Depth of field for birds in flight – good or bad?

Many camera enthusiasts see the wide depth of field (DoF) inherent to Olympus cameras, as a disadvantage. In my humble opinion, wide DoF is a positive advantage for landscape photography, for street, and above all, for birds in flight. Shallow DoF is mostly of use in portraiture, and there is no problem at all in achieving it with the Olympus system, particularly with the wonderful 75mm lens.

Let’s look at DoF for birds in flight in more detail. Because this analysis depends purely on the physics of lens design, it applies to full frame cameras and lenses from all manufacturers.

Why is Depth of Field important for birds in flight?

Depth of Field (DoF) is the distance between the nearest and the farthest objects that are in acceptably sharp focus in an image. The longer the focal length, and the wider the aperture the narrower the range of Depth of Field. To be sure we are getting our bird in focus, its maximum wingspan needs to fit within in the DoF range in any situation. A red kite has a wingspan of 4-5 ft (say 1.5m), and a turkey vulture’s is 6-8ft (say 2.5m). Let’s use 6ft or 1.9m as the bare minimum DoF we need to be sure of getting all of our bird critically in focus.

DoF of Olympus pro telephoto lenses compared to full frame fast telephotos

All Olympus lenses follow the micro four thirds (MFT) standard, which means that they have a full frame equivalent (FFE) focal length of 2x the MFT focal length, and a full frame equivalent aperture from a DoF perspective of 2x the MFT aperture. However, from an exposure equivalent, there is no 2x factor. An MFT f4 lens exposes as an f4 lens.

Depth of Field for 600mm full frame equivalent lenses.

Lets start with the obvious comparison: the Olympus 300mm f4 pro prime, vs a full frame 600mm f4 prime, made by Sony, Canon, or Nikon. The Olympus lens has an FFE focal length of 600mm, and a FFE DoF aperture equivalent of f8. It exposes as an f4 lens however. Now lets see how the depth of field stacks up between the Sony, Nikon, or Canon lenses, and the Olympus. In the table below I have calculated the DoF range for each lens, wide open, by distance from the subject. I have used the commonly used standard calculation for DoF – there are variants dependent on factors like circle of confusion, or output format, but the relative relationships will be unchanged.

Let’s see how far from the bird you need to be to get our 1.9m of depth of field.

For the Olympus 300mm, you need to be at least 38m away for the subject to be in the DoF zone (for a 1.9m bird), a reasonable distance which gives you a good size of the subject in the frame. For the full frame lens, the bird has to be 54m distant, or 42% farther away to get the same Dof, making the bird far smaller in the frame. At 38m the full frame lens has under 1m or 3ft of DoF, making even a medium-sized bird tricky/impossible to get in focus if flying parallel to you. At closer distances than that, some part of the bird is almost certainly going to be out of the DoF zone with the FF lens.

What a pain! If only you could get a 600mm lens that had the same light gathering capability as a full frame one, but had twice the DoF. ………hold on a minute!!!!!

DoF for 400mm Full frame equivalent lenses

Here we can compare the Olympus 40-150mm pro zoom, vs a full frame 400mm f2.8 prime, made by Sony, Canon, or Nikon. With the 1.4 TC, the Olympus has a maximum FFE focal length of 420mm, and a FFE aperture equivalent of f8, but exposes as an f4 lens.

Now lets see how the depth of field stacks up between the Sony, Nikon, or Canon lenses, and the Olympus. In the table below I have calculated the DoF range for each lens, wide open, by distance from the subject.

For the Olympus 40-150mm + 1.4TC, you need to be at least 30m away from the bird to be in the DoF zone (for a 1.9m bird), a reasonable distance which gives you a good size of the subject in the frame. For the full frame lens, the bird has to be 43m distant, or 43% farther away to get the same DoF, making the bird far smaller in the frame. At 30m the full frame lens has 0.94m or 3ft of DoF, making even a medium-sized bird tricky to get in focus if flying parallel to you. At closer distances than that, some part of the bird is almost certainly going to be out of the DoF zone with the FF lens.

DoF for 300mm Full frame equivalent lenses

For completeness, here is the table for a 300mm f2.8 FF lens, compared to the 40-150 f2.8 at 150mm (300mm FFE). Hopefully, I don’t need to explain the results. You get the picture.

Yes but blown out backgrounds are far more important to me than in-focus birds

I hear this one often. But look at the Depth of Field tables above. Any background is likely to be well clear of the Depth of Field region, and fully out of focus. Look at the image on the right for example, shot with the 40-150, or the EM1x Kite or Hawk albums, to see examples of perfectly blown out backgrounds with the 300mm f4.

It seems perverse to me to take immense lengths to get the best possible equipment and then set the physics of your lenses so that you have to choose between the size of the subject in the frame or getting it fully in the Depth of Field zone. Why not have both at once, with, for example, the Olympus 300mm f4?

Depth of Field on Olympus vs Full Frame cameras for birds in flight- what does all this mean?

Pretty straightforward really. Fast MFT lenses are IMHO far better for large birds in flight, from a DoF perspective than fast full frame lenses. There are other disadvantages of the huge FF lenses like a 600mm f4, which I will cover in a later post. But this is why I regard the Sony 200-600 f5.6-6.3 as the best of all the Sony long telephotos, and the same applies to the Nikon 200-500 f5.6, or 500 PF and the Canon RF 100-500 f5.6-7.1. And it’s also why I don’t use or own a fast FF telephoto lens (money doesn’t come into it, as this post might indicate).

Hopefully, I have provided you with some useful information about depth of field on Olympus vs full frame cameras for birds in flight. I have also dealt with similar misconceptions about the Olympus system in earlier posts on focus accuracy, noise, and dynamic range . Have a look at these also if you would like to learn more about this amazing and under-appreciated camera system.