The Digital Photo Guy

Combining Multiple Photos

by on Aug.08, 2010, under gear, Monday Morning Tips, Photo Editing, Photoshop CS2/4, Photoshop Elements

Compositing Photos

As always, whenever I post an article I receive a flurry of e-mails asking, “How’d you do that?” This week, it was about replacing the dull Parisian sky with a bright blue SoCal sky. This is a pretty simple task so I prepared a short 3 minute video.

[swfobj src=”http://www.thedigitalphotoguy.com/wp-content/uploads/2010/08/ReplaceSky1.swf”]

How Your Camera’s AutoFocus Works

(and why you should care)

I posted this as a question on a site I visit from time-to-time and no one either knew the answer or cared. Therefore, I decided my readers were a better audience for something a bit more technical but, nonetheless, important to getting the most out of your camera gear.

Today, most dSLRs use phase detection auto focus systems. Without getting into gory details, phase detection is fast and expensive. This is also the technology that enables predictive AF, also known as Canon AI Servo, Nikon Continuous Servo AF and various other proprietary names.

The key is that contrast detection systems today can only examine a small linear segment of the sensor. I don’t have the exact dimensions but I would guess each segment is only a few pixels wide at most. Sensors in horizontal (landscape) orientation are more sensitive to vertical details while those in vertical (portrait) orientation are more sensitive to horizontal details. This is because the details cut across the sensor at or near 90 degrees. A horizontal object would not offer much detail against a horizontal sensor and vice-versa.

In the early days, all sensors were either horizontal or vertical. Then, cross sensors were developed that combined a horizontal and vertical sensor. Partly for cost and partly for marketing purposes, most cameras only had cross sensors at the center. Today, high-end cameras might also incorporate X-type sensors that are, basically, cross-type sensors turned 45 degrees.

Rather than try to re-write what is essentially a very complex technology, here are some links that explain all this. Bottom line, a good photographer knows what kind of AF sensors he or she has in their camera to get the best performance.

LenRentals.com – Roger Cicala does an excellent job of explaining all this without delving into the gory technical issues. This is particularly amazing considering that Roger is an MD, a profession not usually known for articulate explanations.

CambridgeInColor.com – Sean McHugh, a PhD at the Univesity of Cambridge, does an excellent job of explaining how different lenses effect AF capabilities. Scroll your cursor across the apertures in the illustrations near the middle of the article.

FredMiranda.com – To see photos of an actual AF sensor from a Canon 400D (XTi)/30D, see post #19 (near bottom of page). This will crystallize all the other info.

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2 Comments for this entry

  • Jim

    Hi Lee,

    Could you have placed or dragged layer 2 over layer 3 to get the same result? If not, please explain.

    I noticed you did not feather layer 2 ( an object with blank area above it) or layer 3 (cloud layer).

    Would you explain when you need to feather, where you need to feather and why you need to feather an object in the first place?

    I find feathering confusing because I don’t know when to apply it, how much to apply, or if feathering is needed at all – as you did in this case. Can you clear this up for me?

    Thanks, Jim

    • Lee

      Layers can be dragged up or down. How else would you move a bottom layer up one level? Feather is used to better blend objects. How much and when to feather is a matter of preference and experience.

      There are too many variables to write a full up tutorial here. There are hundreds of free tutorials about layers and feathering on the Web. You can also take a paid class where the instructor will answer your specific question in detail. That’s why my classes come with 90 days free support for the questions you didn’t ask during class.



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