Canon 28mm f/1.8
Canon EF 28mm f/1.8.
SHARPNESS and COMA
It's very sharp, as expected. Also, as expected for a wide lens with a fast aperture, it's a little soft wide open.
Even at f/1.8 it's great for hand-held low-light shooting, but if you're shooting test charts or astronomy, stop down to f/2.8 or spend $1,000 on an L lens like the 24mm /1.4L, 35mm f/1.4L or the Nikon 28mm f/1.4 on a Nikon if you need perfect sharpness wide open.
For regular photography, don't worry: it looks great at every aperture, and if you want perfect test sharpness, shoot at f/2.8 or smaller.
The low contrast wide open comes from spherical aberration, which means that there is always a sharp image, but a veiling haze appears over it wide open.
Stopped down to f/2.8 it's fine for astronomy, too. Here are stars in the moonlit sky.
Resort by Moonlight. Canon 28mm f/1.8 at f/2.8, Canon 5D at ISO 800, 8 seconds.
Here's a complete guide image from my full-frame Canon 5D, from which I will show you crops.
Full image, Canon 28mm f/1.8, full-frame Canon 5D.
Here are crops from the center of the above image at 100%. Remember, the complete image printed at this same magnification would be 44" (1.1m) wide:
Canon 28mm f/1.8 at f/1.8, crop from center of 100% image.
See the slight veiling? That's what happens if you're dumb enough to shoot at f/1.8 in broad daylight. In dark situations where you'd really use f/1.8 hand-held, it looks fine. This is normal and the sort of thing you'd probably never notice until you owned the lens 5 years, then if you saw it for the first time you might worry. Not to worry; this is what this lens does.
Canon 28mm f/1.8 at f/2, crop from center of 100% image.
Even stopping down 1/3 stop to f/2 improves the image.
Canon 28mm f/1.8 at f/2.8, crop from center of 100% image.
By f/2.8 the center of the image is perfectly sharp, so long as your subject is within the small depth of field at f/2.8.
Canon 28mm f/1.8 at f/4, crop from center of 100% image.
This shot's for free: the image is as sharp as it's going to get by f/2.8 in the center, so f/4 looks the same. Diffraction will start to soften the image at f/11 and smaller, as it does on all lenses.
Top Right Corner, Full-Frame
Here are crops from the top right corner of a full-frame image at 100%. Like all wide lenses, performance gets worse as one gets further away from the center, so the corner of a full-frame image is the toughest test. It will be better on a 1.3x (1D Mk III) and much better on a 1.6x (Rebel and 30D) image.
Canon 28mm f/1.8 at f/1.8, crop from top right of 100% image.
This is softer due to the veiling of the spherical aberration, but notice that there is a sharp core as well. The details are here, hiding behind lower contrast. It's also a bit darker in the corner due to the slight falloff.
Canon 28mm f/1.8 at f/2, crop from top right of 100% image.
Stopping down a third of a stop to f/2 is an improvement, both in improving falloff and sharpness.
Canon 28mm f/1.8 at f/2.8, crop from top right of 100% image.
f/2.8 is much better than f/2, but since we're in a far corner, still not as sharp as in the center at f/2.8.
Canon 28mm f/1.8 at f/4, crop from top right of 100% image.
By f/4 even the corners are great. f/2.8 is all you need for perfect sharpness, but if you're looking at it with a microscope like this, then stop down to f/4.
Coma is what makes points of light in the corners look like blobs or comas instead of points. If a lens has coma, it's visible at only the widest apertures. Coma is caused by spherical aberration.
The Canon 28mm f/1.8 has pronounced coma at f/1.8 and f/2, and it goes away at f/2.8.
Full-frame image, Canon 28mm f/1.8 at f/1.8 for 6 seconds, 5D at ISO 200.
See what looks like Saturn in the upper left along the top edge? It's actually Jupiter, and should look like a point, not a blob. The huge bright thing is a 12 day old moon (3/4 full). The blobs at the bottom left and right also are not as we'd like them. These blobs are coma.
Full-frame image, Canon 28mm f/1.8 at f/2 for 8 seconds, 5D at ISO 200.
Stopping down a third of a stop to f/2 is a small improvement.
Full-frame image, Canon 28mm f/1.8 at f/2.8 for 16 seconds, 5D at ISO 200.
By f/2.8 the coma is gone, hooray!
Full-frame image, Canon 28mm f/1.8 at f/4 for 32 seconds, 5D at ISO 200.
f/4 is even a little better, and as you can see under Sunstars, the little points of light are starting to grow points. Jupiter now looks a little bit elongated, but that's due to Earth movement making it appear to move towards the lower right, This longer 32 second exposure brings that out.
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