The mechanical actions of an exposure are: Conversely the more CROSSING a subject is then the higher the frame rate can be and you might still get away with it. So as a basic rule of thumb the more CLOSING a subject is then the LOWER your frame rate needs to be if you are looking for a sharp sequence of shots. Even though most pro-summer and all pro body systems use phase detection auto focus, contrast between the subject to be tracked and its background does impact the efficiency of the overall system.ДЄ swan against a dark background is a lot easier on the auto focus system than a panther in the jungle or a white-tailed eagle against a towering granite cliff in Norway, but the AF system in most cameras is perfectly capable of acquiring, locking on and tracking any of the above subjects. the amount of light on the scene AND the contrast between subject and background ALL effect the ability of the auto focus to do its job. ThatвЂ™s why high grade dSLR auto focus systems have вЂpredictive algorithmsвЂ™ built into them.ДЄlso. So the closer a subject comes to your camera position the greater is the need for the auto focus sensors to see the subject AND react to its changed position вЂ“ and yes, by the time itвЂ™s acquired focus and is ready to take the next frame the subject is now even closer вЂ“ and things get very messy! Once focus is acquired on a CROSSING subject (one thatвЂ™s not changing its distance from the camera) then I might elect to use a faster frame rate as mirror-down-time isnвЂ™t so critical.Д«ut subjects that are either CLOSING or CROSSING & CLOSING are far more common and head on CLOSING subjects are the ones that give our auto focus systems the hardest workout вЂ“ and show the system failures and short-comings the most.Д¬onsider the focus scale on any lens you happen to have handy вЂ“ as you focus closer to you the scale divisions get further apart in other words the lens focus unit has to move further to change from say 10 meters to 5 meters than it does to move from 15 meters to 10 meters вЂ“ itвЂ™s a non-linear scale of change. The Scenario Forces Auto Focus Settings ChoicesДЇrom a photography perspective we are mainly concerned with subjects CROSSING or subjects CLOSING our camera position. When IвЂ™m shooting wildlife using a continuous auto focus mode then I tend to shot at 5 frames per second in Continuous LOW (Nikon-speak) and have the Continuous HIGH setting in reserve set for 9 frames per second. So how do we go about giving the main mirror more вЂњdown timeвЂќ? Simply by slowing the frame rate down is how! Unless the MAIN mirror is fully down, the secondary mirror is not in the correct position to send light to the auto focus sensor array вЂ“ SO GUESS WHAT вЂ“ thatвЂ™s right, your AF ainвЂ™t working or at least itвЂ™s just guessing. The fact that the mirror is still DOWN when they are using the auto focus leads most people into thinking the AF sensor array is elsewhere вЂ“ thatвЂ™s if they can be bothered to think about it in the first place.Д«ecause the center area of the main mirror is only SEMI silvered, and in reality light from the lens does actually pass through it.ДЄs you can now hopefully understand, light passes through the mirror and is reflected downwards by the secondary mirror into the AF sensor array.ДЄs long as the mirror is DOWN the auto focus sensor array can see вЂ“ and so do its job. Most folk thing that the mirror just sits there, reflecting at 45 degrees all the light that comes through the lens up to the focus screen and viewfinder. TheyвЂ™re in the bottom of your cameras mirror box. Well, itвЂ™s simple, and logical if you think about it вЂ“ where are your AF sensors? The effectiveness of auto focus and its ability to track and follow a moving subject IS INFLUENCED by frame rate. There is truly no hopeвЂ¦вЂ¦вЂ¦.! But letвЂ™s get onвЂ¦. Firstly, an apology to my blog followers for the weird blog post notification this morning вЂ“ I had one of those вЂњsenior momentsвЂќ where I confused the Preview button with Publish вЂ“ DOH!

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