Some images just need to be printed large.
Whilst most of us stick with small prints, 4″ x 5″ up to 8″ x 10″, we
are really missing out on things. Photographs, like all other forms of
visual art, have a natural size. Highly detailed, intricate works
perform very well in the smaller sizes, such as 8″ x 10″ or 11″ x 14″,
because they invite the viewer to move in close and become absorbed in
the detail. Other images work much better in a larger size. In
this article we will examine all the issues around how and why to print
in poster size or larger.
How To Print Large
Step 1 – Assess the image
Have a good look at the image in whatever form you have it, whether it
is a normal print from the drug store or a digital file from your
camera. Assess things like its sharpness, level of detail and subject
matter. Subject matter is a highly important issue to how large you
should print. Shocking or confrontational images may be better small.
At a recent exhibition I saw some wonderful photographic images of
Auschwitz that were printed small, around the 8″ x 10″ size. They
worked well in this size because their level of detail invited you in
close. But by being small the subject matter did not push you away.
However such images could also have been printed large. In this case
the emotional response of the viewer would have been very different
because of the impact the larger size would make. Thus you also need to
evaluate what impact you want your image to have on the viewer, what
statement are you trying to make. This is the big difference between a
fine art photographer and a snap shooter. The fine art photographer
will have a clear intent and message that guides how they present their
images. Now this could be as simple as “the beauty of nature” or as
complex and confrontational as “man’s inhumanity to man”, but it will
still be there.
Step 2 – Size and resolution
From the above work out what size you wish to print at and with what
technology. The type of printing you will use determines the resolution
in dots per inch that you must supply to the printer. (see the endbar
Resolution Issues) Continuous tone devices like the digital
photographic printers available at professional bureaus will need
higher resolution input than your ink jet printer may, but they will
produce a smoother result. Tied into these considerations are the
surfaces you will print on. Glossy photographic type paper will need
more resolution than printing on canvas or watercolor paper. This is
because the texture of the material hides many inadequacies in the
image data and adds apparent detail. Plus when you print on artists
materials, like uncoated watercolor paper, the ink dots will spread more, making
the image smoother.
Take your chosen size and multiply it by the dpi rating you need to use
to get the absolute size of the image you require in pixels. For
example a 24″ x 36″ print at 100 dpi requires a 2400 x 3600 pixel
image, or an eight and a half mega pixel image.
Step 3 – Input Options
If you work with a digital camera you simply need to transfer your
images to the computer. With digital cameras I always shoot at the
highest resolution the camera is really capable of, unless I know I
will only ever use the image for a low-res task, like a web site.
Shooting at the highest resolution gives you the most options.
Those of us who still shoot on film and then scan have more to think
about. You can either scan the film or a print. It is always better to
scan the film if possible. A print is a second-generation version of
what is on the film. It will be slightly softer than the original on
film plus there is always some texture to the paper it is printed on
that will also scan in. This said, I have achieved wonderful results
scanning prints on a cheap flatbed scanner. Just make sure that the
scanner is clean and dust-free, and that the print is also clean and
free of fingerprints, etc. Scanning the film will offer the best
results and more potential resolution. 35mm film scanners are
relatively inexpensive and produce lots of resolution. Remember that you can also get
scans done at a bureau at reasonable price with as much resolution as
you need. This is a cost-effective choice if you only need high-res
scans occasionally.
When scanning photos I recommend using the maximum optical resolution
of your scanner. A 1200 x 2400 dpi flatbed scanner has a real optical resolution of 1200 dpi even though
you can set the resolution as high as 9600 dpi. I find it is better to
interpolate up in PhotoShop if necessary. Also if your scanner can
provide more than 24 bits of color data per pixel over to PhotoShop (or
whichever imaging program you use) do so. This gives you more data to
work with. If this is not possible, so as much of the color correction
and contrast adjustment within the scanner software as possible. This
ensures that you will have the most useable color information available
in PhotoShop.
Step 4 – Removing information
There may be some information in the image that you need to remove. At
this stage you should generally cleanup the image by using the clone
tool to remove all dust and hair marks or scratch marks. Another thing
to examine is noise in the image. This may be digital noise in a
digital camera image or film grain in a scanned image. Especially if
you have to enlarge the image substantially this may be very
noticeable, so it is better removed now. One solution is to apply a
Gausian Blur filter, the other being the Speckle filter. Note though
that you have to be careful not to soften the image too much. Since
noise is most noticeable in areas with little detail you could mask
areas and only blur those.
Step 5 – Adjusting resolution
If your input medium provides enough or more resolution than you
require there is not much to do at this stage. Use the Image ->
Image Size dialog in PhotoShop to set the size and resolution you
require, making sure that you select the Resample Image checkbox.
Images of the right resolution will still normally go though this to
adjust the dimensions and dpi rating but without changing the file
size. Say you have scanned a 4″ x 5″ photo at 1200 dpi and you are
printing it at 40″ x 50″ at 120 dpi. The number of pixels won’t change
but you will still need to tell PhotoShop the correct dimensions and
resolution so it can print correctly.
What do you do if you have to increase the
size of the image? Simply
use the Image -> Image Size dialog as above to increase the
resolution with the interpolation method set to Bicubic, which
generally offers the best results. For larger size increases (more than
2x) also do this but it is sometimes better to perform this in a number
of smaller steps (say 1.5x to 2.0x) with some sharpening applied inbetween rather than in one big jump. Try both ways and come the result.
Step 6 – Sharpen the image
All scans and digital camera shots usually benefit from some amount of
sharpening with PhotoShop’s Unsharp Mask filter. This is especially
true if you have increased the resolution of the image. The danger here
is that as well as sharpening detail you want additional detail, like
noise and film grain, will be sharpened. If you got step 4 right this
should not happen. Remember that the dialog box allows you to adjust
the threshold of color difference below which no sharpening occurs. The
radius of the sharpening should vary depending on the size of your
image and beware of going above 150% sharpening. Adjust the settings
until you get the effect you want. Then examine carefully various parts
of the image. If you are not happy with the result use Undo or the
History list to go back and try again. Remember that you can also
sharpen only selected parts of the image.
Step 7 – Adding information
If you have done a major resolution increase you may need to add
information to the image. This all depends on the intended use of the
image. If you are a dab hand with the mouse or graphics tablet you can
draw in fine detail, like small branches in the trees, etc. You may
also be able to clone parts of the image. When doing all this work
first duplicate your image layer, then create a new layer above this
and draw on that. By working with separate layers you can ensure that
you can easily change your mind or even create different images from
the same photo by selectively turning layers on and off.
Resolution Issues (as opposed to issue resolution, which is a job for a psychologist or guru☺)
There is a lot of confusion about resolutions, mainly caused from the
fact that there are two ways to specify it. You can express image
resolution in absolute numbers of pixel terms. This is the best and
least confusing way as you can easily convert to the other form from
here. We know that a 1280 x 1024 pixel image contains 1,310,720 pixels,
and with three bytes or 24 bits of color information per pixel this
will occupy around 3.9Mbytes without compression. The second expression
of resolution is as measurement dimensions and the number of pixels or
dots per unit of measurement. Thus it could be an 8″ x 10″ image at 300
dpi. This in itself is not confusing but its usage can be.
Printers and scanners often express their resolution in dpi. For
scanners this is a reasonable approach. An older Nikon film scanner has a
resolution of 2,700 dpi. Thus a full frame 35mm negative has a size of
24mm x 36mm, or 0.945″ x 1.417″. It thus produces a scan of 2700 x
0.945 by 2700 x 1.417 or 2551 by 3825 pixels. Within PhotoShop, by using
the Image -> Image Size dialog, we can change the dimensions or the
dpi resolution to any setting we like without changing the numbers of
pixels by un-checking the Resample Image checkbox. Thus we can make
this roughly 1″ x 1.4″, 2700dpi image a 8.5″ x 12.7″, 300dpi image
without the need to change anything except how PhotoShop sizes the
image. The important thing is the number of pixels you have. You can
spread these thinly over a large area or pack them tightly into a small
one, but the number can stay the same.
The problems with dpi resolution expressions come with printers. When a
printer says it has a resolution of 1,440dpi all this means is that it
can place 1,440 dots per inch of cyan, magenta, yellow or black ink. It
can not put 1,440 dots of pink, purple or skin tone per inch. Note that
I am not talking here of continuous tone printers, like dye sublimation
printers, only of inkjets and color laser printers. To get colors other
than the actual ink color, numbers of dots need to be laid down close
together in a process called dithering (not the sort a Libra does when
confronted with having to make a decision). If the size and closeness
of the dots is good enough and your eyes are far enough away you see
this as smooth, continuous color. This means that such a 1,440dpi
printer may not be able to print photographic images at much better
than 300dpi. So you don’t need to make your images 1,440dpi resolution
to print them on such a printer. You can easily determine what
resolution you really need by testing. Take an image with lots of
detail at various scales. Change its resolution to that of the
printer’s dpi rating. For example we’ll do this for the author’s 300dpi
large format printer. Then generate additional image files by
resampling this original down to, say, 150, 100 and 50dpi. Print all
the versions and compare. You will notice that, at some point, you
really stop getting more detail even though the resolution sent to the
printer was higher. You can do more at in-between resolutions to
fine-tune this. For my printer it is around 80dpi. This means I gain
nothing by preparing files of higher resolution for printing. They are
just bigger to store, slower to work on and take longer to send to the
printer. Whilst 80dpi might seem very low to you it is important to
remember that most of us never view a huge print up close. So an 80dpi
36″ x 48″ print at a normal viewing distance can look as sharp or
sharper than a 400dpi 8″ x 10″ print in your lap.