Continuing from the earlier post, the next variable to consider is shutter speed.
The foe of sharp images is blur: and this has a number of interesting causes. If, for example, you are trying to make images of fast moving children, then it’s helpful to know that selecting a shutter speed of 1/250″ will slow most human action, and 1/500″ will do an even better job of this. So, the speed of movements of the objects you are shooting need to be considered.
Blur (either movement of the object being shot, or movement of the camera itself while the exposure is being made) will rob your image of its sharpness. Sharpness itself is an illusion: just as a surface that looks polished to the naked eye looks rough under a microscope, so too will apparent sharpness disappear if you magnify any image too much. So we need to consider what the image’s final use will be—if your images appear at small sizes on the internet, or you only show people images off the back of your camera, then you will not need to worry overly about sharpness: if it looks sharp enough on the viewing platform you use, then the image is sharp enough. If, on the other hand, you want to print your images, or see them on a larger computer screen, then sharpness will become a more important aspect of the final work. It’s a very useful exercise to do: download some of your images, and look at them on a big screen: what looked razor sharp, often, is not.
Getting back to the relationship between aperture and shutter speed, and again using two fast-moving kids as an example, then you will need to choose a fast shutter speed (to freeze their movement and, in the process, any movement of the camera itself as you try to keep up with them) and you will need to used a relatively deep (or “thick”) depth of field, to try to ensure that both children are in focus. If you think about this, if you attempt to shoot your children, it is unlikely that they will be playing in the same plane (speaking geometrically here) so if your aperture is shallow (low ƒ stop number) then only a shallow slice of what you are looking at will be in focus. This means that if the camera manages to focus on one child, only she will be in focus, and the other one will be soft. This may suit your intent, but if you want to see both clearly, you need to select your focus point somewhere in between them, and select a thicker ‘slice’ of sharpness by selecting a higher ƒ stop (bigger aperture number).
And the thickness of this slice also depends on the lens’s focal length: wide angle lenses display deep DOF; and longer lenses display shallower DOFs. This is why portrait lenses (the typical head and shoulders with a nicely blurred background) are the tool of choice for most who do this work. The most common portrait length lenses are between 85 and 135mm, but any FL can be used, naturally. In fact there’s an emerging school of ‘environmental portraiture’ that relies on FLs between 24 and 50mm (the latter being the typical lens that came on your Dad’s old film camera.
If you are beginning photographer, I recommend using a FL of between 35 and 50mm (or the equivalent in your system). Let me explain: the numbers I used above when referring to FLs are all relates to the now-old film format; variously called 135, or 35mm. It was the then-new ‘small formate’ popularised by Leica and then other ‘rangefinder’ cameras, and they were described as “small” in relation to the sheet film (4 x 5″, and 10 x 8″) and other larger formats, like ‘medium format’, most of which are still around, like Hasselblad’s 6 x 6cm, Pentax’s 6 x 7, and the larger 6 x 9. I have used all of these in the ‘old days’. If you are using one of the excellent small-sensor cameras that are around these days like the Nikon V1, you will want to get the inexpensive but excellent 18/1.8 (18mm, behaves like a 50mm FL lens, and with a fast aperture of ƒ1.8). This will yield a field of view very similar to the 50 we have been talking about, and it’s a superb lens.
These days, most images are recorded on silicon-based sensors, and these range in size from minuscule (your phone’s sensor) to “full frame” (the size of 35mm film) and larger. So when talking about what size images the sensor will “see”, a language of equivalency has emerged: what FL lens will produce the same look as (say) a 50mm lens used to do (using Dad’s old film camera as the example). And FLs shorter than 50mm are called “wides” or wide angle (and very wide ones called “ultra wides” and FLs longer are called “teles” (or telephoto).
So, to end today, if you have an interchangeable lens camera, I recommend you get a fixed focal length lens (called “primes”) for your and practice learning how to see. Many have written on this, but if your images lack pizazz, you are probably too far away from the object of interest. And primes have another advantage over the cheapo zoom lens that probably came with your camera: they are better corrected optically (less aberrations) and usually have a faster ƒ stop wide open—o can be used to shoot when the light gets low, or to use a faster shutter speed (when things get fast). And primes are almost always smaller, too, so you are more likely to carry the camera with you—and that will always be the best camera to use!
Correction: I changed the FL of the Nikon V1 lens I was talking about; I sold mine to a friend and did not have the actual lens in hand when I was writing. Mea culpa!