Tagged: sensor

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.

28th January 2016 – An evening with Steve Hallam.

Last meeting was the territory of club member and treasurer Steve Hallam, talking through some of his digital history. Steve is an Olympus fan and has been for over ten years. Micro four thirds, the name come from the diameter of the sensor in inches, is the invention of Olympus and as Steve pointed out, the first system to be designed exclusively for digital from scratch. The first Olympus Steve owned had a 5 MP sensor , which when compacts these days can pack 20 Mega Pixels, sounds restrictive. In reality most people would be largely untroubled by 5MP sensors, the key being the quality of rendition not the size. More Mega Pixels give you more room to crop and still get a reasonable image. The ability to resolve reasonably accurate colours and the capacity to restrict noise at higher ISO’s are generally bigger factors in most peoples’ photography . The bigger numbers in terms of Mega Pixels are something driven more by perceived marketing needs (bigger must be better) than actual customer requirements.

 

For so long full frame, as Steve pointed out a tag rather than a technical term of any enlightening feature. The 35mm (actually 36 mm but that is a spurious accuracy) was of course the film size in most SLR’s and has carried over to the digital age as the most common “professional” size. I will come back to the need for the inverted comma’s shortly. In the film age, especially from the late 60’s onwards, 35mm was pretty much everywhere. Unless you were doing advertising or studio work then the frame size went up to 6 x 4.5, 4 x 5, 10 x 8 and so on. Hasselblads used 120 roll film (6 x 7 cm). Just like the Box Brownie. Only there was a bit of difference price wise. Also, it has to be said, there is a slight difference in quality too.

 

The reason for the inverted commas around professional above is that there is no such thing as a camera by which one becomes a professional by being in possession of. There is plenty to be said for the idea of a larger sensor – and some people bang on endlessly about it – but, as been said before in this blog, unless it is predicated on an actual photographic need then there is no reason why a professional has to shoot with a 35mm sensor. Damien Lovegrove doesn’t, as he explained when he visited us back in July 2014, he uses APS-C (among other formats I am sure). Any argument based on the logic of sensor size would have that a 6 x 4.5 medium sensor format has to better than a 35mm and so on. The question always has to be “At what”?

 

Lugging a D800 across Antarctica to photograph polar bears in the wild may seem like hard work, it is, after all a sizeable chunk of Bakelite in its own right. Adding in the heavy duty lenses adds even more bulk and that’s before you realise the nearest wild polar bear is 12,500 miles to the north (it pays to do your research). You had better have a really, really good reason for packing it in the first place. Well that would be weather sealing, shock-proofing, reliability given that it’s 2,500 miles to the nearest camera shop to replace that broken lens (assuming both that you are going North and turning left(ish) and Punta Arenas has a camera shop, otherwise it’s 3,700 miles in a completely different direction to Auckland). You may require very large blow ups at a high dots per inch count, there are any number of reasons you need a full frame camera, but , logically, not one of them is because you are a professional. Steve pointed out the main advantage of the Micro 4/3rds format is the capacity to build smaller, lighter cameras.

 

Smaller lighter cameras with smaller sensors, yet we still think of lenses in 35mm equivalent terms and that does make things easier for comparison reasons, but allows for some confusion. When we talk of crop sensors we are talking about the size of sensors relative to 35mm and as most sensors are smaller than this then we are seeing a smaller image given the same focal length of lens.

 

A confusion creeps in with the idea of “magnification” which a lot of people assume to be a telephoto effect because a 100 mm lens on a 35mm camera shows the same as a 150 mm lens on an APS-C or a 200 mm lens on a micro 4/3rds and the logic goes (off at a tangent but it’s easy to see why) a 200 mm pulls in the image twice as much as a 100 mm lens. Well when you double the focal length on the same size sensor it does, the mistake is to not factor in the change in the size of the sensor. If an image is made with the same lens, but a smaller sensor, it shows a smaller area. Enlarge both your 35 mm and you crop sensor images to, say, 10 x 8 inch print and the degree of enlargement, the magnification if you will, will be greater for the smaller sensor than for a larger one. Hence you might get an inkling of why more Mega Pixels on this year’s sensor than last sounds attractive – you can make larger prints without a loss in quality. Well sort of, as, after a point, those extra pixels start to get in each other’s way.

 

So, our thanks to Steve for bringing up some interesting topics and for sharing his images with us. Much appreciated.

 

N E X T W E E K

NOT AT THE CLUB. Light trails, meet at the fountains on the centre. Bring cameras and tripods we are going to be taking some light trails. 7.30 commencement.