Image Resampling

[neonblitz]

I am slighly confused about resampling my G1 images in Photoshop. For web design I have know problems as 72 dpi is standard.

However for print media, I need to bump up the dpi to 300. Whats the proper way to do this? Do you reduce image size while increasing dpi, this seems obvious, but by how much?

Is it as simple as choosing my output print size and then increasing dpi to 300?

I am shooting all my G1 photos at maximum resolution, btw

[Del K]

First, consider that the number of pixels in a shot is defined by your shooting mode, for example 2048x1536. If you preserve your originals, you will always have this to fall back on.
Second, dpi affects how the image is displayed and how big (height by width) it is. Most screens have 72 pixels per inch. So, you use 72dpi for web display. Dividing 72 into the pixel size gives you the natural physical size for onscreen display.
If you print, you usually use a large dpi, not necessarily 300. It depends on your printer, which will interpolate.
Here is an example. You take a picture at 800x600. If your printer works well at 200dpi and you use that setting, your print will be 4x3 in inches. (You still have 800x600 in pixels.) Now what if you want to print it at 8x6? You have to resample, and your software interpolates and fills in. After resampling, the image has 1600x1200 pixels, some of what the software created. If you take this too far, the image will likely degrade. This is why megapixel size of the sensor affects the size print you can make with high quality.
Finding the best dpi for printing is a matter of experimentation, depending on your printer and paper. I get excellent results for non-professional use at 180dpi on my HP880 printer, if I begin with enough information (pixels) in the image. 8x10 from a Canon RAW or 2048x1536 jpg looks very good. 8x10 from a 640x480 might not, due to too much interpolation in the software. You just need to do the math as you plan your prints.

Del K

neonblitz wrote:
I am slighly confused about resampling my G1 images in Photoshop. For web design I have know problems as 72 dpi is standard.

However for print media, I need to bump up the dpi to 300. Whats the proper way to do this? Do you reduce image size while increasing dpi, this seems obvious, but by how much?

Is it as simple as choosing my output print size and then increasing dpi to 300?

I am shooting all my G1 photos at maximum resolution, btw

[mcaldwell]

As for checking / unchecking "resample", the point is that you do not want to specify a dpi setting but allow the software to do it for you. A RAW image will come out at approximately 180 dpi for an image size of just over 8.5" x 11"; do the math, just divide the pixels in each dimension by the image size in inches. But to make your print a different print size you want to go to the lower part of the dialog box; that is, change the print, not the image. If you leave "resample" checked, the dpi does not change, so you are essentially throwing away pixel data. If instead you uncheck it, then change the image size (I use full sheets cut into quarters, so change the image to 5.5 in the long direction and it comes to 4.125 in the other), PS will recalculate the dpi for you; again, it's just math. Then recheck it and you're done.

[neonblitz]

I am still foggy.

I start with 2048 x 1536 @72 dpi

Now I want to resize this to bring into Quark. And then eventually this will go out of house to a printer to be done at 1200 dpi.

I have talked to graphic designer friends who tell me to bring the graphics into Quark at 300 dpi. So that means I have to convert my G1 photos which are 72 dpi

If I let photoshop resize with resample turned off I can't control dpi. It seems that if I set dpi to 300, first, then set my document dimensions that I can arrive at the right size and dpi ............right?

[Boyd]

This is a confusing subject. Here is my shot at answering it.

Traditional printing (offset litho) can only print in solid colour, and uses just magenta, yellow, cyan and black inks to print everything. The inks are translucent, so printing them on top of one another makes new colours.

To print a colour photograph it needs to break these down into a series of small dots, of varying size, which when printed on top of one another gives the illusion of a continuous tone colour photograph. (One layer each for the cyan, magenta, yellow and black colours). You can't normally see these dots because they are too small, but they become visible under a glass, or when you get really close to a billboard poster.

High quality printing uses more dots per inch (say 200) whereas low quality newsprint might be much cruder at somewhere around 75 dots per inch. A bog standard print will normally have around 133 dots per inch.

So the first thing you need to find out is how many dots per inch (known in commercial printer circles as Lines Per Inch by the way) you work will be printed at.

Then (and as a really rough guide), the dpi of your digital pic needs to be AT LEAST 50% more than the dpi of your printed result. So if you are printing at 133 lpi offset litho, your image (if it is scaled to size) needs to be around 200 dpi. This extra information is needed because there will be degradation as the pic is converted to a printing plate.

So if we assume that the printer will be making plates of 133 lpi, you need to give him an image of roughly 200 dpi at the size it will be used (your friend who advises 300 dpi may be being a bit on the on the safe side). your 2048 x 1536 @72 dpi image therefore works out at an image sixe of 28.4" x 21.3", at 72 dpi. But you want 200 dpi. So divide 200 by 72 and you get 2.7. Therefore in theory, if you convert your pic to 200 dpi, it will have to be 2.7 times smaller, because that is all the information that is there.

Therefore a 28.4" x 21.3" @ 72dpi pic is equivalent to a 10.5" x 7.8" @ 200 dpi pic.

If this size is big enough then you are fine. Just use the photoshop image size command to convert to the dpi and size you want.

However if this is too small then you will need to ask Photoshop to fill in the gaps for you, effectively guessing (though very cleverly) the missing information.

In fact, you do this in exactly the same way using Photoshop's image size command. But just be aware that Image quality will effectively drop, as PS fills in the missing information.

So in short, yes you are right, but there are some things to think about.

A final note. I am guessing, but the fact that your printer will output at 1200 dpi is probibly a red herring. What is being refered to here is the output resolution of the typesetting machine. It means that the typesetter can make a smallest dot of 1200th of an inch. These tiny dots are clumped together to form the actual dots that form your photograph. In short, don't worry about this bit.

Hmm. Confusing stuff eh?

[Mike K]

I agree this is a very confusing topic as the terms are often misused. Here is a fairly short, clear description with pictures of the PS image size screen for clarity.
http://www.luminous-landscape.com/un...resolution.htm
I use the recommended 240 dpi with good results on an Epson 1270.
Regards, Mike K