crop ratios and image size confusion - help!

huntertp

huntertp

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Hi

I have decided to try a couple of A3 size prints with dscl but i'm in total confusion.

I have cropped the image in lightroom 2 using the ratio 1:1.414 which i believe is the correct ratio for A series paper sizes.

I then exported the picture as a jpg in srgb at 300dpi.

When i was getting ready to upload i noticed on the dscl site that they say an image for A3 printing should 3510 x 4950 pixels

I checked my exported image and it was 3272 x 2314 @ 4.2mb :(


So i went back into lightroom - made a copy - and again cropped the image using the same ratio (1:1.414) but then when i exported i put the dimensions 3510 x 4950 in the image size box in the export dialog.

The image is now showing the correct size and is 7.6mb

Now i realise that the difference in my export dialog was the inputting of the size on the 2nd picture but i thought that cropping to the correct ratio was
all i had to do

So - did i miss out an important step and do i need to size the images on export as well as crop to the correct ratio?

So sorry if this is a dumb question - but i have spent hours in the forum trying to make sense of the whole image size/aspect ratio thing

Hope this makes sense to someone. Any help would be very much appreciated

I will post the image up (not full size) as i know we all like pictures :)

Darran
 
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This is the picture

cuban.jpg
 
Might be hard to explain this one:

YOu have cropped it correctly - but that is just a ratio - the same ratio is used for all 'A' shaped pages as you have rightly said. That sets the shape NOT the size. The amount of pixels is the size.

Ignore the 300dpi for now - if they ask for an image to be 3510 x 4950 pixels and yours is 3272 x 2314, your image was simply smaller than what they required for optimum print quality(300dpi). What you have done is resized it while exporting it (magnifying it in a way). It makes the image bigger by guessing what the image would look like at A3 size, based on what yours looks like at 3272x2314.


The dpi is 'dots per inch' - the more dots the better the quality (up to a point). They want to print it at 300dpi, so based on the dimensions of an A3 sheet they have calculated it at 3510 x 4950 pixels (dots).

Confused??

Crackin' pic by the way!!!
 
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Thanks for the reply Rick

So it seems i did miss out a vital step in the export process - ie actually sizing my image!

I am going to have another go - it would be great to get some of my pictures on the wall.

I almost uploaded these last night to dscl, good job i didn't

Thanks again
 
Thanks Danny

Very nice of you to say so.

The pictures were uploaded to DSCL and i have just had an email to tell me they have been despatched.
I'm getting 6 or 7 pictures printed - Cant wait to see if they have turned out ok.
 
huntertp said:
Thanks for the reply Rick

So it seems i did miss out a vital step in the export process - ie actually sizing my image!
Click to expand...

Nope, not really....

What you did before was crop you picture, and leave it at the resolution it was taken at. Now you have done the same things, plus up-sized it using Photoshop. You have effectively created some extra pixels that were not in the original. This shouldn't be necessary at all, unless the printer's machines will only accept 300ppi images AND they can't or wont, correct it themselves. Basically you are fudging the image size to accommodate what makes life easy for the printers, which you shouldn't really have to do if you're ordering 7 A3 prints!

Great shot by the way!
 
Thanks for your input richersea

This was the first time i had uploaded anything to DSCL for printing.
Their image size info page gave me the sizes i needed to upload (@ 300dpi)

I did my ratio cropping and exporting at the correct size from lightroom 2

This was just a test run for me to see if what i see on my macbook is what i get on print outs.

So i ordered 7 or 8 prints at varying sizes of different subjects, some full colour and some black & white. (i ordered some in gloss and some in lustre and chose not to have my prints cropped or auto corrected

I am happy to say i got them delivered within 36 hours and they turned out great. :cool:

I am now getting together another print run of different images for upload.
I am really impressed at how sharp my d90 pictures come out at A3

Thanks for your help lads
 
I'm not sure what options ou get when you resize in PS, but with Paint Shop Pro you get allsorts of options, including the pixel size, printed size and resulution in DPI or Dots per centimetre.

I managed to balls mine up and send DSCL an A3 image sized at 300Dots per cm instead of per inch! Luckily it still printed OK, despite me selecting the pre sized option!

You can let DSCL do all the cropping and sizing, but I prefer to do it here, that way I can see exactly what I'm getting before it's printed. plus I don't want their aut enhance process messing up my images!
 
DPI is actually the wrong term.... In photoshop it's pixels per inch. (PSP incorrectly uses dpi which does cause confusion)! DSCL should not be using that term but they do because it seems everyone does. Images are made of pixels so the correct term for resolution on your computer is PPI. When it comes to printing the printers resolution is measured in dpi as it produces physical dots. I won't go further but there's plenty of reading on this if you search.

Helps a lot if you understand this and can help you size images correctly.

There is a very easy equation to use that helps - it can be written three ways depending on the variables you have.

PS = Px/R
Px = Ps x R
R = Px/Ps

PS = Print Size (In inches)
Px = Pixels (along an axis)
R = Resolution (measured in ppi)

Hunter
To use your example above.....
DCSL say they require a 300ppi image. An A3 print is 11.7" x 16.5" (give or tale a small amount)

So to get the number of pixels you calculate for each axis

Px = R x Ps
Px = 300 x 11.7 = 3510
Px = 300 x 16.5 = 4950

So you can see why they ask for that size however to print at A3 you do not require 300ppi. Any lab that asks for that imo is notr worth it's salt! A lower resolution is perfectly acceptable. Why add random pixels to your image that are not needed? You may not notice on an A3 image though so maybe not worth wrrying too much about. my A3 images are normally printed at between 180 and 240ppi.

if you sent your image to somewhere that accepts a correctly sized image at default resolution you would have had the A3 printed at

R = 3272/16.5 = 198
R = 2314/11.7 = 197

So your image would have been sent at roughly 198ppi - Notice I didn't say printed at 198ppi as the printer's own resolution would have been used (the two are not alike - one is digital the other physical i.e. printed dots on the paper).

As you note you are delighted with the prints. i just prefer to use the camera's resolution up to a point and 198ppi for an A3 would provide just as good results but without the increased file size.

Hope the above at least helps you understand resolution a little better.
 
PS

I love the image!
 
wow - image resizing/printing really is a minefield isn't it.

I did notice that when i was resizing in lightroom it was set as pixels per inch (ppi)
I just assumed that dpi and ppi were one and the same.

Thanks for the kind words re the image - it looks fantastic at A3
 
great explanation, Jim, very helpful.
Dave
 
huntertp said:
wow - image resizing/printing really is a minefield isn't it.

I did notice that when i was resizing in lightroom it was set as pixels per inch (ppi)
I just assumed that dpi and ppi were one and the same.

Thanks for the kind words re the image - it looks fantastic at A3
Click to expand...

ppi relates to an image resolution and the number of pixels that will be crammed into an inch of the final printed output. So 300ppi is a pretty standard size because the human eye apparently cant resolve any more detail than that.

dpi is the physical resolution of the printer in this case. You can send a 72ppi image and print it at 300dpi - so they are both totally different numbers.

Many inkjet printers say they print at very high resolutions but in realty although they lay down a lot of dots their effective resolution is around 300 or 360 dpi.

The equation should help you resize images.
 
No bother
 
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