How high the high resolution can be?

I have canon 20D.
I use photoshop CS v8.
I am trying out Genuine Fractals.
Now I can increase resolution and make a 40x25 image with 350 resolution - looks nice too.
But pixel dimensions is 343.5M. Is this too huge??
I am working on images for dancers who eventually want to make posters out of them.
I am still learning photography, so, please be patient with the questions I have ...

For poster size, you could lower your dpi to 250.
Over 300 is not very usefull. Even less if you upscale your image.
It will lower the image size.

Might want to consider what you have done. The pixel dimensions of a 20d image would be something like 3504 X 2336. That would give you a little over 8,000,000 pixels. In order to get a print 40 X 25 at 350 ppi, you would need an image 40X350 X 25X350. That's 122,500,000 pixels, that is roughly a 15x increase. In other words, only one pixel in 15 is from your original image. Everything else was "made up" by your software. Assuming you had to crop the file to get those dimensions, it would be even worse. Now consider whether your eyes can distinguish the difference in size between a pixel measuring 1/180" and one measuring 1/350" from several feet away. If you re-sampled to 40X25 at 180 ppi, then the resulting file would be around 32,400,000 pixels, only around a 4x increase. It's your call, but I know which way I would go. I used 180 ppi as an example, but anything between 150 and 200 is probably going to give you a better print that "made up" pixels at the higher resolution.

For a 40 inch poster you can safely put 30 pixels per inch, and nobody will see the difference.

30 dpi?

Or did you mean 300? 30 dpi might be ok for a billboard, but not a 40x25 poster.

Not sure if this helps but I recently made up a three sided corroplast sign for a client to be used at a trade show.
Never really did this type of signage so had to rely on the people who did the work and application.
The original file I got was a 24meg 4x6 tiff file, resolution 500dpi. The final size I needed to end up with was 36" wide by 67" high.
So I did what I thought was the correct method by upping the size to my needs at 300dpi in PS. Long story short I ended up with an 800+meg file.
What I found out was they just used the original file as is and plugged in the final size. The resolution was a staggering 50-60dpi???
They informed me it would be fine and sure enough the final result was nothing short of spectacular. This wasn't viewed from a distance away but up close by their booth.

Hellashot wrote in post #2607113
Or did you mean 300? 30 dpi might be ok for a billboard, but not a 40x25 poster.

No, I mean what I say.

For a billboard, one pixel is composed of about 3 golfballs.

All digital printers have a native resolution. Epson printers use 360 dots per inch and Canon and HP printers use 300 dots per inch. If you don't use Genuine Fractals or some other extrapolating program to resize to the printers's native resolution, the printer software will do it for you. Generally you can expect better results if you either do it all under your control or all using the RIP program that most professional print shops have specifically for their printer.

You can send 30 pixels per inch to the printer, but the printer software has to interpolate it to its native resolution in order to print. Don't confuse native resolution of the printer with dots per inch ink dispersion. Each pixel is made up of many dots in order to get the correct colour and shade.

DocFrankenstein wrote in post #2607222
No, I mean what I say.

For a billboard, one pixel is composed of about 3 golfballs.

yup, quite true... the larger the image, the further back most people will stand to look at it. lets assume that you can somehow see a difference above 600dpi @ 4"x6" (you hold it close enough to blind yourself). to get a similar looking image (same amount of eye space is covered) @ 20"x30", you need the same density per area of eye covered (means same number of pixels). that turns out to be 120dpi... hey, that number seems familiar... maybe because that's the number of pixels in a 20d... so you can easily get away with that without any resizing, rather just changing dpi settings... if you properly resize (overshoot using cubic/cubic soft+fix with cubic sharp seems to work decently well), you can get slightly better results, but it will likely increase the noise too (should come out to look more like grain if properly done, grain looks nice)

Man, I don't know what printers you guys use, but when I send a job to mine, be it 200 or 300 ppi, it prints at that resolution. Where did this idea that your printer interpolates to some "native" resolution originate? Pretty easy to test, just take a small crop, say 400 X 500 pixels, set the ppi to something like 20 or 30 and print it at full size. Do those pixels look like they are 1/300" or 1/200"? More like 1/20" or 1/30" to me. "native resolution" may have some meaning when referring to an offset press that has a fixed line screen, but inkjets just don't work that way...

I would not want a 30 dpi 40 x 25 inch poster.

Hellashot wrote in post #2608106
I would not want a 30 dpi 40 x 25 inch poster.

That's what you're getting either way. Interpolation doesn't do anything... if you're far ehough, your eyes will interpolate the poster in a nice analogue fashion.

Bodog wrote in post #2607666
Man, I don't know what printers you guys use, but when I send a job to mine, be it 200 or 300 ppi, it prints at that resolution. Where did this idea that your printer interpolates to some "native" resolution originate? Pretty easy to test, just take a small crop, say 400 X 500 pixels, set the ppi to something like 20 or 30 and print it at full size. Do those pixels look like they are 1/300" or 1/200"? More like 1/20" or 1/30" to me. "native resolution" may have some meaning when referring to an offset press that has a fixed line screen, but inkjets just don't work that way...

Have you tried this on your own printer? On my printer if I crop a 100 x 80 pixel sample with some fine detail from an image, resize the image size to 10"x 8" without interpolation and then look at it page size on my monitor the pixels are very sharply defined. If I then print that image on my Epson inkjet printer I will still see jaggies on diagonal lines and horizontal stripes of colour, but the individual pixels are no longer sharply defined. This is because the printer software has attempted to interpolate the image to the native resolution of the printer, but this was a hopeless task and it only managed to blur the boundries between similar pixels. If the printer had not made any attempt at interpolation, the pixels would have been as sharply defined as they were displayed on the screen.

If I wanted the pixels sharply defined I could interpolate the 100x80 pixel image to 10" x 8", 360 pixels per inch using an interpolation method that just enlarges each pixel rather than bicubic which attempts to interpolate what happens between pixels. The pixels will be sharply defined on my screen and on the print. This is because the printer driver received data at its native 360 dpi resolution and has not had to attempt to do any interpolation. I imagine I could configure my printer so that it will not interpolate, but this is certainly not the default setting and I have never bothered to try to figure out how to change it.

For a long time I assumed that what I printed was what I got, but I read about native resolution of printers, did not believe it, and performed this test to verify it for myself. I encourage you to do a similar test for yourself.

Avoiding printer based interpolation is one of the reasons you can get better results if you use a program like Qimage or a RIP program that sends data to the printer at its native resolution.

Scott, perhaps I don't have a good understanding of interpolation. I take it to mean adding additional data, based on what is already there. If you are saying that just enlarging pixels is a type of interpolation, then we agree on that. That's all the printer does, expand or contract the size to fit the number pixels into the specified dimensions. I also wonder why you believe sharply defined pixels are the ideal. I would think that ideally they would all blend into the other seamlessly. It would seem to me that the delineation is just interpolation errors from the original processing. And yes, I've tried the test I suggested in my previous post. Started somewhere around 100 and went all the way to one, stepping 10 ppi each time. The only obviousdifference was that the pixels kept getting bigger. Viewed from an appropriate distance the perceived detail did not change. I would be interested in reading the information re: native resolution you mentioned. The only thing I've been able to find is opinions. Certainly none of the printer mfrs. mention anything about it. You would think that they would want their customers to get the best possible prints by furnishing that information. My opinion is that this whole idea is another myth like requiring 72 dpi screen resolution and 300 dpi print resolution. But I'm willing to be convinced otherwise.

I can't find it immediately, but there was a very convincing article on the net about the print quality benefit in letting the photo processor enlarge, instead of up-sampling in your software. Perhaps someone can jump in here with reference to that article.