Maximum Apertures and Lenses

I want a "fast lens!"

I hear this often, but without any real explanation or logic behind the statements. "Fast lenses" (or large aperture lenses, like F1) are nice to be sure - if they're any good at that aperture. Few but the most expensive lenses are. There's a bit of physics involved here. A fast lens is BIG. That's not to say that there isn't a time and place for a fast lens.

Back in film days ISO 400 film was about the highest speed you could get before the image would degrade. So, a fast lens was good to keep a decent shutter speed. That's not the issue in digital days and with sophisticaticated sensors, no real ISO's and image stabilization.

Autofocus was pretty rare in film days and you needed as shallow a depth of field as possible to accurately focus - manually - with an SLR, TLR, or view camera, and then you stopped down anyway.

In these digital days a fast lens can assist autofocus but autofocus systems are more geared towards determining the contast of the focus point for accuracy (an out of focus point is lower contrast than an in focus point). A fast lens can help with framing with a through the lens optical viewfinder. With an electronic viewfinder, the view is gained up electronically.

Not all F-stops are good. There's a sweet spot, usually 2 or 3 stops, where you are in the "sweet spot" of the lens, where the optical quirks are at a minimum, and you're not getting diffraction from the aperture blades. A fast lens not optimized for capturing images wide open is not going to give you the best image. Likewise, stopping all the way down for maximum depth of field doesn't guarantee everything will be as sharp as it could be.

So that leaves us with depth of field. True, a faster lens has a shallower depth of field - if you know what depth of field is and how to use it. Today's electronic viewfinders and even the viewfinder in an optical SLR are too small to accurately see the depth of field effect - assuming the camera has a way of previewing the depth of field. In an SLR that would mean stopping down the lens and that would mean making the optical or electronic viewfinder dark.

Where I'm heading with all this tech talk is the near automatic negative reaction to digital lenses with smaller maximum apertures. These lenses are smaller, lighter, easier to carry and typically better wide open than some fast lenses. Granted there's a bit of digital - behind the scenes - tweaking going on, but the laws of physics still apply. A fast lens is bigger and heavier. Unless the lens is optimized to shoot wide open the image may suffer to some degree.

The point is that you shouldn't automatically think "it's only an F4..." and walk away. That lens may be incredibly sharp at F4 compared to an F2.8 or higher lens. Optics are complicated that way. In these digital days it's a lot more complex since the lens and camera electronics, and even the RAW converters, work together to optimize the image. This wasn't so in film days where the lens was everything. If the lens didn't get the image sharp on the film, well, nothing was going to make it sharper.