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.

Back on the streets again

Today I had a good hour on the streets with my 5D and a 50mm lens. It’s been a long time since I used the 50mm now, but it was very pleasant to carry a setup that didn’t scream “PHOTOGRAPHER” two blocks away.

While my Canon zooms, namely the 70-200 f/4L IS, the 24-105 f/4L IS and the 17-40 f/4L are magnificent, they are also huge. That’s a big problem on the street, but even while doing nature photography it’s a problem. Due to some back problems, I am reluctant to carry more than one of those lenses, and even then the setup is heavy on my sholder. Im really thinking more and more of the Pentax gear. So far the starting setup would look something like this.

• K7 body
• The kit zoom for family snaps (doesn’t really add much cost)
• DA 21mm f/3.2
• DA 35mm f/28 Macro
• DA 70mm f/2.4

After that, some potential candidates would be

• DA 15mm f/4
• DA* 50-135mm f/4
• DA* 16-50mm f/4
• DA* 55mm f/1.4

It is always good to have a zoom or two, and the DA* lenses are weather sealed as well, so there is an added benefit.

Anyway, here is some of the pictures from today. Remember, click on them to see a bigger version.

New SSD boot drive

I just installed a new boot drive in my Mac Pro, a OCZ Vertex 120MB SSD drive.

Oh boy how that made a difference. The computer just became so snappy now. Startup, login and application startup is so much faster, and I don’t have to listen to that harddisk sound all the time. I still have ordinary drives for my image files and all the other data I store, as well as for the TimeMachine drives, but the system and applications are on the SSD drive. Below is a table of different operations using the new and the old drive. And by the way, the old boot drive was the standard 320GB drive that came with the computer. It’s a Western Digital 7200rpm drive.

Operation	WD 320GB	OCZ 120GB
Start from pressing button to the login dialog show up	38sec	25sec
From entering password until the desktop is completed	8sec	3sec
Launch Photoshop	18sec	5sec
Launch Firefox	4sec	1sec
Launch Safari	2sec	<1sec
Launch LightRoom	15sec	9sec
Launch iTunes	6sec	2sec
Launch Mail	4sec	1sec

This is not a very accurate or scientific test. I only did it once for each drive, and it is kind of difficult to determine when some operations are finished, but it gives an indication of how it went.

The subjective impression is that things suddenly happens instantly. The icons in the dock hardly gets to bounce when an application is launched. All this joy is of course a result of the close to zero access time on these disks. There is no speedup on cpu intensive operations, like LightRoom development operations or Photoshop work, but all the small daily tasks suddenly became much more enjoyable.

Just a note on the OCZ drive. The Vertix has got some very good reviews, but don’t mix it up with the Apex model. Even if the Apex drive seems to give faster read and write speed, the controller circuit does not handle large amounts of small files very well. The performance drop in such cases are dramatic. The Vertex has a different controller with a cache, which handles this very well.

Pentax nostalgia

My first SLR way back when, was a Pentax. I think it was a Pentax P30n. Then I hit the mark when I got hold af a Pentax LX and a MX along with a 100mm f/2.8 Macro and 35mm f/2.8. I was completely sold. I also owned a couple of autofocus models. First the SF7, which was crap, then the Mz5 which was a really nice camera. Before I sold out, the Mz5 was the companion camera for the LX. It had a traditional shutter wheel, and control ergonomics that felt much like the LX.

The LX was the nicest (is that a word??) camera I’ve ever owned and used. It was small and fairly light. It had moderate weather sealing, and very robust. The auto exposure was very unique, used only by Pentax and Olympus, with TTL reading for ordinary exposures as well as flash. The meter measured the light that reflected off the film during exposure, and closed the shutter when it was enough. Really good for low light shots. It also had a very precise film advance mechanism that enabled you wo rewind to a previous shot on the roll and do a second exposure. On lots of other cameras you could see the spacing between the frames vary, but not on the LX. The handeling was excellent, and the controls was very traditional. It was a camera I could use very fast. Unfortunately I sold it off during my student days. I needed the money to buy into a sailplane (yes, a real plane you sit in and fly, but without an engine).

When I started my quest into digital photography, I instinctively headed over to the Pentax shelves, but at the time it seemed like Pentax had wandered into the dark age with both their dSLRs and their lenses. I went for Canon instead. My Canon gear have seen a lot of use. The 5D is a work horse that produces really nice images, and the lenses are stellar, which shouldn’t come as a surprise as I went for L-glass. But there are some issues. As much as I want to like it, the ergonomics of the knobs and buttons are not quite fit for me. I just can’t get used to it. Also, the gear is very bulky. The lenses, apart from being big and heavy, has only one slight disadvantage. The multicoating is not as good as Pentax SMC. Even the old glass I used to have behaved better when pointed at the sun than the expensive modern Canon glass I own.

Oh, and the viewfinders; The Canon 5D has a very good reputation for a big and bright viewfinder these days. A couple of years back, I found my old Pentax Mz5, which I didn’t sell for some reason, and I was shocked when I looked through it. the Mz5 is a very small camera, smaller than all of todays “small” dSLRs, more like the Panasonic G1. The viewfinder, on the other hand was way bigger and brighter than the Canon 5D. I had a comparable lens on it, a 28-70 f/4 vs. EF24-105 f/4. The prism house was so much smaller to look at than on the Canon, and Pentax also managed to squeeze in a pop up flash as well. But the view …… aaah .. somthing to kill for. And worth mentioning, this was the low end model at the time. I had about the same viewfinder experience when I looked through a Canon 1Ds mkIII the other day, but this camera was also equipped with the 85mm f/1.2 lens.

Well, a couple of years has passed by, and Pentax has found their way back into the sun. The lens lineup is better than ever, and the newly announced K7 seems like a real killer. I’m going to be first in line to try it out when it hit the stores this summer. The K7 with its weather sealing, small size and 100% viewfinder. As much as I like my Canon lenses, I will definitely consider the K7 when it comes. Combine it with a 16-50mm f/2.8 and a 50-135mm/f2.8, it should be a good replacement for my Canon gear. Add the 55mm f1.4 and the 21mm as well, and it may well be a killer kit.

Good thing the camera isn’t in the stores yet so I’m forced to think it through.

If I’m going down that path, maybe I should start with the K7 and only the two mentioned primes. It could be healthy for my photography.

HDR sensor

Fuji has released a new sensor with HDR capabilities. Here is my alternative solution to the problem.

My new snow shoes

Second episode of the “in the pocket” podcast, and this time I’m talking about:

• I got a pair of new snow shoes this christmas, and this saturday I was out using them.
• The battery life on my camera is just too long, and is about to cost me some good shots.
• I little tune to finish off the episode.

An update on the protective spray

A while back, I bought a can of the Hahnemuhle protective spray, which I wrote a piece about here.

Now, I have used much more, and I am still pleased with the result. I only use it on Photorag and Fine Art Pearl, but for those papers I think it improves the final result. The Photorag gets slightly more dense black, which gives the print just a bit more punch to it. It does offset the soft proofing a bit, but would argue that the soft proofing is not that accurate anyway. It will get you very close to the target, but only actual printing can get you spot on.

I have had some mixed feelings when it comes to Fine Art Pearl due to the gloss difference. The whites (or the paper base) is not as glossy as the ink, and I think it is a bit disturbing. This is more or less eliminated by the spray, and I think the paper surface becomes very pleasing. On this paper, the colors are practically unchanged.

I have no idea about longevity, and I don’t really care that much. I usually hang prints in my home without glass, and I haven’t seen any effects on that yet.