John White
by on September 26, 2019
1,339 views

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 and have begun identifying birds based on their photographic records rather than just what they see through their trusty binoculars.

Choosing the correct size (aka focal length or magnification) camera lens for bird photography has therefore become a really important decision. A camera lens for bird photography needs to have sufficient magnification so that you are able to fill a large proportion of the frame with a bird without having to excessively crop the image. Having enough magnification to avoid cropping images will help provide enough detail so that your images 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 described as either fixed focal length (prime lenses) for lenses with no ability to vary the magnification, and variable focal length for lenses where you can rotate a ring on the lens to vary 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 as they are for binoculars and video cameras. At first glance, focal length and magnification seem difficult to compare, however, there is a very easy 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 magnification value of a lens, the focal length value of the lens needs to be divided by 50mm, the 1X equivalent magnification. So, 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 yards.

30 Yards 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. If, however, you are using a crop sensor camera, then 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.

Camera crop factor diagram

 

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 pretty significant difference and is 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 identify birds in long distance photos. I have found my Canon 100-400mm on a Canon 7D mk II really useful for 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 spotting scope.

 

What about using teleconverters to increase magnification?

An easy way to increase a lenses 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 focal length of the lens 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.

Camera Lens Magnification

Lens Focal Length

1.4X Teleconverter

2X Teleconverter

Full Frame

Nikon APS-C

Canon APS-C

Full Frame

Nikon APS-C

Canon APS-C

50mm

1.4X

2.1X

2.2X

2X

3X

3.2X

100mm

2.8X

4.5X

4.2X

4X

6.4X

6X

200mm

5.6X

9X

8.4X

8X

12.8X

12X

300mm

8.4X

13.4X

12.6X

12X

19.2X

18X

400mm

11.2X

17.9X

16.8X

16X

25.6X

24X

500mm

14X

22.4X

21X

20X

32X

30X

600mm

16.8X

25.2X

26.9X

24X

36X

38.4X

 

The table below is the focal length equivalent after factoring in the additional magnification from teleconverters and crop sensors. So, a 600mm lens and 2X teleconverter on a Canon crop sensor camera would give 38.4X magnification and a focal length equivalent of 1,920mm.

Teleconverter Focal Length Equivalent

Lens Focal Length

1.4X Teleconverter

2X Teleconverter

Full Frame

Nikon APS-C

Canon APS-C

Full Frame

Nikon APS-C

Canon APS-C

50mm

70mm

105mm

112mm

100mm

150mm

160mm

100mm

140mm

210mm

224mm

200mm

300mm

320mm

200mm

280mm

420mm

448mm

400mm

600mm

640mm

300mm

420mm

630mm

672mm

600mm

900mm

960mm

400mm

560mm

840mm

896mm

800mm

1200mm

1280mm

500mm

700mm

1050mm

1120mm

1000mm

1500mm

1600mm

600mm

840mm

1260mm

1344mm

1200mm

1800mm

1920mm

 

There are, however, three drawbacks of using teleconverters:

Firstly, adding a teleconverter reduces the maximum aperture by 1 stop for 1.4X teleconverters and by 2 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. Adding more glass, even something as thin as a UV filter, will reduce image quality.

Thirdly, depending on the camera and lens you are using, you may lose autofocus functionality or have reduced autofocus performance. This is because DSLR’s have a minimum aperture that they can autofocus at. Most Canon DSLRs can only focus at an aperture of at least f/5.6 or wider. When the teleconverter is added it reduces a lenses maximum aperture by 1 stop for a 1.4X teleconverter and 2 stops for a 2X teleconverter. So, if you are using a 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 a f4 lens, your maximum aperture is reduced to f8 and autofocus will stop working.

Maximum Lens Aperture

Teleconverter

1.4X

2X

f1.8

f2

f2.8

f2

f2.8

f4

f2.8

f4

f5.6

f4

f5.6

f8

f5.6

f8

f11

 

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 are lenses with 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 wider variety 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 a lot 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 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

Cost

Generally expensive

Wide range dependant on quality

Versatility

Lower

Higher

Image quality

Higher

Lower

Weight >500mm lenses

Lighter

Heavier

Weight <500mm lenses

Heavier

Lighter

 

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