Best Focal Length for Bird Photography?

Digital photography has been a game-changer for birders and given rise to bird photography at a level never seen before. More and more birders have added long lens cameras to their birding equipment arsenal. They have begun identifying birds based on their photographic records rather than just what they see through their trusty binoculars.

Therefore, choosing the correct size (aka focal length or magnification) camera lens for bird photography has become an important decision. A camera lens for bird photography needs to have sufficient magnification so that you can fill a large proportion of the frame with a bird without having to crop the image excessively. Having enough magnification to avoid cropping images will help provide enough detail so that your photos not only look good but are good enough to positively identify what you have photographed. Another important consideration is whether the magnification of the lens is variable, allowing you to zoom in and out to frame your subject to your choosing. With camera lenses, this magnification is either fixed focal length (prime lenses) for lenses with no ability to vary the magnification or variable focal length for lenses where you can rotate a ring on the lens to change the magnification.

Variable focal length lenses with a maximum focal length of between 400mm and 600mm are by far the most popular lenses for bird photography as they provide 8X magnification at 400mm and 12X magnification at 600mm when shooting with a full-frame sensor camera. If you consider that the most popular birdwatching binoculars are in the 8X to 10X range, it makes sense that the same would apply to camera lens magnification. Some of the most popular long lenses are the Canon EF 100-400mm f/4.5-5.6L IS Mk I & II and the Nikon 80-400mm VR Lens, both of which fit neatly into the 8X magnification range for full-frame cameras and 12X magnification range for crop sensor cameras.

What is the magnification value of a camera lens?

Camera lens sizes are measured in mm focal length rather than an 8X or 10X magnification value for binoculars and video cameras. At first glance, focal length and magnification seem challenging to compare; however, there is a straightforward way to calculate an equivalent magnification value. A 50mm camera lens is the equivalent of 1X magnification and comparable to what is seen by the human eye.

camera lens magnification
Source: Nikon

To calculate the lens’s magnification value, the lens’s focal length value needs to be divided by 50mm (the 1X equivalent magnification). For example, a 400mm lens on a full-frame sensor camera has a binocular-equivalent magnification of 8X (400mm divided by 50mm). See the focal length to magnification conversion table below.

Camera Lens Magnification

Lens Focal Length Full Frame Nikon APS-C Canon APS-C
50mm 1X 1.5X 1.6X
100mm 2X 3.2X 3X
200mm 4X 6.4X 6X
300mm 6X 9.6X 9X
400mm 8X 12.8X 12X
500mm 10X 16.0X 15X
600mm 12X 18X 19.2X

The table below shows the 50mm equivalent range of full-frame and crop sensor cameras taking a photo at a range of 30 yards. Remember that a 50mm lens on a full-frame camera has a 1x magnification and is equivalent to the human eye. So, for example, the table shows you that if you are photographing a bird at 30 yards with a Canon APS-C camera and a 400mm lens, it would be the same as standing 2.3 yards away from the same bird with a 50mm lens. This table helps give you a sense of how much magnification you can expect over a reasonable range of 30 metres.

30 Metres Equivalent Distance with a 50mm lens

Lens Focal Length Full Frame Nikon APC-C Canon APC-C
50mm 30m 20m 18.5m
100mm 15m 10m 9.4m
200mm 7.5m 5m 4.7m
300mm 5m 3.3m 3.1m
400mm 3.8m 2.5m 2.3m
500mm 3m 2m 1.9m
600mm 2.5m 1.7m 1.6m

Don’t be confused with the zoom range factor that some camera manufacturers use. This is typically the zoom range of a lens. For example, a Canon 100-400mm lens would have a 4X zoom range (400mm divided by 100mm) and a magnification of 2X at 100m and 8X at 400mm.

Do crop sensor cameras impact lens magnification?

Yes, crop sensor cameras do impact lens magnification. If you are shooting with a full-frame sensor camera, then the lens magnification will be proportional to the focal length value of the lens divided by 50mm, as described above. However, if you are using a crop sensor camera, you will get further magnification over and above that produced by the lens. This is due to the magnification effect of your camera sensor being smaller than a 35mm full-frame sensor. For Nikon APS-C crop sensors, this multiplier effect is 1.5X, whereas, for Canon’s APS-C crop sensor, it is 1.6X. Look at the crop factor diagram below for more information on this.

Best Focal Length For Bird Photography - Full frame vs crop frame

With this in mind, the Canon 100-400mm lens on a crop sensor camera such as the Canon 7D Mark II would have an equivalent focal length of a 160-640mm lens. Use a lens such as the Tamron SP 150-600mm F/5-6.3 Di VC USD G2 on the same crop sensor camera, and you will get an equivalent focal length of a 240-960mm lens. This is a significant difference and a great way to get some serious magnification for those long-distance shots or when you purposefully want to avoid disturbing a bird. The image quality will probably suffer at those distances; however, in most cases, the quality will still be sufficient for you to identify birds in long-distance photos. I have found my Canon 100-400mm on a Canon 7D Mk II helpful in identifying birds that are beyond the range of my 10X binoculars. So, carrying a long lens camera around, especially one with a crop sensor, has some of the benefits of a spotting scope.

What about using teleconverters to increase magnification?

An easy way to increase a lens’s focal length is to use a teleconverter. A teleconverter is a small device that is mounted between the lens and the camera to increase the lens’s focal length by between 1.4X and 2X in most cases. Use a 2X teleconverter and a lens with a maximum focal length of 400mm, and it will be boosted all the way out to 800mm on a full-frame camera and a whopping 1,280mm equivalent on a Canon crop sensor camera. This magnification boost makes teleconverters a handy accessory to keep in your camera bag. Take a look at the lens magnification table below for the magnification achieved by a full-frame and crop sensor cameras using 1.4X and 2X teleconverters.

Teleconverter Camera Lens Magnification

Lens Focal Length1.4X Teleconverter2X Teleconverter
Full FrameNikon APS-CCanon APS-CFull FrameNikon APS-CCanon APS-C

Teleconverter Focal Length Equivalent

Lens Focal Length1.4X Teleconverter2X Teleconverter
Full FrameNikon APS-CCanon APS-CFull FrameNikon APS-CCanon APS-C

There are, however, three drawbacks of using teleconverters:

Firstly, adding a teleconverter reduces the maximum aperture by one stop for 1.4X teleconverters and two stops for 2X teleconverters. If you have paid a small fortune for something like a Canon EF 400mm f/2.8L, you will probably be somewhat loathed to lose the benefits of your f/2.8 aperture.

Secondly, adding a teleconverter to your setup introduces more glass between the incoming light and the camera sensor. Even something as thin as a UV filter will add more glass and reduce image quality.

Thirdly, you may lose autofocus functionality or have reduced autofocus performance depending on your camera and lens. This is because DSLRs have a minimum aperture at which they can autofocus. Most Canon DSLRs can only focus at an aperture of at least f/5.6 or wider. When the teleconverter is used, it reduces a lens’s maximum aperture by one stop for a 1.4X teleconverter and two stops for a 2X teleconverter. So, if you are using an f2.8 lens with a 2X teleconverter, you will still be able to autofocus as your maximum aperture is reduced from f2.8 to f5.6. However, if you were to use the same 2X teleconverter on an f4 lens, your maximum aperture is reduced to f8, and autofocus will stop working.

Maximum Lens Aperture



















So, as a rule of thumb, teleconverters are great for occasional use, but you should consider getting a longer lens if you rely on it for all your photography.

Fixed vs variable focal length lenses

Fixed focal length (prime) lenses have a fixed magnification (focal length), whereas variable focal length lenses allow you to adjust the focal length or magnification. There are a few pros and cons of fixed vs variable focal length lenses that need to be considered.

Probably the first and most important thing to consider is cost. High magnification (long) fixed focal length lenses are generally pretty expensive. In contrast, variable focal length lenses with high focal lengths are available in a broader range of qualities and prices. For example, you can get a lower quality variable focal length lens such as the Canon EF 75-300mm f/4-5.6 III for around $200 or a high-quality lens such as the Canon EF 70-300mm f/4-5.6L IS USM PRICE. On the other hand, the best value for money 300mm fixed focal length lens would probably be the Canon EF 300mm f/4L IS USM at around $1,350.

The second most important consideration would probably be versatility. A variable focal length lens gives you many more options in the field. With something like the Canon EF 100-400mm, you could be snapping away at a bird at your feet one minute and then zoom into a bird at a distance the next minute. Variable lenses also allow you to keep a single lens on your camera, which means you carry less gear and spend less time changing lenses when you could be taking photos. This is probably what makes lenses such as the Canon EF 100-400mm best sellers.

The third consideration is image quality. Because fixed focal length lenses have fewer moving parts, they tend to deliver higher quality results when compared to variable focal length lenses. This difference in quality is especially noticeable when it comes to cheaper, lower end variable focal length lenses.

Finally, weight is another factor in a decision between fixed vs variable focal length lenses. For shorter focal length lenses below 500mm, fixed lenses tend to be lighter than variable lenses. E.g. the Canon EF 400mm f/5.6L USM Super Telephoto lens weighs about 2.8lb compared to about 3.46 lbs for the Canon EF 100-400mm f/4.5-5.6L IS II. For longer focal length lenses above 500mm, fixed lenses tend to be heavier than variable lenses. E.g. The Canon EF 500mm f/4L IS USM Super Telephoto lens weighs about 5.5lb compared to about 6.31 lbs for the Sigma 150-600mm 5-6.3 Sports DG OS HSM Lens.


Fixed Focal Length

Variable Focal Length


Generally expensive

Wide range dependant on quality




Image quality



Weight >500mm lenses



Weight <500mm lenses



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