It is not necessary to understand any of the technical aspects of photography to create great images - any more than it is necessary to be at ease with binary notation and Boolean logic in order to use a computer. There are leading photographers who claim complete ignorance of all but the creative (and business!) aspects of photography. However, these are invariably mainstream photographers, not those specializing in close-up or high-speed photography, where the laws of optics are frequently pushed to their outer limits.

It sometimes seems that the laws of optics are very poorly conceived! - reduce aperture to improve depth of field, for example, and you lose resolution, reduce exposure time to freeze movement and you lose color balance. Success in photography often means mastering the art of compromise - learning to balance drawback with payoff. The more you understand of the basic principles, the easier it becomes to make informed decisions - hence the seemingly heavy information presented here.

Although many people distrust, even fear, equations and formulae, they do provide the most concise and logical way to express relationships. I hope they will be viewed here as a help rather than a hindrance - I've tried to keep it simple!.

There is greater overlap between these two photographic disciplines than might at first be supposed, for both can conflict with the laws of optics in similar ways. Bear in mind that in general, the smaller the subject the greater the need for short exposure times. Vibration of both camera and subject become increasingly critical as magnification increases. Moreover, many of the phenomena for which high-speed photography is well suited - drips & splashes for example - already fall into the realm of close-ups.

Magnification is the relationship of subject size to its image on film. It can be expressed in three ways::

One of the most basic relationships in optics is expressed as:

Rearrange this (remembering high school algebra) and you eventually get:

This tells you that you may change magnification in just two ways- either by altering the focal length of the camera lens or by altering its distance to the film plane, which means adding either an extension tube or a diopter (supplementary) lens.   Each has advantages and disadvantages.

Under optimum viewing conditions a person with good vision may resolve objects whose diameter is 1/3000 of the viewing distance, meaning that lines 1 mm across can be distinguished at 3 meters. Photographic convention, however, works on the assumption that at 10 inches, the eye cannot separate image patches less than one hundredth of an inch across. This means that image patches less than 1/100" in diameter will be perceived as sharp. Hence, an acceptable circle of confusion in these circumstances is deemed to be 1/100".

Quite empirically the acceptable circle of confusion for a 35mm transparency has been codified as 0.03mm (0.06mm on 120 or 0.125mm on 4x5 ). These figures serve only as a useful guide to sharpness, which can also be influenced by other factors such as accutance - the contrast between adjacent image areas. Ultimately you must rely on what your eyes tell you is sharp.

Ever wondered about those strange numbers on the barrel of your lens? Why the curious sequence: 2.8, 4, 5.6, 8, 11,22 etc.? The explanation is quite simple. The aperture or f/number is defined as the relationship between focal length and effective lens diameter. This is defined in terms of the cone subtended by the effective lens diameter at the focal point, measured as the semi-angle (q ), namely as:

Thus there is a theoretical maximum f/number of f/0.5, namely ½ sin 90°. So here we have a starting value for the f/number series - 0.5. In practice apertures rarely reach as low as f/1.0.

To compare the light-transmission properties of f/numbers one must compare their squares.

Thus the ratio of f/5.6 to  f/4.0 is the ratio (5.6)² : (4.0)² = 31.36:16.00 = 2:1

From this it is apparent that f/0.5, 1,2,4,8,16 each represent a fourfold increase in exposure time. To get a doubling in exposure time one must interpolate the sequence f/0.7,1.4,2.8,5.6,11,22, whence we get our familiar f/number series:

The reason for elaborating on f/stops here is because in the world of close-up photography one sometimes encounters lenses with unfamiliar markings.

Zeiss Luminars are perhaps the finest close-up lenses available. They are marked with an Aperture Scale running 1,2,4,8,15,30. These are not f/numbers. The f/number equivalents vary with different lenses in the Luminar family, running either f/2.5, 3.5, 5, 7, 10 (for the 16mm and 25mm focal lengths) or f/4.5, 6.3, 9, 12.5, 18, 25 (for the 40mm and 63mm focal lengths). These values, of course, lie one third of a stop above and below the usual f/stop sequence. 

Leitz Summar macro lenses use the totally unfamiliar Stoltze guide numbers, which run: 1, 2, 3, 4, 6, 12, 24, 48, 96. These equate to "f/number squared/10" and are equivalent to the series: f/3.2, 4.4, 5.6, 6.3, 8, 11, 16, 22, 32.

Boring, perhaps, but these are great lenses and this  vital information about them is not to be found readily elsewhere.

In theory a lens can only focus in a single plane, meaning that its depth of field is infinitely thin. However, in practice the depth of field is related to the acceptable circle of confusion. By doubling the aperture (f/number) one doubles the depth of field, but one also reduces resolution for the following reason.

As light rays pass by the iris diaphragm within the camera lens they are slightly diverted from their straight path by diffraction. This only occurs in a narrow region close to the diaphragm. Under normal circumstances the distortion is not visible because the affected area is small in relation to the total lens area and is swamped out. As the lens is stopped down and the total area reduced, diffraction around the periphery becomes proportionately more significant and finally dominates.

What this means in practice is that depth of field falls off sharply with increased magnification. At natural size (1:1) one can shoot at f/22 without perceptible loss of resolution but depth of field is only 3mm. At twice natural size (2:1) f/16 is just acceptable but depth of field has dropped to less than 1 mm. At ten times natural size (10:1) one must shoot at f/4, when the depth of field will be only 0.03mm.

The message here is that one should not shoot at higher magnifications than absolutely necessary. It is better to have the subject small in frame and then crop & enlarge.

Most photographers have to trick their strobes into producing very short-duration flashes in order to obtain high-speed images. Modern thyristor-controlled flash units can be made to produce flashes of 1/20,000 -1/40,000-second duration. However, this is at the expense of light output. For this reason you want to use a flash duration no shorter than is necessary to freeze your subject so that you have maximum light to work with.

Instead of asking the question "What flash duration should I use", it is often more useful to ask; "At what magnification should I shoot". This is particularly true if one has a flash unit that gives a light pulse of fixed duration. At 25 microseconds it would be possible to get William Tell's entire arrow traveling at 100 meters-per-second acceptably sharp as it passed through an apple. However, increasing magnification to get just the arrowhead as it emerged from the apple would give a slightly blurred image. This probably wouldn't be a pressing concern to the model on whose head the apple was placed, but illustrates the importance of selecting an appropriate magnification at which to shoot.