Explain some numbers please???

[erik_argotti]

Ok, I am relatively new to the whole digital thing, and i have a few

questions. i am getting a new scanner, so i have been reading up on

all the terminology, so words sound more familiar when i finally

start scanning.Also so I know what i am doing.

From my understanding to make a good print without interpolation, say

8x10, you want to have an output of 300dpi. calculating this out, it

comes to 2400x3000 d.p.i.. this is 7.2 mega pixels. so if most of the

digital cameras on the market today are 6 megapixels, does this mean

that these cameras are only capable of producing a good quality

(without interpolation)8in.x8in.(5.8 megapixels)?

i don't understand how this could be true, because i have heard that

these cameras are capable of producing quality 16x24inch prints. What

am i missing here? Are these 16x24 prints interpolated? And of not

that great of quality. do the words on the camera (6 megapixels),

mean something else that I don't understand?

Can someone please, make this clear to me.

Thank you

ERik

[jon_austin]

The short answer is that dpi (dots per inch) are not equivalent to pixels. A single pixel in a printed image may be represented by many dots in the printing process.

 

I've read a wide range of opinions about how many pixels per inch (ppi) are required for a quality, realistic photographic print. The low end seems to be around 150 ppi, however, I've made some excellent 13x19 prints from 3.1 mp images (2160 x 1440), which works out to about 110 ppi.

 

The quality of the output depends a great deal on the printer and its drivers: how well it interpolates image data into the tiny droplets of ink it spits onto the paper. And any printer, combined with its paper and ink, comprises a "system;" you'll see lots of discussions in the Digital Darkroom forum about which inks and papers work best with which printers.

 

Finally, consider the distance from which the print will be viewed. Common practice (and common sense) dictates that larger prints will be viewed from a greater distance than smaller prints. The pixels in billboard-sized photographs may be the size of Post-It notes, but the images look fine from the intended viewing distance (hundreds of feet away). But just as "pixel peepers" (thanks to Michael Reichmann) and "measurbators" (thanks to ??) are fond of viewing their images at the pixel level, I'm sure there are plenty of folks looking at 8 x 10 and larger prints under a loupe, and bitching about any visible pixelation they may find.

 

HTH.

[ellis_vener_photography]

That was a good answer. Another part ofthe equation is that an image made

directly with a digital camera is much superior in image quality to an image

made on film and then scanned.

[Ed_Ingold]

It's a mistake to play with numbers; the proof is in the results. I presume you have seen a print from a high-end DSLR, and merely wonder why it looks so good.

 

The large-sized sensor in a DSLR, for example, has very little bleed from one pixel to another. Therefore the acutance (contrast) is very high compared to film, even though the ultimate resolution is lower. The sensor, combined with first-rate glass, can produce very high quality images.

 

Certain details are rendered clearly in a way that seem unlikely. In a previous thread, someone asked if a 6Mp DSLR could show a guitar string (~0.003") in a full-length portrait. The response (with examples) shows this to be the case, even though the string is theoretically less than one pixel wide!

 

Interpolation serves to render the angular nature of pixels invisible, even though the resolution is unchanged. With interpolation, an image from a 6Mp DSLR produces an excellent 11x14 print, which is the largest size you are likely to observe at an arm's length. Since 16x24 prints are usually observed from 3-4' or so, the same image can be enlarged to that extent as well, or even to billboard-size.

[mark u]

I think Ellis over-generalises in his enthusiasm for digital! The quality of the film and scanner (or enlarger/darkroom process)and the quality of the digital camera and image processing software/technique can allow either to be apparently "better".

[erik_argotti]

sorry, I am really confusing the two (ppi and dpi), but now I understand them more. So if I just changed my original question a little bit, I might be able to get the answer that I am looking for.

If i am looking to make and image of 11in.x 17in, at final output resolution of 300 ppi, The total # if pixels is 3300x5100.

from this i can multiply these two to figure out and i have 16.8 mega pixels. Correct? why do i need soo many, when most cameras are only shooting at 6 megapixels?

[david_walker1]

Erik,

 

The short answer is that you don't need 300ppi for a good print, especially a large one that will be viewed from normal distances. You can either deliver a lower resolution file to the printer and let the driver interpolate or you can interplotate yourself using Photoshop or any other of a number of programs.

[m_.]

Erik:

 

Another short answer is that a 6 MP digital camera will give you a file size of about 18MB. If you print this file at 300 dpi, it will allow you to print to size up to 12x16 in (3600 x 4800 = 17.3 MB). If you are using a 6 MP DSLR (12-bit color depth), the file size is about 28MB (I believe) which will allow you to print even bigger size.