Lens Sharpness and Contrast
Tamron did a good job with the Tamron SP 45mm f/1.8 lens – it is pretty sharp at large apertures, and its contrast is also excellent. Take a look at the below chart, as measured by Imatest:
The lens is quite sharp wide open – something many other primes struggle with. As you stop down, sharpness improves significantly, giving the best results between the f/2.8 and f/5.6 range. The sweet spot of the lens for landscapes is f/5.6 – that’s where you get the best overall performance, even in the extreme corners.
The lens suffers from little field curvature issues, but there is a moderate amount of focus shift. So if you want the best results, make sure to stop down the lens to your desired aperture before you focus (preferably in live view).
Here is the MTF chart provided by Tamron for comparison:
Lens Construction
And if you are wondering about the optical design of the lens, here it is:
Bokeh
The Tamron SP 45mm f/1.8 Di VC USD yields smooth, natural-looking backgrounds in images, as can be seen from the image sample below:
However, the bokeh performance (the way out-of-focus highlight appear in images) isn’t great, which is expected from a lens with aspherical elements. The onion-shaped background highlights have distinct outer shapes, which are quite visible when there are bright light sources in front of and behind your subject.
On the positive side, the 9-bladed diaphragm is rounded, resulting in circular bokeh even when stopped down.
Vignetting
Tamron did a great job with controlling vignetting on the lens, but it varies quite a bit depending on whether your subject is close or at infinity. Take a look at the below vignetting chart:
As you can see, the lens exhibits about 0.6 stops of fall-off at f/1.8 in the extreme corners, but at infinity focus, the effect more than doubles at 1.45 EV. Stopping the lens down to f/2.8 reduces vignetting significantly and anything smaller than f/4 is not an issue. Here is the worst-case scenario at f/1.8, infinity focus:
If vignetting bothers you, it is easy to fix in post-processing software like Lightroom.
Ghosting and Flare
Thanks to Tamron’s special eBAND and BBAR coating technologies, the Tamron SP 45mm f/1.8 Di VC USD performs admirably well when shooting against bright sources of light. But keep in mind that ghosting and flare performance can vary depending on the size and the location of the light source. In some situations, you might see some ghosting in your images, and flare will reduce general contrast. When shooting with the sun overhead, I would recommend using the included petal-shaped hood – it does a great job of making sure that the sun rays don’t reach the front element of your lens.
Distortion
Distortion is certainly there, but it is not bad. Imatest measured 0.8% barrel distortion, which makes it somewhat noticeable when shooting straight lines. It does not bother me, especially knowing I can quickly fix it in Lightroom and other post-processing software.
Chromatic Aberrations
The biggest issue with the Tamron SP 45mm f/1.8 Di VC USD I have noticed so far is its purple and green fringing due to strong Longitudinal Chromatic Aberration (LoCA), which is unfortunate. Take a look at the below image:
As you can see, the de-focused areas in the back show very strong green fringing, while the foreground shows strong purple fringing. The strange thing is, green/purple fringing shows up in even slightly blurred areas of the frame, which probably frustrates most owners of this lens. Unfortunately, LoCA is not an easy issue to address in post-processing.
On the other hand, lateral chromatic aberrations aren’t as bad, with the lens showing less than a pixel of CA at all apertures. Here is the chart that shows lateral CA at different apertures:
And if that bothers you, lateral CA is relatively easy to deal with in post-processing software.