What does dpi mean?


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Oct 4, 2006
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#1
I've read up on entry level DSLR quite a bit since I'm intending to get one. I've narrowed down to 2 choices.D70s and 350D. I know the pros and cons of both, but what i dun understand is the link btw dpi and resolution(MP).350d has 8MP but when i look at the EXIF of the file,it shows 72dpi.D70s has 6.1MP but 300 dpi. Will dpi affect print quality?
 

Megadark

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Feb 29, 2004
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#2
Dots per inch if I am not wrong. Higher dpi means better quality. Dimension of image also affects quality. As a rule of the thumb, I always try to get 300 dpi for every each of photo I print, for eg, a 4R print is 4 inches by 6 iches. So what I'll do is to make sure that the dimensions of the image is at least 1200 pixels by 100 inches. Someone please correct me if I am wrong.
 

zoossh

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Nov 29, 2005
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#4
I've read up on entry level DSLR quite a bit since I'm intending to get one. I've narrowed down to 2 choices.D70s and 350D. I know the pros and cons of both, but what i dun understand is the link btw dpi and resolution(MP).350d has 8MP but when i look at the EXIF of the file,it shows 72dpi.D70s has 6.1MP but 300 dpi. Will dpi affect print quality?
resolution refers to the number of pixel that is captured in 1 frame of picture. more pixel means more data you can take in each frame. the maximum resolution is determined by the sensor, and the various lower resolution is available by the camera jpeg setting, e.g. a 6.1MP (megapixel) camera can shoot a 3.2MP picture.

dpi refers to how far you want to space out your data. you will have the same amt of data determined by the resolution. but you can decide if you want to show/print a bigger picture by spacing out the data e.g. 72dpi for web posting, or you can show/print a smaller picture by packing the data more tightly e.g. 300dpi for printing photoquality.
 

zoossh

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#5
i still feel that there are still certain things that get left out in his discussion.

1. less resolution gives rise to more alteration distortions when you post process.
2. larger resolution files can take up significant more file space.
 

Terence

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Nov 16, 2003
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#6
i still feel that there are still certain things that get left out in his discussion.

1. less resolution gives rise to more alteration distortions when you post process.
2. larger resolution files can take up significant more file space.
What has those 2 points got to do with the definition of dpi and resolution?
 

ortega

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Nov 2, 2004
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#8
dpi = dots per inch

in a 300dpi image

there are 300 pixels (dots) in 1 physical inch
so for a 8 " x 10" image it would be (300 x 8) x (300 x 10)
 

Terence

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Nov 16, 2003
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#9
it affects people's perception of how does resolution matters.
I think the article addresses the issue in a rather engaging and understandable manner without further need to talk about file sizes and post processing. I think that will only serve to confuse people more as it's meant to be a basic tutorial into the understanding of dpi and ppi, and the myth behind the two.
 

lsisaxon

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Nov 29, 2004
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#10
I've read up on entry level DSLR quite a bit since I'm intending to get one. I've narrowed down to 2 choices.D70s and 350D. I know the pros and cons of both, but what i dun understand is the link btw dpi and resolution(MP).350d has 8MP but when i look at the EXIF of the file,it shows 72dpi.D70s has 6.1MP but 300 dpi. Will dpi affect print quality?
On a body, dpi doesn't mean anything. Just that it is encoded at 300dpi to show you the physical size you can print at 300 dpi resolution. So for 3000x2000 at 300 dpi, the print size would be 10" x 6.67". At 72dpi, the same number of pixels can give you a print size of 41.67" x 27.78".

What's more important is the number of pixels. When you change the dpi, you don't change the amount of information, you only change the output size you can achieve. That's why I say it doesn't mean anything in the context of a camera. DPI will only have meaning when you are outputting the image.
 

Apr 12, 2005
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#11
I've read up on entry level DSLR quite a bit since I'm intending to get one. I've narrowed down to 2 choices.D70s and 350D. I know the pros and cons of both, but what i dun understand is the link btw dpi and resolution(MP).350d has 8MP but when i look at the EXIF of the file,it shows 72dpi.D70s has 6.1MP but 300 dpi. Will dpi affect print quality?
The link between DPI (dots per inch) and resolution is that if you have a picture with a resolution of 1800 x 1200,

@300 DPI, your printed picture size will be 6" x 4"
@72 DPI, your printed picture size will be 25" x 16.7"

Therefore at lower DPI, the same picture will be larger in print when compared to higher DPI.

At higher DPI, more dots are squeezed into each Inch and so more is squeezed into the same picture size.

As for the D70s vs 350D, you don't have to worry about the difference in DPI embedded in the EXIF because the DPI can be changed easily (such as in editing software) to a number you want, depending on the picture size you want to print physically.

The important thing is having enough resolution in the picture so that when you use a resonable DPI (such as at least 150 DPI), you are still able to get the picture size you want without losing too much details.

For e.g. if you want a 4R (4" x 6") size picture at 300 DPI, then your picture must have a resolution of 1200 x 1800.

If you want a 8R (8" x 10") picture at 150 DPI, then your picture must have a resolution of 1200 x 1500.

Of course, given a particular photo size, more DPI means more details because of higher resolution picture used but it also means bigger file and sometimes your cropped picture may not have enough resolution to print the size you want at higher DPI.

For e.g. if you want to print S12R (12" x 18") @300 DPI, you would need a picture with resolution of 3,600 x 5,400 = 19.440 Mega Pixels which is beyond most digi cameras currently.

In any case, big photo size are meant to be viewed from much further than arm's length and so a lower DPI is still acceptable. A S12R @150 DPI would need a picture with resolution of 1,800 x 2,700 = 4.86 MP only (1/4 that @300 DPI, half the DPI for length and breadth means 1/2 x 1/2 = 1/4).

For printng at shops, the usual requirement is at least 150 DPI. Try to use higher DPI (200-300) when the picture print size wanted is small (such as 4R) since the required resolution for such a size is not demanding : 4" x 6" @300 DPI = 1200 x 1800 = 2.16 MP only.

E.g. http://www.so-net.com.hk/imagestation/faq/Faq05.php

In any case, beyond a certain DPI for a given photo size meant to be normally viewed from a certain distance, there is a decreasing marginal gains in picture quality. For e.g. most people won't notice significant difference in a 4R picture viewed at the usual distance when printed at 240 DPI or 300 DPI. Almost all people won't notice any difference beyond 300 DPI for 4R when viewed at the usual distance.

In summary :
1) You don't have to bother about the DPI embedded in the pictures of D70s vs 350D because you can change it to any number you want. You only need to take note of the resolution difference in the image sensor.

2) Yes, DPI affects picture quality. Higher = better because the printing dots are much closer to one another. However, higher DPI printing requires higher resolution picture. If the photo size you want is very huge, you may not have enough resolution to print at very high DPI.
 

creampuff

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Jul 11, 2006
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#12
As an aside, coming from the print business, I take what Ken Rockwell says with a grain of salt. More megapixels do matter especially when one needs to output beyond 8 x 10 at 300 dpi (about 7.2 megapixels). Check with professional stock picture libraries and you will find that most won't accept submissions that are at least this size.
 

wanzw

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Aug 15, 2006
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#13
to threadstarter you need not worry about the DPI. if you shoot in raw, you can adjust the DPI as you well.

just enjoy shooting, that's the key!
 

Oct 4, 2006
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#14
hey, many thanks to all those who actually bothered to help me out here. As of now, i'm more settled on the 350D after comparing the iso noise,focusing speed and price with the D70s. Anybody out here had a 350D and had other complaints other than its small and uncomfortable grip?
 

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