The Great Journey in Photography

A Fine Line Between Clever and Stupid

Snow Goose at Bosque Del Apache

700mm f5.6 320iso 1/500sec

It is not easy to use long focal length lenses. I have said right here that it took several months to get dialed in to using the 300mm f2.8. One of the reasons why it takes so long is the shallow depth of field such a lens will produce. Being able to control what and how much is in focus is important in all photograph and especially critical for bird photography.

The picture above was made with a 500mm lens, a 1.4x teleconverter, and a 1.5x crop DX sensor camera and as you can see the depth of field is only a few inches. Only the head of this snow goose is in focus. Lets take a look at the math and see why that is so. First multiply the  focal length of the lens by the magnification factor of the teleconverter (500mm * 1.4) and you get 700mm from the lens optics. Now take that number and multiply it by the crop factor of the camera sensor, in effect also a magnification factor, (700mm * 1.5) and you get 1050mm. That is the effective focal length, in other words, if I were shooting 35mm film I would be using a 1050mm lens. Lets look at the depth of field using that focal length. I was using an aperture of 5.6 which is wide open for this set up and the subject was about 25 yards out. Lets head over to a depth of field calculator. I like but there are a number of them on the web, and plug-in the numbers. OK-about 1,000mm, well say 70 feet out at the aperture of f5.6 using Nikon D300s.
Crunch, crunch, crunch.
Total Depth of Field 0.32ft.
Mmmm, what’s that? Times 12 it is 3.84 inches.
Holy Smokes, less than 4″ depth of field! The bad part is you are working with less than a 4″ DoF.
The good part is you are working with less than a 4″ DoF! Seriously, talk about selective focus, and check out the background, it is as creamy and blurred as it is going to get. So was this the look I was going for? Not really, in all honesty I was really trying to keep the shutter speed up by opening the aperture all the way. My intention that day was to be around f8 but the light was low. I am low ISO wide aperture type of photographer so I always tend to default in that direction. If your intention i to have all the bird in focus you will want to stop down and boost the ISO.

As you might imagine hitting a 4″ target on a moving object from 70 feet out is not a trivial matter and it is just one of the reasons why it takes a while to handle long lenses. You have to know what your image is going to look like and adjust for it on the fly and that comes from a deep understanding of the optics and a fluid and quick thought process that becomes sub-conscience intuition.

Thanks for reading everyone. You can see a full res image of this goose over on my Smug Mug page.


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