How Lens Length Affects Apparent Background Size: An Example Using the Moon
Lens compression. You’ve probably heard that phrase used before, along with “flattening” of an image with a telephoto lens. So what is it and why would you want to use it?
In order to keep a subject the same relative size in a photo when using different focal length lenses you need to physically move closer to the subject as you go from a telephoto lens to a wide angle lens. The wider the lens, the smaller and further away the background appears from the subject when in reality the distance between the subject and background has not changed. For my example photos I have our model, Q, on a motorcycle and I am using the moon to show how longer focal lengths appear to flatten or compress the image. This full moon also happened to be the Strawberry Moon on the Summer Solstice.
I used a Canon 1D Mark IV along with several Canon lenses ranging from 24mm to 560mm in focal length. All images were taken using the same camera settings, so only the focal length changed. Q is stationary in all of the photos as the moon rose behind him. I, on the other hand, moved closer to Q as I changed from the longest focal length of 560mm (with the Canon 200-400mm using its built-in 1.4x teleconverter), to the shortest focal length of 24mm with the Canon 24-70mm. All photos were taken in a span of about 8 minutes as the moon rose above the horizon.
Far left is Q. I am on the right with the Canon 200-400 f/4 IS. At this distance and with this focal range, you get a result like this:
To achieve the same kind of framing with a short lens, I have to move in much closer to Q.
At this distance and with this focal range, you get a result like this:
I attempted to keep Q approximately the same size and placement in frame in each of the photos. Again, the only thing that actually moved was myself. The closer to Q I moved, the smaller the moon appeared. I started the series of photos with the longest focal length since the moon was rising and I wouldn’t have been able to keep both Q and the moon in frame as the moon rose higher in the sky.
Why is the Moon Higher in the Telephoto Shot?
One thing thing you may notice is that the moon seems higher in the sky in the longer focal length images (see the entire series below). That is partly because the ground I was standing on was not completely flat. You’ll notice a large drop in the moon from 200mm to 135mm and it was caused by me being slightly higher up in the shorter focal length photos. That huge drop does not have to do with the focal length of the lenses.
Why is the Moon Softer in the Telephoto Shot?
The moon in the longer focal length images seems much more out of focus than the standard and wide angle photos. That is partly due to the focal lengths of the lenses. As you use longer focal lengths, it is harder to keep both the subject and the background both in focus. Usually that is what’s preferred – blur out the background and create beautiful bokeh. If you are trying to keep both the subject and the background in sharp focus you will need to use a smaller aperture. Remember that a smaller aperture is actually a larger number, such as f/16. A larger aperture is a smaller number, such as f/2. For these photos I used f/11 as my aperture. In the first two images the moon is also partially blurred from the atmosphere as it was just rising above the horizon.
See How Focal Length Affects Perceptual Background Size in Relation to Foreground Subject
You can see how striking the difference is in the appearance of the moon from 560mm to 24mm. So why would you want to compress or flatten an image? The longer focal lengths really accentuate the moon making it much larger in frame and appears to flatten the image making the moon seem very close to Q. That leads to a much more dramatic image. You could use this technique in your portraits to highlight parts of the background such as mountains, the moon, or any other object that may be far off in the distance.
In contrast, you can use wider lenses to accentuate your close-up subject and the foreground. Sometimes you don’t have the best background to work with so using a wider lens can make it appear further away and less of a focal point to your image.
These are just a few examples of how you can use telephoto lens compression in your photography. I often photograph vast panoramic landscapes and some of my favorites have been taken with longer lenses at 100mm or longer. You get a very different perspective on a mountain taken further away with a long lens than you do up close with a wide angle lens. There are many other creative ways to use telephoto lens compression to make your photos really stand out.