A fast lens with terrible corners

Like the other posters in this thread i too like the EF 28mm f/1.8 USM even with it's flaws.

I got it originally for my 650D for a walk-around prime [i first got the 50mm f/1.4 but the field-of-view was too tight for general use] and it looked right on the crop body and handled well ergonomically. As for IQ the images looked good and in low-light i was always amazed how bright the images could be thanks to the wide aperture! But, yes, at those very wide apertures the images can look a bit soft but in some of those low-light situations it could actually sometimes add a more pleasing look to them.

Although the size of the 28mm f/1.8 is quite compact even on the crop-body i eventually purchased the EF-S 24mm f/2.8 pancake lens for the even more smaller size. [When i purchased the 28mm the 24mm pancake was not in production at the time, had it been i probably would have gone for it in the first place]. So that kinda made the 28mm redundant... but then soon after i bought a Canon 6D!

Putting the 28 1.8 on the 6D bought it back to life because it now became it's true-self of a 'wide angle' lens instead of a "standard" FOV and is currently the widest aperture lens i have for my full-frame body. So now i have a wide aperture, FF set-up i thought i'd see what it'd be like for astrophotography - but first i read-up a bit to see if it would be okay before venturing out, and i was a little disappointed with what i read.

It turns out that the lens suffers from coma, particulary sagittal astigmatisim. This causes stars or other points of lights to look stretched around the edges and the only way to counter this is to stop-down to around f/2.8, of course by then it loses some of that good light collecting ability. Nevertheless it can be still used for astro - especially if you don't have any other suitable lens - and some good images can still be had, in fact atmospheric conditions & location are often more of an issue!

Below are images of what i recently took to test for the astigmatism, i didn't use a perfect set-up, i just leaned against some railings at night pointing at some lights shooting a range of apertures and, indeed, the abberation is there and, for practical purposes, is corrected by f/2.8.

f1.8 - look at those stretched light points away from the center!! The very center point looks okay, round, but is actually blurred - soft.

f/2.0 - getting better but still soft overall.

f2.2 - Center, good. Edge, still poor.

f/2.5 - a lot better, but edge still a touch soft.

f/2.8 - looks clear, to me. All point around round and in focus even with no tripod used!

f3.2 - is it sharper too? Perhaps not, tripod & remote shutter not being used means imperfect test but i don't think it's sharpened, really, from f/2.8 but you can noticed the 'star' point lines beginning to grow.

f/3.5

f/4.0 - Stars on the ground.

Example of one of my astro practice shots with the EF 28mm f/1.8 @ f/2.8 on the 6D with tripod, remote release and a touch of breeze. Exposure +1 in post.