Experienced photographers know that the humble, inexpensive 50 is a little powerhouse. But should you get the faster f/1.4 or the less expensive f/1.8? Lab test results reveal interesting surprises and practical advice.
All lab test results courtesy DxOMark.com; used with permission.
50mm lenses get a bum rap. They’re not as versatile as zoom lenses and, because many lower-end cameras are bundled with kit zoom lenses that cover the 50mm focal length, are overlooked. And yet…they’re fast, cheap, and can produce some of the sharpest shots you can get. Hey, Henri Cartier-Bresson used a 50mm lens almost exclusively for over 50 years. That worked out pretty well for him!
In the digital age, a 50mm lens on a DSLR with an APS-sized sensor will deliver 35mm equivalent angle of coverage of a 75mm lens, making it well-suited for portraits. A 50mm fixed focal length (“prime”) lens will deliver better image quality than a zoom lens set at 50mm. A 28-70mm lens set at 50mm, for example, will produce more linear distortion, lower resolution (measured as lines per millimeter), and have more optical anomalies than a 50mm prime.
Prime lenses have simpler optical design requirements than zoom lenses and as a result, so they generally produce sharper pictures. 50mm lenses are faster—where the fastest mid-range zoom might be f/2.8, 50mm lenses go to f/1.8, f/1.4, and even (for a princely price) f/1.2! Couple any 50mm prime with the latest low-light high ISO DSLR and you’ve got an impressive low-light shooting kit.
Which 50mm should you buy?
But which 50mm lens is best? A 50mm f/1.8 lens tends to cost in the $100-150 range—the same as a kit lens—while an f/1.4 lens costs at least twice as much. At their widest apertures, these lenses produce a very narrow depth of focus and create separation between subject and background (or foreground) that adds dimensionality to your photos for flattering portraits.
There’s so little difference, spec-wise, between an f/1.4 and an f/1.8, but the difference in price is huge. How does that translate as far as image quality? All settings being equal, does one lens outperform the other?
I’ve studied the lab test results comparing Nikon and Canon’s f/1.4 and f/1.8 50mm lens test results provided by DxOMark.com; you can scroll down to "Dig deeper: MTF charts and other test results compared" at the end of this article, and peruse the details. For those who don’t want to geek out, let me give you the summary and spoil the ending: For both Nikon and Canon 50mm lenses, overall image quality is generally sharper, with less vignetting and fewer aberrations over the range of apertures with the less expensive f/1.8 version. In most (but not all) cases, this makes the f/1.8 a more desirable lens. Why? Read on!
A surprising result
One of the problems with faster lenses is loss of light at wider apertures. “We have been very surprised,” says Frédéric Guichard, chief scientist at DxO Labs, “to find out that some of the gain from wider lens openings seems to be offset by the present state of sensor technology. Our measurements all point in the same direction: As you go further than f/4,to f/2 and wider, the accrued quantity of light falls marginally onto the sensor. A stronger and stronger part of this additional light is blocked or lost. I am therefore inclined to question the real benefit of faster lenses.”
While image quality in the middle apertures (usually f/5.6-f/11) is uniformly outstanding in the f/1.8 lenses, I observed that there is more falloff in corner sharpness and vignetting in both f/1.4 lenses that DxO tested. This differences are very subtle. They’re more pronounced at the wide apertures, but the lens’s faults are still somewhat apparent beyond either extreme setting. At f/5.6, for example, lab tests recorded more vignetting, lower lines per millimeter ratings and other problems at the corners when shooting with an f/1.4 lens than were observed when shooting at the same aperture with an f/1.8.
(Both Canon and Nikon also make a 50mm f/1.2 lenses. The manual-focus Nikon costs around $700; DxOMark does not have test results for this lens, but for the Canon AF f/1.2, it does. While resolution is very good in the middle apertures, it is much less so at the extremes f/stops and in the corners; chromatic aberration is quite high, distortion is fairly high.)
Why the difference?
There are several theories, and lens behavior on digital cameras may be different than when shooting on film. “We can suspect,” Guichard says, “that sensors collect the incoming light all the more improperly, in that this light comes from a more oblique angle. Since faster lens have, by definition, a wider opening, they raise the proportion of oblique light, hence the proportion of lost energy which never lands on the pixels.”
In other words, the light lost when shooting at the widest aperture may negate the benefits of shooting with the wider aperture, especially when shooting with a digital camera.
Conclusion and recommendation
The main reason I can see to invest in a 50mm f/1.4 is that the extremely narrow depth of field creates a very desirable effect that, in some circumstances, leads to a desirable look and feel that can’t be measured in the lab. The overall softness at f/2 or lower, along with the more pronounced edge softness and vignetting, may lend itself well to portrait work and this may be well worth the higher price for some photographers.
But if you’re a stickler for sharpness and exposure accuracy, you tend to shoot in the middle apertures, and your budget is tight, I recommend saving the money and investing in a 50mm f/1.8 lens (especially a lens with shake reduction) for general shooting and low-light photography instead.
In fact, at around $100, the 50mm f/1.8 lens may be one of the best values in photography.
Dig deeper: MTF charts and other test results compared
Using the results of DxOMark’s lab test results, let’s compare the Canon 50mm f/1.4 USM to the EF f/1.8 II lens, and the Nikon 50mm f/1.4 Nikkor to the 50mm f/1.8 Nikkor performed under DxOMark’s lab testing, using DSLRs that produced the best overall image quality for each company.
Canon 50mm lens test results
Canon 50mm EF f/1.8 II vs. 50mm f/1.4 USM
Tested on a Canon EOS 1Ds Mark III
Lab report
Starting with overall scores, the f/1.8 scored two points higher than the f/1.4. Chromatic aberrations were lower on the f/1.8 although the actual resolution was 63 lp/mm for the f/1.4 and 59 lp/mm for the f/1.8. In vignetting, chromatic aberrations, light transmission and distortion, the 50mm f/1.8 edged out the 50mm f/1.4.
MTF Charts:
f/1.4 and f/1.8 Corners compared at f/2.8: This chart shows both results at the corners of the frame when both lenses were shot at f/2.8. The light green (f/1.4) lp/mm lines dip deeper towards the middle, indicating lower resolution.
At f/5.6, both lenses’ resolution at the corners are identical.
At f/22, the smallest aperture, the f/1.4 lens shows slightly worse resolution. Other measurements of center and corner resolution bore similar results to this set of charts.
While I only show corer results here, DxO has published extensive test results at corners, center of image field and other positions. You can delve into the lab data here.
Nikon 50mm f/1.4D and f/1.8D lens test results compared
Tested on a Nikon D3x
Lab report
Both Nikon 50mm lenses tested very close when it came to overall score, with the f/1.8 edging out the f/1.4 in resolution. The f/1.8 exhibited almost no distortion, while the f/1.4 showed slight distortion as well as more vignetting (up to a 2.4-stop difference from center to corners at full aperture). Even at f/4 the f/1.4 lens exhibited more vignetting than the f/1.8 Light transmission on the f/1.4 was nearly a half stop less than the f/1.8. Both lenses scored very high in chromatic aberration correction. Interestingly, the pricier Nikon 50mm f/1.4G lens had a better overall score than the 50mm f/1.8D, but scored lower in the lp/mm and vignetting realms.
MTF Charts
f/1.4 and f/1.8 Center and corner compared at f/2.8: While both lenses performed equally in the corners, the f/1.8 (dark green lines) delivered higher lp/mm scores throughout when shooting at f/2.8.
In the middle apertures, performance is almost equal; as you can see here at f/5.6, the two solid lines converge for most of the chart.
At f/16, the smallest aperture for both lenses, the results flipped, and the f/1.4 actually delivered clearly better resolution than the f/1.8. If you’re shooting for extremely deep depth of field, the f/1.4 lens has the clear advantage.
While I only show center results here, DxO has published extensive test results at corners, center of image field and other positions. You can find them here.
