The poinsettia as a Christmas flower allegedly traces its roots to the 16th century in Mexico, it's popularity in America is largely a 20th century phenom. In many parts of the world, such as Mexico, India and parts of Nepal, the flower grows in the wild and is more of a shrub or a tree. And it was on a hike in Nepal, specifically Pokhara, at the base of the Himalayas that we came across a huge plant towering over us with the sky behind. Though I am used to seeing poinsettias in December in America, I was not used to a large plant hanging ten feet overhead. I had my new Tamron SP 70-200 f2.8 VC lens with image stabilization already on the camera and was able to take this image at 181mm at 1/40th of a second hand held. The aperture was f7.1 and the ISO 125. The camera was set in Manual mode.
Depending on the situation, I use my camera in different modes. In this case Manual not only allowed me to control the camera to properly expose the flower, but also to maximize my depth of field and image quality with a lower ISO. In general the lower the ISO the better the image quality but because there is a relationship between ISO and shutter speed, one has to make choices to optimize your picture. Having Tamron's VC (Vibration Compensation) stabilizer in the lens allowed a shutter speed of 1/40 of a second hand held at 181mm. 10 years ago this would have been almost impossible to get without a tripod.
Even though my lens can open to an aperture of f2.8, sometimes you need additional depth of field to get the image you want. In order to get the central poinsettia sharp and the surrounding ones a little soft, at 181mm the aperture was stopped down to f7.1.
When exposing an image that is back lit, it is necessary to over-expose, from what the cameras meter will tell you. This is because the meter, if you leave it in an evaluative or matrix setting, is looking at the entire frame and does not know you want to expose for a back lit subject. By overexposing, the blue sky turns white and the black flowers turn red. The image below is how the camera wants to expose it automatically. Often, great images are all around you, even if you are not in Nepal, but sometimes you have to manipulate the camera to make pictures the way you see them. Until next year...