Saturday, June 23, 2007

Depth of Field

I have a new camera and I have had the same problem over and over - insufficient depth of field (DOF) So I decided to try to figure it out. Here is the result.

Depth of Field is the distance from the focal plane of the camera over which a picture is in focus. With insufficient DOF, sometimes my subject is either out of focus, or partly out of focus. So I did a quick study for myself to see how DOF is controlled with my new camera. If anyone technical is reading this, here are the important specs on my camera. It is a Nikon D200, 10.2 Mpix, with a 17-35 mm f2.8 lense. The lense is designed for a film camera and due to some other techincal stuff, this lense performs like a 24 - 52 mm lense on my digital camera. All the other stuff will be cited below. Remember, I need a deeper depth of field in order to overcome my inexperience as a photographer. That's the goal.

For Scenario 1 I set a pencil on a ruler approximately 30 inches from the camera's focal plane. With my lense, that means the pencil is about 24 inches from the front edge of the lense. I took pics on Apperture Priority with onboard flash. Here are the results

At f2.8 (apperture wide open - pictured on the right here) the pencil is in focus and you can see from the ruler that the pic starts coming into focus at about 26 inches and stays in focus until about 33 inches. That means two things. First, the camera did not actually focus on the pencil at 30 inches. The area around 28 - 29 inches is actually the sharpest. Second, at 30 inches with a wide open apperture, I get a 6 or 7 inch depth of field. That should normally be sufficient but if I am taking a picture of Brenda, her face might be in focus, but some of her hair might be fuzzy. Not good. Of course it depends on the subject being photographed.

The other end of the DOF spectrum is with the lense stopped all the way down, f22 in this case. At this point the apperture is letting in as little light as possible, so shutter speeds have to slow down, but look what happens to DOF. This pic is in focus basically for the entire photo. Nothing is fuzzy.

The other two pics in this series are at F10 and F5 (left to right below). F10 seems to be a good compromise to get sufficient DOF and still let enough light in to get a fast shutter speed.

Scenario 2 is with the pencil at 12 inches, about 6 inches from the end of my lense. This lense is pretty big and you can see that the lense obscures the flash in the forground of the pic (that dark shadow on the ruler.) Big problem for tight shots with a flash, but there are ways to fix this little issue.

First pic is F2.8 (on right) and look how tight the DOF is. The pencil point is in focus at 12 inches, but the 11 on the ruler is blurred. Similarly, the 13 is in focus, but not the 14. If you had a high resolution pic like I have on my computer here, you would see that the DOF goes from about 11 3/4 inches to 13 inches. So I got the pencil into the sharp area in this pic by only 1/4 inch and total DOF was only 1 1/4 inch overall. Not much tolerance in this set-up.

Now look at F22 (stopped down) Again, everything is in focus, even the cup holder in the forground and the banannas in the background (might be too dark on the web to see, but it is clear on my computer.) Nice. The F10 and F5 settings gave similar results as in the 30 inch scenario.

So my conclusion is that with this new, very nice camera, a wide open apperture (f2.8) gives me the fastest shutter speed possible but gives me a DOF of about 10%-20% of the distance from the camera's focal plane to the subject. At 12 inches, I get a 1.2 inch DOF, at 30 inches I get a 6 inch DOF. For pics beyond 30 inches, DOF becomes less important with this lense. So to get my subjects in focus, I need to use Apperture priority more often and try to shoot most of my subjects in the f10- f5 range. If it is a very brightly lit area, I can go to F22 and get everything in the field of view in focus. As I get better, maybe I can start opening the aperture more and reducing the DOF.

fun stuff



It has been awhile since I posted here, and that was primarily because I had run out of interesting things to do in Bangkok. I don't want to bore anyone, least of all myself by writing "Went to Burger King for lunch today...." so I have just held off on the posts. But Last week I found something new, at least to me and it turned out to be a pretty interesting week. I got certified as an Open Water SCUBA diver.

To make a really long story short, I studied the book one night and took written tests the next day. Aced them all. The following day I had the pool sessions. No problems there either. And the next day I went diving in the Gulf of Thailand near Pattaya, Thailand. That trip was not so problem free.

Much to my surprise I became quite uneasy as I descended for the first time into 10 meters of wild water. Visibility was poor and I could not see the sandy bottom until I was well over half way down. Of course that means that I also could not see the surface once I was down past 10 feet or so. I have never been anxious in the water, but this experience rattled me. My anxiety was exacerbated by some equipment problems too. My rented bouyancy vest was old and not functioning correctly (it actually sprung a leak later in the day and had to be scrapped) and I had on too little weight on my weight belt. These two things and my inexperience meant that I had a very difficult time keeping myself at the proper depth to stay with my group. My anxiety and the exertion used staying at depth lead to heavy, anxious breathing that used up my air tank in about 45 minutes. They should last a lot longer than that. But my first dive was complete and I was on track for certification. The pic to the right is a black sea urchin. It stings.

During my second dive, I still did not have my weight belt configured properly and I was too light to stay on bottom again. Well, it was actually a couple of things. During this dive my bouancy vest gave up the ghost. It sprung a leak and refused to hold air for more than 30 seconds. It was also too big for me (too fat, not too tall) and would cantankerously hold air in folds when I didn't want it to be there. To make matters worse, this was a skills dive in which I had to do things like flood and clear my mask, ascend from the bottom with no air supply and what not. To do all that I had to be able to stay in position so my instructor could see me. But after clearing my mask for the second time, I had to signal my instructor that I had to surface. I was fighting my bouyancy vest so hard I just could not stay down. We surfaced and added a Kilogram to my belt and went down for more tests. I got them all done, but I was not getting any more comfortable in the water. It seemed like nothing was going well, and I was thinking that maybe this wasn't such a good idea. But the second and final dive for the day was over and I headed to the surface after about 1 hour and 10 minutes on the bottom. Better than the 45 minute first dive.

My dive group consisted of Eleven age 30 something Thais who invited me to spend the evening with them. We went to a Thai seafood resturant where they passed plate after plate of "not too spicy" foods over for me to sample. They all laughed heartily as the expression on my face indicated that some of the dishes were maybe a "little too spicey" after all. We had a good time and I ate things that I have not eaten before. I may have some of them again. Some I may not.

In the morning our group went to a Thai breakfast place for another meal like I have not had before. Thai breakfast apparently focuses on soup. Not what I am used to but it was hearty and satisfied my hunger.

After breakfast, and a quick stop at Starbucks we got back to the boat for the second day of diving tests. I had a new bouyancy vest (that worked) and I had finally found the right set-up for my weight belt so the first dive this day was perfect. I was comfortable and performed all the skills easily, just like in the pool. Finally a good dive that boosted my confidence. Back on track. At the end of this dive we found a nearby coral reef and poked around a little. A fishing net was tangled on the reef and my instructor freed it while I kept back a little and rolled it up to carry back to the boat. I hope the reef is happier now. My instructor got stung a little bit, but she obviously felt that her effort was worthwhile.

A rain squally blew in while we ate lunch and it looked like it would not clear very soon so we took the dive boat to the other side of the island and prepared to dive the abandoned pier near an Island resort. Unfortunatly, I am a big guy and we took nine novice divers into this pretty confined space with very poor visibility. There were lots of fish to see, and corals, sea fans and all sorts of things growing on every piling. But I cannot yet put myself in postion and maintain it very long, so as the instructor would find interesting things, by the time all nine of us tried to get a glimpse of them, I found myself more often than not drifting into something that would sting me if I didn't get myself out of there. Another less than perfect dive, but this time it was just because of my insufficient skills and a group that was too large for the dive site.

Well, I am certified for open water diving now, but I have alot of practicing to do before I am competent. I intend to go back for another practice dive next weekend and hopefully I will be ready for a real dive into clear water with lots to see in August. Might even dive at night. We'll see.