Some Thoughts on Sensor Sizes for Birding

  Lesser Goldfinch, Fuji X-Pro2, Fuji 100-400mm f/4.5-5.6 @ 400mm

Lesser Goldfinch, Fuji X-Pro2, Fuji 100-400mm f/4.5-5.6 @ 400mm

Large camera sensors have always been associated with high quality output. As the theory goes, the more data you collect, the more detail and dynamic range you'll have in the final image. For still subjects that are relatively close to the camera, this is most certainly true. And since a large majority of photos are taken of nearby people, it's almost become a Universal Truth in photographic circles that "Full Frame" cameras are the best.  

Taking photos of backyard birds offers a unique set of challenges. Because they're wild, they're difficult to get close to. They're small, fast, and devilishly unpredictable. In other words, they behave a lot like other wild creatures. To be successful at photographing these reluctant subjects, photographers must set aside their preconceived notions about what constitutes the "best" gear and build an outfit specifically optimized for bird photography. 

Sensors and Lenses

In general, given a particular lens specification, the larger the sensor, the larger the lens. An obvious example would be to compare a 50mm lens on a full frame DSLR to a 50mm lens on a smartphone. While lenses can be designed with compactness as a priority, there's no escaping the physics of the relationship between sensor size and lens size.

With all other things being equal, there's also no escaping the fact that larger lenses require larger lens elements (glass), which makes larger lenses more expensive. So it naturally follows that large sensor=large lenses=high price.

This issue becomes even more acute when we start talking about long, fast, telephoto zoom lenses, which are generally the most difficult to manufacture, and most expensive, of all lenses. These are the lenses most commonly used in bird photography. 

A Comparison

It might be worthwhile to compare the cost of a couple of birding rigs, one on a full frame sensor, and one on a smaller APS-C sensor. The goal with each system below is to have the equivalent of 600mm reach, combined with a maximum aperture of at least f/5.6. I consider these specs a minimum for photographing birds. I also chose these systems for their comparable image quality. It's possible to buy less expensive third-party lenses (at least for the Canon), but for the most part the image quality will not come close to the quality of these native lenses.

Canon 5D MkIV - $3499
Canon 100-400 f/4.5-5.6 IS L - $1999
$5498

Fuji X-T2 - $1599
Fuji XF100-400 f/4.5-5.6 - $1899
$3498

At first glance these systems seem comparable. The focal lengths and maximum apertures on both zooms are the same. The cameras provide similar image quality, though the Canon's larger sensor may provide some subtle advantages in difficult lighting situations. Of course, the Canon body is also more expensive due to the full frame sensor. 

There's more to the story than meets the eye though. Because the Fuji uses a smaller APS-C sensor, the stated focal length of the lens is multiplied by 1.5 (this is called the "crop factor"). So even though the lens is a 100-400, the field of view is comparable to a 150-600 (100-400 x 1.5 = 150-600) on a full frame camera. In other words, to get comparable reach to a 100-400 on an APS-C sensor, one would need to purchase a 600mm lens for their full frame body. 

Let's look at the above comparison again but with a 600mm lens on the Canon. 

Canon 5D MkIV - $3499
Canon 600mm f/4 L - $11,499
$14,998

Fuji X-T2 - $1599
Fuji XF100-400 f/4.5-5.6 - $1899
$3498

Ouch. 

Obviously, the Canon 600 f/4 is out of reach for a large majority of photographers, but it's the only professional 600mm lens Canon offers. There are alternatives, including using either a 1.4x or 2.0x teleconverter on the 100-400, or going to a third-party lens like the Tamron 150-600. The problems with teleconverters are many, including soft images, maximum apertures reduced by a factor of the converter (one stop for a 1.4, 2 stops for a 2.0), and poor autofocus performance. If you thought using long telephotos was difficult, try using one with a teleconverter sometime. Where budgets don't allow for an $11,000 lens, third-party lenses are probably the best bet on a full frame body. 

The Sweet Spot

As I stated above, all things being equal, larger sensors almost always produce higher image quality. But, of course, all things are never equal, and everything that has to do with camera performance involves compromises. In the case of photographing birds, it's hard to overstate the importance of reach. And because of the physics involved in the lens/sensor relationship, smaller sensors make possible more reasonably priced lenses with long reach. The $64 question is, how small should one go before the loss of image quality offsets the advantages of reach? Some take it so far as to photograph birds with "superzoom" cameras with tiny sensors, many of which have the equivalent of 600-1200mm maximum zooms. In my opinion, these cameras give up too much in image quality in trade for reach. Others use shorter lenses and use stealth to get as close as possible to the birds (I wrote about this in my article, Pixels on the Bird). This can be an effective strategy, but I'm finding out it has major limitations with bird species that are spooky.

So what is the sweet spot sensor size for birding? I would argue that it's either APS-C or Micro 4/3 with their respective 1.5 and 2.0 crop factors. Both formats provide very good image quality on today's modern sensors, while also having available excellent lenses with up to 600mm equivalent reach. The best lenses in these formats are still relatively expensive, but they're downright cheap when compared to equivalent quality lenses for full frame, which, as we've seen above, can cost as much as a small car. These smaller systems are also lighter, which makes them easier to carry and use hand-held. 

Conclusion

Among the many factors to consider when assembling an outfit for birding, I believe it's best to start with sensor size and build from there. And when considering sensor sizes for bird photography, it pays to remember that bigger is not necessarily better. The goal is to choose a size that best balances image quality with reach, size, and cost. I'd argue that sensors between 1" and full frame (which means either APS-C or Micro 4/3) best manage this balance.