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Old Apr 24, 2010, 1:35 PM   #1
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Default low light shooting

not really sure where to post this, so if this is the wrong place i'm sorry.
anyway, after reading Understanding Exposure, i have two specific questions (hoping Mark1616 and JohnG see this, since they have been involved in my informal education a lot )

1. In the Understanding Exposure book, the author (metering chapter) seems to meter with a wide open lens on a background/sky and then manually adjusts/calculates the shutter speed for a higher f-stop number. Why not meter with the target f-stop right away? why is he doing math in his head? As i am writing this i think it dawned onto me, but i hope somebody answers this anyway.

2. I was repeatedly told by people much smarter than me, that freezing action in low light situations depends on the flash rather than on the shutter speed. I dont quite get the math in this case, but that is the only think i am looking at (math). if the flash lasts anywhere between 1/1000 and 1/4000 and the shutter speed is lets say 1/8 sec, how can that freeze action since the sensor is exposed to the low light longer than to the flash? is it voodoo?

as usual, thanx for help
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Old Apr 24, 2010, 1:49 PM   #2
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I wrote the following below as a tutorial for another forum, hoefully you will find it of some use!

I have noticed that recently there has been a number of threads or questions posted about how to acheive correct exposure. As I have taught the basics of exposure control to many people over the years I thought I would try to put down on paper (or screen if you prefer) my take on the subject.

Hopefully some of you may find it useful.

Understanding exposure is not that difficult!

Camera Controls:

On the more advanced cameras with fully manual modes there are three main variable controls which you can adjust to get the correct exposure, although adjusting each will have some effect on the image. These are the Shutter Speed Control, the Aperture Control and the ISO setting control.

If your camera has Auto modes such as 'Aperture Priority (A or AV)', 'Shutter Priority (S or TV)' or 'Programmed (P)' auto modes you will usually have an exposure compensation facility which you can use to control the exposure.

We will look at how making adjustments to the above controls effect the final image a little later. As with most things in life, you don't get something for nothing and each adjustment will have a bearing on the final image.

Understanding how the Camera's Exposure Meter works:

Almost every camera these days has a built in exposure meter, and most problems with exposure are caused by the camera user not understanding that the camera does not know what you are pointing it at when you are taking an exposure reading. The camera's exposure meter is calibrated to a mid grey of between 12% and 18% grey, and therefore if you point it at anything other than a mid grey, or if the multi-matrix metering doesn't average it out to a mid grey then an incorrect exposure will result.

Imagine therefore the following two scenarios:

What does the meter do when you point it at a bright subject, and what does the meter do when you point it at a dark subject?

If you point your camera at a predominantly ‘light’ subject the camera will say 'that's a bright mid grey' and suggest (or adjust if in an auto mode) that the exposure is reduced and under-exposure will result. This will result in grey whites and blocked out dark tones.

To try this for yourself, take a picture of a white piece of paper with the cameras controls set to the reading obtained from metering from the paper.

1/160 @ f16 at ISO400


If you point your camera at a predominantly ‘dark’ subject, the camera will say 'that's a dark mid grey' and suggest (or adjust if in an auto mode) that the exposure is increased and over-exposure will result. This will result in grey blacks and burnt out white and highlights.

To try this for yourself, take a picture of a black piece of paper with the cameras controls set to the reading obtained from metering from the paper.

1/160 @ f4 at ISO400


Now take a picture of the white paper, black paper and a grey paper/card with the exposure reading selected being that taken from the grey card.

1/160 @ f8 at ISO400


So knowing this, we can, by ensuring we are aware of the tone we are metering from, compensate for the camera's lack of intelligence: To this end we can follow the steps below and use the information provided to give you a basis to begin to understand how to get a correct exposure.

The most important thing is that you are 'fully aware' what TONE you are metering from and where you want it to fall on the tonal scale. The important tones to remember are the brightest tone in which you wish to retain detail (white feathers on a bird, wedding dress etc), the darkest tone in which you wish to retain detail (texture of rocks in shadow, tyre tread etc) and a mid grey (green grass, deep blue midday sky, light tarmac etc). To this end it is an advantage to use the 'spot metering' mode if your camera supports it.

If you are metering from 'the brightest tone in which you wish to retain detail' then it is important that you increase the exposure by 2 full stops to compensate for the meter assuming that the tone is a mid grey.

If you are metering from a 'mid grey' then use the reading obtained.

If you are metering from 'the darkest tone in which you wish to retain detail' then it is important that you decrease the exposure by 2 full stops to compensate for the meter assuming that the tone is a mid grey.

If you have a camera which uses or has the facility for Auto modes then you can achieve the same results as above by using your exposure compensation facility to increase or decrease the exposure as required.

Relationship between the camera controls and a given exposure:

The camera controls mentioned earlier all relate to one another to control the exposure, and each have individual effects on the final image, so a conscious decision must be made as to which you adjust to achieve the result you require.

Shutter Speed: The shutter speed control does as it says on the tin, it sets the duration that the sensor will be exposed to light. A faster shutter speed will let in less light than a slower shutter speed, and the range of speeds can usually be set on an advanced camera between the ranges of 30 seconds – 1/8000th of a second. These can be adjusted in 1/3, 1/2 or full stops depending on your camera model. Faster speeds have the potential to freeze action but require more light whilst slower speeds can be used to blur movement.

Aperture Control: The aperture control sets the diaphragm contained within the lens which controls the amount of light which is able to pass through it during the exposure. The lens aperture is usually specified as an f no which is the ratio of the focal length to the effective aperture diameter. The Aperture control will adjust a number of f-stops which can be set. A lower f-number denotes a greater aperture opening which allows more light to reach the film or image sensor. The photography term one f-stop refers to a factor of √2 (approx. 1.41) change in f-number, which in turn corresponds to a factor of 2 (doubling or halving) change in light intensity. Lenses can again be adjusted in 1/3, 1/2 or full stops depending on your camera model. The higher the f number used the greater the depth of field* and the lower the amount of light which is passed through the lens. The lower the f number the greater the amount of light which is passed through the lens and a corresponding reduction in the amount of depth of field*.

* Depth of field is the amount of the image which appears to be in focus either side of the actual point of focus.

ISO control: This controls the sensitivity of the sensor/film to light. Typically higher ISO settings will result in lower quality images suffering from increased grain or noise, although on many of the newer digital cameras the sensitivity of the sensors has been improved allowing higher ISO setting with little loss of quality.

Exposure variations for a meter reading based on a reading from a mid grey tone.

ISO setting 200
f number f8
shutter speed 1/125

If a faster shutter speed was required to stop action then we have to adjust like for like

Increasing the shutter speed with a corresponding increase in ISO whilst maintaining the f number.

ISO setting 800
f number f8
shutter speed 1/500

Increasing the shutter speed with a corresponding increase of the size of the aperture
ISO setting 200
f number f4
shutter speed 1/500

Decreasing the aperture to increase the depth of field whilst keeping the ISO low to maintain quality, maintaining the shutter speed not an issue as camera mounted on a tripod.

ISO setting 200
f number f16
shutter speed 1/30

Increasing the aperture to give limited depth of field whilst maintaining the ISO

ISO setting 200
f number f1.4
shutter speed 1/4000

All the above settings give the same exposure but with different results increased or decreased depth of field or increased or decreased shutter speed or increased ISO.
If the original readings were not from a mid grey tone but from either ‘the brightest tone in which we wish to retain detail’ or the ‘darkest tone in which we wish to retain detail’, then the original exposure would have to be adjusted to compensate:

Original meter reading.....................Brightest Tone.................... Darkest Tone
ISO setting 200...........................ISO setting 200...................ISO setting 200
f number f8..................................f number f5.6.......................f number f11
shutter speed 1/125..................shutter speed 1/60.................shutter speed 1/250

The above compensations are just one variant of the possible variances between the different controls to achieve the required result. Provided the controls are adjusted in the same proportions to maintain the exposure you can then decide how to alter the controls to get the photographic effects you require.

One further note, if the range of tones between light and dark are too great for the dynamic range of the sensor, then you have to make a decision on where to base your exposure. There are four main options:

1. Meter and correctly expose for the subject and let the other tones fall where they will.

2. Meter from the brightest tone in which you wish to retain detail and open up by 2 stops, this will preserve the highlights. This is not recommended if the main subject is dark as the tones will be blocked out!

3. Take more than one exposure of the same subject at different exposures and blend them in an HDR program.

4. Use a graduated ND filter to balance the bright and dark areas (if possible) typically used to balance the sky against the ground in landscape and seascape shots.

All photos are straight jpgs out of the camera, no PP at all.
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Old Apr 24, 2010, 1:59 PM   #3
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Point 2.

The flash duration freezes action providing that the subject is fully exposed by the flash only. If you include a degree of ambient light in the exposure then it is likely there will be some subject movement visible.

The amount of movement will be proportional to both the slowness of the shutter and the ratio of flash to ambient light!
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Old Apr 24, 2010, 4:24 PM   #4
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Quote:
Originally Posted by Ed Bray View Post
Point 2.

The flash duration freezes action providing that the subject is fully exposed by the flash only. If you include a degree of ambient light in the exposure then it is likely there will be some subject movement visible.

The amount of movement will be proportional to both the slowness of the shutter and the ratio of flash to ambient light!

so if i understand this correctly: i would use spot metering and meter of the dark areas of the frame, recompose on the moving subject and shoot. Effectively, even if the camera says that the that SSis lets say 1/8, the EFFECTIVE shutter speed will be much faster, because it is only recording the "lit up" subject.

btw, thank you very much for the tutorial above on metering. it certainly filled some knowledge gaps as well.
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Old Apr 24, 2010, 4:29 PM   #5
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pretty much.

If you check out my thread, and see the difference between the first 2 and the 3rd shot. You will see if you meter on the brighter spot, you will lose the details of the darker areas by having it over exposed.

http://forums.steves-digicams.com/tr...auderdale.html

PS I used my old light meter on the 3rd shot, since I packed it with my wife's epl-1. They really help out in night photography
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Old Apr 24, 2010, 5:33 PM   #6
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shoturtle, i'm guessing the 2nd one was metered on the bright areas, or do i have this flipflopped
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Old Apr 24, 2010, 5:39 PM   #7
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Yup on the second one I was lazy and let the camera meter on the building and did not compensate for it correctly, so you did not get the details of the sky like the 3rd one.

This is what happens when you let the camera do everything, and not correct for it like the 2 from my thread. You turn night into twilight and get the purple fringe from the lights.
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Old Apr 25, 2010, 5:57 AM   #8
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It is a pity there is no tutorial section on these forums where something like this topic could be placed as I suspect many people could benefit from it, but will never see it as they do not look at the Canon forum!
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Old Apr 25, 2010, 6:02 AM   #9
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Quote:
Originally Posted by Ed Bray View Post
It is a pity there is no tutorial section on these forums where something like this topic could be placed as I suspect many people could benefit from it, but will never see it as they do not look at the Canon forum!
Haven't read the whole thread - but a "Tutorial"-section would be something I'd really love to see here at Steve's
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Old Apr 25, 2010, 11:10 AM   #10
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Thank you Ed Bray, as someone new to photography that tutorial is just the kind of information I'm looking for right now.
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