Your lens doesn't blur the background: focal length in dogs in action photography
Last week we looked at what sits behind the dog. The next question is always about gear: which lens blurs the background best?
It is the wrong question, and it is an expensive one.
A long lens does not blur your background. It chooses it. In dogs in action photography that difference decides where you stand, and once you see it you stop trying to buy your way out of the problem and start solving it with your feet.
What focal length really does in dogs in action photography
Three things happen when you go longer, and only one of them is the one people talk about.
It narrows the slice of world behind your dog. This is the big one. At 70mm you are showing about thirty degrees of everything: the treeline, the car park, the bin, the bright gap between two trunks. At 300mm you are showing about seven. Same dog, same spot. You did not blur the car park. You excluded it.
It magnifies whatever you kept. The strip you chose gets bigger in the frame. A far treeline spreads into a wash of soft colour, which is exactly what you want. A bright gap in that treeline also gets bigger, which is exactly what you do not.
It does give you more blur, for a reason that has nothing to do with the number on the barrel. At the same framing and the same f-number, a longer lens has a physically wider opening. A 300mm at f/2.8 opens to about 107mm, a 100mm at f/2.8 to about 36mm. Wider opening, more defocus. The blur is real. It is just the third effect, not the first.
The number that actually decides your background
It is not the focal length. It is the gap between the dog and whatever stands behind him.
Put a hedge three metres behind the dog and reach for a 300mm. You get a softer hedge, but still a hedge: its bright patches and its colour are all there, still competing for the eye. Blur takes away detail. It does not take away brightness or colour.
Put a treeline eighty metres behind the same dog and almost any focal length gives you a clean frame, because the distance does the work for free.
Here are the numbers. The fixed point is the dog: same size in the frame every time, so with each lens you stand at a different distance. Seven lenses, all at f/2.8:
Lens | Angle | Distance to dog | Hedge 3 m back | Trees 80 m back |
35mm | 54° | 3 m | 13 px | 24 px |
70mm | 29° | 6 m | 17 px | 47 px |
85mm | 24° | 7 m | 18 px | 56 px |
105mm | 19° | 9 m | 19 px | 68 px |
135mm | 15° | 11 m | 20 px | 85 px |
200mm | 10° | 17 m | 22 px | 118 px |
300mm | 7° | 25 m | 23 px | 163 px |
Full frame at f/2.8, the dog filling a frame three metres wide. Angle is how much world you see behind the dog. The last two columns show how big a single point of the background becomes once blurred, on a photo 6000 pixels wide.
From 35mm to 300mm the hedge goes from 13 to 23 pixels of blur. It is still a hedge. With the trees far away, the same change of lens goes from 24 to 163.
The lens multiplies the distance you give it. It cannot create it.
This is why a photographer with a 70-200 who chose the ground well goes home with cleaner backgrounds than a photographer with a 300mm who did not. No lens can buy you a gap that is not there.
The trap nobody warns you about
A long lens enlarges your mistakes at exactly the same rate it enlarges everything else.
Last week I wrote about the autumn canopy, those bright holes of sky that open up as the leaves come down. At 100mm they are small scattered dots behind the dog. At 300mm a single hole can take a big part of the frame, and now you do not have confetti, you have a white wall with a dog in front of it.
The same goes for a distant roof, a parked car, a person in a red jacket far across the field. A wide lens buries them in the clutter. A long lens finds them, magnifies them, and places them right behind your subject's head.
Going longer does not forgive a background you did not look at. It judges it.
Why a 70-200 wins more often than people admit
Working distance. At 300mm on a running dog you are standing a long way back. You cannot read his face, you cannot talk to him, and you cannot take two steps to your left without losing the frame.
Those two steps are the whole point. Your feet are the most powerful background tool you own, and the longer the lens, the more every step costs you. A 200mm lets you move and recompose between runs. A 300mm pins you to a spot you picked before you knew what the background was going to do.
The dog. Dogs run differently toward a person they can see and read. At twelve metres you are part of the session. At twenty-five you are a tripod.
Not sure which lens suits the way you shoot? The free lesson on choosing a lens for dog action work goes through focal length, aperture and working distance, and shows where each of them stops helping. Watch the free lesson
What to do on your next session
Choose the lens last.
Walk to the spot you plan to shoot from. Look at what sits behind the place where the dog will be, without the dog in it. Then ask one question: how much of that do I want in the frame?
If the answer is almost none, go long, and check what that narrow strip actually contains before the first throw. If the background is already far and clean, your shorter lens is the better tool, because it buys you movement and closeness, and those are worth more than two stops of blur.
The lens is a decision about how much world to let in. Make it after you have looked at the world.
Everything else, how to read the dog's line a second before he commits, how to work the light you have instead of the light you wanted, how to finish the frame in Lightroom so the separation you built on location survives the edit, is in the full course.
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