Month: July 2015

Today’s Question: I saw your (very nice) photo of a crop duster, and wonder how you go about making sure the propeller is actually shown to be moving rather than being “frozen” by a fast shutter speed. Do you just stop the lens down to the smallest aperture?

Tim’s Quick Answer: Photographing propeller-driven aircraft is a perfect example of a rare case when I will actually use shutter priority (or time value) exposure mode. I determine an appropriate shutter speed for the subject, and set that as a fixed exposure setting in shutter priority mode, ensuring a degree of blur for the propeller in each photo.

More Detail: In the case of the photo you are referring to, the crop duster is powered by a turboprop engine that turns the propeller at a relative fast rate. Therefore, a shutter speed of no faster than about 1/250th of a second or so will ensure a small amount of motion blur for the propeller, while still providing a shutter speed that is fast enough to help achieve a photo where the airplane actually appears sharp.

For those who didn’t see the photo referenced in the question, by the way, you can view it on my 500px page here:

https://500px.com/photo/112385163/crop-duster-spraying-in-palouse-by-tim-grey

When you are using shutter priority mode (as opposed to aperture priority mode) there is an increased risk of “running out” of appropriate exposure settings. In other words, there is a risk that the shutter speed you have set will require an aperture that goes beyond what your lens can actually achieve. On a bright sunny day that might mean even with the lens fully stopped down the exposure is too bright, or that even with the aperture fully open the exposure is too dark.

Because of this issue, when I set the camera to shutter priority mode I will hold the shutter release button halfway down to enable the meter on the camera, and then point the lens at various areas within the scene before me, being sure to check the brightest and darkest areas to confirm that an appropriate aperture is available.

If the subject will remain under relatively fixed lighting conditions, I might also use the manual exposure mode with specific exposure settings that work for the subject. However, when photographing aircraft it is very likely that the lighting conditions will change on the subject. For example, in some cases the aircraft may be lit from the front, while in other cases it may be backlit. In these types of situations I find that shutter priority mode helps ensure more consistent results, though it may also be necessary to apply some degree of exposure compensation.

Again, the key to photographing propeller-driven aircraft is to use the fastest shutter speed you can (generally speaking) that will still ensure at least some degree of motion blur for the propeller. The specific shutter speed will vary based on the rate at which the propeller is spinning, but in most cases shutter speeds in a range from about 1/125th of a second to about 1/350th of a second will produce excellent results.

Today’s Question: In Photoshop I drew a grid  made up of one-inch squares on a transparent layer. I use it for my drawings placing the grid over a photo. However, when I place my ruler on the monitor screen to see if the squares are one inch, I find they are not and no matter what I do (fit screen, print size, actual pixels, zooming in and out), the squares are never one inch. In other words, the monitor screen inch does not match my ruler inch.

Tim’s Quick Answer: The solution here is to set the Screen Resolution in the Preferences dialog for Photoshop to the true pixel per inch resolution of your monitor display, and then make use of the Print Size display option from the View menu.

More Detail: By default Photoshop assumes a display resolution of 72 pixels per inch. That does not match the vast majority of monitor displays in use today. To calculate your actual pixel per inch (ppi) resolution, you need to know what pixel dimensions you have set as the resolution for your monitor display. You can then measure the horizontal display size for your monitor, and calculate the actual pixel per inch resolution.

For example, let’s assume a display that is operating at 1920 by 1080 pixels (full high definition resolution). Let’s further assume the width of the actual display area for the monitor is 14 inches. With 1920 pixels across and a width of 14 inches, there are 137 pixels per inch across the display.

To set the resolution you’ve calculated for the display in Photoshop, you’ll need to first bring up the Preferences dialog. To do so, choose Photoshop > Preferences > Units & Rulers on the Macintosh version of Photoshop, or Edit > Preferences > Units & Rulers on the Windows version of Photoshop. In the Units section, set the value for Screen Resolution to the number you calculated (137 in the example above) and set the popup to the unit of measure you used (“Pixels/Inch” in this example). Then click OK to close the Preferences dialog.

With this setting established, when you choose View > Print Size from the menu in Photoshop, the image will be resized based on the pixel per inch resolution so that one inch on Photoshop’s ruler display (View > Rulers from the menu) will represent one inch on your monitor, and the size of the image will match what you can expect for the image if it is printed at the current resolution setting for the image.