Archive for July, 2009


Maybe LightRoom wasn't that slow

A while back I wrote about how slow LR was when using gradients and brush tools. Even on my 8-core MacPro, the beach ball kept on spinning forever. Today, as I was watching the L-L video “Where are my #$$$”% images”, I realized that I had turned on the “Automatically write changes into XMP” option. This makes LR write every single movement on my sliders into the XMP sidecar file (or the DNG file) as I do the movement, and the slowdown is caused by the disk bandwidth. When I turned off this option, LR in snappy, and I am happy.

So, go into the catalog settings and uncheck the “Automatically write changes into XMP” option inside the “Metadata” tab. You should also go into the “View Options” under the “View” menu, and tick off the “Unsaved Metadata” checkbox. That way you will get a small arrow on each photo that contains unsaved metadata. Every once in a while, and at least every time you quit the session, select those photos and hit Cmd-S. You will not lose work even if you don’t save, but then the metadata is only stored in the LR database file, and not as part of the image file. If the metadata is stored in the image files, you will get back everything if you reimport them into a new catalog, which is good if the lightroom catalog gets corrupted.


Auto focus is confusing?

I while back, I looked through the archives at Cambrigde In Colour, and found this article about auto focus technologies. The author tried to describe how the autofocus of dSLR’s are working, when in fact he was describing the contrast based autofocus most commonly used in compact cameras. Later I found Tim Jacksons D70 site, where he also describes a focus system that I don’t really think exists in any camera on the market. After looking around on the net, it occurred to me that there are many attempts to explain autofocus out there that are just plain wrong. It seems like the phase dection autofocus system used by all SLR cameras, both digital and film, is a bit hard to grasp, so most authors explains the conseptually more easy contrast based autofocus instead. Or maybe they just fail to see that there is a difference.

Contrast based auto focus

To start with contrast based auto focus. This type of system is most commonly used in compact cameras, and on some dSLR’s when the Live View function is used. It uses the actual imaging sensor to determine focus, and here is how it works:

  1. It shoots a series of images at different focus settings.
  2. For each of these images, it will determine how  much contrast there is in the designated focus areas.
  3. It selects the image with the most contrast, and reset the focus to the value it used to capture this image.

This process is relatively slow since the camera must take a large number of shots and process them before finding the focus. On many cameras you can observe this if you look at the lens while trying to focus. It will cycle focus from one endpoint to the other before it snaps back to somewhere in between which hopefully is the right focus setting.

A major drawback with this kind of focus is that the camera has no way of predicting if the focus is in front of, or back of the current setting. It must just try in one direction and hope that it is right, and if not, it must go the other way.

Phase detection auto focus

This type of auto focus works kind of like the good old split prism in the days of manual focus. The focus system will take a part of the light coming from one side of lens and project it on one sensor, and a part of the light coming from the opposite side of the lens and project it on another sensor. These sensors are dedicated AF sensors, and are usually formed as narrow strips. When the image is in focus, the two images are perfectly aligned, but they will get more and more misaligned as the system gets more and more out of focus. Just like the split prism, all the AF system has to do is to rotate the focus until the two images are aligned.

A major benefit with this type of AF is that it can predict which way the focus shall be turned. Not only that, but it can even predict how far it shall be turned. This is accomplished by detecting to which side the misalignment is, and how much the images are off.

Here is a short drill down how it works:

  1. The AF sensor detects the direction and amount of misalignment.
  2. Based on the information about the lens, if predicts how far the focus must be turned.
  3. The focus is then adjusted accordingly (focus is not measured when this happens since the focus changes too fast).
  4. When the focus motor stops, a new measurement is performed. If the focus is not perfect the drill (1 through 3) is performed over again, but now the adjustments are much smaller, and therefor more precise.
  5. This goes on until the focus is good, or the system times out because it is unable to find focus.

The big misconception is that this system optimizes contrast, when in fact it correlates two one-dimensional images. When the contrast based system relies on a sharp edge to focus, this system only need some sort of recognizable pattern. In theory, it should be able to focus on a smooth gradient that goes along the focus sensor.

Here is an illustration by Canon (go to the bottom of the page).

And here is the Wikipedia article.

So, this is an overview of my understanding of these systems. If anyone more knowledgeable than me finds this to be wrong, please write me a comment.

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