INTRODUCTION

In my college years, I learned a Latin term from a psychology professor that I later applied to an aspect of HDR photography. The latin phrase Ceteris Paribus, literally translated as “with other things held constant,” is a term often used for scientific experiments because it plays an important role in determining causation for complex situations. While one variable is allowed to change, all other variables that may potentially interfere with determining causation are held constant.

So what does Ceteris Paribus have to do with HDR photography? What I’m getting at is this: When adjusting the exposure value between a bracketed sequence, the only changing variable should be the shutter speed. There are important reasons for this. We will discuss what would result if you were to:

1. Change the ISO to control bracketing exposures
2. Change the Aperture to control bracketing exposures

Granted if you are using the Automatic Exposure Bracketing (AEB) Function, your camera will automatically bracket your images using shutter speed adjustments. However, it is good to know the reasons why you wouldn’t want to adjust stops of light any other way when bracketing for an HDR photograph.

CHANGING THE ISO

Although the interactions between different value combinations of the aperture, ISO, and shutter speed can result in equal exposures, the act of changing the ISO or the aperture will produce unwanted effects on our images.

The effect of changing the ISO value is going to directly interfere with the whole purpose of creating an HDR image. When creating an HDR image, the goal is to create an image with more detail and dynamic range than possible normally. By raising the ISO, you are increasing grain which destroys detail. In addition, higher ISO levels reduces the dynamic range which again defeats the purpose of HDR photography. If you absolutely must, it is possible to raise the ISO one to two stops beyond your native ISO setting, however anything beyond will generally result in an unusable sequence. By shooting with the lowest possible native ISO you are able to use the maximum amount of color and detail that your sensor will provide. This is why we do not use the ISO to control the exposure values in a bracketed sequence.

CHANGING THE APERTURE

Since the aperture of a camera determines the image depth-of-field, adjusting the aperture to control the exposure value will change the composition and detail within each shot. A large aperture will create a shorter DoF and softer details while a closed down aperture will create a wider DoF and sharper details. Adjustments in the aperture with each exposure will create images with different compositions and detail. Bringing each of these inconsistent exposures together would dilute the overall detail in the final photograph which again contradicts the purpose of high dynamic range photography since we are trying to enhance detail.

CONCLUSION

The shutter speed is the only variable that should be altered when bracketing your images for HDR photography. Because we are using the shutter speed as our changing variable for adjusting exposure, it’s possible that we may run into a couple problems such as motion blur and ghosting. But, we will discuss these problems and their respective solutions in a later article.

Again, remember the AEB Function does this automatically for you in camera. If your camera has AEB, let it do the heavy lifting for you. The purpose of this article is to hopefully provide you with some understanding of the reasons why the shutter speed should be the only changing variable in an HDR bracketed sequence in case one day (heaven forbid) you need to shoot a manual bracketed sequence. Stay tuned for the next article.