photo:light_values
light values and exposure metering
Digital exposure using the histogram method
thus, most digital photographers prefer to check the histograms to ensure none of the main image peak goes past the right side of the histogram in any of the 3 channels - although, inevitably with high contrast situations where there are 2 peaks, the right peak for very bright sky, reflections or the light source itself, will usually be to the right of the limit of the histogram and is thus ignored, while the right edge of the left peak representing your subject does not reach the right limit.
light values:
given that flash photography will be practically useless, and tripods not feasible, one will generally need to rely on hand-held photography, so will your camera/lens combination cut it in taking sharp photos without camera shake?
you could just buy the most expensive camera and hope it will do, but the alternative is if you know the light value that you are likely to be working with (which I give below), then you can determine your requirements more accurately.
given my calculations for paintings in the Louvre of -1 to +2 light value, this will mean that you can use ISO 200, f/2.8 and shutter speed will need to be 1/4 to 1/15th second which is too long hand held unless you have an image stabiliser or you use a higher ISO or wider aperture. Thus most consumer cameras would not be adequate to get good results with low noise.
the difference of 1 for a light value equates to 1 f-stop difference.
if you double the distance of a light source, this will result in the light reducing to 1/4 which equates to 2 f-stops less or 2LV less.
camera manufacturers often give exposure range for their light metering, auto focus capability, etc in EV ranges, to determine exposure value (EV) from LV, just add 5 to LV assuming you are using ISO value of 100 for determining EV.
The S+L = A+T exposure system:
An EV (exposure value) is equivalent to a combination of f/ratio, shutter speed at a given ISO value (usually 100ISO), with a value of 0 being 1 second at f/1.0., thus EV = S+L = A+T.
The S+L=A+T exposure system uses this concept, but expands on it to create a value for each of the 4 variables of Speed (film speed), Light intensity, Aperture & Time (shutter speed) such that an integer difference in any of the values equates to 1 f stop difference, and the sum of S+L equals the sum of A+T which equals the EV at ISO 100:
Speed: ISO film rating of 100 has a value of 5, each time you double the ISO value, you increase this value by 1, conversely, halving it drops the value by 1, thus ISO 1600 = 9. (S = log2 (0.3 x ISO))
ISO 1600 = 9
ISO 800 = 8
ISO 400 = 7
ISO 200 = 6
ISO 100 = 5
Light: bright sunlight = 10, for more see below (I use abbreviation LV for this) also LV = log2 (brightness in foot-Lamberts)
Aperture: f/4 = 4, each f-stop change gives an integer change in the value, thus f/8 = 6 as it is 2 stops different to f/4. A = log2 (f-stop^2)
f/16 = 8
f/11 = 7
f/8 = 6
f/5.6 = 5
f/4 = 4
f/2.8 = 3
f/2.0 = 2
f/1.4 = 1
Time: 1sec = 0, each time you halve the shutter speed, you decrease exposure by 1 f-stop, and you add an integer to the T value, so that 1/125th = 7 and 1/1000th = 10. T = log2 (1/exposure time in seconds).
1/1000th = 10
1/500th = 9
1/250th = 8
1/125th = 7
1/60th = 6
1/30th = 5
1/15th = 4
1/8th = 3
1/4 = 2
1/2 = 1
1sec = 0
2 secs = -1
examples of light values:
light value | example ambient lighting scenes |
10 | bright sunlight |
9 | subjects backlit by sun with light background to give a silhouette (try 7-10LV); photos of the moon +/- 1stops; |
8 | |
7 | outdoor shade (light from midday blue sky) |
6 | overcast sky (6.5LV ~1000lux; subjects backlit by sun with dark background (try 5.5-7LV) - may need a warming filter; |
5 | light from blue sky at sunset; crescent moon; ie. landscapes immediately after sunset; |
4 | sports ground night lighting (3.5-4 eg. ISO 3200, f/2.8, 1/1600th sec); indoor lighting (3-5); Paris' Dome Church ceiling in daylight; Church stained glass window in daylight; twilight 30-45min after sunset of mid-toned subject - bracket +/- 2 fstops; neon lights; |
3 | under a 300W reading lamp |
2 | office lighting, Westcott Ice Light 2 at 1m 1700lux |
1 | average living room lighting; heavy cloud, rain (0.6LV ~100lux); slit lamp for corneal flourescein; |
0 | the Louvre paintings (most are -1 to +2, eg. Mona Lisa 0.5, although some are very dimly lit at -2.5 to -2) |
-1 | night scenes of street lighting on buildings/streets (-2.5 to +0.5 eg. 1/15th sec ISO 2500 f/4); twilight (~10lux) St Peter's Basilica interior (1/15th sec ISO 400 f/2) |
-2 | light from a candle at 1 foot = 1 foot-candle; church cathedral interiors (eg. Notre Dame -3 to -1); fireworks; |
-3 | lightning +/- 2 stops; |
-4 | light from a candle at 2 feet = 0.25 foot-candle; distant view of city skyline lights; |
-5 | light from a candle at 1 meter = 1 lux = 1 meter-candle = 0.09 foot-candle; deep twilight; |
-6 | |
-7 | night scene lit by full moon overhead (~0.27lux); photo of bright stars (~0.0001lux for -1.4 magnitude) |
-8 | Milky Way |
-9 | |
-10 | |
-11 | photo of deep sky objects such as comet tails, nebulae (~0.0000001lux for 6th magnitude objects) - see astrophotography |
-12 | |
Zone system for reflective light metering
A few more concepts:
Very brief duration light sources:
the above S+L=A+T exposure system only applies to light sources that have a duration longer than the shutter speeds used.
electronic flash units have a duration of 1/300th sec to 1/20,000th sec depending on the unit and output selected, thus the camera shutter speed selected only determines ambient lighting effect and does not affect the flash lighting which is dependent on aperture and ISO, hence the use of the guide number (GN) system for determining flash output.
lightning also is very brief and its brightness is dependent on many factors including its intensity, distance and the amount of cloud or rain between the lightning and your camera. For lightning a few kilometres away, consider using f stop of f/5.6 to f/8 at ISO 50 and then set shutter speed according to desired ambient lighting effects needed (eg. for night streetscapes try 8-30 secs). A long shutter speed is usually needed to maximise the chance a lightning strike will occur during the exposure, unless you have a light-sensitive shutter trigger that takes the exposure immediately lightning occurs.
What is the true ISO?:
Illuminance:
Reciprocity failure compensation for most films:
1/10th sec ⇒ + 0.5 stop
1 sec ⇒ + 1 stop
10 sec ⇒ + 1.5-2 stops
120 sec ⇒ + 2.5-3 stops
Dealing with high contrast scenes:
digital sensors, like transparency film can only deal with 5-6 f-stops of dynamic range, thus for high contrast scenes such as landscapes when one wishes to show detail in the clouds/sky as well as in the scene shadows then one must resort to other means of obtaining it such as:
use a higher dynamic range film such as B&W (expose for the shadows, adjust development for the highlight range needed) or color negative film
use a higher dynamic range sensor such as medium format sensors (but these are very expensive) and save as HDR file.
use a gradient filter on the lens - but this adds to lens flare and may not match the geometry of the highlight region
-
see
Gary Maico's pics - US seascapes using Singh-Ray variND filter (~$US399), ND grad filter, Clor Grad filter, Blue & Gold Polariser.
use a polarising filter to darken the sky - but only if sky is approx. 90deg to the sun
use infra-red filters to darken the sky - but this then creates an infra-red effect
contrast mask blend a single image in Photoshop:
use HDR techniques by taking 3-5 bracketed exposures 2-3 f-stops apart and blend images in Photoshop:
photo/light_values.txt · Last modified: 2018/01/28 05:21 by gary1