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F Numbers

Lesson 12 from: Mastering Your Digital Camera

Chris Weston

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Lesson Info

12. F Numbers

Aperture is measured in f-stops or f-numbers. Decipher the numbers and learn how to create a balanced exposure when adjusting the shutter speed or aperture.


Lesson Info

F Numbers

So what do all the different exposure numbers mean? Well, shutter speed, the pretty obvious. They're measures of seconds and fractions of seconds. The numbers relating to lens aperture, however, are a bit more confusing. But honestly, they do make sense. They equate to the area of the hole in the lens, the aperture through which light passes Now. In the old days, each number related to a whole one stop change on the scales looked a bit like this. In the simplest terms, each number is a doubling or harding of the exposure. For example, if I change lens aperture from F A 12 11 I'm having the quantity of light entering through the lens. If I do the opposite and change from F eight, where 5. I'm doubling the quantity of light similarly with shutter speed, if I change the shutter speed from 1 to 50th to 1 500 I'm having the length of time the shutter is open. If I go the other way and change from 1 to 50 with the 11 25th I'm doubling the time now to confuse things. Modern cameras enable adj...

ustments be made in smaller increments. 1/2 or 1/3 stops, and the scales now look more like this and notice the addition of the intermittent markers. But whatever the incremental change, the principle is the same. Each change increases or reduces exposure by a fixed amount. So now you understand the principle. Let's look at how this works. In practice, I'm starting with a portrait shot of the horse and rider. I've said the lens after a 5.6 and a shutter speed of 1 2/50 and this gives me a correctly exposed image. Next, I want an action shot. But my shutter speed 1 2/50 is too slow to freeze the movement of the horses, so I need to increase it. I'm going to put it upto 1 1/1000 which means the shutter is open for 1/4 of the time compared to my original setting of 1 to 50. To compensate, I need to open the apogee of in my original setting of 5.6 F four F 2.8, which quadruples the quantity of light passing through the lens. So now, even though my shutter speed and aperture have changed, the exposure value is the same which will give me the same correct exposure. Finally, I want an image that captures a sense of where we are now. I need a smaller aperture to get some detail in the background. I'm going to adjust my aperture from F 2.88 which is a three stop change. Again, I need to compensate for the loss of light by reducing the shutter speed by three stops that takes it from 1 1/1000 toe 11 25th. As before, my exposure settings have changed, with the total amount of light affecting. The sensor is exactly the same, resulting in the correct exposure. And you can see this if we compare all three images and look at the sky. The brightness is the same across each photograph. When it comes to exposure, you are constantly changing you the lens aperture or shutter speed. The question is, does it matter which of the two you change? The answer is yes, it does. In fact, it's about the only thing that matters

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Ratings and Reviews

mark jacobson

What a marvelous course! What a marvelous teacher! When I went to college, my father would always ask me about my professors, more than the courses themselves. He was passionate about learning and although too busy with earning an income to go beyond an undergrad degree, continued to read 50 books a year. I still remember how he'd get almost visibly excited when I'd tell him about some special professor who taught with such enthusiasm and, more than just passion, evident delight and joy in the subject. 'Ah they're the best, son. How wonderful you have such a teacher." Well, he passed away decades ago but if he were still around I'd get a kick out of telling him about Chris Weston, the 'Prof' of this course. He's one of the very special ones: a teacher who's loved and lived his vocation--his avocation--since he was a boy--and still is as excited about it now as he was then. The result: a course that seems to be more a labor of love--of pouring far more energy and thought into the details then one typically finds in these courses--than anything else. Bravo Chris! I'm already on to your next one.


Chris is an amazing instructor who dissects theory giving amazing analogies that bring concepts to life. I have rarely been able to sit through most video course for more than a half-hour but watched this one from beginning to end. A good refresher course if you've been away from the camera for awhile or there are some concepts that still illude you. I highly recommend this course and look forward to watching his others. Thank you for the clarity and great explanations.

Sky Bergman

This was an amazing class. I have looked at a number of basic photography classes. This one was by far the best I have seen. Chris is an exceptional teacher. He breaks things down into digestible information and then inspires you to be creative. Thank you!

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