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I have made a collage with 5120 x 2880 px size using picasa.

Now I want to order a print of it.

What would be the best option to make a canvas print or a poster print?

I want the best size and picture quality.

And what can be the max size of my print in inches, considering the aspect ratio is intact... I dont want any cropping of my lovely collage. :)

Second question... the collage is composed of many pictures of people on it.

So what would be more suitable, a print on canvas or on poster?

I think a canvas is a cloth and poster is paper? Am I correct? and that canvas would give

a rough finish and poster a glossy one?

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Although the question it's attached to is specifically about inkjet prints, this awesome answer is worth reading: photo.stackexchange.com/questions/1715/… –  mattdm Feb 22 '11 at 17:25
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possible duplicate of Image size vs. print size –  mattdm Feb 22 '11 at 17:29

6 Answers 6

A common recommendation for printing is 300 ppi, so approximately 17x10 in. (16x9 is probably easier to find)

For a poster or canvas you can often go a little lower, into the 200-250 ppi range without too much difference in quality.

If you dropped all the way down to 160 ppi, then you could do a 32 x 18 in. print.

The best path, however, is to ask your printer what they recommend.

In regard to printing on poster vs canvas:

In this situation, it being a collage at an aspect ratio that you want to maintain, I feel that you will have better luck with a poster. A poster will not have the texture that canvas has, but in this situation I think that would be fine. Posters are easier to have printed at a custom size, and you often have semi-gloss or high-gloss options.

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Also, for more explanation on ppi, jrista has a good answer written: photo.stackexchange.com/questions/456/image-size-vs-print-size/… –  chills42 Feb 22 '11 at 17:04
    
... and, depending on the image content, it might survive a good upscaling to be printed at the equivalent of 100 original pixels per inch (or 28.8 by 51.2 inches, which is going to have to be cropped as a canvas -- they aren't going to custom-fabricate stretchers to odd fractions of an inch -- although it can be trimmed to actual size as a poster). Again, that's a good rescaling -- Image->Size in your favorite editor won't cut that brand of mustard. –  user2719 Feb 22 '11 at 17:14
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The caveat to the "bigger prints can do ok with lower pixel density" mantra (which I subscribe to) is that it is based on a single image. When you do a collage people tend to get in closer to see the individual images, negating that benefit. –  cabbey Feb 22 '11 at 17:15
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It is important to note that the PPI you print at should ALWAYS be evenly divisible into the printers native resolution. For Epson, that would be 720ppi, for other ink jet printers, that is usually 600ppi. If you use other technologies, such as dye-sub, you may have an odd native resolution, such as 360.21ppi, and you should always print at that resolution for best results. Printing at oddball PPI, such as 250 or 160, will usually cause the printer to do its own "nearest-neighbor" scaling to normalize the image with its native resolution. (cont...) –  jrista Feb 23 '11 at 18:46
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When printing at non-divisible PPI, the crappy scaling the printer driver (or hardware) does usually results in undesirable artifacts and pixel filtering. You would probably get better results printing at 150ppi (which is evenly divisible into 600ppi for Canon) than 160ppi. Same would be the case if you used 240ppi rather than 250ppi. For Epson printers, 180ppi and 288ppi would be the appropriate resolutions. The "common" value of 300ppi is only valid for printer-native 600ppi, while 360ppi should be used on Epsons. –  jrista Feb 23 '11 at 18:48

If you want a big print, but aren't too fussed about viewing it close up you can print it at 180ppi (pixels per inch), so a quick bit of maths would give you: 28.4 x 16 (which isn't a standard size), so you could get a bigger print and trim it.

This would be fine viewed from a couple of feet away.

However, my best adivice would really be to speak to your printer and find out what they can do for you.

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It should be noted that 180ppi only works for Epson printers, which have a native resolution of 720ppi. You would need to use either 150ppi or 200ppi for other printers, which have a native resolution of 600ppi. Printing at a PPI that does not evenly divide into the native resolution can produce undesirable results. –  jrista Feb 23 '11 at 18:38
    
Yes, I should have been clearer with that. Hence if you aren't printing yourself it is really best to speak to your printer/lab to see what they suggest. –  LC1983 Feb 23 '11 at 19:31

It depends on the results you want. You can have it printed at Fedex Kinkos, 3 feet wide, at 600 dpi. This will look good from a 3-4 feet, but up close you will be able to see imperfections. At 1200 dpi, the pixels in the image will be the limiting factor until you get down around 2 feet wide. These numbers are very rough, and depend on the printer quality, paper, and image content.

Your best bet will be to print a small sample at a few sizes to see what you're really going to get and how well it fits your needs.

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Two drive-by downvotes with no comment. +1 just to spite them. –  Ron Warholic Feb 22 '11 at 19:42
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@Ron — I didn't downvote, but commenting is not required in order to downvote. It's perfectly legitimate to vote down if you feel the answer contains misinformation (which this one basically does, because its use of dpi is either confused or confusing). Voting answers up "just to spite" people is malicious and actively breaks the site. –  mattdm Feb 22 '11 at 20:06
    
@Ron - I didn't downvote either, but I have to concur with @mattdm. This answer is 'self-evident' in its incorrectness and it was probably voted down as a result. On the other hand, your upvoting a clearly incorrect answer is far more 'harmful' to the site than a downvote with no explanation because an upvote promotes bad information... –  Jay Lance Photography Feb 22 '11 at 21:31
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Ok let me try. This comment makes no sense; the dpi values are way wrong. 3 feet wide at 600 pixels per inch means the image would have to be 21,600 pixels wide, but the question's about one that's 5,120 pixels wide. 600 pixels per inch is insane for a poster - 120 to 300 is more normal, and the recommendations I've seen for 180 or 160ppi are probably most appropriate. The statement about seeing imperfections at 600 or 1200 ppi are insane. Firstly the OP's image would only be 8 or 4 inches wide at those figures. This would be more believable if it was something more like 120 and 240 ppi. –  thomasrutter Feb 23 '11 at 3:35
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There is also the confusion between dpi (dots per inch) and ppi (pixels per inch). So maybe this answer just misunderstood something along those lines because it does mention dpi and not ppi, where ppi would be relevant here. –  thomasrutter Feb 23 '11 at 3:42

quick answer: As big as you want.

Useless answer: It depends.

I've printed 2006x2507 at 16x20 and been happy with the results. There were two factors that allowed me to do this. First, the original shot was tack sharp. this meant that I could up-res a bit with minimal loss in quality. And, second, you don't view a 16x20 from 10" away.

Larger prints are generally meant to be viewed from further away so you can get by with less resolution. This is especially true of a canvas print, the maximum resolution of these depends on the weave of the canvas which is probably way less that 300tpi.

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There really is no "optimal" or "best" resolution or surface because there are several competing variables at play here. You'll need to figure out what the values of some of these variables are before deciding what the resolution should be.

Distance

One of the main questions you'll need to answer is "what distance will the image be viewed from?" If you print at very a high resolution, the individual images making up the collage will be smaller and anyone viewing the images will feel the need to get closer so they can actually see them. For example if you print at 600 ppi, you'll have a very sharp image (indeed many people could view it from only 8-10 inches and still be unable to resolve the individual pixels), but the long side of your collage will only be 8.5 inches. Each individual image in the collage will be smaller still. If you hang such a print on the wall, many people wouldn't even really be able to view it well without getting fairly close to the image. Depending on how you want to display the image, this may not be desirable (then again it may be exactly what you want). On the other hand if people will likely want to get within 18 inches or so of the image, you'll probably not want to go below 200 ppi.

Paper Type

Yes, canvas is rough (textured). If you print on a textured surface, you typically lose ability to resolve fine detail because the detail in the texture will interfere with finer detail in the print. This isn't a bad thing, it simply means (as others have already pointed out) that you can afford to drop the resolution down a little bit and get a larger print. It also adds a nice artistic effect that may be desirable.

Eye Acuity

As other's have pointed out, jrista gave a rather detailed answer explaining how the eye's resolving power influences the optimal printing resolution. In jrista's post, he gave the resolving power of the human eye for a person with 20/20 vision as 1 arc-minute. Doing some more research, I discovered a book that made a claim that appeared to suggest the resolving power was more like 1.2 arc-minutes. Reading further, I found a certain R. N. Clark making a claim of 0.7 arc-minutes (i.e. 42 arc-seconds). If you want to play it on the safe side, you could use a visual acuity value of 42 arc-seconds to find out the best resolution. I used the basic formula below to get my estimates above (α is the angle of visual acuity (calculators typically expect this to be in radians), d is the distance from the image (in inches), and r is the resolution (in ppi)):

r = 1 / (2 * d * tan(α/2))
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I did some additional research myself, and one can figure that, worst case, human visual acuity (corrected or natural, obviously uncorrected is a different case) is 20/20, or about 1 arc-minute. Human vision ranges from 20/20 down to 20/10, with 20/16-20/12 being fairly common. If you intend to produce a print that services those of the community with the best eyesight, 20/10 vision, or that 0.7 arc-minute value you stated, should be used. Anything below 0.5 arc-minute, and you are beyond the physical resolving power of the retina, and any additional detail will be unresolvable. –  jrista Feb 23 '11 at 18:42
    
Yeah, there seemed to be a lot of conflicting information, but I would agree that from what I could find, 0.5 arc-minute is a good limit on what the human eye can resolve. –  Benjamin Cutler Feb 23 '11 at 19:40
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I wish my eye had a resolving ability of .7 arc minutes! Mine's more like 5. –  Nick Bedford Feb 24 '11 at 1:10

If you wish to produce a very large print with the maximum amount of detail, so that each image in your collage is as clear and detailed as possible, you should do a couple of things. First, the direct, "native" print size for an image 5120x2880 pixels in size, printed on a Canon or HP ink jet (native 600ppi resolution) at the "standard" 300ppi is:

5120 pixels / 300ppi = 17.06"
2880 pixels / 300ppi = 9.6"

Conversely, on an Epson printer, which use a unique 720ppi native print resolution, the "standard" ppi would be 360, resulting in:

5120 pixels / 360ppi = 14.22"
2880 pixels / 360ppi = 8"

There are a couple things to note here. First, if you have a collage with many images, neither of those two print sizes are very large. Second, if you really want to preserve the detail in your image, you will want to print at a higher resolution. Preferably, you would print at 600ppi for Canon/HP, or 720ppi for Epson. At maximum resolution, your physical print size drops considerably:

5120 pixels / 600ppi = 8.53"
2880 pixels / 600ppi = 4.8"

5120 pixels / 720ppi = 7.11"
2880 pixels / 720ppi = 4""

If you really want to print something poster size, you will need more resolution. It should be noted that when you print on Canvas, the maximum amount of detail that can be maintained will generally be less than if you print on another form of paper. You mentioned that you are printing a collage, which I assume means you merged together multiple images. If the original images are large, you might want to rebuild your collage at a higher native size. For normal papers, I would print at 600/720ppi, and for canvas, I would print 400/480ppi. For a very large print, say 48"x27" (thats four feet wide), you would need an image that was at least this large for normal paper:

48" * 600ppi = 28800 pixels
27" * 600ppi = 16200 pixels

48" * 720ppi = 34560 pixels
27" * 720ppi = 19440 pixels

Or at least this large for canvas paper (note that, while these resolutions do not integrally divide into the native resolutions, they do divide "nicely" at 1.5x):

48" * 400ppi = 19200 pixels
27" * 400ppi = 10800 pixels

48" * 480ppi = 23040 pixels
27" * 480ppi = 12960 pixels

Generally speaking, for such a large print, I would recommend 300/360ppi. In the case of a collage, generated from other detailed images, however, I definitely recommend printing at a higher resolution. Every image in the collage can maintain quite a bit of detail, and while at first glance your viewers may look at the print from several feet and "take it all in", there is a lot more detail that can easily draw your viewers in for a closer look. Detail that is not normally present in a poser-size print of, say, a landscape. If you have a physical print size in mind, you can determine how many pixels the image will need to have to print out correctly at that size, without loss of detail. Simply multiply the size in inches by the PPI you intend to print at, to determine the pixel size of your image:

W" * PPI = Wpixel
H" * PPI = Hpixel

Finally, it is important to know the native resolution of what you will be printing with. In the ink jet world, Epson is unique with their 720ppi native resolution, and 2880x1440 dpi print heads. Canon and HP, two other common printer brands, have a native resolution of 600ppi, with either 2400x1200 dpi print heads, or in the case of some Canon printers (such as their professional PIXMA Pro9500 II 13x19" printer) 4800x2400 dpi. Many dyesub (dye sublimation) printers have oddball native resolutions, which also usually match their dpi, which may be something like 320.19ppi/dpi. In the case of such printers, you will want to scale your image to exactly the correct PPI before printing.

Many professional print shops use commercial HP printers, while fewer use Canon or Epson. Regardless, you should try to find out what printers are used, choose a paper type, and scale or rebuild your image accordingly to get the best results. Make sure you manually scale your image to the right PPI and pixel size, to prevent the printer from scaling with unsightly artifacts.

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