1st and 8th September – We’re back and a Q&A

Aaaaaand we’re back. No wait, did that one last year. Try this. Hello, again.  The first of season 2016 – 2017 we did out annual what-I-did-on-my-summer-holidays session. No, wait, it was way better than that, at least after I’d done my stint. What he we had was a good variety of people, landscapes, details, street, and what else taken over the break by the membership. Even those club shoots we did, where there are a lot of views of the same thing, showed a refreshing variety of angles and interpretations of some very stimulating evenings shooting. Quite energising, as it is always a range even when taken of the same subject. Angle, crop, the way the light falls all make up some subtle differences and that is before you get to the mechanics of the way different firms sensors render colours, or, indeed whether the photographer has chosen black and white.

We followed that with a Q&A in week two, with varying degrees of success and it is this that I want to follow up but because at least two of these answers is about the length of an average post in itself, then I am going to split these over approximately the next month in association with content from that week.


What difference does it make when I change the DPI and its relation to pixels?

The simplest answer to this is none whatsoever, but simple answers don’t always mean helpful answers. It can cause confusion, not least because we are seeing the term PPI banded about more frequently of late. Add to that the fact that you can set both pixel count and DPI often on the same menu and club photographers most often come across these when resizing for competitions and voila we have a strong correlation by association. They are not, however, interchangeable in any meaningful way.

Let us start with DPI or Dots Per Inch. This term has been around longer than the common notion of pixels and has been associated with different aspects of image quality over the years. DPI has come to mean more than it actually does in this context and can be the source of confusion. In fact DPI refers to prints only. It is not directly related to image quality at the core, that is to say on the sensor. Your printer creates the final image through spraying jets of ink – it’s sort of in the name – on to a surface. The resulting dots are engineered to be thinner individually, than a human hair. It is a sort of the ultimate in Pointillism and is philosophically absolutely nothing to do with that school of art and so isn’t, if you see what I mean. No? Move on.

What it practically has in common with pointillism is  the capacity of the human optical and processing system (that is your eyes and your brain) to create a fuller tonal range from the arrangement of dots of colours of far less actual colours being applied. Hence RGBY  (Red Green Blue Yellow) in one cartridge and black in another on your basic home printer can give you a fair rendition of  a much, much wider tonal range.

So, why do we even have the dots per inch option built into our editing suites print menus? The answer is tied up with that tonal range and the viewing distance. Another confusing label attached to this is “Professional”. “Professional” prints are printed at around 300 DPI (depends upon the printer system being used). It’s to do with quality, we are told. Yes it is, very much. Doesn’t explain why your average billboard poster is printed at around 6 DPI and they are professional too. The key to understanding that is the viewing distance. Billboards are, most often, not viewed at arm’s length. If you have to stand back further to get the whole image in then you need less dots. If you have ever bought printer ink then you know that it is, per litre, one of the most expensive things on Earth to buy. Someone, I forget who, calculated that to fill your average family car petrol tank with the stuff would cost you around £90,000 at the pump. And then it wouldn’t go. So that’s £90k + a garage bill + some very awkward questions to answer from your other half and the bank.

It should be noted that this is all about the final quality of the print. If the quality isn’t in the file you are trying to print in the first place then daubing more paint onto your canvas just (expensively) glorifies the mess you have started with. Which brings us to pixels.

Pixel is a compound word made up from the term Picture Element. It is not purely a photographic term by any means. It is the smallest programmable element in an electronic visual display. It is a single point in that display.  Your TV works on pixels. Your computer screen works on pixels and yes, your digital camera sensor works on pixels. Except …. well we photographers in particular, it being such a large element in the marketing of the quality of capture our cameras can make (at least according to the industry’s marketing departments), actually confuse  the physical elements on the sensors which are, more correctly, photo sites, the receptors that sit behind tiny , tiny lenses and convert light into binary electronic signals with something that is fixed in size.

Logically that is not the case (necessarily). The actual, physical, size of these picture elements is determined by the resolution the designer is engineering for. So if you are looking at this on a 800 x 600 screen the answer is, half the size of looking at this on a 1600 x 1200 screen. Or if you have 16 mega pixels of an APSC sensor Nikon D3100  the sensor on the 24 mega pixel Nikon D500 has 50% more pixels, but they have to be smaller to fit in the same physical space. You have more points, so, like having more dots in your final print, you have more chance at rendering gradations etc more accurately and subtly.   So there is no definitive answer to how big is a pixel, it depends upon the medium you are using for the display, and the display size itself is dependent upon how far it is expected to be viewed from. The rough dimensions are one and a half to two times the diagonal (on the diagonal is how TV screen sizes are measured).

The basic rule – and I mean rule – is that the initial quality of the file you are displaying dictates the maximum overall quality you can achieve. So, being both Pro cake and pro eating it both the Get-It-Right-In-The-Camera-istas and Ye-Accolytes-Of-Photoshop are right. Firstly you need to get your picture as right as you can at source and if you have more information to work with you can do more at the extremes of editing. This is important, because a low resolution produced-for-the-web image is not going to make much resizing before it becomes useless. No amount of expensive dots per inch squirting is going to change that. Interpolation, where your editing programme takes each pixel in relation to the pixel bordering it makes a decision on shade and contrast of it and fills in the space with a guess-of-a-pixel (we will look at that in a little more depth another time) has its limits. It also takes up space – the very reason that your average web image is of low resolution, it’s good enough for the purpose its designed for given data limits and user load times.

Oh, I could go on, but I won’t.


N E X T  M E E T I N G

In the Footsteps of Shackleton.