Module 22 Section2 
 

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Module 22:

 Basics of Photography, Part 2
 

Section 2:

 Flash and Focal Length
 

 

    
 

 

Flash photography

Flash power, coverage, and range

Flash sychronization

Flash Modes

Camera Lens Focal Lengths
     
       
 

Flash photography

 

As we found out in modules 21 and 22, good image exposure is very much dependent upon just the right amount of light exposing the film or the digital sensor.  Having a sufficient amount of light gives us more choices of shutter speeds and f-stops, and may allow us to use a slower speed film or digital sensitivity rating for better image quality.   When available light is lacking, we can use an electronic flash to illuminate the scene.  Although we would not be able to get an image without a flash in some situations, there are some undesirable qualities of electronic flash, especially when used in medical photography.

 

Flash power, coverage, and range

 

Most light sources, including electronic flash, give off diverging light. This means, that as the light travels away from the source, the light rays get farther and farther part, as demonstrated here:

 

This means that the objects close to the flash are relatively brightly illuminated, and objects farther away are more dimly illuminated.  And the progression is geometric, meaning objects become more dim dramatically as you move away from the flash.  So, as you might expect, each flash has an effective range.

 

The effective range depends upon how powerful the flash tube is, the focus of the flash lens (this is the plastic piece in front of the flash tube), the ISO of the film or sensor, and the f-stop setting of the camera lens.

 

Obviously, the more powerful your flash is, the better the range will be for any given camera/film combination.  Of course, there is always a tradeoff.  The higher the light output of the flash, generally the larger the flash tends to be, and the more power each flash consumes.  This translates into more batteries and shorter battery life.
 

The built-in flash on the point-and-shoot camera has very limited range, typically under 15 feet.  This flash has to be small to fit in the limited space, and power is limited because it draws its power from the same battery that powers the camera electronics and wind mechanisms.
  The close proximity of the flash to the lens produces the red-eye flash reflex. 

 

 

We know from retina photography that the red-eye reflex is a reflection of the retina back through the pupil that only occurs when the light source being reflected is close to the axis of the observers eye or the lens of camera.  Some manufactures have tried to minimize this effect with "red-eye reduction".  The camera produces two closely timed flashes.  The first flash is supposed to reduce the pupil size so that the effect is minimized.

  
  More expensive cameras have a "hot shoe", which will accept an accessory flash.  These flash units are more powerful, they use their own batteries instead of the camera system battery, and the increased distance from the lens eliminates red-eye reflexes.

 

 

Many professional photographers use flash units that are handle mounted for even more power and flexibility.  Batteries in the handle or in an external pack provide more flash power, and the unit can be mounted or held away from the camera for more variety in illumination.

 
 

 

Flash synchronization

 

When using an electronic flash, the flash illumination must be synchronized with the camera shutter.  In other words, if the flash goes off and the lens shutter is not wide open, you will either get no picture, or only a portion of the picture.

 

The photo on the right below demonstrates a problem with flash synchronization.  The shutter speed is too fast for the flash.  The shutter has started to close when the flash has gone off, causing part of the frame to be cut off.

 
 
       
 

If the flash unit is built into the camera, flash synchronization is not a problem, but if an accessory or external flash unit is used, the shutter speed must be synchronized with the flash.  For most camera bodies, this is a shutter speed of 1/60 or slower, or a shutter speed setting called the "X" synch.  On many camera bodies these settings are marked in red as a reminder.

 

So, when using electronic flash, shutter speed is usually not a variable when determining proper exposure because it is usually set on 60 or the "X" synch position.  That leaves the lens f-stop and the flash power to determine the exposure for any given ISO rating.

 

Flash Modes

 

Cameras with built in flash usually set the exposure automatically, giving you some control with different flash modes.  

  • The "automatic" mode trips the flash any time the exposure meter reads low light levels. 

  • The "fill flash" mode trips the flash no matter what the exposure meter reads.  This is useful for giving extra exposure to subjects in front of a relatively bright background.

  • The "red-eye" mode double flashes the subject to reduce pupil size, as discussed earlier.

  • The "slow-sync" mode slows the shutter speed below 1/60 for extra background exposure when using the flash in night scenes.

Accessory or external flash units usually have a manual mode and several semi-automatic modes.  

  • The manual mode fires the flash full power.  The flash unit has a table of values that tell you which f-stop to use for a given flash-to-subject distance and a given !SO rating.

  • The semi-automatic modes allow you to choose between several f-stop values.  The higher the f-stop, the shorter the flash range will be.  The flash unit has its own light meter system that adjusts the flash power for any flash-to-subject distance within the chosen range.

With the shutter speed set to a relatively slow level when using a flash, you might think that there is a danger of a blurry image due to the motion of your subject or camera shake.  Actually, the flash is of such short duration that the image is frozen.

 
  Even though the shutter speed is set at a relatively slow 1/60 sec for this flash photo, the action is frozen because of the short duration of the flash.

   

 
 

Exposure with a retinal imaging system is controlled only with changing the flash power output.  The shutter is set to synchronize with the flash and the lens system does not have an iris to control the f-stop.
   
 

 

Camera Lens Focal Lengths

The focal length is the distance from the center of the lens to the image it forms of an object at an infinite distance in front of the lens. With a simple plus lens it can be diagramed as follows:

With a 35mm camera it is the distance from the film plane to a point within the system of lenses that makes up the 35mm lens.
  Camera lenses come in a variety of focal lengths.

       
 

The "normal" lens for a 35mm camera is a 50mm lens. This gives a "normal" perspective to a two person photograph.
       
  Shorter focal length lenses from about 20 to 40mm are called "wide angle" lenses in the 35mm format. These lenses tend to make an object look smaller in the frame and they expand the angle of view. 

       
 

Longer focal length lenses are called telephoto lenses (70-200mm). These lenses narrow the angle of view and make objects appear larger in the frame. They allow you to get closer to the subject matter visually, while physically being farther away. Remember that for any lens, the higher the magnification is, the narrower the view will be.
       
 

There is also an inverse relationship between the focal length and the depth of field. Wide angle lenses have a greater depth of field as compared to telephoto lenses.
       
  Because of excellent depth-of-field, foreground and background in this wide angle photo are in sharp focus.
       
 

The shallow depth-of-field in this photo taken with a telephoto lens is evident as the highway center line quickly blurrs out in the distance
     
 

Some lenses have fixed focal lengths.  Many lenses sold today are "zoom" lenses, meaning the focal length of the lens can be changed.  The front of the lens is usually marked with the zoom range of the lens.  When comparing cameras, remember that the zoom ranges of the lenses are only comparable if the format is the same.  For example, a 24mm focal length lens in a full frame 35mm film format camera will not have the same angle of view as a 24mm lens in a 3 megapixel digital camera.  Generally, the smaller the frame format is, the narrower the angle of view will be for a given focal length.

 

The ratio of the focal length to the diameter of a lens determines its light-gathering power. This characteristic is referred to as the "speed" of a lens. Because of the shorter focal lengths, wide angle lenses are "faster" than telephoto lenses for a given lens diameter. That is, they gather more light in low light situations. A "1:1.2" lens is faster than a "1:3.5" lens.  This is usually also marked on the front of the lens.  For a given focal length, faster lenses are usually better quality lenses.

 

 

Most camera lenses will not focus closer than 2-3 feet.  "Maco" lenses are designed to focus much closer, within perhaps a few inches of the subject.  Some conventional lenses have a macro setting that allows close focus.  

A downside of a macro focusing lens is the shallow depth-of-field.  Notice that the bug is in focus, but the background is not in sharp focus.

       
  The focal length doesn’t have much significance for the fundus photographer. The fundus camera either has only one lens system (e.g. Zeiss FF4), or it has a lens system that varies according to the angle of coverage (e.g. Topcon). 
  However, notice that the focal length rules still apply. The more magnified image has the narrower angle of view.
       
  Focal length becomes particularly useful in external eye photography. "Macro" lenses for close focusing can be of a shorter focal length (55mm) or a longer focal length (105mm). For a given level of magnification, the longer focal length lens focuses at a greater distance away from the subject. This permits greater control of lighting, is less intimidating to the patient, and it reduces the size of the flash reflection on the cornea (the "corneal reflex"). When taking photos during surgery, the extra working distance keeps the camera lens a safe distance away from the sterile field.
   
 

This is an external eye macro photo of an ectopic pupil taken with a camera setup similar to the one pictured above.  The 105mm lens allows the camera to be about a foot from the eye.  A 55mm macro lens would have to be much closer.

 

The flash mounted on the front of the lens allows shadowless coverage and maximum flash power so that a small aperture (f-32) can be used for maximum depth of field.

 

  Notice the corneal reflex from the flash on the temporal side of the cornea.  If the ectopic pupil had been on the temporal side, the reflex would have obscured part of it.  It is important to have a setup with a flash that can be repositioned to avoid such a situation.
   
   
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