Friday, February 27, 2009

Lens reviews/ Chromatic Aberration

One of the most awesome aspects of the  1Ds Mark III and the new 5 D Mark II is the fact that the resolution of the new cameras actually outstrips the performance of (some ) older Canon L series lenses. This is why there have been so many new " II " series additions to the Canon lens line.

Chromatic aberration in particular was never a huge problem in the past when a print from a 35mm negative was seldom enlarged past 11 X 14.  We just never looked at stuff with that kind of magnification.

Now I check almost every image I submit at 100%.  Why?  Because the client probably will.

I just sent some photos to a designer who called back wondering why there was a little red/green fringing on an image, even though the photo was slated to be used on a website at about 5 X 7 at 72 dpi.  People are really picky about digital images.

Given the amazing low light abilities of the Mark II,  I've been  daydreaming about buying a really stupid fast lens, but the 24mm f 1.4 (just upgraded) is almost $ 1700, so I wondered if there were any bargains to be had that were really good from secondary suppliers like Sigma or Zeiss.  

I came across this great website, The that has the most extensive and in depth lens reviews I have found. Unfortunately he only reviews Canon related gear but I always check it when I am looking to buy a new lens.  Alas, there are no bargains in the fast wide angle category, but it did save me from buying a Sigma 24 or a Canon 28 mm 1.8 that I am sure I would have been disappointed with.  However the tests do show that are some really outstanding lenses for Canon cameras that are real bargains, like the 85 F 1.8, the 100mm F 2.0, and the very surprising 18-200 and 10-22 EF-S lenses. 

So what is Chromatic Aberration and why is it suddenly such a big deal?

The easy answer is that it's just the inability of the lens to focus all of the wavelengths of visible light to the same point.  However with digital imaging there are more things to consider:

When photographers shot color film the different colors were recorded on different layers so lenses could actually be optimized to take advantage of the slightly different focal planes for each color layer.   Film had a certain amount of physical depth that lens designers could use.  However a digital sensor is a truly flat plane, so CA is more noticeable.  We also shoot a lot more with zoom lenses, and it's tough to correct a zoom lens so that it performs perfectly throughout the range.  Smaller chips also mean shorter focal lengths and CA is much more of a problem in short focal length lenses.

Finally, the imaging sensors in digital cameras also have micro lenses on each photosite in the chip itself.  These small micro lenses introduce another form of CA because they are tuned to more accurately focus green wavelengths, at the expense of red and blue wavelengths (perhaps because it's the midpoint in the spectrum?) and this can result in purple fringing.

I've actually accentuated the CA in this photo to make it easier to see.

CA is also more visible in digital images because the digital sensors responds better to light rays that hit the focal plane perpendicularly.  Film didn't care.  When you add it all up it is easy to see why Canon has been revamping their L series lens line.  The old lenses were optimized for film and all of the new lenses are marked by less CA when used for digital capture.

While it is possible to mitigate the effects of CA in Lightroom and Camera RAW, it doesn't always work perfectly and it is really time consuming, so good lenses are still the most important factor to overall image quality.

One other interesting side note:  Like almost everyone else I use skylight filters on my lenses to protect them.  I recently replaced the $ 20 Sunpack filter on my 17-40 mm lens with a primo, $ 100 Rodenstock skylight filter.  The very small amount of CA that lens had was virtually eliminated by simply upgrading the filter, and lens flare was also reduced to a negligible amount.   It was really worth the extra cash.

I might start a Rodenstock dealership.  Maybe then I 'll be able to afford the $1700 24mm L series II.

By the way, there's a nifty little tool in Lightroom 2 that can help with any residual CA you can't fix with the Chromatic Aberration sliders:  Grab the paintbrush tool and just de-saturate the edges.  

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