A most compelling evening (well if you are a camera club member) had courtesy of Keynsham Photographic Centre. Simon and Neil took us through papers, sizes and types of processes KPC offer and their knowledge and enthusiasm went a long way to explaining why KPC is a destination of choice of so many serious photographers here-abouts. Around 2 in 3 of those present were already users but we all had something new to learn about the services they offer, which are quite extensive. I have always found them to be helpful and friendly and I am happy to pass on a recommendation to use them.
Why print? There is an emotional and a logical answer to this. The emotional one is that we do seem to react differently to a print than to a projected image or one on a phone or other screen. This is certainly the case with the written word. With pictures it might be an age thing, with those of us of a certain maturity having an emotional tie to what we started with, the family albums and so on, but as these are family history it is not uncertain that these emotions are passed on. It might be the way we view what is art. Even slides are, these days, often rendered digital. Prints have a longevity that digital files cannot match.
The logical one is more existential. Our photographs aren’t photographs until they are printed. They are computer files that are capable of being rendered as images given the right, seldom very cheap, equipment. To that extent they are, at best, semi permanent records. They exist not as pictures but as 1’s and 0’s and that makes them vulnerable to damage, loss and redundancy in many different ways.
One of the questions that came up in discussions was the notion of the minimum resolution (pixels per inch or PPI) as a ratio to print size that is compatible with a reasonable quality print. In order to look at this we need to look at and differentiate between two measurements that the computer file we turn into a picture holds – and which can be reset via (photo editing) software. The first is Pixels Per Inch and the second is Dots Per Inch, or as we will refer to them PPI and DPI.
There is a previous post dedicated to this and a good start if your interest is more than passing. Essentially the PPI is a measure of the resolution of the computer file when shown on a screen and the DPI a measure of the resolution that a printer will reproduce the computer file as a digital print. They are not the same thing, they are not interchangeable, the connection between them is that you cannot use DPI to overcome low resolution in the PPI. That might not seem like much of a connection but it is a common error, at least it is a common misunderstanding.
Just as one is not strictly reliant on another there is an optimum setting for both, but it is not a single magic number or formula. This is because different manufacturers of camera sensors and printers construct their wares in slightly different ways to slightly different priorities. The only rock solid constant you can apply is garbage in garbage out aka GIGO. What doesn’t help is that there are always going to be subjective elements to the ideas of output and acceptable, but we can set these aside.
A slight diversion, but one that is worth taking, is the purpose for which we are taking our images in the first place. The digital age has brought with it a number of outlets that, even 20 years ago, were pipedreams or even unimaginable to most photographers. Film cameras would enjoy their predominance for another 10 years in the “serious” and professional markets. A decent living could still be made from stock photography. These days there are far more outlets and they demand different things of the data files they publish. What remains is the fact that proper prior planning prevents poor photography. Start with the end in mind and choose formats etc accordingly.
The argument for recording images in camera RAW is that all the information is left in. The argument for JPEG is its universality and space saving compression. The fact is you can shoot in both and convert your final image from RAW to JPEG or any other format (has to be some other format as you cannot save in RAW). In your final version of the image is the information that other devices are going to use to present your image, including colour space, PPI and a whole lot else. The point here is that you are going to have to make, or let programme defaults make, these decisions to determine the look of your output (as best you can).
So, long way round to the answer we were looking for but the fact remains, that for all the reasons above, there simply is no answer to the question of a ratio of file resolution to print size because Pixels are not the same size on all cameras – 24 mega pixels on a 35mm sized sensor take up more space than 24 mega pixels on an APSC sensor. There are approximations, guidelines and above all, experience. Let’s look at rules of thumb, but first a caveat.
Pixel peaking, the urge to zoom to the maximum and suck the teeth at the lack of relative clarity, is neither helpful nor useful, outside of certain photographic publications desperate for some comparators in a world where you really have to go some to find a “bad” lens or sensor. It’s the aggregate of the pixels used that we view and a badly composed photograph is a badly composed photograph regardless of the amount of money you have spent or your equipment’s DXO rating. We are here looking at making a print of what you have got, regardless of your motive to print it.
Lets first talk of PPI. Smaller sized prints, say up to 10 x 8 (ish) we can get a reasonable print at 125 ppi. That is to say at arm’s length it’s going to look OK. Press your nose against it or use a magnifying glass it is going to look pretty rubbish. I have to say that such a view point rather spoils the whole point as, again you are looking at the aggregate of the pixels after all, or as I like to call it, “The picture”. This is a fall-back position only, If your default is 72ppi (i.e. it’s designed to show up on your computer or phone screen) then you are not going to get much above a 6 x 4, maybe 7 x 5 at a pinch, out of it (feel free to prove me wrong because you are only going to find out by using your combination of equipment and the printer you choose). The best lies between 200 and 300 ppi, by more or less common agreement. KPC ask for 305 PPI. Basically of you use the 300 end of the range you are likely to be close to the optimum for most commercial printers at standard sizes. Refer to your supplier for the necessary information. To change images PPI we use editing software like Lightroom, Gimp or Affinity.
But we were looking for a rule of thumb. Any minimum figure is a result of the combination of equipment particulars and specifications and materials used.
So, take the longest edge of the image (measured in pixels) and divide by the longest edge of the desired print size (measured in inches). If an image measures 3,840 x 5,760 pixels and you want an 8 x 10 inch print. 5,760 pixels ÷ 10 inches = 576 ppi. That’s more than enough resolution. If you want make a 30 x 20 inch poster out of that image, you’d have a resolution of 192 ppi (5,760 ÷ 30), which isn’t high enough for optimum, if we are looking at least at 200 PPI. This is a minimum remember, so adjust accordingly.
Alternatively, and my personal method, size the image in inches at a ratio of 1:1. So an 18 x 12 print is sized as an 18 x 12 image in my editing software (Gimp). The resolution is set at 305 by 305 dots per inch as I use KPC, and the image, in pixels, is 5490 x 4118. The down side is the file size, the upside is its going to fit without faffing around.
So http://www.keynshamphoto.co.uk/ and start printing!