Why tele lenses with low “f”?

3 Feb

When we shoot tele (using a long lens to bring our subject closer, or, “zoom in”), we may initially find we struggle to get enough depth of field into our subjects, even more so when shooting macro. So the fact that my lens, let’s say a 50-200mm or 70-300mm, narrows down to f/5.6 or f/6.3 at the “long” end may seem to be no problem at all, and might make you wonder what esoteric applications might make you buy into an f/2.8 for that focal length.

In order to understand the answer, we should consider at least one additional property of modern photographic systems: The lens narrows down to its intended f-value (or aperture) only when the shutter is “released” (that means, a picture is being taken). So before that, when looking through an optical viewfinder, we get the benefit of the full amount of light that comes through the lens.* Getting a clear, bright image could be the reason for preferring a pentaprism viewfinder over a pentamirror one, so under the same reasoning, you may want to pick a lens with constant, low “f”.

At this point you might think, “well, I’m using an electronic viewfinder, what difference is it to me whether my image is a little noisier or has a slightly lower refresh rate?” However, consider that you’re not the only part of the system that requires a lot of light to function well – autofocus does as well. And it can’t get it in any way other than through the lens. The more light is available to the autofocus, the more accurate it will be, and the less “hunting” it will take to achieve a “lock”. By the same token, a lens with wider aperture will allow the autofocus to work deeper into darkness – for instance near sunrise or sunset.

Moreover, depth of field will be shallower with wider apertures, allowing greater autofocus accuracy as there will be a greater difference in blur** between two given settings of your focus ring. This effect is self-cancelling in real world applications, however, because large depths of field are more forgiving of small focus deviations.

As a third relevant point, many cameras now use a combination of differently spaced phase detection modules – a larger spacing gives more autofocus accuracy, but requires the lens to be wide open – typically at f/2.8. For f/2.8 autofocus points, there is usually a f/4.0 or f/5.6 fallback point at the same location, which allows somewhat less accurate autofocus when a narrower lens is used. However, when the widest aperture of the lens is narrower than f/5.6, autofocus ability may be more limited.

Everything so far applies equally to any lens, not just to tele. However, with tele, additional thought should be given to the possibility that you might at some point use a teleconverter (TC) to get even more reach out of your lens. Using a TC makes your image darker due to it narrowing your available aperture by an amount that depends on the magnification given by the teleconverter – commonly 1.4, 1.7 or 2 times magnified. The stronger the magnification, the greater the loss in aperture width.

So when you plug the teleconverter into your optical array, your viewfinder image gets dark, and you may as a result find it harder to focus manually.*** Well, guess what, your autofocus has the same problem. All autofocus systems, even “live view” autofocus****, require a certain amount of incident light to operate well – not least because you want your autofocus decision quickly, which means that less light can be captured before the decision is made.

I hope that I’ve been able to outline the main advantages a fast lens offers. Many high-end compact cameras have recently raced towards wider apertures, not least the Panasonic FZ200 with its constant f/2.8 aperture. Perhaps this article will inspire some to also look for fast glass in their system cameras.

* Some cameras also have a “preview” functionality where the lens stops down to the intended aperture when you hold down a designated button.

** Phase detection autofocus measures “blurriness” in an indirect way, by considering whether two beams emerging from the same point on the subject, converge on the sensor plane. This is achieved by having the AF sensor at the same distance from the back of the lens as the sensor, via a mirror.

*** You’re additionally losing light because with every bit of glass added, your “transmission” (measured in units called T) decreases, because all glass is imperfect in stopping some of your light getting through, either by absorbing, or by reflecting it – usually a little bit of both.

**** technically called contrast detection autofocus


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