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Shine Gonzalvez
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DPI

Postby Shine Gonzalvez » Fri Jan 16, 2015 6:52 pm

can someone explain the following....for printing pics i am using 300dpi...for uploading onto this forum i am using 72 dpi.....
I understand one file sae is bigger than the other..
question
1) why does there appear to be no loss of definition when reducing dpi to 72
2)what would happen if i got pics blown up and printed in 72dpi
3)if i was uploading a pic to Facebook what dpi should i use and why?
4) are any other dpi"s i should be considering and when should i use them and why
thanks in advance
5)does dpi have an affect on distortion of a pic
6) when should i not check the resample image box
thanks in advance
shine

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Re: DPI

Postby TomCofer » Fri Jan 16, 2015 9:51 pm

Nice questions Shine. :)
I can't give you good in-depth and accurate answers here.
I believe you can find some of them if you do a search on "dpi and ppi"

2- It wouldn't look too good. That's why you see a lot of folks upload their photos in low-res, people can't get decent prints from them but they will still look pretty good on screen. The bonus being that they are a smaller file to upload.

3- Might not be the "right" answer but... I have high-speed internet. I believe FB resamples/resizes your photo uploads unless you specifically load or choose "high quality". Ok, and I tend to be a bit lazy. When editing, I tend to do a 4"x 6" 300dpi version for prints of that size. I'll usually just use those for uploading onto FB. I suspect with things picking up here, I will be adding a 72dpi 1700 pixel max width version to the workflow on all future edits.

5- I believe it will only affect the level of detail and pixilation, not distortion.

---
For uploading photos as attachments here, I've been taking those 3x6 300dpi prints and running them thru "Save for web..." option in Photoshop. There, I've been dropping their max width down to 1700 pixels and the quality to about 50-60 percent. I don't know that it's the best way, but it's been working.
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Re: DPI

Postby TomCofer » Sat Jan 17, 2015 1:46 am

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Re: DPI

Postby Onslow » Sat Jan 17, 2015 3:19 am

Shine Gonzalvez wrote:can someone explain the following....for printing pics i am using 300dpi...for uploading onto this forum i am using 72 dpi.....
I understand one file sae is bigger than the other..
question
1) why does there appear to be no loss of definition when reducing dpi to 72
2)what would happen if i got pics blown up and printed in 72dpi
3)if i was uploading a pic to Facebook what dpi should i use and why?
4) are any other dpi"s i should be considering and when should i use them and why
thanks in advance
5)does dpi have an affect on distortion of a pic
6) when should i not check the resample image box
thanks in advance
shine


Shine, the problem is directly related to the destination of the image.

On Displays, they typically have between 72 and 96 pixels per inch. There will be little difference between monitors. The image size will change on screen only if you go from a small to a large monitor or from monitors with different resolutions.For example, I have an NEC 26" with a resolution 1920(w) x 1200(h). My monitor for retouching is my 27" Eizo with a resolution of 2560(w) x 1440(h). At pM we suggest 1700 pixels on the long side. Now, a pixel has a physical size. The information in the pixel though has no physical size. It can be small or large depending on the use. Monitor pixels do have a physical size all different depending on the quality etc. If I have a landscape image at 1700 pixels on the horizontal, we can see that both monitors will display the full image with using scroll bars. If we change the horizontal pixels to 2300, my eizo will still display the full image, however, now my nec won't and will use scroll bars to see the full image. So, monitor size AND the display resolution affect how the image is displayed.

For Print, it depends.. Due to the method of manufacture, differrent printer makers have different native resolutions. Native means what the printer will actually print at regardless of the ppi you send it. For my Epson printer its native resolution is 360ppi. This means that 360 pixels in the image will be printed in one inch. Of course, there will be much more to it than that but this is a simple explanation. If I send it an image at 180 ppi, the printer has to interpolate (makeup essentially) the inbetween bits of data to make it fit the 360 dpi. So, since my printer prints at 360dpi, if I send it an image at 180ppi, it makes up information and resamples it on the fly. It means that the PHYSICAL size of the print will be twice as big. This is because now we are talking actual physical sizes, as opposed to displays. If you send a 72 dpi image to a printer, the printer will print it 5 times larger than if the same image had been sent as 360ppi. 360/5 = 5
As a result of my printers native resolution of 360ppi, I set my Camera Raw and Photoshop/Lightroom prefernces to 360 ppi as well. This then gives me the correct size of the print automatically in the software.

For example, I have an image below. 3744x5616 pixels. At 360 ppi, the print size is 10.4" x 15.6"
DPI 360 Resample.JPG
DPI 360 Resample.JPG (41.24 KiB) Viewed 691 times


However, by NOT resampling and telling PS that the resolution is 300ppi (for Canon and most other printers) we see that the same image not altered apart from changing the ppi willl print out at 12.48" x 18.72"
DPI 300 No Resample.JPG
DPI 300 No Resample.JPG (41.23 KiB) Viewed 691 times


The fun starts when we want to print at my printers native resolution of 360ppi. AND I want it larger than 10"x 15". I could leave it to the printer to do that BUT, I feel that if I resize it myself in PS, I have control over the resizing, rather than the print driver which does a soso job.

In the image below, you can see that I specified a print 22" wide AND specified the 360ppi. The only thing that could then be changed was the number of pizels required. PS as a result worked out that for an image 22" wide, I needed 7920 pixels instead of the original 3744 pixels.
DPI 360 Resample and Resized.JPG
DPI 360 Resample and Resized.JPG (40.98 KiB) Viewed 691 times


In the image below, I specified a print width of 22" and deselected the Resample checkbox. This resulted in the ppi dropping to 170.182 instead of 360. However, this will now be resized by the print driver which is not what I want.
DPI 300 No Resample and Resized.JPG
DPI 300 No Resample and Resized.JPG (40.44 KiB) Viewed 691 times


Q1) why does there appear to be no loss of definition when reducing dpi to 72
For displays, the image is not changed whether it is set to 72 or 300 dpi. It is irrevelant for displays. Only printers.

Q2), what would happen if i got pics blown up and printed in 72dpi?
Depending on how the image is printed you MAY end up with very large images. It does depend on how you set the software up.

Q3) if i was uploading a pic to Facebook what dpi should i use and why?
Again, it's irrevelant. I use 1920 x 1080 in LR for export to FB. On High Def 1920x1080 displays, the picture can be shown full size without FB screwing it up. Others suggest different numbers. I use this and it works.

Q4) are any other dpi"s i should be considering and when should i use them and why.
DPIs are for printing use. Set your Epson to 360 in software. eg, LR, ACR and PS. For Canon and most others, set it to 300. Check your printer documentation to confirm the native resolution.

Q5) does dpi have an affect on distortion of a pic?
No, aspect ratio does, an image 3" wide and 2" tall has an aspect raio of 3:2. If you CHANGE that ratio, it will distort the image. Otherwise, PS and LR are pretty smart on resizing and will keep the aspect ratio of the image.

Q6) when should i not check the resample image box?
When you want to lower the ppi and have the printer do the resizing for you. I don't recomend this.

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Re: DPI

Postby TomCofer » Sat Jan 17, 2015 3:26 am

Thanks John, I knew you or Duck could explain all this better than I could.
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Re: DPI

Postby Duck » Sat Jan 17, 2015 4:48 am

DPI and PPI are two separate animals that, unfortunately, get intertwined with each other in conversation. DPI stands for Dots per Inch and is a term used in printing to specify dot density on paper. PPI stands for Pixels per Inch and refers to the density of light emitting diodes on a monitor screen. Each "dot" represents a "pixel" in a matrix of rows and columns. Mind you that this is a rather elementary description as the mechanics behind PPI and DPI are a bit more complex, but to give you an understanding here is a brief rundown.

Modern desktop monitors used to operate at a resolution of 72 PPI. Higher end monitors were 96 PPI. These were the two most common industry standards. As we move forward we have better monitors, smart phones and tablets that operate at varying resolutions and digital televisions. My Samsung Galaxy Tab has about 150 PPI for example while my desktop monitor can be adjusted for several resolutions though natively it's at 89 PPI. PPI simply translates to the number of pixels that can fit in a single row that measures one inch in length.

DPI is similar but it's related to printer output. Like screens, different printers have different native dot densities. Most common household or office printers run approximately 150 dots per inch but they are getting better with a more common DPI of about 200-220 DPI. Photo printers and higher end commercial printers have a relatively higher DPI however, since most printers use a minimum of four colors to print, the dot patterns vary in order to avoid moire patterning (where dot patterns align causing unusual visual effects). They also don't print one dot row at a time but rather several dot rows in an overlapping pattern. For this printers usually reference the Lines per Inch rather than the Dots per Inch.

Still keeping it simplified let me explain why it's good to understand the relationship between DPI and PPI. Since you now understand that you can infer that one pixel can represent one dot of ink (not really but we'll keep it simple) you can extrapolate how large a print you can create from an image that is X pixels wide. For example, if your image is 1,000 pixels wide and you print it on a printer that is 150 DPI you can expect a printed image size of a little over 6.5 inches wide while maintaining good resolution. However, if you wanted to print it as an 8x10 (10" being the long edge) you would need a minimum of 1,500 pixels across.

Now, here is the kicker... If you have an image that is 1500 pixels wide and you save it at 72dpi from your editing program what will it display at on your monitor? This one's simple, 1500 pixels wide. Now, let' say you saved the same 1500 pixel image at 300dpi, what will it display as on your monitor? Well, since I can't pause for an answer I'll tell you that this is a trick question. It displays at the same size; 1500 pixels wide. Monitors don't care if you saved an image at 72dpi or 300dpi. It just knows it's 1500 pixels wide. Where it matters is when you want to print it.

So let's answer your questions and hopefully you can now understand the answers a bit better.

why does there appear to be no loss of definition when reducing dpi to 72


You will not visually notice any change to the image if all you are doing is changing from 72dpi to 300dpi if the image's pixel size remains the same. The reason being is that you are not changing the physical size of the images as it displays on your monitor.

what would happen if i got pics blown up and printed in 72dpi


I am assuming you are specifying the dpi on file output from a program like Photoshop and also assuming the printer prints natively at 72dpi. In this case it really depends on whether you also changed the physical dimensions from the original image file. For example; if the original image was 3400 pixels wide at 300dpi and in changing it to 72dpi the program altered the file size in order to retain the same dimensions then the simple answer is there would be a visual difference (but other conditions come into play here that are too complex to get into here). As an example I'll explain it like this... At 3400 pixels wide with a resolution of 300dpi you would expect an image size of 11.3 inches wide. When converted to 72dpi the image size would then be 816 pixels wide but would still give you a print size of 11.3 inches wide. The difference would be in resolution. At 300dpi you would have a sharper image with better detail while the 72dpi image, even though it's the same size, would look like a collection of dots across the page. However (and this is why I said there were other conditions) if you looked at that same 72dpi image from 10 feet away you wouldn't notice the dots but rather see the photo better.

if i was uploading a pic to Facebook what dpi should i use and why?


It doesn't matter. What matters is the final width and height in pixels. Hopefully you now understand why. :)

are any other dpi"s i should be considering and when should i use them and why


Yes. When you go to print, you need to understand what resolution you will be printing at and offer up a file that can produce a suitable print at that resolution. For simplicity sake you don't need to calculate the exact resolution for a specific printer, you can let the printer profiles handle that. All you need to have is a file size that is at least the size, or larger, than what you are printing.

does dpi have an affect on distortion of a pic


Yes, but only if the dpi is below the capabilities of a printer. For example; if you take a 1000 pixel wide image that would normally give you a 6.5 inch photo and you decided to print a wall poster. There would be a noticeable softening or pixilation to the image.

when should i not check the resample image box


Resampling is a process a program uses to determine what pixels to keep and what to throw away when it reduces the number of pixels. This is determined by the image content and complex algorithms are used so that the tree in your photo still looks like a tree when reduced. There are a variety of resampling algorithms and there's nothing wrong with playing around with them to see if there are any noticeable differences. Personally I keep it at the default which is (I believe) bycubic.

Hopefully I answered your questions satisfactorily enough. There are many great resources on line if you want to delve deeper into the subject. Just realize that it can be headache inducing. :wink:
"If you didn't learn something new today, you wasted a day."
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Re: DPI

Postby Onslow » Sat Jan 17, 2015 6:00 am

Shine, I hope you see that whilst printing is easy, it can quickly get complicated. As Duck says, DPI and PPI are used interchangeably while they do refer to different things. PPI though is actually used by PS, ACR and LR as can be seen in the printer dialogs I produced. I have had discussions online where people use the wrong terms for the wrong areas and this can quickly lead to heated arguments because of this interuse of terms. Printers can indeed print up to 2880 DPI, My Epson 3880 and 7800 both offer printing at 2880 dpi. As you start drilling down in depth, you see how much common terms are misused. Still, it does make for interesting discussions. :)
So, my advice is, there's a lot of info out there. If you wish to print, welcome to my nightmare. :) For the screen, it's a lot easier.
For a different perspective again have a look at
http://www.photoshopessentials.com/essentials/the-72-ppi-web-resolution-myth/

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Shine Gonzalvez
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Re: DPI

Postby Shine Gonzalvez » Sat Jan 17, 2015 1:35 pm

Onslow/Duck.....this is super super clear...thank you so much....I have been studying this word by word for over 2hrs.(yes I'm at work) and I understand it all...however this will take time to sink in...I often revisit posts like these over 20 times until it becomes basic knowledge....


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