Most manufacturers would have you believe that the reason why you want to get the top of the line fast zoom lens is to shoot in absolute darkness or to get gorgeous bokeh balls. Although there is some inkling of truth in there, the idea is absolutely absurd. The name photography suggests painting with light. Why then would you want to shoot in absolute darkness if the concept is contrary to what photography stands for? Further, are we going to shoot every image so that it has a milky smooth background and gorgeous bokeh balls. Granted, if the idea was to impress your friends and persons who are not knowledgeable enough in photography, you will certainly win some points. But, why by gear often times costing thousands of dollars just to shoot in low light or show how great you can blur the background ? Having set that premise, I will try to describe a practical reason why you should get a fast zoom lens.
To explain, lets have a look at a consumer grade and a professional lens with similar zoom lengths – the 55-200mm f4-5.6 Nikkor and the 70-200mm f2.8 Nikkor. For the sake of this explanation we will also assume that these lenses are placed on a crop body as the the former is a DX sensor lens and the latter is an FX sensor lens. Certainly, the DX lens is lighter it can certainly allow for zooming within the same range as the 70-200mm. The 70-200mm however allows the user to maintain an aperture of f2.8 throughout the length. The 55-200mm changes its aperture through the process of zooming in and out. This can lead to some difficulty when it comes to maintaining the exposure of your image across different zoom lengths.
If you recall the exposure triangle which is made up of the ISO, aperture and shutter speed, as you adjust one area of the triangle, you are certainly going to have to adjust one or two of the other areas to compensate. So, lets have a look at the specifications of two images
- ISO 200, aperture 4, shutter speed 1/200s
- ISO 200, aperture 5.6, shutter speed 1/100s
Both images will have the same exposure, or have the same amount of light applied to them. As the aperture changed, the shutter speed was adjusted to compensate for the quantity of light coming through the lens.
Getting back to the comparison of our two lenses. As you zoom the length of the 55-200mm lens the aperture will change resulting in a change in exposure whereas the 70-200mm would not change the aperture at all throughout the length. Thus for a fast moving event like a sports game or animals in a zoo, this is ideal as you never have to adjust any of your other settings. You simply shoot and forget about it.
Now, I know what you are thinking. Why not set the camera in aperture priority and let the camera figure out the shutter speed as you zoom through the lengths. That will certainly allow the lens to behave as if it has a fixed aperture through the changes in its distance from the sensor. Please note that the 55-200mm will still adjust the aperture at its widest position regardless of being placed in aperture priority. But, for the sake of this example we will assume that it does not. If we took our two photograph examples above. This means that the shutter speed will be adjusted accordingly from 1/200 – 1/100 seconds and vice versa. This is great if you don’t zoom past 100mm. But beyond that, you have the possibility of introducing camera shake unless you are on a tripod. As a rule of thumb, it is expected that your shutter speed be the reciprocal of the lenses distance from the sensor. At the extreme of 200mm, the 55-200mm succumbs to the limitations of its design. A limitation which can only be compensated by a fast zoom lens or boosting the ISO of the camera. Boosting the ISO however leads to other issues in the quality of the resulting photograph.
So, just to recap, you should not be fooled by the marketing gimmick of shooting in low light and bokeh balls. That is not a significant enough reason why you should get a fast zoom lens. A fast zoom lens should be purchased because it helps you overcome the limitations of consumer lenses where the aperture varies and and as a result, the exposure of the photograph. Thus, this allows you to have consistent well exposed photographs throughout your zoom range.