Choosing the right lens, depends upon knowing the intended use of the lens.
Available-Light Photography
The F1.4 lens is 2/3 stops brighter (66 percent brighter). It is sometimes mistakenly assumed to be 1/2 stops brighter than the F1.8. The 1/2 stop is actually at F1.7.
2/3 stops can be critical when shooting in low light and without flash. It is possible to require slow 1/15 or 1/30 second shutter speeds at ISO 1600 and F1.4. Which is about the limit for hand holding the camera without a tripod.
Even with image-stabilization (few, if any, 50mm lenses are stabilized), fast-moving subjects will be less blurred at higher shutter speeds.
Blurred Out Background (Bokeh)
50mm lenses are often used at maximum aperture to throw the background out of focus, while keeping the subject sharply in focus. This is especially effective for portraits of people.
For this application, there is little difference between the two lenses.
Whether the F1.4 or the F1.8 has the better "bokeh" or pleasing out-of-focus images, will depend on the manufacturer (Sigma, Nikon, Canon, Sony, Pentax, Olympus etc.) and individual preference.
Image Quality
50mm lenses are often chosen because of the sharp images they can produce, considered better than zooms.
Different reviewers rate the lenses differently. Some say the F1.8 is sharper and with higher contrast (this is a good thing), others disagree. This may be due to differences between manufacturers - one may have better F1.4 lenses, another may have better F1.8 lenses.
It also matters whether the comparison is made at maximum aperture (F1.4 and F1.8), or stopped down (F2 and F2, F2.8 and F2.8 etc.).
The F1.8 is universally regarded as having lower distortion - straight lines will appear straight. The F1.4 will have slightly more distortion.
Taken all together, any differences are minor, noticeable only in direct side by side comparisons of enlarged images.
The photographer's technique will likely be the limiting factor, not the lens. This applies in such areas as:
- focus accuracy, can be tricky even with autofocus (need to wait for focus lock before releasing shutter)
- supporting the camera or steady hand-holding
- panning to reduce subject motion-blur
- choice of aperture for depth of field
- choice of high-ISO noise versus shutter speed trade-off
- use of lens hood to reduce flare
New Generation 50mm Lenses
Photographer and reviewer Ken Rockwell says that the basic Planar / Gaussian optical design of current 50mm lenses is decades old.
50mm F1.4 lenses using redesigned optics for better image quality, have recently been introduced. Some even use expensive aspherical elements. Photographers looking for the best image quality should consider
- Sigma F1.4 EX DG HSM
- Nikon F1.4 AF-S G
F1.8 lenses are considered budget (but still good) lenses, not worth the cost of a redesign. They will likely continue to be offered only in classic Planar / Gaussian versions.
50mm F1.4 Compared With F1.8
Both F1.4 and F1.8 lenses are excellent additions to any photographer's toolkit.
Anyone uncertain as to which to get can start with an F1.8, and will probably be happy to stick with it. Available-light enthusiasts will prefer an F1.4.
Resources
- Nikon and Canon Digital SLR Lenses. Overview of large-aperture prime lenses for DSLR Cameras.
- Canon EF 50mm F1.4 vs F1.8 MK II, by Wee Keng Hor.
- Nikon 50mm Lens Comparison, by Ken Rockwell.
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