ppi (not dpi) for good sized photos to hang on the wall...

arad85

arad85

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Andy
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Hi,

I'm just about to order a set of prints from DSCL. The pictures will be crops of images I've taken from the 5Dmk2 and as Christmas presents for people. Now, I totally understand the dpi (at DSCL it's 300dpi, so if I have a 3000 x 3000 crop that is 10" square without resizing). What I don't understand is what is an acceptable ppi (pixels per inch) to get a good quality print. I.e. if I have 3 arbitrary crops:

3000 x 3000
1500 x 1500
750 x 750

and I want to print them all at 10"x10" at 300dpi, I'll need to provide a file that's 3000x3000 pixels in size. That will lead, in the 3 examples above, to ppi of, 300, 150 and 75 respectively. I understand that it depends how far away you will view the print but what is deemed an "acceptable" ppi for a (say) 12" square print viewed from a few feet (as you would be if you were looking at said photo on the wall).

Thanks
 
I would just upscale all of the images to 3000 x 3000 pixels, and they should come out pretty well when viewed up on a wall.
DCSL are cheap enough anyway, so you might as well just order them and see what they come out like.
 
Thanks.

I was kind of hoping there was an "acceptable" rescale size though. Kind of a rule of thumb...
 
There probably is somewhere. For 55p a print it's worth having a try to see what they come out like.
 
Yes, but I have a few to do and would like to do them only once - it is only 75p per photo, but x10 and you're at £7.50 already.

I'd like to get them as big as possible (within reason). I would know what sizes I could go up to given my original crop if there were some rule of thumb. It might even lead me to choose a different crop or photo in the first place...
 
DPI refers to the resolution of the printer and will not affect image size
 
Dogfish_magnet said:
DPI refers to the resolution of the printer and will not affect image size
Click to expand...

And a printer's dpi wouldn't be 300.

Me thinks there's some confusion here between dpi and ppi.

Edit: Given that you have crops that have pixel dimensions of,

3000 x 3000
1500 x 1500
750 x 750

Then to print at 300 ppi with no interpolation your prints would need to be,

10" x 10"
5" x 5"
2.5" x 2.5"

Though you could probably get away with some interpolation, what's the general rule? 1.5x or something?
 
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<sigh> I understand DPI and PPI ;) What I'm asking is how much can I resize the image by to get acceptable results when printing at, say, 12" x 12" for viewing at a couple of feet from the wall. 10% (making it 110% of the original size and making it 272.7 original ppi), 20% (making it 120% and 250 original ppi), 100% (making it 200% and 150 original ppi)?
 
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arad85 said:
<sigh> I understand DPI and PPI ;) What I'm asking is how much can I resize the image by to get acceptable results when printing at, say, 12" x 12" for viewing at a couple of feet from the wall. 10% (making it 110% of the original size and making it 270 original ppi), 20% (making it 120% and 240 original ppi), 100% (making it 200% and 150 original ppi)?
Click to expand...

I really don't understand this.
You can't make something a different original ppi.
 
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andephotographic said:
Ok, if you understand dpi and ppi then away and print your xmas pressies at 300 dpi then.
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<sigh> again ;) The printer - DSCL - accepts 300dpi (when in fact it is ppi) and does no resizing on them as far as I can see. I don't care what they print them at in terms of real dpi as that's whatever his printer prints in terms of ink dots. It's just that most people use dpi and ppi interchangeably and I was just trying to head off the inevitable discussion over ppi/dpi which I seemed to have failed to do!

All I need to know is how much I can resize images and still get very good results. I know it would be best to print 1:1, but unless I'm extremely lucky, or have rubbish framing, I'll have to resize the images to get a different number of pixels in the image I send to be printed when compared to the crop that I've used. All I'm asking is what is the generally accepted limit for this (like the generally accepted limit for shooting handheld is shutter speed = 1/focal length).
 
andephotographic said:
You can't make something a different original ppi.
Click to expand...
Lets assume I don't do any resizing and I have a 3000x3000 image. If I send it away to a printer and they print it at 300ppi, I'll have a picture 10" x 10". If I want it 11" x 11", I'll have to upsample it to 3300 x 3300 and send it off to be printed. It will come back 11" x 11". What I have done is effectively stretched 3000 pixels into 3300 pixels. That means in each inch of the upsampled image, I have effectively stretched 300 pixels of my original image to be 330 pixels in my new image but still printing at 300 pixels per inch which gives me 272.7 of my original pixels now contributing to an inch of print in the resized pic.
 
Do DSCL only accept 300 ppi (or dpi as they want to call it) files?

Is there any difference in the end result print quality between upsampling your pixel dimensions and reducing your ppi?

Someone on here should know what the acceptable rule of thumb for interpolation/upsampling is.
 
Hi. My take on this is that for a 'standard' size print (10x8 or thereabouts) somewhere between 240 and 300 ppi on the original image is what to aim for. The larger the print, the less ppi you need. If the image is meant to be viewed from a distance (over the fireplace for instance) it doesn't need as many ppi as one you might pick up and look at closely. Think of those large media posters you see, the resolution on them is quite poor when you look closely, but you arent meant to look closely.

I'd suggest you upsample ONLY if you have no other option. Its probably better to reduce the ppi slightly instead.
 
andephotographic said:
Do DSCL only accept 300 ppi (or dpi as they want to call it) files?
Click to expand...
Yes, I believe so:

DSCL said:
All images should be supplied as SRGB JPEG files or with our own profiles at 300 dpi and sized to the correct output size required.
Click to expand...

andephotographic said:
Is there any difference in the end result print quality between upsampling your pixel dimensions and reducing your ppi?
Click to expand...
Basically, something has to do the resize. I'd rather it were me, rather than the printer as I have control over the resizing...
 
Photodiva said:
Hi. My take on this is that for a 'standard' size print (10x8 or thereabouts) somewhere between 240 and 300 ppi on the original image is what to aim for. The larger the print, the less ppi you need. If the image is meant to be viewed from a distance (over the fireplace for instance) it doesn't need as many ppi as one you might pick up and look at closely. Think of those large media posters you see, the resolution on them is quite poor when you look closely, but you arent meant to look closely.
Click to expand...
But what's the rule? There must be a pixel per inch per viewing distance...

Photodiva said:
I'd suggest you upsample ONLY if you have no other option. Its probably better to reduce the ppi slightly instead.
Click to expand...
Either process involves resampling the image for the printer anyway - it's just where it is done... You change dpi (or is that ppi) for the printer, then the printer does the resize. If you do it yourself, you have the control to ensure the most appropriate algorithm is used.
 
I was also trying to work out the answer.
Still grasping, so I sent off some photos to test with on the canvases.

I took a 1800x1800 pixel image, and scaled it to 3600x3600 pixels, giving 12" at 300dpi, printed onto canvas without any problems.
So that would be 150ppi source image. It is possible that 75ppi source would have worked, it depends how far away you are viewing it (I have hung this one above my computer monitor, so it could be 2' viewing distance).
Hope this helps a bit
 
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Interesting information here: http://gadgetopia.com/post/5381

I guess you view a monitor at around 2" usually, so so long as your source image >70ppi, then you are probably ok for many situations, apart from gallery hanging.

The resolution of an image is highly dependant on the dpi of the output device for which it is intended. For example, some typical numbers for output devices:

monitors ~72dpi magazines ~300dpi billboards ~45dpi
Click to expand...
 
arad85 said:
But what's the rule? There must be a pixel per inch per viewing distance...
.
Click to expand...

You ain't gonna like it, to calculate it, you would use the following: (from http://clarkvision.com/imagedetail/eye-resolution.html)

The acuity of 1.7 corresponds to 0.59 arc minute PER LINE PAIR
....

Thus, one needs two pixels per line pair, and that means pixel spacing of 0.3 arc-minute!

Blackwell (1046) derived the eye's resolution, which he called the critical visual angle as a function of brightness and contrast. In bright light (e.g. typical office light to full sunlight), the critical visual angle is 0.7 arc-minute (see Clark, 1990, for additional analysis of the Blackwell data). The number above, 0.7 arc-minute, corresponds to the resolution of a spot as non-point source. Again you need two pixels to say it is not a point, thus the pixels must be 0.35 arc-minute (or smaller) at the limit of visual acuity, in close agreement with the line pairs. Line pairs are easier to detect than spots, so this too is consistent, but is closer than I thought it would be.

In modern studies, like Curcio et al. (1990), acuity is measured in cycles per degree. Curcio et al. derived 77 cycles per degree, or 0.78 arc-minute/cycle. Again, you need an minimum of 2 pixels to define a cycle, so the pixel spacing is 0.78/2 = 0.39 arc-minute, close to the above numbers.
Click to expand...
 
Great stuff CP... Some reading to do there - thanks :)
 
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