and not just from your home printer.

Your home printer (the machine again) is capable of amazing things at lower resolutions because there's no screen involved, but once you go to an actual printing press, you need that resolution in there because of what the screen dots do to the final product when ink is put to paper.

07-06-2010, 08:51 AM   #5
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Well I have a canvas print at home, from my K-7 (all the pixels inside used) and it is 45 x 67 inch. Razorsharp portrait. I love it. so a lot is possible as long as your pictures are sharp enough.
07-06-2010, 09:28 AM   #6
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QuoteOriginally posted by Ira Quote
Actually, when you have resample checked, what it does is "guess" at the pixels and add ADDITIONAL ones around each one to make it your new size. A pixel size is always a pixel.

By resampling, you're adding more pixels to the image, and since your image editing program doesn't really know what it's re-rendering, resampling up in size always results in a downgrading of quality, although 10%, even 20% up resizing can be acceptable. However, on your monitor, always view at 100% of image size to see the actual quality. It can look fine viewing at 50%, but once you view at 100%, you'll see the boxiness--and that boxiness will PRINT at any size. Downsize resampling is fine.

The whole business is complicated and not something you should concern yourself with too much just yet. The main thing is when someone asks you to supply a file, ask for their specs, and if it seems suspicious, ask here.

For example, your JPEG images are RGB, put printers (the machines) use CMYK inks. So what happens is depending on both the application the file is printed from (Photoshop, a layout program such as InDesign, etc.), and depending on the printer driver, this software makes the RGB to CMYK conversion for you. However, when I'm printing for myself directly from Photoshop, I always first convert my RGB images to CMYK, and save as a TIFF.

In a similar vein, let's say you have a 20" by 20" image at 72dpi, but I place it/use it in a layout program at only 5" by 5". Normally, I would go to PS and resize it to 5 by 5 without resampling, thus upping my resolution up to 288. Nowadays, most layout programs "fix" these kind of resolution problems on the fly, but it's nice to know the "right" way to prepare your files if they're going to be used for actual, real offset printing, and not just from your home printer.

Your home printer (the machine again) is capable of amazing things at lower resolutions because there's no screen involved, but once you go to an actual printing press, you need that resolution in there because of what the screen dots do to the final product when ink is put to paper.
Pixel size changes. It is not always the same size. 72 pixels per inch means that each pixel is 1/72". 300 pixels per inch means each pixel is 1/300".

Why would you change from RGB to CMYK when going to your home machine? The processor in your "little" printer can do a better job than PS can in this instance. It expects an RGB file. Ira I print to high en, high dollar machines and I don't convert to CMYK unless I am going to press with it. That RIP can do a better job. After all it has only one job to do. For one specific machine.

And in some way there is some type of screen involved in every printer. Any printer's print head can be in one of 2 states. It either drops ink or it doesn't. And every paper absorbs ink differently. That's what paper profiles are designed to tell software.

With CS4 they changed the algorithms for resampling also. Upsizing even 200% (twice the size) looks pretty darn good IIRC.

And an 8 x 10 at 72 dpi is going to look mighty ragged. Lowest I'd go is 240 depending on the paper.
07-06-2010, 09:42 AM   #7
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QuoteOriginally posted by menessie Quote
I've got the opportunity to post images to a gallery that produces large canvas wall art prints and I'm really keen to do it. They have asked for high res images. When I use my K20 even at the highest setting I get image sizes of 4672 x 3104 pixels at 72 dpi. Is 72 dpi going to be a high enough res for printing large canvases? How can I get a higher res than that without losing quality?
Thanks in advance for much appreciated help
I recently ordered a canvas print for a customer at a 20x30 inch size. The original cropped image was 4451 pixels and i resized it larger using Lighroom ver 2.6 to 12,000 pixels on the long side, i.e. 30". Thats a 270 percent increase. (my cam is K20)

The canvas company offered a 50% reduction for a second copy, so i ordered 2, one for my own wall. They came out beautiful, customer is happy and i got to take my copy down to my photo club and get excessive compliments

About 8 months back or so, there was a post of these forums somewhere of a guy who printed canvas 32 x 48" i believe, from a K20.

I think the old thumb rule of 20% has been made obsolete by new resizing algorithms and personally don't pay any attention to it anymore.

Recommend you carefully examine your print image on your screen first, and do some test prints to find any dust spots or trash that may have gotten into your scene before you send in the final order.

best wishes,
07-06-2010, 09:49 AM   #8
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I incorrectly described how pixels operate, will concede that point, but I think I still explained the concept of resampling.

I convert to CMYK because I don't have a "little" printer, it's an Epson 5500, and I use RIP software to make it postcript (iProof).

Also, how an inkjet printer operates no way reflects how offset, or web printing, operates. Any image at 72 dpi will print just fine on a home printer. Use that file for offset printing with a 133 screen, or even an 85 screen, and you have what's called total garbage.

07-06-2010, 11:53 AM   #9
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QuoteOriginally posted by menessie Quote
I've got the opportunity to post images to a gallery that produces large canvas wall art prints and I'm really keen to do it. They have asked for high res images.
Have they said what exactly they mean by "high res"? They should be able to give you specific numbers in actual pixels for a given print size. If all they tell is pixels per inch, then you'll have to do the math yourself to figure out how many inches that means you can print with the number of pixels you have.

But in either case, the numbers you care about the actual number of pixels and the print size - *these* are what you will use to determine the actual resolution in dpi. The number arbitrarily stuffed into the EXIF does *not* tell you the actual resolution of a print you make. The camera puts 72 into the EXIF because the EXIF standard dictates that cameras do this, nbot because images taken from digital cameras can only be printed at the particular size that yields 72dpi. The field was originally meant for scanners, not cameras, and scanners have a meaningful dpi figure they can put there, because they know the actual document size. but your camera does not know the print size, so there's no way it can calculate a meaningful dpi figure. The 72 placed in the EXIF is just a placeholder. When you go to print, the *actual* resolution will be number of pixels (dots) divided by numbers of inches - very literally what "dpi" stands for. You can either fiddle weith those numbers in Photoshop if you like, or ignore them completely and it won't make a whit of difference. An image containing X pixels that is printed Y inches is X/Y dpi, period, end of story.

So your image *can* be printed at 72dpi, *if* you happen to print at exactly the size that would make the math work out that way (number of pixels divided by print size = dpi, plain and simple). But you could also print at 300dpi if you printed a lot smaller. Or, you could print even bigger, but at only maybe 10dpi. Pick a print size, and that determines dpi. Not some number stuffed into an EXIF field by a camera that has no idea what size you will print. Remove the EXIF info if it bothers you to see 72 there even though you know you are printing small enough to get much higher resolution than that.

See about a zillion other previous posts for tons more on the whole issue if you still find it confusing (as many people do). It's mathematically quite simple; no different from calculating MPH given a distance in miles and a time in hours. dpi is pixels divided by inches just as surely as MPH is miles divided by hours.

Last edited by Marc Sabatella; 07-06-2010 at 05:22 PM.
07-06-2010, 01:44 PM   #10
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Thanks. guys, for all the replies

Confusing? Certainly is but I think I understand it slightly better and, as several of you suggest, I'll ask for more info from the print company and take it from there. I did find on the web some software called Akvas Magnifier which wasn't prohibitively expensive so might try the 10 day trial to see how it does - unless anyone out there has any experience of it?
07-07-2010, 07:24 AM   #11
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QuoteOriginally posted by Ira Quote
I incorrectly described how pixels operate, will concede that point, but I think I still explained the concept of resampling.

I convert to CMYK because I don't have a "little" printer, it's an Epson 5500, and I use RIP software to make it postcript (iProof).

Also, how an inkjet printer operates no way reflects how offset, or web printing, operates. Any image at 72 dpi will print just fine on a home printer. Use that file for offset printing with a 133 screen, or even an 85 screen, and you have what's called total garbage.
I've got an old epson printer (was a decent one at the time) and also a Dell at home plus the Kodak. 72 dpi looks like, well, 72 dpi crap no matter how you slice it.
07-07-2010, 09:12 AM   #12
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Agreed, 72dpi looks bad printed, at least at the sizes a home printer can print. Your basic home inkjet printer - including the cheapest ones they often give away free with a camera or comuter - can do 300dpi. Well, 300ppi - pixels per inch, not dots - if we need to make that distinction here. You'll often see printers bragging about much higher dpi figures, but they are basically just multiply the 300ppi by the number of ink tanks they use. That is, a pixel is made of several dots of different colored ink. So a 300ppi printer that uses 4 colors of ink might says it 1200dpi, but that's still just 300ppi as far as you image is concerned. This distinction is usually made only for printers, not for digital images per se - so the figures reported in Photoshop or in the EXIF of an image as "dpi" are really "ppi".

Anyhow, yes, 72dpi is terrible for printing. But again, we're talking 72 *actual* dots per inch. If you've got 3000x4500 pixels to start with (nice round numbers that are somewhat close to the actual numbers), a 72dpi print is 3000/72 x 4500/72, which is a little over 40x60". That is, you'd have to make a print that big to get resolution as low as 72dpi from a K20D. Once more, I don't care if the EXIF says 72dpi. A print from the K20D is only 72dpi if you actually print 40x60". If you print at any smaller size, the actual resolution is correspondingly higher, EXIF or no EXIF. If you print 10x15", for example, the actual resolution is 300dpi, because you cannot possibly make 3000 divided by 10 not equal 10. Try it; you'll fail. If you take 3000 pixels (the short dimension of the K20D image) and print them on a piece of canvas that is 10 inches wide, that's 3000/10 = 300dpi. Cannot possibly be anything else.

Now, since you talk about "large" canvas prints, maybe you are talking about something approaching 40x60", and are concerned now that 72dpi will look terrible. It actually won't, because nobody looks at a 40x60" print from as close a distance as they do a 10x15" print, much less a 4x6" print. very large prints are routinely done at very low resolution, if for no reason other than that no one has cameras that could possible produce, say, a billboard at 300dpi.

Last edited by Marc Sabatella; 07-07-2010 at 09:18 AM.
07-07-2010, 07:33 PM   #13
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A couple of years ago, either here or on another website (or possibly both) - benjikan had a number of his photos (from a K20 as I recall) printed to something like 12 feet by 6 feet for a fashion show over in Europe. If I remember he had some samples on his web site at the time, and they looked wonderful. My point is that with good quality file, the printers can figure everything out for you. You job is to capture the original image. I think that will possibly be the more difficult problem here.

QuoteQuote:
When I use my K20 even at the highest setting
This implies your taking JPG images. I would think that RAW would be more appropriate. Do you have a process to take the images, straight on (perpendicular) to the canvas, and what ever mechanical support gear you will need (camera, canvas and lighting)? Lighting (real white) with out capturing any glare from the paintings and without washing out the canvas' colors - white balance, accurate color capture? Size of the paintings versus the distance from the paintings (to capture the entire canvas), with respect to the lens that you are planning to use? On the lens that you are planning on using, do you know what aperture it is sharpest at, with the least distortion across the lens center, edges and corners? How much time are they going to give you with each canvas - and will it be sufficient for your setup and everything? Are they going to provide you with space at their gallery (if not a transport problem - insurance for possible damage)? Have you practiced all of this and have everything down? Done some test runs? If the canvas' are too large, have you considered stitching?

I am thinking out loud here, because I do not know, but possibly a good lens would be the FA 77 or DA70, as this may be similar to portrait photography. Depending on the size of the canvas, you might be able to get everything in a single frame with sufficient resolution and detail and avoid possible lighting problems associated with stitching. Just a thought.

Another idea is to take several images, one with a color chart in the image and one with out, this way you have a color control. You just need to make sure that the only difference between the two images is the color chart. You might also wish to take several images of each canvas with a variety of exposure settings. Maybe bracket for HDR there by stretching the dynamic range - however this would increase your post production work. Just another thought.

Post production process. Do you have your work flow and tools. How are you going to check for accurate color capture?

Unless you have all of this figured out, printing is going to be the least of your problems.


Last edited by interested_observer; 07-07-2010 at 10:00 PM.
07-07-2010, 11:02 PM   #14
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they may be requesting 'high res images' so they don't get web sized images sent. It's amazing what some people with no idea will send and expect that because it looked good on screen (not even full screen) then it should print full page fine. At least these days screens have more pixels and the gap between screen and print has narrowed.

Your K20D files might be more than enough. Do they only accept files from >15MP cameras? I doubt it.
07-14-2010, 01:32 PM   #15
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QuoteOriginally posted by menessie Quote
I get image sizes of 4672 x 3104 pixels at 72 dpi. Is 72 dpi going to be a high enough res for printing large canvases? How can I get a higher res than that without losing quality?
Hey there,

I just struggled through the process of printing my first image for a gallery this weekend.

Are you shooting JPEG or RAW? Because I shoot raw, and when I go into GIMP (I know, I know) my image is 4672 * 3104 at 240 DPI.

I was only hanging an 8 * 12, so I went and increased the DPI to 300. This action is non-destructive so long as you allow the image size to decrease.

In order to get the highest quality prints, they say you should go for a DPI of 300.

In your case a DPI of 300 may be impossible without resampling, but the native resolution of 240 DPI is not bad either.

Ask to speak to whoever does the printing and see what they hav to say about the image size and DPI. Remember - if you have a larger image with a smaller DPI, it will be the same resolution as a smaller image with a larger DPI.

All I know is that a DPI of 72 is only appropriate for web viewing. So have a closer look at your images to ensure your DPI is actually 72. If it is, you are likely shooting at a lower quality JPEG setting, which I would strongly recommend you stop doing with such beautiful equipment .

PS I'm a novice so don't take my word for it, but this is how I understand things to be...
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