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The whole field of photography is full of matters like, ''exposure'', ''f stops'', ''shutter speeds'', ''depth of field'' ..... and so on. Because of so much automation now available in the modern camera, much of this does not need to be very deeply understood.

However - many cameras, including some digital, do give options for a certain amount of manual control. This then can put the photographer more in creative control and for that reason some of the more ''technical'' aspects are worth understanding so as to better exploit the manual options.

Many books have been written on this subject - it is also possible to find much information on the web. I will though here just try as briefly as I can, to touch on various aspects through example and analogy - after which further reading is recommended.

Try and read through this section first (sorry, I know - more text!) and then take a look at some diagrams that will hopefully further clarify what we are discussing.

What is an ''f'' stop?? -

Simply put, it is a size of aperture (hole!) in a lens - mostly controlled by a mechanical iris mechanism.

If a lens of 50mm focal length was wide open, then it would have an aperture of f1 and transmit its maximum amount of light. Reduce that aperture to 25mm and we have f2, reduce further again, to 12.5mm and this will be f4, etc.

Now, if we halve the diameter this way in each step, we in fact invoke the inverse square law .... meaning that f2 will transmit 1/4 the amount of light compared with f1. By the time we get to f4 we are down to 1/16 the amount of light.

This is too large a jump and so an intermediate aperture designation is needed, so that each step halves the light each time. As this is a logarithmic process, we can multiply the cross sectional area of the 50mm (f1) aperture (1963.5 mm squared) by .71, which gives us 1402 sq mm. If we use that to now work out the new aperture diameter, (2 x [sq root, area divided by Pi]) we arrive at 42mm. The inverse of .71 is 1.4 and this is the new f number ...... f1.4 ...... letting through half the light compared with f1 but twice the light of f2.

The progression of these ''intermediate'' f stop numbers follows on by being a series modified (nominally) by factor 2 ..... so, 1.4, 2.8, 5.6, 11, 22 ... each altering the light transmission by 1/4, as with the whole numbers we looked at first. The whole sequence then of usual and familiar f stop numbers would be ........

1, 1.4, 2 ,2.8, 4, 5.6, 8, 11, 16, 22, 32 .......

This is the common scale ... rarely going smaller than f32, but sometimes intermediate numbers are added if required. So - go up the scale in numerical value and you go down the light transmission scale by factors of 1/2 for each step. Obviously the reverse is also true.

Shutter Speed -

This is a little less involved compared with apertures! Quite simply, shutter opening times are either halved or doubled .... corresponding to halving light input or doubling light input. Easy!

The usual scale starts conveniently from 1 second ... and then down in halving increments ..... 1/2 sec, 1/4 sec, 1/8 sec, 1/15 sec, 1/30 sec, 1/60 sec, 1/125 sec, 1/250 sec, 1/500 sec and 1/1000 sec. Each halving of time, halves light input - but you will notice here, there is a small concession to ''rounding off'' the figures, to maintain a convenient scale.

How though do we make choices for shutter speed if our camera has manual options? This is discussed later on this page where we consider the importance of movement (camera and subject) and the effects of different focal length lenses.

Aperture vs shutter -

We can now hopefully consolidate a vital relationship here. If we halve the light entering through the lens but double the duration of shutter opening - we have not changed the overall light transmitted. For example .......

We have a setting on the camera of 1/125 at f8 ...... one of the all round ideal ''compromize'' settings. We could halve the amount of light the aperture lets in and then double the amount the shutter allows through ...... giving a new exposure set of - 1/60 at f11 sec.

Going the other way - we might double the light allowed through the aperture but half the time the shutter is open - so our new exposure set would then be 1/250 at f5.5. All these three settings will let exactly the same amount of light through - the exposure value is the same.

The differences tho will affect how well movement is stopped (shutter speed) ...... and how much depth of field/focus we have (aperture), see next section. Thus, choices need made for any given subject matter based on those requirements. These factors are further expanded below.

Depth of Field (Zone of sharpness) -

When taking distance shots with normal focal length lenses, this is rarely an issue. As soon however as we close in on subject matter we have to become more concerned as to what will and what will not be in good focus.

Imagine you wish to photograph a snubbie revolver with the muzzle in front, but want also to try and keep the cylinder in focus and even too, the grips. To obtain the best depth of field you will need the smallest aperture available and the shortest focal length of lens ... no zoom. Even then it may be difficult. Sometimes the best solution is to shoot at high quality, move back a ways and then later crop the picture to select the central portion of required subject matter - this will have improved (''stretched'', or expanded) your depth of field.

At the other end of the scale, we may in fact want shallow focus, such that the subject is sharp but what is behind is well blurred - for this we choose a much larger aperture if possible. Add to that choosing to zoom in so that lens focal length increases and at the same time probably moving away somewhat.

For an example which well illustrates the relationships between aperture size and depth of field, take a look here.

All this juggling with aperture will mean compensating through shutter exposure, the ''time'' factor. This may induce other potential problems - see following section.

Shutter Speeds, Movement and Lenses -

If as in the above description of depth of field relative to aperture - we have selected a small aperture to maximize depth of field - we will be necessarily obliged to use much slower shutter speeds.

If the subject is stationary and does not need to be ''frozen'' by a fast shutter speed, all may be well. Except - there is now still serious potential for picture blurring due to camera shake. The way around this will usually be to support the camera somehow, a sturdy tripod being far and away the best solution.

Camera shake and low shutter speeds also become more serious as lens focal length increases - remember how shakey a target looks when viewed through a scope!! Long focal length (zoomed in) shots always present problems because you not only want enough shutter speed to compensate for camera shake when hand held but - as you increase shutter speed, so you must further open the aperture - and that will shorten your focal depth!

There might be occasions when a fast shutter speed is needed however - for instance you want to take a picture of your buddy firing his auto, and maybe hope to catch a case in the air! Now - you will have to use the fastest speed available in the hopes that it will ''freeze'' motion enough. This would tho necessitate a large aperture in order to still have a suitable exposure value - and bearing in mind what is written above re depth of field .... this just might make your focus very critical. Something to remember.

It will possibly be apparent by now - there is much need for compromize ...... you have to decide which aspect of your picture matters most for a given exposure - focal depth or, stopping movement and camera shake. Not always easy!

"Film Speed" -

This is only really relevant when related to films. They are usually rated by ''DIN'' number or ''ASA'' rating. For example, 100 ASA film is quite slow but good definition. 200 ASA is twice as fast but with slightly less resolution. And 400 ASA is twice as fast again ..... with resolution being further reduced.

With digital cameras however, we are not too interested in this as most functions are automated. However, some cameras do have a setting which works like an equivalent of film ASA, and gives some sort of guide to choice of flash distances and powers, as well as sometimes giving the option to make a choice based on desired results.

After plodding though the above, and if you have reached here - congratulations! I hope maybe you have learned a little more, even if somewhat fatigued!

If your camera makes provision for manual settings, including aperture and shutter priority options - you can maybe now relate what you have read, to how best those options might assist you in taking the shots you want.

For some diagramatic reinforcement of all this, go look at this page and perhaps it will further help consolidate the subject.

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