Wednesday, November 30, 2011

Tools of the Trade- More on Lenses

Greetings, Hohenfels, welcome to Hohenfels Volks, THE place for our place. Here’s hoping today sees you starting on the downhill run of a great week!

Today it’s time to talk a little about lens attributes and traits.

We’ve already talked a little about lenses, so today we’re going to concentrate on some of the quality issues and features of your lenses. It’s a little long, for which I apologize.

One of the first things most folks need to know is that your old film lenses will work on digital cameras. They may have a crop factor, for instance 1.6X for APS-C, but if they can mount on your camera, you can use them. On the other hand, your newer digital lenses are unusable on full frame or film cameras. When you put a film lens on an APS-C sensor, the crop factor comes from the distance from the back of the lens to the sensor. The film lenses cover a larger area than the sensor; this in turn causes an apparent increase in focal length and the crop. Canon’s recent systems, the EOS cameras, use EF lenses, which have a 1.6X crop on your APS-C sensor. Their digital line of lenses for the EOS system is the EF-S line. They require no crop factor and apparent focal length is actual focal length. The reason the digital series of lenses don’t work on full frame or film cameras is due to their smaller projection of the image onto a smaller sensor. They won’t fill the frame or film, and most likely won’t even focus properly even if they could be mounted.

Another concern about your lens on zoom lenses is often the aperture. Less expensive lenses read something like 28-135 f/3.5-5.6. This is because the aperture size doesn’t change during zooming, that is the largest it can go. The area of the aperture remains constant, requiring a change in f-number. If you remember, your f-number is a ratio of focal length and aperture. It represents the focal length divided by that number, that’s why it’s written f/2, f/5.6 etc. The longer a lens is, the less light reaches the sensor. That’s why the f/number changes throughout the range of zooms. If your area doesn’t change, your f-number must. The reason for this is the cost and weight added to vary the aperture size throughout the zoom range. That doesn’t mean a constant aperture means a cheap lens, it doesn’t mean less quality, it just means less light as you zoom in.

A great feature of lenses over the past few years is the addition of IS. The affordability of technology has made it possible to use feature that used to be unavailable to the hobbyist. IS allows slower shutter speeds when enabled. Using it hand held, you can get down to about 3 stops lower that the handheld limits. The systems work by compensating for motion with motion in the opposite direction. When hand holding your shot, using proper shooting styles, with arms tucked in etc, will enable the IS to really slug it out with vibration. The most important thing to remember is turning it on for handheld, and ALWAYS turn it off for tripod shots. When on a tripod, the IS searches for motion in the lens and can cause vibration rather than reducing it. The big drawback to IS is that it uses your camera’s battery for power.

The last thing I’ll bring up for now is a quality issue. The problem is chromatic aberration or CA. This is distortion caused by different colors, or wavelengths, of light focusing at differing areas on your sensor. There are several types and names, but we’re not going into it that deep here. It often causes the purple fringing that you see along borders with bright highlights and dark shadows together. Using a smaller aperture can help mitigate this, as can a longer lens. Low dispersion glass and good coatings can combat this effectively. There seems to be many complexities involved in CA, including the types and calculations. The best practice when purchasing a lens, if possible, is to take a test shot using that lens of a high contrast area, and zoom in looking for fringing. CA can also cause blurriness in Black and White photography, so keep that in mind. You can see examples on Google to get more information and some idea of what to look for. According to the reviews I’ve read, Canon makes the most advanced software correction to mitigate CA and other distortions available. Because Canon’s EF lens system is actually all electronic, and records the information on individual lens models, Digital Photo Pro can compensate for it. Yet another advantage of shooting RAW!

Whenever you look to purchase a lens, make sure you read a variety of reviews, and look at loads of test photos taken with that lens, it could save you some headaches! If you have a lens that you favor and would like to write it up here, let me know, and I’ll get it posted for you!

Now on to other things, remember to get your votes in for next week’s theme. This week our theme is “Morning Moments.” Dazzle me with your work! Get shooting and start posting at the Hohenfels Volks Facebook page. Of course, commenting here is always welcome, too!

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