Sunday, July 26, 2015

Correcting Back Focus

I really liked this image when I made it on July 3. The lens, a Nikon 20-35mm 2.8 D, was purchased for under $300.00 from Fireside Camera in San Francisco. I was warned that one of the glass elements in the lens had a small scratch on it. It turned out to be a tiny and could only be seen if a flashlight was used to light it.

On this day, my goal for the day's shooting was to experiment with large shooting aperture settings using my formula for flash-fill. To limit the depth of field, I set the ISO to an equivalent of 100 and installed a 3-stop neutral density filter. The lead shot was made at F 3.5, which for all intents and purposes was wide open. Again, I was happy with the shot, until I took a closer look.

You can see that the foreground subject is very soft, while the spectators in the background are noticeably sharper. Now I KNOW I had focus properly, but obviously something was amiss. This condition, where there is a disagreement between the actual plane of focus and the locked autofocus setting is called Back Focus, or Front Focus if you prefer. In my case, there was clearly a problem, exacerbated by my nearly wide-open shooting aperture. Photographers who shoot with fast prime lenses will do well to check their lenses, since these large aperture wonders leave the shooter with a very, very narrow margin of focusing error.
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To test the lens, I used a Datacolor SpyderLensCal Autofocus Calibration Aid, a relatively inexpensive device that gives a photographer a convenient way to easily check for back or front focus. First, you'll need to set the device the Calibration Aid at the proper distance from the film plane of the camera. I found that the website of a similar device suggested that the distance between the file plane and the focusing spot should be 25 times the focal length of the lens. In my case, the 20mm short setting suggested a distance of 500 mm, or about 20 inches. Once the distance is set, you photograph the high-contrast focusing spot in the center of the device and check the inclined ruler on the side. By examining the sharpness of increments, you could see exactly where you camera was locking focus. 

When set up, you'll get a photo that looks something like this. Incidentally, I used a Nikon speedlight for illumination. After making the photo, you can check the offset by simply zooming in during the image preview. I've found that it's pretty easy to see where the plane of critical focus is. Once you've established how a particular lens responds during autofocus, you can save a custom focus setting in the camera body for up to 12 individual lenses.

This sample image shows the 20-35 lens with the focus locked on the focusing spot. It was alarming to see an out-of-focus reference point, and discovering a plane of sharp focus was behind the zero setting. Notice that the numbers between 2 and 6, located behind the focusing spot, are in sharp focus, while the 1 and the 0 are not.

You can see a short YouTube video here. Don't be discouraged, as making the adjustments gets easier each time you do it. The focus offset for my 20-35mm lens was set to -20, the most offset I could dial in. When I assigned it to Custom Setting #3, I was surprised to find that the camera was able to extract the aperture and zoom range directly from the lens itself and include those specifications in the custom setting. 

You can see the improvement it made. Just so that you know, there will always be a bit more depth of field behind the critical focal point than in front. At any rate, this would allow me to establish critical focus on my subjects eyes, exactly what one needs to do when photographing people.

The proof came when I photographed this woman during International Night here at the Adult School. While the settings were different from those in the lead photos, the focusing problems appear to have been addressed. The eyes are sharp, and there is plenty of detail in the rest of the photo.

These before and after back-focus comparison shots are very much an apples and oranges affair. But the improvement is there to see, even though the shooting conditions of the two sample shots weren't identical.
Some final caveats: You have to remember to turn the back focus setting "off" when you change to another lens. Also, this feature is only available on the "prosumer" level cameras, which means the D300 series, the D7000 series, and all of the full frame bodies.

Sunday, July 12, 2015

Meters Possess Knowledge, Not Wisdom

Manual Setting: 1/125 of a second, F 5.0, ISO 2000, Incandescent White Balance
This post's title is another Bob Schwalberg quote from one of the many articles he wrote for Popular Photography in the 1970's. A great time for photography. The Nikon F single lens reflex ruled supreme, Acufine allowed film to be exposed at ASA speeds of 1600 and greater, and in the darkroom, Kodak's resin coated printing paper allowed shorter print washing times. Practical autofocus and automated exposure settings had yet to be invented.

Many things have come of age in photography's Digital Age. Take exposure metering. Today, we take the accuracy of our built-in, through-the-lens metering systems very much for granted. Under even lighting conditions, they accurately assess the brightest regions (highlights), the darkest (shadows), and those regions in between (midtones). For the most part, the system works well, and when it doesn't, there are measurement options (spot, average, center-weighted) which can select a specific region for exposure evaluation, or skew the results towards a specific region.

For the most part, it's important to remember that the camera listens to every pixel in the frame, to some extent. It may listen more closely to the regions closest to the center (Center Weighted) or the tiny region defined by one of the focusing brackets (Spot). In the end, the final exposure determined by the camera  is a mashup of different values, and sometimes can be fooled.

This talented young cellist is Erica Mulkey performing as Unwoman at my nephew's birthday party. Just for kicks (and for the sake of my back) I brought only a Fuji X-E1 with a 18-55 2.8-4.0 kit zoom lens, along with a Fuji flash. I thought about bring some real lenses but decided to not get too involved. After all, it was a birthday party.

Under questionable lighting conditions, I always plan on establishing the "correct" exposure through sophisticated form of guesstimation. Normally, I establish a base exposure by:
  • Setting the ISO to the lowest value I think I can get away with. When I'm indoors, I usually start with ISO 800, always hopeful that there's enough light.
  • Setting the White Balance to one of the camera's presets. I chose Incandescent because of the hot stage lighting.
  • Setting the camera to Aperture Priority, and then setting the aperture to "wide open" for the lens I'm using. This is less complicated when one uses constant aperture lenses, as I do on "real" assignments. Here, I set the lens to F 4.0, my largest aperture at the longest focal length of my kit zoom. This way I know that my aperture will be constant throughout the zoom range of the lens.
  • Next, I mentally calculated my Minimum Shutter Speed. A Rule of Thumb says the longest hand-holdable exposure time (shutter speed) will to be the inverse of the focal length of the lens. This means that if you used a 50mm lens on a film camera,  the the longest exposure time you could use without resorting to a tripod would be 1/50 of a second. When using an APS sized (cropped frame) sensor digital camera, I multiply this number by a factor of 1.5, giving me 1/75 of a second, or thereabouts.
  • Finally, I took a shot. When I found I was getting excessively long exposure times, I increased the ISO to 2000. I shot again.
Aperture Priority: 1/34 of a second, F 4.0, ISO 2000, Incandescent White Balance
In theory, the camera's meter properly evaluated all of the values (shadow, midtones, and highlights), crunched the values, and chose this as the proper exposure setting for the 2000 ISO setting. Judging from the burned (or "hot") highlights, I needed to decrease my exposure time. I set the camera to the Manual exposure mode setting. I knew the camera chose 1/32 of a second for this shot, and decided to drop the exposure time by a factor of 4 (2 F-Stops). After a bit of playing, I arrived at my final setting of  1/125 of a second, F 5.0, ISO 2000. Somewhere along the way, I decreased the aperture by 1/3 of a stop. The 1/125 was briefer than my calculated maximum exposure time of  1/75 of a second, so my images should be reasonably sharp. This doesn't take subject motion into account, so I still have to wait for lulls in the action when I press the shutter release.
Manual Setting: 1/125 of a second, F 5.0, ISO 2000, Incandescent White Balance
You can see that allowing the background to go completely black works, in spite of my meter's best efforts to interpret it as "gray". Dropping the suggest exposure makes the image truer to life and allows me to keep lots of detail in the midtones and highlights. I still have some exposure latitude in post production, so the highlights could be darkened a bit, if I wished.

Manual Setting: 1/125 of a second, F 5.0, ISO 2000, Incandescent White Balance

The beauty of working manually is the consistency of the exposure. When I recomposed the image with much more "dark space", the appearance of the flesh tones remained constant. And since I was pretty sure there would be no variation in the intensity or location of the stage lights, I was free to shoot without giving my exposure another thought. Yeah, I wasn't happy about the "Exit" sign, but it was there, and I didn't clone it out.

The nice thing about the X-E1 is the ability to preview the image without taking the camera from my eye. This allowed me to monitor my results in the viewfinder, thus avoiding the "shoot and chimp" routine. This saved a lot of time. I really like the Fuji in available light situations when I have lots of time to set my controls. It really sucks as a "run and gun" camera. Still, this doesn't keep me from looking at fast Fuji prime  lenses, however.

Saturday, July 4, 2015

I Looked At Clouds That Way...

Photo #1
Happy Fourth of July! The Adult School celebrates the Fourth of July be creating a school-wide picnic that emphasizes games more than the food. Individual classrooms may choose to serve hot dogs and such, but the big gathering in all about having fun, as shown by these two competitors preparing for the Three Legged Race (Photo #1). Today I decided to make some quick photos using a fill technique that  I often use, and a lens which I've never used on a assignment.

The Equipment: By setup included the following:
  • Camera ISO: I set the camera (in this case a D600) to the lowest possible ISO setting, which in this case is equivalent to 100. 
  • Lens: An untried Nikon 20-35mm 2.8 AF zoom. A great lens when it was introduced in 1993 and a good lens today.
  • Filtration: I installed a 3-stop Neutral Density, just to see how my images would look with a shallow depth of field.
  • Speedlight: I used a Nikon SB-900 for its improved output over the SB-800. I didn't add a supplementary battery pack.
The Exposure: My starting point for flash supplemented outdoor photographs starts out with a few simple camera and flash adjustments
  • Shutter Priority Exposure Mode: I set the camera body to Shutter Priority and the Shutter speed to 1/200 of a second, the fastest flash synchronization speed I can use without slipping into Auto High Speed Synchronization.
  • Exposure Compensation, Camera: I set this to -1.0 stop. This would essentially underexpose my background.
  • Flash Compensation: This setting is a little squishy. I normally set it to -2/3 stop, and tweak it, up or down, as the situation requires.
  • Flash Beam Angle: The SB-900 is more adaptable to the FX (Full Frame) shooter. I set the beam angle to 35 degrees to concentrate the light in the center of the photo.
The Composition: The clouds were especially fetching that morning, so they became the background for most of the shots. Underexposure allows the subtle form of the clouds to appear, outlined by a slightly darkened blue sky.
Photo #2
Things Are Looking Up: I shot from a low angle to include the cloud-filled sky a a backdrop. You can see that the single stop of underexposure compensation darkens the sky, allowing the texture of the clouds to show through.

Had I not used some form of supplementary lighting, my foreground subject would also be underexposed. In Photo #2, you can see that the camera-mounted speed light is providing nearly all of the necessary lighting. Without the use of flash, his face would have been as dark as the shadow under his chin.
Photo #3
Moving Subjects: There are several ways to handle fast moving subjects. In this case, my subject would have to cross in front of the the orange pylon. By focusing on the pylon and waiting my subject to enter into the same plane of focus, I could get her in sharp focus without relying on the camera's auto-focus (Photo #3). 
Photo #4
Egg Racers: When it's safe to do so, I'll sometimes establish focus on a approaching subject a walk backwards as I photograph. This essentially keeps the subject-to-camera distance constant, allowing me to keep shooting without waiting to re-establish proper focus with each shot. It helps to use a small aperture so that depth of field will help you maintain foreground sharpness (Photo #4).

Be careful when you try this. I crashed into Cissie shortly after I made this photo.
Photo #5
Musical Chairs: The chairs are in a circle surrounding the flagpole. I made some quick photos with my back to the pole, but moved to the outside of the circle when the chairs were fewer in number. As always, everybody is relaxed and friendly (Photo #5). Then it got ugly.
Photo #6
The thrill of victory (Photo #6).
Photo #7
Aren't you going to let me have the chair? I'm a GIRL! (Photo #7)
Photo #8
Well that didn't work (Photo #8).
Photo #9
MY Chair! (Photo #9)
Photo #10
Or not... (Photo #10).
Photo #11
Last Woman Sitting! (Photo #11).

A bit of photo-fluff for the Fourth. Enjoy it safely!