|
Hi Stu,
You will see that on all lenses they have an f/stop range shown, typically f/3.5-5.6 or f/2.8-4.5 for instance. Now when your lens is at the 18mm end of it's range you will be able to open the aperture up to the first value shown, but when you've got the lens at it's maximum telephoto you will only be able to get a maximum aperture of the second value shown. This means that you are unable to get as much light through the lens and on to the sensor whatever shutterspeed you are using.
An example that is often used is say you have a bucket and you want to pour a litre of water into it throught a funnel. If you use a small funnel the water will take longer to pass throught it and go into the bucket. So if you want to increase the speed of the flow into the bucket you need to use a wider funnel. Then the water will stream through it without even touching the sides. If you imagine the bucket as your sensor, the aperture of your lens as the funnel and the water is the light. So by setting your lens to a higher focal length you close the aperture of the funnel and the light takes longer to pass through it and on to the sensor.
Therefore, if you want to use a fast shutter speed with your lens at 135mm or 180mm you will need to probably up the ISO a little to say 800 or 1200, so your sensor is more sensitive to the light coming through the lens to stop your images being under-exposed if conditions are a bit dull. However if you're shooting on a bright sunny day you may still be ok at a lower ISO setting of say 400.
Now you mentioned your images were blurred. Were you trying to pan the camera to follow the action? If so, you must keep the camera moving whilst the shutter is open. If you were holding the camera in your hand and zoomed in close to the action and it came out blurred, this was probably caused by hand shake and you need to use a shutter speed that is no less than 1.5 times the focal length you are using with you lens. So if you were zoomed in at 50mm you need to use a shutter speed of no less than 125th of a second to stop the camera shake. If you had the camera mounted on a tripod and the image is blurred, itc could either be a bit of vibration caused by the mirror flipping up, so lock it up if you can and that should stop it. The other thing that causes blurring is a focusing error. As in when you take your shot you need the autofocus (AF) point to be on the rider as you squeeze the shutter otherwise it maybe focusing on something else in the picture than your subject and what you want is out of focus. To get round this, you need to judge where your subject is going to be when they appear and frame your shot accordingly so they will be either slightly left or right of centre of the image and set your AF point accordingly. So when they appear you'll have the AF point trained on them and it should focus correctly. Don't have it set in the auto mode, where the camera has to pick what it focuses on as it may make the wrong choice. Don't leave it to chance!
Anyway, best of luck with it and I hope you have a successful day with the pro photographer.
|