Delayed Shutter Action With Digital Cameras

‘Shutter Delay’ or ‘Shutter Lag’ is a known problem with most digital cameras. The problem has been addressed by the manufacturers in recent times as a consequence of critical comment and is subsequently becoming less of a problem with more recent models.  SLR (single lens reflex) digital cameras have a different shutter mechanism and the problem has always been less with this type of digital camera.

Almost all digital cameras have a ‘point and shoot’ option.  This camera setting is sometimes referred to as ‘automatic’ or ‘program’ etc. depending on camera model.  This option utilises both the automatic focus and automatic exposure functions and is normally activated by pressing a 2 stage trigger down to the first detent and then holding it momentarily before continuing with the final press to release the shutter.  This process of determining the correct settings for focus and exposure is an essential step and it introduces a delay which can present a problem with action shots.  The delay varies depending on a number of things including the level of lighting, (poor lighting is notorious for slowing down the auto focus function).  The use of the LCD monitor instead of the viewfinder also adds slightly to the delay.  The actual delay is difficult to quantify as there is quite a variation across the range of cameras.  However, I have seen figures quoted ranging from 1/10 of a second to more than one whole second.

Shutter delay should be looked at in two parts;

1) The first part, as just explained, is the delay incurred following the first press of the trigger to the detent position as the electronic circuitry of the camera calculates and sets the focus and the exposure.

2) The second part is the delay that follows the second press of the trigger until the shutter is fully open.  This second consideration is quite small, around 1/10 to 1/4 of one second but is perhaps a little insidious as the expectation is that the second and final press of the trigger would be ‘instantaneous’.  A horse going over a hurdle could travel a couple of metres in 1/4 of a second and so be out of position in the photo unless an allowance was made.  So this second component, although quite small, is also worthy of consideration. (Interestingly, a comparative value for SLR digital cameras of this second component is around 1/20 second or better and in practice can be treated as zero.)

The use of ‘pre-flash’ which is part of the operation of some cameras and optional after-market flash attachments also has a delaying effect if used, but this is really a separate consideration.  (For those interested, the pre-flash occurs after the second trigger press but just prior to the lens opening.  This enables a value to be calculated for the optimum flash intensity which is then used by the actual flash.  Some of the more expensive cameras and external flash units also incorporate a discreet light beam to assist auto focusing in poor light conditions.)

Red-eye reduction options associated with both the inbuilt flash function and with add on flash units is something that should be viewed quite separately again.  When used, the red eye reduction function adds significantly to shutter delay because of the use of pre-flash.  The use of this option involves a number of less intense flashes, in very quick succession immediately prior to the normal flash.  This technique relies simply on a contraction of the pupil in the eye caused by the successive, less intensive pre flashes. The actual ‘red’ that causes the problem is the flash reflecting of the concentration of blood vessels in the back of the eye.  The phrase “red eye reduction” is an apt phrase as most times the ‘red eye’ effect is reduced rather than completely removed.

So, the practical considerations are;

·         It is possible to take a photo by simply pressing the 2 stage trigger all the way down in the one motion, however, this method is not recommended.

·         The 2 stage shutter release technique is obviously the preferred method as camera movement is reduced to a minimum and the major component of shutter delay is removed from further deliberation.

·         Where an action shot is planned, capture the focus and the exposure in anticipation.

For example; when photographing a horse jumping over a hurdle with the intention of capturing the horse exactly mid-way over the hurdle.  Anticipate and capture the focus and exposure just before the horse reaches the hurdle. Assuming that you were to be positioned perpendicular to the line of the horse this could be done by panning on the horse and taking the first trigger pressure just before the horse reached the hurdle with the trigger pressure being held in readiness for the final press a seconds or two later.  If it were too difficult to pan the horse, it may suffice to capture the focus and exposure on the hurdle a few seconds before the arrival of the horse.

 

 

 

Exposure & Depth of Field

 

Point and Shoot cameras take care of different lighting situations by automatically setting values for aperture and shutter speed.  This is usually done in a very adequate manner, however it is interesting to delve into the detail.

 A photographic image is captured by passing light through the lens of a camera which is then recorded on either a light sensitive, receptive, chemically coated surface (film) or on a light sensitive, receptive Silicon Chip (digital). There are many different types of film and there are a few variations in basic chip design [1] but the fundamental process is common to both film and digital.

 There is an obvious requirement for the recorded image to be properly captured - that is, to be  properly ‘imprinted’ or ‘burned’ onto the receptive surface to just the right extent (not too dark, not too light).  This part of the process involves the matching up of 2 basic variables;

1) the first is the sensitivity of the receptive surface;

that is, the extent to which the receptive surface will react to a given amount of light.  Conventional film is available in a range of sensitivities, each being identified with a distinct ISO rating number.  Digital cameras mimic the same situation by providing a range if ISO settings.  The ISO references commonly used range around 80 ~ 400 with 100 being common.

2) the second of the two basic variables is the ‘quantity’ of light that strikes the receptive surface;

that is, the quantity being the net measure of light that results from;

         1) the brightness of the light source,

         2) the amount of light that is allowed in (aperture setting) and

         3) the length of time that the light falls on the receptive surface (shutter speed).

So, it follows that for a ‘given level of lighting’, the perfectly exposed image will result from a particular summing of the following three factors;

         1) The ISO setting, (the sensitivity of the receptive surface),

         2) The f setting (aperture) and

         3) the shutter speed (shutter open time expressed in fractions of a second).

If one value is altered but then compensated for by altering another to maintain the sum, then optimum exposure will be maintained.  This is not strictly correct as there are other considerations involved here, with one such being that ‘noise’ [2] increases along with increases in ISO values. [3]

It can be seen now that options are available which accommodate a variety of lighting situations while still allowing for optimum exposure.  To make full use of these options it is, of course, necessary to use the manual mode rather than the point and shoot automatic mode. There would be many situations where a manual setting would provide an advantage.  For example, a particular high-speed sporting action requiring a very short exposure.  Another example might be where a change in the ISO setting was preferred over a change in aperture to enable the same shutter speed to be maintained in a poorer light.  There are obviously many variables to experiment with and with a digital camera there is virtually no costs involved until you decide to print.

A further consideration which should be included when studying ‘Exposure’ is the relationship between ‘Aperture’ and ‘Depth of Field’. A smaller apertures (higher f number) results in a deeper, (longer) depth of field and conversely a larger aperture (lower f number) results in a shorter depth of field. (see info text box at bottom of page)

There are many factors involved in this consideration due to the differences in camera design and the variety of lens types so I am providing only a very general explanation here.

What is ‘Depth of Field’ (?)

Depth of Field defines the depth, or distance as measured from the nearest point of ‘acceptable clear focus’ to the ‘furthermost point of acceptable clear focus’.  Or put another way ‘the area contained between these two points’. (see page 3)

Under some circumstances the depth of field (the area having acceptable clear focus) may be well defined and obvious and have the potential to enhance or to degrade a photograph.

For a given aperture setting, the use of a wide angle lens (the shortest focal length on a camera with zoom) will result in a greater (longer) depth of field whereas the use of a lens with increased focal length (zoom/telephoto) will result in a shorter and more obviously defined depth of field.  Also, the depth of field increases as the distance of the subject in focus increases.  From what I make of it, when using fairly short focal length lenses for landscape subjects, the depth of field consideration is not an issue.  However, the effect becomes quite noticeable when using zoom, especially if the subject of focus is close.

 

End Notes ---------------------------------------

[1] CCD (Charge Coupled Device) and CMOS (Complementary Metal Oxide Semiconductor) image sensors are two different technologies for capturing images digitally. Each has unique strengths and weaknesses giving advantages in different applications. Neither is categorically superior to the other, although vendors selling only one technology have usually claimed otherwise. (More info on this subject from “http://www.dalsa.com/markets/ccd_vs_cmos.asp”)

[2] In most ccd chips noise shows up as squiggly random patterns of colour spread throughout the image. The best way to avoid this is to use good lighting where signal to noise in very high. Don't try to push the ISO value of the ccd chip to higher values than it was designed to use, or it will produce this type of noise.

[3] Larger chip sizes (higher pixel count) which are becoming increasingly common, substantially reduce this problem.

 

 

 

Illustration of well defined Depth of Field created by using a large aperture (f2.4).

 

 

 

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