Fast Lenses on (D)SLRs
Introduction
The fascination with fast lenses is as old as photography. But what are the implications of using fast lenses with digital and analog single lens reflex cameras?
There is no universal definition of what constitutes a fast lens, but personally I consider lenses in the focal range of 24 to 135mm with an aperture of f/2 or less as “fast”.
Advantages
Manual Focusing
Manual focusing with fast lenses is generally easier as the depth of field visible in the viewfinder is almost a preview of the plane of critical focus, where maximum sharpness is achieved.
Compared to slower lenses, the brighter viewfinder image is very helpful when shooting in darker environments. In my experience, even some photo studios can be quite dark, so this advantage is not only relevant for available light photography.
Auto Focus
Auto focus systems of DSLRs operate with the aperture wide open. The aperture is only closed in the moment the picture is taken. More light usually results in faster and more reliable AF operation, if the lens provides enough contrast wide open.
Low-Light Imaging Performance
When working with older DSLRs or film SLRs, the usable sensitivity range is somewhat limited. A faster lens can therefore mean the difference between getting the shot or not.
Subject Separation & Softness
Fast apertures allow for better subject separation, which can be a desired look for portraiture.
In addition, most older fast lenses are not very sharp wide open. This results in less detail and a slight overall softness.
Both traits combined make some fast lenses the ideal portrait tools, as they act as a sort of “in camera retouching”. This is especially relevant when sticking to an all-analog workflow, as retouching in post production can be very tedious.
Disadvantages
Size and Weight
Fast aperture lenses are almost always heavier and bigger than their slower counterparts of the same focal length. Rare exceptions are found in Nikon’s F-mount lineup, where the 50mm f/1.4 AF-D for example is smaller than the 50mm f/1.8 AF-S due to lack of an integrated AF motor.
Maximum Sharpness
Comparing a slow lens with a fast lens at the same aperture almost always shows that the slower lens outperforms the faster lens in terms of micro-contrast and sharpness. Some new lens designs almost overcome this issue but at the cost of significant increase in size, weight and cost.
Price
Fast aperture lenses are almost always more expensive than slower lenses, especially if they are designed with optical excellence at all apertures in mind.
Focus Shift
All lenses with spherical aberrations exhibit focus shift to a certain degree when stopped down. Slower lenses sometimes “hide” this effect as they provide sufficient depth of field to start with.
Fine tuning the AF for optimum sharpness at the working aperture is almost always necessary. Instead, the lens can be stopped down to the working aperture and focused manually. This is of course a major inconvenience.
Conclusion
I hope I could demystify fast lenses a bit.
On the one hand, they provide unique capabilities and often more interesting optical character – some call that optical defects - than their slower counterparts.
On the other hand, they are more expensive, heavier, more prone to user error and offer worse imaging performance in terms of sharpness and contrast compared to their slower peers.
I recommend every photographer to have at least one fast prime lens. Notwithstanding, not every lens in your camera bag has to be an f/1.4.
