The Addition of Sports Photography: The Limits of Lenses
For some photographers taking a picture is as simple as depressing the shutter release half-way to auto focus, and then all the way to take the photograph.
For other photographers focusing is a more involved. They understand that focusing is done by focus points on the camera’s sensor. The photographer selects which points directs the focus to compose the image.
But fewer photographers understand that not all sensor focus points are created equal, and for a given camera that changes from lens to lens. For the sports photographer this understanding is the difference between capturing a tack-sharp image of a player making the play, or a tack-sharp image of a player watching a play be made.
The sophistication of the auto focus system is probably the least understood aspect of digital photography. That mystery is revealed here. For the purposes of this conversation we will use Canon cameras and lenses as the example. A similar analysis applies to all camera and lens makers.
Phase Detection Focusing System
The most common auto focusing system for sports photography is also fastest to acquire and focus the image. The phase detection auto focus system works by splitting the light into two images and then focusing the lens until the two images converge as a single image on the sensor. Phase detection is found in all most all DSLR cameras and is a component of the focusing system use in some mirrorless cameras.
Look through the viewfinder of most digital cameras and you will see several dots, or squares, that represent individual points at which the camera’s sensor is capable of focusing. The number and location of the focus points is a feature that changes by camera.
First auto focus cameras had one pair of points along the horizontal plain. This was adequate for photographing something that was horizontal, but not when the point was aligned to the vertical plain.
A second set of points was added to the vertical plain that intersected with the horizontal plain. This was called a cross-type focusing sensor and work to focus on images with horizontal and vertical patterns.
Some cameras have taken this a step forward by adding a second cross-type sensor oriented on the diagonal and is called a dual cross-type focusing sensors.
The camera’s focusing speed, as well as the focus accuracy, depends on the type of focusing points: single type, cross-type, or dual cross-type. Understanding the differences helps you to take better photos.
For example, the table below shows the number of dual cross-type and cross-type sensor points available to a Canon 1DX II using a 70-200mm lens. The number and quality of sensor points decreases significantly as the f/stop increases (i.e., aperture decreases).
f/stop # Dual Cross-Type # Cross-Type Total
2.8 5 36 41
4.0 0 41 41
8.0 (1) 0 21 21
(1) 70-200mm f/2.8 with 2x extender
Canon has greater than 90 lenses available for use with their cameras. These lenses are divided into 11 Groups based on compatible with auto focus focal point arrays. We’re going to review four of these groups as they apply to the most popular sports photography lenses using the Canon 1DX as an example.
f/2.8 Lenses (Group A)
The Group A Lenses includes our fastest glass, lenses with an aperture of f/2.8, and have focal lengths ranging for 24mm to 400mm.
Some of the abbreviations used in the lens description are:
L – Luxury
IS – Image Stabilization
USM – Ultra Sonic Motor (faster, smoother autofocus than STM)
STM – Stepper Motor
This collection of lenses makes use of the greatest number of dual cross-type focal points available. To get the maximum benefit of the dual cross-type sensors, you must focus using focal points from the center column. The remaining focal points work with Group A lenses because the f/2.8 aperture is larger than the minimum f/stop for those sensor points.
Figure 4 is an example using a EF70-200mm F2.8L IS II USM, a group A lens. On the left is a photo of a player celebrating a touchdown. The photo is centered on the celebrating player making use of the dual-cross type sensors that are centered in the frame. This shot by its self is an interesting image, but it needs some editing.
The image on the right has been cropped, moving the players to the right. The image wouldn’t have gotten the benefit of the dual cross-type sensor points if it originally composed as edited. In reality, sports photography is so fast moving that you don’t often have the time to make decisions on composition on the fly.
Older f/2.8 Lenses (Group B)
The Group B Lenses are different from the Group A lenses solely in the number of available dual cross-type focal points. There is only one sports lenses in this group, the older version EF24-70mm F2.8L USM.
The number of available dual cross-type and cross-type focal points decreases as we progress into subsequent lens groups. This becomes an issue for the speed and quality of the focus.
The images presented in Figure 6 is an example when there is time to make decisions on composition using the rule of thirds.
Fans are invited to watch the Texas Stars hockey team warm-up from the player’s bench. In this shot, I wanted the fan to be in focus while watching the players. I used the EF24-70mm F2.8L USM lens, the only sports lens one in Group B. To get the desired affect, I manually moved the focal point to use the cross-type focal points on the right side of the frame.
On the right is the image after cropping. The cropping tightens the frame, but doesn’t really change too much about the shot.
f/4.0 Lenses (Group C)
The Group C Lenses are comprised of f/4.0 lenses or f/2.8 lenses using a 1.4x extender, making it an f/4.0 lens. The dual cross-type focal points are not available to Group C lenses because the dual cross-type requires a maximum aperture of an f/2.8 lens.
Figure 8 is an example of a Group C lens combination: an EF300mm F2.8L IS II USM with an EF1.4x estender. The extender changes the f/2.8 lens to a 420mm f/4.0 lens, and the dual cross-type focal points are not available.
In this case I used the cross-type focal points on the right to focus the image, although I could have easily used the points on the left. This original shot could have worked nicely, but the University of Texas (right) was hosting the track meet and I removed the Texas A&M athlete (left).
This is a pretty significant cropping: you wouldn’t want to do too much more than this. But in reality, I didn’t have a choice because of restrictions on where the photographers could be positioned.
f/5.6 Lenses (Group E)
The final group of lenses that might be considered for sports are the Group E Lenses. These lenses have an effective aperture of f/5.6 and should only be considered when shooting sports in daylight. The lenses of interest included f/2.8 lens with a 2.0x extender, or a f/4.0 lens with a 1.4x extender.
There are other groups of lenses but they are well suited to fast moving sport photography.
Available Focal Points
This complicated theory is much simpler in the field. The available focal points can be seen by looking through the view finder and pressing the “AF Point Selection Button.” The dual cross-type and cross-type focal points remain solid while the horizontal focal points flash.
Contrast Detection and Hybrid Focusing Systems
Most mirrorless cameras, point-and-shoots, and mobile phones use a contrast-detect focusing system. A contrast detection works by analyzing pixels on the camera’s sensor. The sensor tells the camera to keep changing focus until the contrast from one pixel to the next is the highest possible. The subject is in focus when contrast is highest.
The contrast detection system provides a sharper focus. However, the system is also slower because camera must push the focus point of the lens back and forth to hone in on the the maximum contrast. The back and forth actions also consumes more battery power.
More advanced mirrorless cameras employ a hybrid focusing system, using phase detection and contract detection in tandem. The phase detection sensors acquire the initial focus and the contract detection sensors fine tune the focus.
Auto focus systems are extremely sophisticated and continue to evolve. It is essential to understand the capabilities of your camera and how it
interacts with your lenses. Most important is knowing that not all focal points are created equal, and those capabilities change from lens to lens. This understanding enables you to produce the highest quality digital images.
About the Author
Andy Nietupski founded TTL Sport Media in 2015 after a corporate career of business start-ups and turn-arounds. TTL Sports Media helps sports organizations optimize their business results using the latest digital sales and marketing techniques. TTL Sports Media publishes thousands of pieces of content annually and curates a catalog of more than 100,000 items. On behalf of its client interests, TTL Sports Media annually publishes nearly 200 articles and makes 1000’s of social media posts.
Copyright 2020 Andy Nietupski and TTL Sports Media