Near my house this winter is an open pond with lots of fish. I’ve seen as many as 9 bald eagles working the pond from nearby cottonwood trees…Chilkat comes to Colorado! Yesterday I was there talking with the numerous photographers lined up to get an eagle photo. The topic of long lens depth of field came up, and I realized there was a lot of misunderstanding. Most folks were using one of the popular super telephoto zooms (200-500mm range), and some were shooting at F8 to ‘get more depth of field’. But is it really worth it? Let’s consider a few factors.
Using my handy Depth of Field calculator on my phone, I considered my situation shooting the eagles. I was shooting a 600mm F4 lens and shooting at F4. At 75 feet, about the distance to the eagles, my depth of field was .76 feet, or about 9 inches. That is a narrow plane of focus to capture a flying eagle. If I went to F5.6, my depth of field was about12 inches, so I gain 3 inches. If I went to f8, my depth of field was about 18 inches, so double the plane of focus from F4. But what did I lose?
This is where you have to make some decisions. If I was shooting at F4 at 1/1000, then at F8 I would be shooting 1/250…way too slow for moving birds. If I had been shooting at F4 at 1/4000, then f8 would have put me at 1/1000, my minimum speed for birds in flight (when I want them sharp). You could increase your ISO; auto ISO is a great tool and works well for photographing birds. But there is a difference in image quality, color fidelity and sharpness from ISO 200 to ISO 800 (an change in two stops to reflect the change to F8 from F4). Depending on your camera, this ISO change might be minimal to plainly visible.
Yes, going to F8 will gain you some depth of field, but with telephoto lenses this is minimal. If I had my 24mm lens attached, and changed from F4 to F8 my depth of field would increase from approximately 18 feet to infinity in-focus to 12 feet to infinity in-focus, a huge jump. The other aspect to consider is what is the sharpest aperture for your lens. Generally wide open at F4 or F5.6 is not as sharp as F8; this difference can be significant and is a good reason to choose a midrange aperture. One benefit of prime fast telephotos is they have excellent sharpness wide open (but you pay for it!).
Back to the eagles. I almost always shoot wide open with my 600mm at F4. Not only is the lens sharp at this aperture, but I love the bokeh it produces. When it comes to choosing an aperture for your telephoto lens, remember it isn’t always about depth of field . Other variables like lens performance/sharpness, ISO performance and bokeh all weigh into this decision. If you want to know just how much depth of field you are gaining, download one of the many depth of field apps to your phone ( I used Depth Of Field Calculator) and check it out.