| | |  Vallarta Living | Art Talk | May 2009 
Photo Tip of the Week: Learning F/Stop or Aperture - Part 2
 Larry and Linda Bennett - PVNN
This week, we continue addressing the F/Stop or aperture. You may want to review Part One of my Learning F/Stop or Aperture series before you begin this week's article.
What is the area of aperture at F/2.8? Well let’s look at it from a mathematical perspective. F/stop is a ratio of the lenses focal length to the diameter. A 50mm lens at F/2.8 would have a diameter of 50mm/2.8=17.86 mm. The area of the circle thus formed would be n x (17.86/2)2 or 250.5 square mm. That’s about 250 sq mm at 2.8 and 500 at F/2, it’s a double half relationship. Here it is in a nut shell, the area of the aperture (or hole) doubles and halves; it’s just represented by a ratio on the lens.
If you have a 50mm lens on your camera and you say, I shot this image with a 50mm at 1/125th and an aperture of 63 square millimeters, no one will be able to understand that one. It’s much easier to say, "I shot this image at 1/125th at f 5/6." (F 5/6 is the same as 63 square millimeters.) This is getting more confusing than helpful, so let’s back out of this paragraph gracefully and go on to lenses.
Lenses and What Makes a Lens Fast?
Lenses are referred to by their maximum aperture. Remember it’s the biggest the hole will open, but is the smallest F number. Say you have a camera and lens with the following: a 50mm/F 3.5 and a 50mm F/2.8. The lens with F/3.5 will let in 1/3 less light than the lens with F/2.8. And it keeps going, a 50mm lens with F/2.8 will only let in half the light of a 50mm lens with F/2.0 at its maximum aperture, remember it's one big math problem...
However, the camera manufacturers have helped us all by putting the F number on the lens, and TTL (through the lens) on to your camera. In photographic circles a lens with a wide maximum aperture that lets in lots of light, say with a F/2.0 would be considered fast, a lens with a F/2.8 -3.5 would be mid-range as far as speed and anything over a F/3.5 would be considered to be a slow lens. There is some compensation for a slower lens that needs more light, it’s called ISO. We can talk about ISO in the following weeks.
Why do you want a fast lens when they weigh a lot and are much more expensive? Let’s look at this more closely to find out why you would want a faster lens...
To get those larger diameter apertures means you need a larger piece of glass, mounted in a correspondingly larger lens barrel, and they’re harder to make due to their optical design and glass quality. This all equates to more money; but to me it’s worth the extra money.
There is a difference in the quality of the image and the ability to shoot in low lighting conditions that will surely justify the cost to some, but in the end, it’s what you want to do with the final image that’s important. Most professional photographers will almost always have one lens that will have the capability to shoot an F/2.8 or wider.
Let’s talk briefly about the lens image stabilization (IS) and how it works with F/stop. Some people get confused about losing the F/stop if they’re using image stabilization. No, you won’t loose any of the F/stop; it will stay where you have it set.
IS does not move or change your F/stop. IS works like a mini-gyro in that it will hold your lens element or in some cases your sensor in place and by doing so will allow your camera to compensate for natural body movement and unsteadiness.
IS will also allow you to hand hold your camera using a slower shutter speed without any movement induced blur in your images. For example, you have a 200mm lens on your camera, with IS you can now hand hold and shot without blur in most cases. If you were starting to blur at 1/250 with IS it might be 1/125 or if you blur at 1/125 it now might be 1/60 or 1/30 while still maintaining a sharp quality image.
You hear the term "stopping down a lens" often in photography circles. "Stopping down a lens" simply means you are going to a larger F number (or a smaller aperture) and therefore you will need less light for the correct image exposure. For example, going from F/5.6 to F/8 is stopping down.
"Opening up or stopping up a lens" is just the opposite, going from F/11 to F/6 is moving towards the smaller number but the larger or wider aperture therefore allowing more light on to your sensor or film.
Remember F/stop is a doubling and halving relationship that works hand in hand with shutter speed and exposure. This is important since the shutter speed and F/stop you choose will have implications on how your final image will turn out.
You need to remember that as you stop down you get more depth of field. For example an F/22 will give you more depth of field than F/11. Try this test. Sit on your patio or balcony and start with the largest or widest stop, maybe F/3..5, and shoot an image. Now continue taking images of the same subject each time and change only the F stop each time working your way all the way down to F/22 or F/30. Keep your same shutter speed, ISO and white balance for each image. When you are finished, download your images and you will see what I have been saying all along, F/stop is depth of field.
Okay, so far we have learned: 1) The F/stop and shutter speed control the amount of light that goes onto your sensor or film; and 2) That shutter speed and F/stop both double and half themselves. Therefore, we know we can open up an F/stop letting in twice the light, moving the shutter one step faster by cutting the time in half, and have the same amount of light on our sensor or film.
If we were to use a light meter on the scene of our subject and it shows that 1/125th at F/8 is the correct exposure, any of the following combinations will work to get close to the same results of a perfect image.You are not going to have one lens which takes you from F/¼ to F/45 and your camera will not have the higher shutter speeds. Also, if you are without a tripod, there are limits as to how slow you can handle your camera with slower shutter speeds before body movement and blur begins to appear in your images.
There are some constraints, but the point remains that all of the combinations will yield the same amount of light on the sensor or film yielding the same image in the terms of light and brightness. What will vary is your ability to stop action and your ??? Come on... guess.
If you said, "depth of field," you are absolutely correct.
A short explanation on depth of field is, the amount focus either in front of or in the rear of your subject. Again, these combinations (mentioned above) will give you all the control on the outcome of a successful image. Study it, practice it, use it, once you understand F/stop you are on your way to becoming a better and successful photographer.
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 Photo Tips of the week are written by Larry Bennett, a professional photographer living in Puerto Vallarta. These tips are to be just tips, refer to your cameras owner's manual for specifics on your camera. Readers are welcome to enjoy Larry's website at LarryBennettPhotography.com.
Click HERE for more Photo Tips from Larry Bennett. |
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