What is a Fast Lens? And What does Lens Speed mean?
Lenses that have wider aperture openings (smaller f numbers, f/1.4, f/2.8 and so on) let in more light and thus enable the use of faster shutter speeds at the same ISO setting.
Consider a situation where the camera meter says f/4 at 1/30 at ISO 800. By looking at the variables we could easily see that it’s a low light scene and we are almost maxed out on ISO (considering ISO 800 as a good compromise between noise and camera shake). Having a faster lens for e.g one with an f/2.8 maximum aperture will let us expose the scene with double the shutter speed which is 1/60. And this is the reason why lenses with wider aperture openings are called fast lenses. For more information on this topic do take a look at some of our previous articles – What is a Fast Lens and Advantages of Using a Fast Lens.
Is Lens Speed an Issue in Architectural Photography?
It is a fact that fast lenses cost significantly more than slower lenses. For example a lens with a maximum aperture of f/2.8 could cost nearly twice as much as that of a lens with the same specifications (focal length, focusing systems, image stabilization etc) but with a maximum aperture of f/4 or narrower.
But for the purpose of architectural photography, do we really need fast lenses?
The answer is both No and Yes.
No we don’t necessarily need fast lenses for Architectural Photography.
Fast shutter speeds is rarely a priority when shooting architecture as most of the work involves shooting a stationary subject with the camera mount on a solid tripod. One important factor in architecture is the requirement for large depth of field. Architectural photography demands both elements in the foreground and background to be in sharp focus on most occasions. To achieve this we do most of our work with very narrow aperture openings like f/16 and f/22. If we need more light we could always keep the shutter open for longer durations without having to resort to increasing ISO settings of our camera.
Yes Fast Lenses are Advantageous for Architecture Photography
Yes because fast lenses allow us to see a much brighter view of the scene in front of us, they make focusing faster and more accurate especially in low light conditions, they do offer better image quality when compared to slower lenses.
An f/2.8 lens lets in double the light than an f/4 lens which lets in double the light than an f/5.6 lens and so on. In order to double the quantity of light passing through the lens, the performance of that lens has to be substantially improved to take into account the wider aperture. This is because the worst aberrations, or optical errors, of a lens tend to occur towards the outer limits of the lens image field. Unlike narrow apertures which use only the center portion of the lens to form the image, wider apertures make use of the most part of the lens. So in order to render a perfect image at wider aperture the lens quality need to be considerably better.
So for the extra money you spend on the faster lens, you get a brighter/clearer viewfinder image, faster/more accurate focusing, better low light shooting ability (when handholding), better optics (they are often better corrected for distortions and aberrations) and in most cases also better build quality.
Now that you know the pros and cons of each your decision should be based on the nature of your work and your budget.