Week 2 Lecture notes -image capture and lossy/vs lossless file formats

ACM202 Week 2 notes

Did we bring our cameras?  No!

Digital capture and Camera Raw

Today is the last ‘catch up’ session of work covered in ACM102.

I.e. Revision – but in more detail than in 101

See link on Cloud to slides for today’s class

Lecture

Basic introduction to Digital image capture

(n.b. see Cloud for ‘Digital Darkroom cheat sheet’ – worth printing out)

Pre-capture decisions

  • File format – RAW or Jpeg
  • Exposure variables – depth of field (aperture), image blur (speed), Signal/noise ratio: ISO/light sensitivity

It is vital to have a well-exposed image from the camera – this gives most scope for post-capture manipulation

Sean gave a quick explanation of the relationship of fstop to shutter speed (stopping), and ISO settings (relative sensitivity of sensor to light)-

Framing

  • Focus

Digital images – basic guide

rule of thumb

  • Expose for highlights (expose to the right)
  • Process for shadows

Pixels and their historical relationship to making images with light – Camera Obscura –

Cam. Obsc. – a projection of an image in a darkened room.  – inverted/back to front   (exploitation of this – Abelardo Morell – on the ACM202 blog – uses a tent in ‘camera obscura’ mode (also his Italian works) – projections on the ground, also James McArdell – using his t-shirt to create ‘camera obscura’ images)

Permanent camera obscura (photography) – a light sensitive chemical/array to capture the light

See week 1 notes for the way the Bayer Array works to record photons.

When photographing, we attempt to capture as much information as possible without blowing shadows or highlights (clipping)

Exposure is subjective (can be, to some extent, adjusted in Camera Raw)

Histograms

A histogram is a graph of the tonal range – shadows to left, highlights to right (n.b. 255 levels in 8 bit image)

It is worth learning to use the histogram when capturing images.

Most on slides for week 2.

NB slide 15 represents ‘coning’ when there are peaks with no information in between (tonal square grid)

Other histograms

Luminosity – vital to our capture.  – also for each colour or for colour RGB

Using the camera

after capturing an image

–       Look at histograms – 1st is luminosity, 2nd is RGB

Slide 28 to give e.g. of depth of field variation

Digital cameras may have ½ stops in depth of field

The user chooses priority – shutter speed or depth of field (also ISO setting)

NB – for both aperture and shutter speed, progressive steps either double or halve the amount of light entering the camera – but some cameras will have ½ values

ISO (relative light sensitivity) effects image noise when applied to digital imaging

–       The lower the ISO (film speed) the less image noise (c.f. film – where higher ISO/faster film gave greater graininess – and some films were developed specifically to give grain)

What is Digital image Noise?

–       The camera’s attempt to replace missing information – usually speckled colour

–       (ISO=International Standards Organisation)

there is a loss in low light – and using a higher ISO will help but the low light will lead to more digital noise.

Rule of thumb – use the lowest ISO that will still give you an acceptable image in the light available.

(See slide 34 for bar graph to demonstrate this)  – the signal noise remains constant, if you need a higher ISO to get the same amount of light, the ratio to signal noise increases.

About camera settings – slide 40 –

For this unit, use the ‘creative zone’ – Av, Tv, manual etc.

JPEGs and other file types

JPEG – a format for automatic compression and basic in-camera treatment of images.  Nearest neighbor algorithm – the JPEG algorithm adds pixels of adjacent colours – which will add to a muddying of images.  – see JPEG artifacts in blog (to see the JPEG artifact halos) – JPEGs degrade with each use – every time an image is saved.

Lossy vs lossless compression

–       two types of image compression algorithms – lossy and lossless

–       Lossless compression – reduce file size – no loss of image quality – use when image quality is more important than file size.  (e.g. tiff)

–       Lossy compression – discard information  (based on what human eye can see and what not at a particular size) – there are variable amounts of compression available –lots compression leads to ‘compression artifacting’ – (e.g. JPEG)

RAW formatting – (what we use in this unit)

–       Gives better post camera processing and is lossless

Colour balance

–       What is white?  This varies depending on the light source –

–       The eye adjusts for a white – the camera does not – unless we tell it to…

–       Slide 46 gives Kelvin temperature for various types of light source

(aside – some women have extra acuity to yellow – can see more definition – due to an extra type of colour receptor – based on the xx chromosome combination)

–       Daylight – about 6,500 kelvin – midday – summer’s day – but varies depending on latitude, full sun/overcast etc.

–       Flouro – nowadays are adjusted in colour more like daylight, used to give a green cast, tungsten – red/yellow cast, candle – red cast.

Camera Raw

Clipping warnings (can be turned off and on) – red for over-exp., blue for under exp.

–       Colour temperature may be changed in raw to improve clipping

Sequence of slides to show this – Slides 47-50.

Sean uses camera’s auto white balance, then manipulates colour balance in Camera RAW.

Saving files for different purposes:

For print: – psd, or Tiff – (lossless compression)

For screen – JPEG (less problem with compression –lossy)

–          DNG format – a Raw format – digital negative  – if you convert your raw files to DNG they are then readable/openable on any device whether your file type is loaded or not.

Suggestion – watch the video on post-processing (Cloud)

In Camera Raw – the presets button (2nd from tight on icons above histogram) will allow you to use the preview/unpreview button to show changes in all areas of Camera Raw.

Global vs. specific adjustment

Global adjustments – in camera raw – less damage to histogram (i.e. image levels)  – do as much as you can in Camera Raw.

Specific adjustments (e.g. for specific parts of the image) – in e.g. PS.

Adobe Lightroom is also on the Deakin computers and will handle Raw files

Also Apple program – Aperture program  (but not on these computers – Brad will deal with this during the week)

Assignment Brief for Assignment 2 – Due week 5 –

Next week – Looking at Photoshop layers etc.

Inside Out  – HDR editing – High Dynamic Range images –

Possible programs – HDR pro – with Photoshop

+ commercial programs

Read the reading – the written part of this assignments – interior spaces give clues – impart ideologies – how does your environment define you?  We project ourselves to the world – by our dress, demeanor etc.

500 word statement – comes from the readings (assigned and extra research)

NB – for blog entry – some examples of good HDR – Craig Sillitoe website – http://www.csillitoe.com/

Workshopping of completed images – discussion of own and other’s work in Week 5 – bring completed images.

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