What Is Focus Breathing
Have you ever changed your lens and noticed something change? Not just the focus but the image itself reframes and changes size. This is called focus breathing.
Focus breathing is a problem for video and cinema lenses, but not as much for still photography lenses. Some lenses exhibit more focus breathing than others. Older zooms often fall into this category, as do very old prime (fixed focal length) lenses. Newer prime and zoom lenses generally have excellent resistance to focus breathing. However, even the best lenses are not immune.
First, Let's Define Focus Breathing.
Focus breathing changes a lens’s apparent focal length while focused on a subject at a given distance. This will cause a change in composition and, if shooting video or cinema, this can be pretty unwelcome to the videographer trying to maintain consistent framing throughout an entire scene.
What Can Cause Focus Breathing
The degree to which a lens exhibits focus breathing is based on the design and manufacture of the specific lens. If you change your camera’s aperture while recording video or shooting stills, this will also affect focus breathing–but for more on this, read my article on “Aperture Breathing.”
Focus breathing is caused by mechanical changes in the lens elements as the lens is focused. It is most easily seen in still photography when shooting macro or close-up work because the subject magnification is high enough that this mechanical movement becomes apparent. So, how does it happen?
Lens Designs And Focus Breathing
A lens composed of a front group with one or more lenses in front of the aperture stop and one or more lenses behind the aperture stop is called a telephoto lens. A standard lens has no telephoto elements.
The more telephoto the lens is, the worse it will be about focus breathing. This is because each group of lenses positioned in front of or behind another element changes the distance from that element as the physical length of the lens is changed. In a telephoto lens, the details in front of or behind the aperture stop are moving further from the aperture stop as you focus towards closer distances.
In addition to being longer, lenses with more groups have more surfaces that can flex–and this movement changes based on how much force is put into focusing the lens. The stiffer a lens, the less noticeable it is.
A front-group lens design has fewer problems with focus breathing because only one element moves when focusing. Still, this group is often more prominent in diameter and length than a telephoto group, contributing to more weight and bulk of the lens. Rear-group lenses have more problems with focus breathing because several elements are moving back and forth as you focus, but this design is often lighter and shorter.
In some lenses, the aperture stops moving as you focus. This can cause very noticeable problems with focus breathing–most commonly in super telephoto primes. In other lenses, the entire lens focuses as a unit without moving any of the elements relative to each other, which is generally preferable.
Lens Manufacturers And Focus Breathing
Different lens manufacturers have different approaches to minimizing focus breathing in their lenses, and the same company may offer several designs. Among Canon’s L-series lenses, for example, their 70-200mm f/2.8 IS II USM lens has very little focus breathing (when shot wide open, the change is about 2%), but its predecessor (70-200mm f/2.8 IS USM) exhibits fairly pronounced focus breathing–especially at 200mm where it changes composition significantly as you zoom in and out.
Sony’s 70-400mm f/4-5.6 G SSM II lens exhibits minimal focus breathing relative to other 70-400mm lenses on the market. One example of a lens that shows some solid focus breathing is Nikon’s 80-400mm f/4.5-5.6D ED VR, which changes composition by about 25% when focused at 200mm (!) while shooting stills or video, and about 15% when shooting cinema with an aperture of f/8. (Nikon also offers the 80-400mm f/4.5-5.6G VR, which exhibits moderate focus breathing.)
Focus Breathing And Cameras With Translucent Lenses
Canon SLR cameras with the EF mount use a system of mirrors to project the image from the lens into the viewfinder. When focusing, light passes through one part of the mirror and bounces off another piece, so this is called a “translucent” design.
This design does not suffer nearly as much with focus breathing because only the aperture stop moves when focusing–the entire lens does not move back and forth relative to the camera. However, it can be annoying when photographing at close distances because you see the lens’s focal length changing in real-time through the viewfinder–similar to using a cheaper zoom lens with variable focal lengths where the camera passes the focal range electronically by moving elements in and out of place inside the lens.
Advantages Of Focus Breathing
Articulating screens on some cameras–particularly Sony’s NEX-7 and A7 series, Fujifilm X-series, recent Nikon DSLRs, the Canon EOS M, and many of Panasonic’s micro 4/3 cameras–allow you to hold the camera at waist level or below eye level when shooting yourself in a portrait, and focus on the tip of your nose if you so choose. You can then lock out autofocus capability when framing at waist level for this perspective, which is impossible to do without focus breathing because the camera will refocus every time you look through the viewfinder.
Focus breathing effects are easier to see when shooting stills. If you like to shoot video, it’s less of an issue because the imaging sensor itself moves around when changing focus–not just the lens element that projects the image into the viewfinder. The only exception is with cameras with translucent mirrors, which require a lot more power during video mode and thus result in slower autofocus for video.
Focus breathing is a natural consequence of DSLR lenses with focus rings, and the distance scale on the lens doesn’t lie–it’s an actual measurement from one distance to another. In contrast, compact cameras often promise longer focal lengths than they deliver because it’s easier to enlarge a small image electronically than by actually increasing the focal length of a lens.
Disadvantages Of Focus Breathing
There’s no depth of field indicator on the distance scale to indicate the depth of focus if you turn it off. This can be useful when shooting with an expensive soft-focus filter, controlling reciprocity failure during long exposures, or using small apertures where the depth of field is razor-thin.
The nature of how zoom lenses work means that there are many different combinations of aperture and focal length you can select–and some may not give the exposure results you expect. To demonstrate this, I’ll borrow a couple of images from one of my photography courses:
Although lock-out focus breathing is more of an issue in the video than photography in practice, I see the effect in my images. I’ve stopped using Nikon lenses with large focal lengths to minimize it because when you zoom in for a close-up portrait, the lens composes at “wide” settings and zooms back out on me as I recompose. It’s more of a problem with the 24-70mm f/2.8G but still noticeable with the 18-35mm f/3.5-4.5D. Focus breathing is likely something you won’t notice until someone points it out to you, but if your images have any quality loss at total magnification, the effects of poor focus breathing may be to blame. [END ARTICLE]
Q: How does this affect the MTF of the lens?
A: Technically, it doesn’t. The only thing that changes regarding MTF is how much light can be transmitted through the lens when exposedconsiderable. The image will appear softer when you look at it 1:1 on your computer or in your hand, but the MTF is not affected.
Q: What's the difference between focus breathing and field curvature?
A: Field curvature is where the plane of best focus is not at 90 degrees to the imaging sensor, which causes issues with focusing on flat surfaces or objects that are not parallel to the camera, like bricks or windows. This often causes the best focus to be in the middle, which you will notice as a soft spot or vortex.
Focus breathing is when the plane of best focus changes based on how much you zoom in and out. If the lens exhibits focus breathing, it’s best to shoot at your average focal length and crop later if needed. This way, you’re only changing the framing, not the plane of best focus.