#1. "RE: Why Stop down to F8?" In response to Reply # 0 Sat 19-Mar-11 04:43 AM by JPJ
I would disagree that the focus point is clearly on the dog, let me explain why.
First, I would not put too much stock into where ViewNX puts that focus box. It is a graphic overlay that is not necessarily an accurate representation of where focus locked when the photo was taken. There are a few interesting threads about this on the internet if you want to read up on it further, but sufficed to say it gives you an approximation of where the focus box was at the time the shot was taken, not necessarily where focus was.
Even if we use the focus box, you can see the entire box does not cover the dog. In fact part of it, the lower left, is covering the ground behind the dog, which is what (along with part of the tree just behind it) looks sharpest. One thing you need to be aware of about the focus sensors on your camera is that the actual sensor is bigger than the focus box on the camera. It is important that you completely fill the sensor with your subject that you want to focus on, which means ensuring the whole box covers it and then some. If not, it is possible for the camera to be confused about what you are trying to focus on. Here the camera clearly focused on the ground in front of the tree imo, in fact it is sharp along the entire plane parallel to that point.
Ultimately, this appears to be a case where you aimed for one subject but ended up getting focus on another. Without knowing more about the technique you used to AF here I can say much else. It may be that given the fact that your subject was running that improper AF technique lead to this problem. It may also be that there was proper technique and you just missed it, that happens with moving subjects sometimes. Are you set to release or focus priority. One critical issue if you were using AF-C is whether you had focus tracking on and if so set to what level (setting A3)? Finally, you were using 9 points, where did you start trying to lock focus on your subject and with which AF point (eg. the center?)?
I personally would have stopped down a little here as F/2.8 doesn't leave you much room for error even if you nailed the focus, but I don't think you need to stop down to f/8, f/4-5.6 would suffice. Even if you had done that here, given that AF did not lock on the dog this would not have helped much.
#2. "RE: Why Stop down to F8?" In response to Reply # 0
Seattle, WA, US
Jason gives a great answer for the focus points.
For the f/8 suggestion, it probably comes down to two things. (1) f/8 is usually in the sharpness sweet spot for just about every lens. (2) The depth of field at f/8 is normally big enough to hide most focus problems.
---------+---------+---------+---------+ Joseph K Seattle, WA, USA
#3. "RE: Why Stop down to F8?" In response to Reply # 0 Sat 19-Mar-11 11:05 AM by chris_platt
As mentioned above, in addition to the DOF considerations, the vast majority of lenses perform better when stopped down.
All lenses suffer from things like vignetting - darkening or fall-off as you move to the edge of the image; variations in sharpness (contrast and resolution) at different f stops and as you move to the edge of the frame; and chromatic aberration - red or blue fringing which often gets worse towards the edges and corners. Sometimes those issues are obvious, and sometimes they are more subtle, but still contribute to "softness" or make marginal focus appear to be worse. As you close down the f stop, those problems tend to reduce dramatically.
The www.dpreview.com website provides great graphic representations of those effects and the impact of changing the f stop. Graphs and charts don't always provide a reliable prediction of lens performance in the field, but they do illustrate the concept.
I'm not sure which 50mm lens you used, but here is a link to one 50mm test that illustrates the point.
#5. "RE: Why Stop down to F8?" In response to Reply # 3
South Lake Tahoe, US
One question. You said that you were shooting at 2.8, yet I looked at your info box in the upper left hand corner of your image and is shows 1.8 with a shutter of 3200 which gives you a depth of field of 3.58 feet. And Looking at the ground I appears to me that you have about that much in focus on the ground based on a Germanshepherd being about 44inches long. I have gone Thur all this as well and choose to stop down the aperture to f8 or larger to allow for the focusing mistakes by me or the camera. I also use single point to help me focus on moving subjects. If you trust your settings and do the math as you have done, you can figure out where the camera really did focus. I looked like it focused on the tree or just in front of it which would put your dog out of focus. Don
#6. "RE: Why Stop down to F8?" In response to Reply # 5
Brownstown Twp, US
There is always a trade offs with lots of factors.
Shooting moving subjects is alway a tough one.
There is a famous saying "F8 and be there!" which is highly debated and attributed to different people.
It was explained to me when I was in school for the Air Force to become a Combat Photographer, that F8 is typically where to start from. Everyone needs a starting point and a reference point. We used F8. It got to where we would be walking around and the instructor would say okay without looking through the camera or using a light meter, what is the current exposure for X- being any number of things.
We got pretty good at being able to guesstimate an acceptable exposure. Notice I said Acceptable not best. So f8 being an aperture that most any lens does well with and in a reasonable shutter range in normal circumstances. F8 was our starting point.
You should try it sometime. Walk around and guess what an exposure would be for a given subject. Put you camera in Manual and shoot it. Then look at it. You will see and learn very quickly and tune your eye.
Did you know your eye can be fooled into seeing better than your normal vision? Shooting on the Air Force marksman team I had our top shooter explain why a peep sight works for people who are far and near sighted. The aperture of the peep sight forces the eye to dilate into a narrower/smaller pupil. This decreases the amount of light but also increases the level of detail at distances.
My father (before he passed) was having difficulty in shooting firearms and being able to see the target and the front sight. So I had a peep sight put on one of his rifles and he was amazed. Suddenly he could see both target and sight again.
Light effects our eyes and our lenses much the same way. If the aperture or pupil is dilated or closed down then we actually can see better details and focus for a greater range. We can fool the brain and the camera to increase the detail within the nearest field that is in sharp focus as well as the furthest field and everything in between.
Plus F8 is an aperture that typically will give acceptable results - irregardless of the camera or lens being used. It is just a starting point!