Sigma 20 mm f/1.8 lens test

[Tom W]

I ran my new (well, almost new) 20 mm through a few tests today just to get an idea on how well it performed.

I was a little leery of this lens at first - a wide lens with a shallow DOF is a difficult lens to make and to use. I was getting mixed results in the two days of ownership so I thought I might need to run it through a few tests.

I set up a table with some props that would give some indication of the abilities of the lens, both for sharpness, contrast, and color. Then I set the camera on the tripod to keep things as consistent as possible.

All images are 100% crops!

The first set of images were taken with the lens wide open at f/1.8:

First, center, then left, then right side (note that the Duracell batteries are on the same focus plane as the CD's and calculator)

http://images.fotopic.net/?iid=y3igh7&outx=600&noresize=1&nostamp=1
http://images.fotopic.net/?iid=y3ighg&outx=600&noresize=1&nostamp=1
http://images.fotopic.net/?iid=y3ighj&outx=600&noresize=1&nostamp=1

Note that the center image was slightly soft, and the two side images were softer yet with the right side being worse than the left. Its almost as if the focus plane bends a little towards the camera at the right side. Anyway, for f/1.8 at 20 mm, this isn't too bad.

My rating of the Sigma at f/1.8:

Center - fair
edge - poor

Next, I stopped down to f/2.8 and ran the same three tests. Results were much better:

http://images.fotopic.net/?iid=y3igh8&outx=600&noresize=1&nostamp=1
http://images.fotopic.net/?iid=y3ighh&outx=600&noresize=1&nostamp=1
http://images.fotopic.net/?iid=y3ighk&outx=600&noresize=1&nostamp=1

Once again, the center is better than either edge and the right side lags a bit.

My rating at f/2.8:

Center - good-very good
Edges - fair

The third test was done at f/4 with the same three objects. This time, however, I had something else to compare it with - the Canon 17-40L. This time, I'll post both center images first - you might want to open them in separate windows to compare. First, the Sigma:

http://images.fotopic.net/?iid=y3ighc&outx=600&noresize=1&nostamp=1

Then, the Canon:

http://images.fotopic.net/?iid=y3igh1&outx=600&noresize=1&nostamp=1

Both edge shots will be in the same order(Sigma, Canon), first left, then right:

http://images.fotopic.net/?iid=y3ighi&outx=600&noresize=1&nostamp=1
http://images.fotopic.net/?iid=y3igh3&outx=600&noresize=1&nostamp=1

http://images.fotopic.net/?iid=y3ighl&outx=600&noresize=1&nostamp=1
http://images.fotopic.net/?iid=y3igh5&outx=600&noresize=1&nostamp=1

In this case, with the Sigma stopped down to f/4, it barely edges out the Canon in center sharpness, and falls just short on edge sharpness (left edge is pretty close, but the Canon is superior on the right side)

My ratings - Sigma @ f/4.0

Center - Very good
Edge - fair - good

The Canon @ f/4.0

Center - Very good
Edge - good

And finally, f/5.6. Again, both lenses are tested in the same order - Sigma, Canon , first at center:

http://images.fotopic.net/?iid=y3igha&outx=600&noresize=1&nostamp=1
http://images.fotopic.net/?iid=y3igh2&outx=600&noresize=1&nostamp=1

And for the edges, left and then right:

http://images.fotopic.net/?iid=y3ighx&outx=600&noresize=1&nostamp=1
http://images.fotopic.net/?iid=y3ighf&outx=600&noresize=1&nostamp=1

http://images.fotopic.net/?iid=y3ighm&outx=600&noresize=1&nostamp=1
http://images.fotopic.net/?iid=y3ighs&outx=600&noresize=1&nostamp=1

Things are very close at f/5.6. In the center, both lenses are about equal in sharpness, while on the edges, the Sigma has a small advantage on the left side while they are about equal on the right.

My rating for f/5.6:

Sigma -

Center - Very good - Excellent
Edges - Good - very good

Canon -

Center - Very good - Excellent
Edges - Good - very good

I looked at contrast and color rendition and found the Canon to have an advantage in the color department, though contrast was pretty close.

Overall, I would say that the Sigma is a good lens, producing images of similar quality to the Canon 17-40L at similar apertures. I did not get a chance to test for flair with the Sigma, but I already know that the Canon is excellent in this regard for a wide lens.

To me, the biggest issue to resolve is whether I really need a faster lens at the wide end. Once stopped down, ther Sigma prime doesn't show a significant advantage to the zoom in this case, except at f/4. On the other hand, it looks pretty sharp at f/2.8 and even wide open in the center of the lens.

I guess I'll just have to experiment a bit more and then decide.

[Kevin M]

Hi Tom

I haven't had a chance to compare the Sigma 20 with a similar wide angle - but i did compare it for sharpness with two other well regarded primes - the ef 50 1.8 and 85 1.8

http://homepage.eircom.net/~bot/paint/lenstest.htm


Kevin

[Tom W]

Nice test, Kevin. It looks to me like the 85 is the one to beat so far. I have it and the 50 f/1.4 and I am more fond of the 85.

I'll note though that it is more difficult to make a super-wide f/1.8 lens. I suspect that the focus "plane" is a bit curved on the wide lens. There's just a ton of correction to be made to get the plane of focus flat.

[Kevin M]

Hi Tom

I agree - the 85 is a superb lens at a bargain price.

The Sigma is difficult to focus spot on at F/1.8 - but is useful for those very low light shots where critical sharpness across the frame may not be your main concern. Stopped down a bit it performs very well - and as a landscape lens it is hard to beat.

Kevin

[Tom W]

Hi Tom

I agree - the 85 is a superb lens at a bargain price.

The Sigma is difficult to focus spot on at F/1.8 - but is useful for those very low light shots where critical sharpness across the frame may not be your main concern. Stopped down a bit it performs very well - and as a landscape lens it is hard to beat.

Kevin

It seems to really crispen up at f/2.8. I didn't check the intermediate stops (f/2.2, f/2.5), but at 2.8, its pretty nice in the center and not too bad on the edges.

Right now, I'm just wondering how I'll use it. I wanted to cover a decent range with a couple of fast primes, but now that I have a fast one at the wide end, I'm not sure of its value. Image-wise, I don't think it has much if anything on the 17-40L zoom, particularly in the normal outdoor aperture range. Indoors, I'll have to give it some exercise and see how well it does in more realistic situations. I think that its value will show up there.

I got it for a respectable price, so I'll probably hang on to it for a while.

[Kevin M]

Since you already have that fov covered by the 17-40 - the only advantage to the Sigma 20 for you would be the extra speed in poor light. It is probably slightly sharper - being a prime - but given the overall versatility of the 17-40 not of real concern.

[CyberDyneSystems]

I'm amazed at the difference in the Sigma from 1.8 to f/4... the F/4 positively looks better than the Canon zoom at f/4.. and well it should... being a prime.. but the edges at f/1.8? yucko!