On this page I've posted some center and corner crops from a couple of lenses. These are 100% crops from the full size images, i.e. 1 pixel on your screen is one pixel in the original image.
The first lens is a Samyang 18-28/4 zoom. This is a manual focus lens which I mounted on an EOS 40D via an M42 -> EOS adapter. I chose this lens because it's an excellent example of a cheap and fairly nasty zoom when used wide open. It shows you the type of images that are indicative of a pretty low quality lens - or a really bad sample of a better quality lens. The Samyang is very well built, with smooth focusing and solid metal construction. It looks nice but optically it's not a very impressive lens, especially when used wide open.
The second lens I used was the Canon EF 50/1.8. This one happened to be the MkI version with a metal lens mount, but in the past I've looked at the 50/1.8 MkII with the plastic lensmount and the performance of the two lenses was very similar indeed. I've included images taken wide open at f1.8 and stopped down to f5.6 where the resolution probably peaks. It's an example of what a good lens looks like (especially at f5.6). If your lens produces images like the 50/1.8 at f5.6, there's nothing wrong with it!
|This is the center of the image shot with the Samyang 18-28 at 18mm and f4. It's soft and contrast is low and there's a hint of chromatic aberration even near the center of the image.|
|This is the corner of the image shot with the Samyang 18-28 at 18mm and f4. It's the poster boy for bad images! It's very soft, the contrast is low and chromatic aberration is very obvious. If you see something like this you know you have a bad lens. The siemens star and line patterns both indicate significant astigmatism.|
|Here's the center of the image shot with the same Samyang 18-28, but this time at 28mm and f4. The most obvious problem is low contrast, plus it's soft and again you can see chromatic aberration creeping in even close to the center as shown by the purple/yellow fringing on the back bar on the left.|
|Here's the corner of the image at 28mm and f4. Vignetting has lowered the exposure which actually makes the image look a bit better! However it's still soft and chromatic aberration is very evident.|
|Here's the center of the image Canon EF 50/1.8 shot at f1.8. What a difference! Higher contrast, much better sharpness and no real hint of chromatic aberration.|
|Here's the corner of the image shot with the 50/1.8 at f1.8. A little darker due to vignetting, but sharpness and contrast are still good. chromatic aberration is well controlled|
|Finally here are two shots taken with the 50/1.8 at f5.6. This sample is from the center of the image. Going from f1.8 to 5.6 has increased sharpness just a touch and has improved contrast. Compare the checkerboard pattern with that of the shot at f1.8 and you'll see the blacks are much blacker. especially around the edges of the black squares.|
|Finally here's the corner of the image shot with the 50/1.8 at f5.6. Vignetting is obviously much less than at f1.8, contrast is higher and resolution is slightly better. Again compare the black squares in the checkerboard patterns in the images at f1.8 and f5.6 and you'll see a significant difference.|
Below are two samples from an image shot with the Canon EOS 40D and the Canon EF300/4L, a very good telephoto. As you can see, the image quality in the center is very similar to that of the 50/1.8 at f5.6.
|This is the center from an image shot using the EOS 40D and the Canon EF 300/4L (non-IS version), wide open at f4|
|This is the corner of the image shot with the EOS 40D and the Canon EF 300/4L (non-IS) at f4. Not quite as sharp as the center, but still very good.|
Of course it's reasonable to ask why buy expensive, fast lenses if the 50/1.8 ($85) gives images which are pretty much just as good? Well, the reason to buy an expensive, fast, "L" series lens is that (a) you're not limited to 50mm, (b) in the 50mm lens series, you're not limited to f1.8, (c) it's going to be better constructed and focus faster, (d) if you want a fast lens, you have to pay for it (unless it's a 50/1.8!) and (e) some people want zooms and high performance zooms are more expensive to manufacture than primes.
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