Exercise: Higher and Lower Sensitivity

In the old days my camera bag was cluttered with various rolls of film from luscious colour slide, through bog standard ASA200 colour print to highly sensitive and big grain Black and White.  Many rolls were half used and rewound waiting for an opportunity to finish them off.

Of course nowadays my camera can replicate many of the results I obtained with my 'wet' medium at the flick of a dial.  Using a good digital has, however, made me a bit lazy and it was good to go through this exercise to remind me of the flexibility available.

Most of the time I shoot with as low an ISO setting as I can get away with so that my photos have as little noise as possible and can be easily blown up to full size without deteriorating.  When I am faced with a low light situation I usually try to get away with using my image stabiliser or a tripod if I am lucky enough to have brought one.  Some situations, however, need good control of depth of field or speed and I need to wind up the ISO sensitivity to get the speed and aperture settings that I need.

1/30  f5.6 ISO 1600

For example, this shot of the Guggenheim Museum in New York needed to be hand held so I selected the lowest speed I could get away with, an f stop that would just about give me enough depth of field and an ISO that exposed correctly.  All were a compromise as I would have loved to have shot at 1/200, f11 and ISO 100 but without a tripod and shooting quickly as compositions presented themselves that was not an option!

1/350  f11 ISO800

 Here I was shooting in the early morning light, working fast to get what I wanted before the sun rose too high so I preset ISO800 and worked without worrying too much as I knew that I would have enough sensitivity to cope with a reasonable shutter speed and f stop.  As can be seen I could easily have lowered the ISO as I had plenty of light at this point.

Closeup

At ISO800, as the closeup shows, noise wasn't a problem as my camera copes well with quite high ISO setting.  The following exercise asked us to shoot scenes at high and low ISO settings to compare the pros and cons.

 20sec f14 ISO200

 This image of a Miami Beach hotel is taken on a tripod using an insensitive ISO of 200 with a good f stop for depth of field and an appropriate time to expose properly.  The long exposure produces the smooth water on the fountain and the light streak of a passing car.  The depth of field is great and it makes a good image.

Closeup

  The 100% closeup doesn't show any noise and it is a nice clean image that could be enlarged to poster size without any concerns except for a number of artefacts that would need to be removed before printing.  These artefacts, seen below, show up as bright specks of white or coloured light are a result of the long exposure required for the shot.  With low light some pixels on the sensor prove to be inappropriately sensitive and display themselves as bright spots, they are often referred to as 'hot pixels'.  Their output is interpreted as real light and the camera's sensor logic highlights the area.  The appearance of these artefacts can be reduced by using the camera's long exposure or low noise settings or they can be manually removed post production.

 Closeup

1/25 f4 ISO6400

Shot again off the tripod, the hotel looks very similar, particularly when displayed at this size and only at 72 DPI.  The shutter speed is much faster having gone from 20 seconds to 1/25th of a second which has given a different effect to the water fountain, partly freezing the water.  The depth of field is much smaller but since most of the image is approaching infinity it all looks well focussed.  It isn't until we examine the close up that the differences can really be seen.

Closeup

Now we can see that the smooth and even colours of the image look mottled with noise and false artefacts when the sensor is set to such a high ISO.  This is the trade off between exposing for a long time with a low ISO setting and exposing for a short time with a high ISO setting.  It is the equivalent of using fast film which employs large silver halide salts to absorb the light quickly enough but has a large and visible grain when enlarged.   In a digital camera the noise comes from the amplification of the sensor signal (Gaussian noise) needed to increase its sensitivity to allow for the low light levels. The problem with noise on digital images is that the natural and uneven look to a grainy print is replaced by an much more regular and multicoloured effect.  We have grown used, over the years, to accept a grainy black and white image as a natural, indeed full of atmosphere and emotion which can enhance an image.  Noise on a digital image just looks like an electronic failure and detracts from the images purity and quality.  

  25sec f13 ISO200

Taken across a busy street, the exposure duration has much more impact on this image as  25 seconds with the shutter open gives rise to ghost images and streaks of light that add to the feelings of activity and light in the composition.  Some cars appear less blurred than others as they were waiting at traffic lights and moved late on in the exposure.

Closeup

The lack of noise at ISO200 is apparent here in the closeup and if people remain still enough they appear as normal as opposed to a blurred ghost.

 1/30 f4 ISO6400

 The advantages of shooting at a high ISO and therefore a faster speed is that the motion blur becomes manageable and the cars are frozen allowing us to include a limo into the image which would otherwise have been just a streak of light.

  Closeup

The drawback is, as I have previously mentioned, the level of noise that builds up at high ISO settings.

15sec f14 ISO200

Again here a long exposure has allowed good depth of field but given a strange look to the sitting man's feet as he moved them during the exposure.

 Closeup

No noise or artefacts which makes this an excellent choice of exposure for an enlargement.

 1/50 f5 ISO6400

At ISO6400 the sensitivity allows the speed to rise to 1/50th which gave me the chance to freeze the movement of some passers by, something impossible when the exposure is measured in seconds.

 Closeup

The level of coloured noise that occurs at a high ISO setting is very apparent against the white of the church front.

1/60 f5 ISO6400

Although noise is ugly and rarely acceptable in a colour image, when the image is converted to black and white the noise resembles our old friend film grain which we love.  This gives an acceptable alternative to a high ISO blow up that might otherwise be unusable.

 

Although in this case I rather miss the blue palm tree!

Darcy Padilla - The Julie Project

I was browsing my fellow students blogs looking at some of the great work out there and came across a link to the Julie Project.  My thanks to Dave Whenham (FatherPie) for pointing me there.

I am finding it hard to get my thoughts together after looking through Darcy's work with Julie as she documented the last 18 years of her life.  It puts my own photographic efforts into perspective when I see what an impact a photograph can achieve.  The heart rending and tragic story of Julie's life is portrayed with such feeling and skill I am in awe of her as a photographer and a person.  Her photo journalistic ability is, in my mind, only matched by such stars as Bill Brandt and Bert Hardey and I hope that the recent awarding of 2010 W. Eugene Smith Award in Humanistic Photography is the first of many she receives in recognition of her efforts.  It is apt that I am working through the TAOP module in Light as it is Darcy's ability to use light and exposure in the most difficult and trying of circumstances, both technical and emotional that I find so amazing.  In particular her shot of the birth of Julie's last child Elyssa found here is a truly great example of the skill of light exposure and of finding the decisive moment.

I am sure that I am only one amongst many who will be moved by Darcy's photographs and story telling and I urge all to visit her web site and see for yourselves.

Exercise: Measuring Exposure part 2

The second part of this exercise asked us to photograph a number of subjects shooting each 5 times from -1 stop, through EV0 to +1 stop at half stop intervals in order to see which exposure level looked best and which gave unexpected but good results.  My choice for the best exposure level is the larger example.

 -1

-1/2

EV0

+1/2

+1

The doves are one of the few subjects that I shot which more or less work at all exposures from -1 to +1 stops.  The -1/2 stop to EV0 work best as the background colour is nicely saturated and the details of the doves are at their best but they are acceptable even when slightly overexposed.

-1

 

 -1/2

EV0

+1/2

+1

Like the doves, the drum kit also managed to look quite good all the way from -1 to +1 stop of EV range.  I believe it is because that neither composition has any large extremes of light value.  As we can see from the histograms below, both images show a fairly constant light level that can accommodate a small movement in exposure without becoming disastrously under or over exposed. We will see later on that when there are extremes of light values then a movement from the ideal exposure results in very poor results.

Dove histogram EV0

Drum histogram EV0

 

-1

-1/2

EV0

 +1/2

+1

This gloomy drizzling day on the moor walking the dogs was a day where the camera was having difficulty in coping with the low levels of reflected light which meant that the cloud was just about the only source of relative brightness.  As a consequence, everything below +1 stop was underexposed.

-1

-1/2

 EV0

 +1/2

 +1

The Japanese snow lantern here is in an area of fairly constant tonal values so with no particular highlights or deep shadows EV0 is a very good exposure value.  Because I prefer the greens to look a little more saturated I prefer the -1/2 stop value but there is little to tell between the exposures until we get to +1 stop which looks decidedly washed out.

 

 -1

 -1/2

 EV0

+1/2

+1

The white barked tree here is shot against a pale sky just after sunrise but despite the low light levels the sky still completely dominates the tree and it can only really be seen properly at +1 stop.  Any lower EV than this starts to turn the tree into a silhouette which, given the attractive detail, wouldn't be good.  The histogram for this shot at EV0 shows the large hump representing the sky and the smaller hump which is the tree.  Any lower EV and the graph moves left until the tree is completely black.  Even at EV0 the tree is a long way left on the graph and to be exposed properly it needs to be moved closer to the centre by increasing the exposure value.

 Tree histogram

 -1

 -1/2

 EV0

 +1/2

 +1

Despite being a similar composition to the tree above, this clock tower occupies a larger area of the sensor so that the camera has tried to expose it as the main subject despite the brighter sky around it.  The nature of the clock face, however, benefits from slight under exposure to highlight the rusty streaks that line it and to give it a more dingy feel.  So a -1 stop exposure suits it much better than the +1 stop exposure that I chose for the earlier tree.  Out of interest the histogram for the clock looks like this:

 Clock histogram EV0

 Clock histogram -1