Author Topic: 300 pixels?  (Read 3798 times)

Offline Chris J

  • obsessive maniac
  • ***
  • Posts: 358
  • BEC - Everything to Excess!!
    • Casj.co.uk
300 pixels?
« on: June 30, 2009, 08:11:31 pm »
Hi all,

Have been told that I need to provide an image of quality of 300 pixels. My digital camera is a 6.2 mega pixel job.

From doing some googling my guess is that they mean 300 pixles per inch as a resolution? and that a 6 mega pixel camera can do this.... for a certain size of image. E.g. if I increase the pixel resolution in the picture in photoshop then I'll decrease the size (dimensions) of the image? (can I only do it once I've taken the picture - not a setting on the camera?)

So does my question back to the person who told me that I need 300 pixels need to be - for what size of image?

Sorry if that doesn't make sense - not really into this camera stuff.

cheers,
Chris 

Offline Steve Sharp

  • Axbridge Caving Group - Chelsea Spelaeological Society
  • menacing presence
  • **
  • Posts: 178
    • My Caving Photo's
Re: 300 pixels?
« Reply #1 on: June 30, 2009, 09:29:10 pm »
You need to go to image/image size in photoshop and change to 300 pixels per inch for decent printing

Below this and the prints wont be so good, you should be fine with your camera of 6.2 Millions pixels (not per inch)

un-tick resample image

Have fun


Offline Chris J

  • obsessive maniac
  • ***
  • Posts: 358
  • BEC - Everything to Excess!!
    • Casj.co.uk
Re: 300 pixels?
« Reply #2 on: June 30, 2009, 11:21:00 pm »
Hi Steve,

What I don't understand is that I've created the pictures on my digital camera, saved them on my PC (only does Jpeg) then sent them to someone - who has said they aren't 300 pixels resolution...?

Well I'm pretty sure they were taken at the max setting on my camera - so do I then change the jpeg in photoshop? if so presumably the size of the image (dimensions) has to reduce to increase the resolution?

Thanks,
Chris

Offline ian mckenzie

  • obsessive maniac
  • ***
  • Posts: 340
  • Alberta Speleological Society (Canada)
    • Karst Almighty
Re: 300 pixels?
« Reply #3 on: June 30, 2009, 11:37:11 pm »
Most cameras have an Auto function which will select a lower resolution if lighting or other conditions warrant it.  Sometimes I end up with only a res of 72, and in order to boost that to 300 I have to choose the Resize function in Photoshop and select 300 px/inch and a corresponding physical size reduction (inches or cm) that results in the same total pixels.  So you end up with a smaller physical image but with a higher resolution.  Make sure you save this altered photo as a new file and don't irreversibly alter the original.

It is also possible that your photo was dumbed-down during the process of emailing it in order to comply with some server's maximum file sizes.  I use the Save For Web function in Photoshop when sending round pics which makes them small - and useless for printing.

Online graham

  • Retired
  • forum hero
  • *****
  • Posts: 10368
  • UBSS, Speleo-Club de Perigueux, GSG, SUI
    • UBSS
Re: 300 pixels?
« Reply #4 on: July 01, 2009, 06:08:18 am »
First point, yes we are talking about "pixels per inch". Most "ordinary" printers work at that resolution, though even that simple statement isn't entirely true.

Next, stating the bleeding obvious, although programs like Photoshop can alter the resolution, they cannot improve the resolution of the original photograph.

Most ordinary cameras will save files as jpegs, which are compressed files. These lose some quality in the compression process. Save in a non-compressed format (and need more storage media).

But lastly, an SLR with a better (more expensive) lens and - equally as important - a physically bigger CCD will produce a better quality photo than a pocket camera (or worse, phone) with a much smaller CCD whatever the notional number of megapixels quoted.

If you are really serious, buy one of these:

Caving is for Life not just for Christmas

Offline Steve Sharp

  • Axbridge Caving Group - Chelsea Spelaeological Society
  • menacing presence
  • **
  • Posts: 178
    • My Caving Photo's
Re: 300 pixels?
« Reply #5 on: July 01, 2009, 06:45:47 am »
Not to over complicate things

I have a Canon A620 (7.1 Million pixels) - When loaded in Photoshop the pixel per inch is 180 (I cannot change this in the camera) I change it manually in Photoshop to 300 pixels / inch without changing the size (un-tick all the boxes) Your average home printer will print 1200 pixels and above but the eye only sees about 300, that's why they have asked for this - Its very simple but very easy to forget, Web images only need to be around 72 pixels / inch saving you memory space.

If you want good quality cheap prints try Asda - Get them to print one of your pics 6 x 9 inch at 180 and one at 300 and see for yourself 30p each

Offline sluka

  • menacing presence
  • **
  • Posts: 206
Re: 300 pixels?
« Reply #6 on: July 01, 2009, 06:59:00 am »

What I don't understand is that I've created the pictures on my digital camera, saved them on my PC (only does Jpeg) then sent them to someone - who has said they aren't 300 pixels resolution...?

I apologize myself, but those "magic 300 dpi" is the most stupid superstition in printing I know. For high quality inkjet printing aproximately 150 dpi is enough exception if the picture obtain minute periodic structures - such as feathers of birds. In that case there is highly recommended to print pictures in the native resolution of printer (360 for EPSONs, 300/600 dpi for Canon and HP). The same if there are minute characters. But it all depends on concrete situation. The good quality data which are not destroyed by JPEG compression is possible to extend up to 300-400 % of original size/resolution without problems. Data destroyed by JPEG is possible to extend and add there artificial photographic grain and derange the JPEG squares. (I printed 1x1,5 m picture from 280 kB JPEG and it was "only the real exhibition photography because of photographic grain" :) .

For traditional printing the data should have resolution 1,5 time higher than is frequency of printing raster - for 225 lpi (line per inch) press is 150 dpi enough, for 200 lpi is 300 dpi enough. There is a calculator for this values in any Photoshop.

Well I'm pretty sure they were taken at the max setting on my camera - so do I then change the jpeg in photoshop? if so presumably the size of the image (dimensions) has to reduce to increase the resolution?

You are right. But if possible don't save the resized picture in JPEG because your data will be JPEGed again. If you want to send the picture just as a preview, JPEG it without any problems.

Offline SamT

  • Global Moderator
  • forum hero
  • *****
  • Posts: 5359
    • The Eldon Pothole Club
Re: 300 pixels?
« Reply #7 on: July 01, 2009, 07:41:25 am »

Still confused chris...

I certainly am  :shrug:   :wall:


Offline Roger W

  • forum hero
  • *****
  • Posts: 1452
Re: 300 pixels?
« Reply #8 on: July 01, 2009, 12:57:58 pm »
It was much easier when I was using my Olympus OM40 and Fujichrome!

This here picture was taken using an Olympus SP350 digital camerai.



The actual image - in its original format -  is made up of 8,000,000 little dots, spots or pixels, and so is an 8-megapixel picture. (Though it's only 240 by 180 pixels now that it's been into Flickr and out again onto the forum...)

The original on my computer measures 3,268 pixels one way and 2448 pixels the other (making 8,000,064 pixels altogether).

If you display it or print that out as a 10" by 8" picture, that should give you about 300 pixels per inch.

That's about as far as I can go - 'cos I'm quite confused too     :)

If anyone can tell me how to make the pictures bigger when I import them...



"That, of course, is the dangerous part about caves:  you don't know how far they go back, sometimes... or what is waiting for you inside."   JRR Tolkein: "The Hobbit"

Offline sluka

  • menacing presence
  • **
  • Posts: 206
Re: 300 pixels?
« Reply #9 on: July 01, 2009, 02:25:20 pm »
If you display it or print that out as a 10" by 8" picture, that should give you about 300 pixels per inch.

It simply means you may print picture 20" by 16" with no observable difference in quality.

If I understood you correctly, you want to increase the size of picture. The recipe is to open the picture in an image editor (Photoshop) and increase the size or resolution in 20-30 % steps with Resample Image on. You may to sharp it a bit and in many cases the simple trick helps: use Unsharp mask and set it to:15-20/50/0 (area contrast).

Offline zippy

  • addict
  • **
  • Posts: 121
    • Caves, volcanoes, and outdoor adventure: the Winster Cavers
Re: 300 pixels?
« Reply #10 on: July 01, 2009, 04:29:49 pm »
Should this thread be titled  "how to make something simple difficult" ????

Yes, DPI, Dots (aka pixles) per inch. 

The "resolution" setting seen in photoshop and similar programs is a bit misleading, to say the least.  At the end of the day, it all depends on how big you want to print/display the image.  The image from your (6MP) camera will have a finite number of pixels - errr ~6 million of the little fellows!  Changing the "resolution" setting photoshop will not change the number of pixels on your camera!  It's little more than a couple of "tags" added to your image file to indicate the physical size of the image at a given resolution.


For arguments sake (to make the maths easy) let's say this 6MP has 3000 x 2000 pixels.  IF you wish to print at 300DPI, then the largest you can go is:

3000/300 = 10"
2000/300 = 6.67"

If you want to print larger, you'll be printing at a lower resolution and MAY see some pixelation / loss of fine detail, depending on how far you go, on your printer, your printer settings, etc, etc...
If you want to print smaller, you could potentially downsample the image, but this in itself may lose you some image quality - especially if the downsize agorithm is weak.

Printers specify 300dpi to maintain consistent print quality, although the true requirement can vary depending on the nature of the image in question.  Many printers actually ask for 600dpi.


As for up-sizing images... well.  Every process you perform on an image will LOSE data, so upsizing images (for print or otherwise) needs to be treated with care.  If you don't believe me, take an image, upsize it, upsize it again.... 'n' times as suggested, and then downsize it to the original size.  Compare the two.  This is not to say that there are not cases where upsizing or other procesing is not good & needed, just that you should always keep your original, lest you need to go back to it!!

Hope this helps!




"The world is a book.  Those who do not travel read only one page"  St. Augustine.

Online graham

  • Retired
  • forum hero
  • *****
  • Posts: 10368
  • UBSS, Speleo-Club de Perigueux, GSG, SUI
    • UBSS
Re: 300 pixels?
« Reply #11 on: July 01, 2009, 04:32:42 pm »
... just that you should always keep your original, lest you need to go back to it!!

Most important, yes.  :thumbsup: :thumbsup: :thumbsup:
Caving is for Life not just for Christmas

Offline Chris J

  • obsessive maniac
  • ***
  • Posts: 358
  • BEC - Everything to Excess!!
    • Casj.co.uk
Re: 300 pixels?
« Reply #12 on: July 01, 2009, 05:09:34 pm »
ok - I think I get this.... but let me be specific -

I have taken a photo on my camera (6MP), saved it to my PC (it can ONLY be a Jpeg) and then emailed it to someone - they say it is low res... but I've taken it with the highest quality I can.

The image I've given them is 2848  x 2136

If I open this in photoshop and change the resolution to 300 pixels - then it will be (largest)

9.5" x 7.12"?

I hit save as - to create a new image then email it to them and they will be happy?

Online graham

  • Retired
  • forum hero
  • *****
  • Posts: 10368
  • UBSS, Speleo-Club de Perigueux, GSG, SUI
    • UBSS
Re: 300 pixels?
« Reply #13 on: July 01, 2009, 05:58:03 pm »
Well, yes. Out of interest if you open it now, what does the program say about resolution and size?
Caving is for Life not just for Christmas

Offline DavidGibson

  • lurker
  • addict
  • **
  • Posts: 115
    • My web site
Re: 300 pixels?
« Reply #14 on: July 17, 2009, 09:06:35 pm »
I have taken a photo on my camera (6MP), saved it to my PC (it can ONLY be a Jpeg) and then emailed it to someone - they say it is low res... but I've taken it with the highest quality I can.

The image I've given them is 2848  x 2136

That is a high resolution image.  The people you are dealing with appear to be a little dumb.

Quote
If I open this in photoshop and change the resolution to 300 pixels - then it will be (largest)
9.5" x 7.12"?

Errr. no. It doesnt work like that, really.

Quote
I hit save as - to create a new image then email it to them and they will be happy?

Quite possibly. They are dumb. Who knows what will make them happy?

Pixels per inch is one of the most misunderstood things about photos.  Its important for the printed image but it is largely irrelevant as an attribute of your image file.

Your image file has a certain size - in pixels.  It also has an attribute that you can set in your image-editing program called "pixels per inch". But altering this on its own does not necessarily do anything at all to the image. The number of pixels is the same - all you have done is stuck a virtual post-it note on the image saying "Dear printer; if you can, print this at so-many pixels per inch".

Photoshop also lets you specify an actual size for the image. But this can be confusing, as the image doesnt have a "size" - its an array of pixels, not a physical thing. (And, as far as I know, physical dimension is not a parameter of your image file - it only keeps a note of the pixels/inch). Clearly, if you have three parameters - pixel number, physical dimensions and pixels/inch - they wont all necessarily match, and something has to "give". The thing that gives is the actual number of pixels - your image editing program will delete data, or interpolate data to make the number of pixels match your specified physical dimensions and pixels/inch.

The only parameter about your image that is actually important is the overall number of pixels. Think of the pixels/inch as an "advisory" virtual post-it note, and the physical dimensions as something that is used to allow your editing program to decide whether to change the overall number of pixels.

When preparing photos for Speleology, I only ever use the actual pixel size. If you send me an image that is, say, 2848  x 2136 and I want to print it three inches wide, I will reduce it to 1050 pixels wide which at 3" is 350 dpi which is our standard resolution that we work to (it allows for some leeway in sizing amongst other, rather technical reasons).  But I dont do that by altering the dpi or the size - I manually change the number of pixels because that's what Im actually interested in: I want the ultimate control over what Im doing to the pixels.

I would never say to you  "send me an image at 350 dpi", because that ... well, it just doesnt make sense if I dont tell you the size Im going to print the image as well!

You may be wondering why it is necessary to re-size the images "manually" at all. Well, I dont need to: If I just paste your 2848  x 2136 image into my DTP software, and make it look 1" wide, what happens at a later stage of the process (e.g. when I make a PDF or when the publisher outputs the file to his plate-maker) is that the additional pixels in the embedded image are thrown away. You'll see this if you paste a whole lot of photos into Word and then make a PDF. The PDF is usually a lot smaller than the Word document.  So why do I bother to manually re-size and throw the pixels away myself? The answer is that it gives me a much better control over the operation. Some processes such as the unsharp mask, which is an essential part of the operation, only work well if they are applied to an image that is at its final printing size. Another reason, of course, is that the files are smaller if theyre the correct size. You wouldnt fill your web page with 1MByte photos and rely on the visitor's browser to re-size them (which it will happily do). Instead you reduce them to a sensible size to start with so that they download quicker.

Where this leaves you, Im not sure, because you have been given a request that is basically nonsensical. If they mean 300 pixels/inch then applying that setting to your image does not, in itself, do anything at all. If you apply it at the same time as specifying a physical size then the number of pixels in your photo could be altered (irreparably) to fit the request. Clearly if you said to PhotoShop "300 dpi and 2 inches wide, please" then the photo would be re-sized to be 600 pixels wide and, if they want to print it at 6" wide, its going to look a little sub-standard.

Of course they might really have meant 300 pixels but if that's to be printed on paper, that's a very small image!  300 pixels on a computer monitor is a decent size.  If your monitor is 1280 x 1024 for example, its about 30% of the screen width.
BCRA Secretary from 1/1/2010.

Offline sluka

  • menacing presence
  • **
  • Posts: 206
Re: 300 pixels?
« Reply #15 on: July 17, 2009, 09:52:30 pm »
I have taken a photo on my camera (6MP), saved it to my PC (it can ONLY be a Jpeg) and then emailed it to someone - they say it is low res... but I've taken it with the highest quality I can.

The image I've given them is 2848  x 2136

That is a high resolution image.  The people you are dealing with appear to be a little dumb.

Quote
If I open this in photoshop and change the resolution to 300 pixels - then it will be (largest)
9.5" x 7.12"?

Errr. no. It doesnt work like that, really.

Quote
I hit save as - to create a new image then email it to them and they will be happy?

It is exactly as Chris wrote. 2848 / 300 = 9.49 inch.

But what I absolutely doesn't understand is those 300 or 350 (!!!) pixels per inch. Really, is there any visible difference you print image on inkjet in 150 dpi and 300 or 350 dpi?

By my 10 years experience there is not. With one exception - subtle structure which may produce moire if one print it out not in native resolution of printer. And native resolution of EPSONs printers is 360 dpi and 300/600 dpi for Canon and HP.

So from which hell that 350 dpi came?

Offline DavidGibson

  • lurker
  • addict
  • **
  • Posts: 115
    • My web site
Re: 300 pixels?
« Reply #16 on: July 18, 2009, 10:59:59 pm »
It is exactly as Chris wrote. 2848 / 300 = 9.49 inch.

When I said "It doesnt work like that, really" I didnt mean the calculation was wrong, I meant that the whole process of changing the resolution in the way Chris described is flawed. It is not useful in practice. Im writing has someone who has to deal with photos that are sent to me for publication. Having the photographer mess around with the resolution in pixels/inch is not helpful. As I hoped I had explained in my last message, changing the resolution does not, in itself actually do anything useful to the image. (Or it does something unexpected or undesired.

Quote
But what I absolutely doesn't understand is those 300 or 350 (!!!) pixels per inch. Really, is there any visible difference you print image on inkjet in 150 dpi and 300 or 350 dpi?

By my 10 years experience there is not. With one exception - subtle structure which may produce moire if one print it out not in native resolution of printer. And native resolution of EPSONs printers is 360 dpi and 300/600 dpi for Canon and HP.

So from which hell that 350 dpi came?

Ah, well now youre getting into quite murky depths. Tossing terms around like "native dpi" might confuse people even more. Speleology magazine is printed at 2400 dots per inch IIRC. That's the resolution of the plate-making machine.  As Im sure you know, those dots are grouped together to simulate the half-tone screen of a conventional photographic process. An 8 x 8 group of dots could make a blob that can accept 256 levels of brightness at a resolution of 300 "blobs" per inch. (Im oversimplifying dramatically). In fact, our printer uses a screen of 175 lines per inch IIRC, so the "blobs per inch" is 175. 

Your argument that you only need an image resolution to match this - 175 pixels per inch - is a sensible one, but it does not allow any leeway for the digital pre-press - sharpness adjustment, tonal adjustment and so on. Or indeed any leeway to suddenly decide to change the image size, so a good rule of thumb is to use a resolution of twice the line pitch - i.e. in our case 350 dpi.

Youre right that people tend to chant this as it if is set in stone somewhere - it isnt, its just a useful practical rule of thumb for someone in my position. If youre printing your own photos on your own printer ... well, the rule of thumb is not designed for that situation.  (Although you might find, when applying that final unsharp mask that you coould still benefit from a bit of leeway).

Its worth pointing out, though, that this rule of thumb only applies to photos. If you send us an image that contains text or lines - e.g. a cave survey - then it has to go in at 1200 dpi.  You said "is there any visible difference [if] you print image on inkjet in 150 dpi and 300" and the answer is "yes there bloody well is, if the image is a graphical one rather than a photo!".

We have a particular problem if we want to reproduce the cover of a book as part of, say a book review.  If it goes in at 300-odd dpi because its a "photo" of the book cover, it can look crap, because the text on the front cover of the book is noticeably jagged or fuzzy. The solution is ... well the solution is usually that we dont bother to do anything about it because we dont have time, but if I do have time, I do a tedious amount of processing so that the greyscale image goes in at 300-odd dpi without the text elements and it is overlaid with a bi-level image (i.e. just containing black and white and no shades in-between) of the text elements at 1200 dpi.  The 300 dpi image gets "half-toned", if you like, and the 1200 dpi image doesnt.

sorry - thats a bit of a digression - but I hope it is interesting :-)
BCRA Secretary from 1/1/2010.

Offline sluka

  • menacing presence
  • **
  • Posts: 206
Re: 300 pixels?
« Reply #17 on: July 19, 2009, 06:24:02 am »
sorry - thats a bit of a digression - but I hope it is interesting :-)

I apologize myself. I work in publishing from year 1992. And that is the reason I may say the 300/350 dpi or 1200 dpi are simply and only superstitions.

For standard printing technology which use autotypic screening (normal ofset) you simply don't need 2 times resolution of frequency of raster. The 1.5 times is enough. Check Photoshop calculator for it. For special purposes (art books, special photographics books) is 2 times valid. For special technics one should use higher ratio. For graphics in normal offset printing 600 dpi is absolutely enough, we used 400 dpi without any problem. 

For inkjet printing 150 dpi is enough. And if I print cave maps on my large format Epsons I use 360 dpi or if the map is very large 180 dpi. Yes, the subtle lines are not the sharpest one, but for normal observer it is not so important.

There are two kinds of visitors on printing fairs: One is very excited of any colourful piece of paper laying near printing machines. Second one, armed with strong loupe, looks on such a print out from several inches, with turning his head he say "Awfully, terribly, ... , look on this registration, the dots are not sharp enough, ...".

I know very well that 1 TB disk is normal today, but difference between 300 and 150 dpi is four times size of image file. And create 4 times bigger data files just for superstitions? Or print photos quarter of size because the larger ones will be "not enough high-quality"? Way?


Online graham

  • Retired
  • forum hero
  • *****
  • Posts: 10368
  • UBSS, Speleo-Club de Perigueux, GSG, SUI
    • UBSS
Re: 300 pixels?
« Reply #18 on: July 19, 2009, 08:43:59 am »
The simple answer for most contributors (as opposed to printers) is to produce your photos with the maximum amount of information that your camera is capable of, to store them in a non-compressed format so that none of that information is lost, to hand that file to your printer and to negotiate with him as to the best size to reproduce it in terms of both image size and layout.

And then to kick shit out of him if the final results are crap.  >:(

One thing to remember is that most printers will give you proofs that come from some sort of ink jet machine that is streets away from what they will produce the final product with, so don't be fooled by the quality (or otherwise) of the pictures in those proofs.
Caving is for Life not just for Christmas

Offline DavidGibson

  • lurker
  • addict
  • **
  • Posts: 115
    • My web site
Re: 300 pixels?
« Reply #19 on: July 19, 2009, 05:07:56 pm »

For inkjet printing 150 dpi is enough. And if I print cave maps on my large format Epsons I use 360 dpi or if the map is very large 180 dpi. Yes, the subtle lines are not the sharpest one, but for normal observer it is not so important.

Hmm.  I would say that it depends a lot on the context.  Do you remember 9-pin dot matrix printers?  Their quality was "ok" until 24-pin printers because available and suddenly, all those 9-pin documents you had produced looked terrible. But 24-pin printers were amazing... until laser printers came along, and suddenly all those 24-pin documents you had produced looked terrible.... and so it goes.

That 72 dpi photo on your computer monitor looks great ... but 72 dpi on paper is less impressive.  Its the same with cave surveys - your paper printout at 360 dpi or 180 dpi, may look fine to you but ... well ... my view is that if you looked at it alongside a survey printed at 1200 dpi in Descent or Speleology (actually, it might not be 1200, it could be 900 we use, I cant remember) it would look rather tacky.

Of course, if we're talking about surveys, the main thing to to is to try to convince the author that supplying a compressed JPG is not on. :)

Also, of course, the resolution depends on the viewing distance. If you start from the assumption (which I do) that 1200 dpi is advantageous on an A4 magazine page, for a cave survey then, if you were to reproduce that as an A2 poster, 600 dpi would be acceptable because its twice as big and you would stand twice as far back to view it. An A0 poster might get away with 300 dpi lines. Although, as you say, there is a group of people who will peer at it close-up with a loupe and make a comment.

That's why large plasma screen TVs only have the same number of pixels as a computer monitor  - youre supposed to sit further away :)
BCRA Secretary from 1/1/2010.

Online graham

  • Retired
  • forum hero
  • *****
  • Posts: 10368
  • UBSS, Speleo-Club de Perigueux, GSG, SUI
    • UBSS
Re: 300 pixels?
« Reply #20 on: July 19, 2009, 06:50:33 pm »
... for a cave survey then, if you were to reproduce that as an A2 poster, 600 dpi would be acceptable because its twice as big and you would stand twice as far back to view it. An A0 poster might get away with 300 dpi lines. ...

I think that generations of cavers who have stood in the long common room at Penwyllt and squinted at the detail of the OFD survey, which is in effect somewhat larger than A0, will disagree with you on viewing distances, Dave.
Caving is for Life not just for Christmas