LESSON 2: Aperture Priority is Your Friend! Focal length, aperture, and shutter speed are the three biggest choices we have to make when capturing a subject or scene exactly as we want the rest of the world to see it. All are equally important in expressing our own creative visions, and all must be thoughtfully chosen before we can create that “Perfect Picture”. In this lesson, we are going to cover aperture and more specifically, aperture priority mode, and show you why this can cover most shooting situations. I find I have my camera set to Av (Aperture Priority) about 90% of the time as I feel it allows me the most creative flexibility for my style of shooting. About the only time I don’t use it is when I am shooting sports and want to freeze action, and in the studio using strobes where the camera’s flash sync speed is limited to between 1/125-1/250 sec. There are several reasons for this and the first is that it allows me to totally concentrate on my subject and composition without having to worry about changing light conditions and improper exposure. I only need to be concerned with depth of field, and point of focus. Both of which I can change quickly and easily without taking my eye off the subject or checking my exposure meter. Shooting in natural light, the conditions can change very quickly. Not only is our light source, in this case the sun, moving but we have to deal with other factors such as clouds moving through, shadows from trees and other objects, etc, that are always changing as the sun moves or the wind blows. Portland Head Light House f/10, 1/25sec, 70-200mm lens@70mm Av I can’t tell you how many times I’ve been completely lost behind the viewfinder, entranced by my subject, that I didn’t notice my exposure meter moving and either under or over exposed an image. The reason for this was my camera was set to manual and while caught up in the moment, I simply forgot to adjust my shutter speed for the changing light. I hate to admit that but it does happen to just about every photographer I know, no matter their experience level. To use Av to its fullest potential, we must first have a THOROUGH understanding of how aperture size, lens type, and even lens construction, effect depth of field. First of all, we need to know that DOF (depth of field) is not split evenly in front of and behind the focus point. It’s is closer to 1/3 in front, 2/3 behind the point of focus. This can be extremely critical when shooting with very shallow DOF! Many times, manual focus will be needed to ensure we have exactly what we want sharp in an image. Second, and this is the biggest issue we see in classes and on workshops, is that even at the same aperture, DOF is different for each type of lens! Many times we have had students ask why they didn’t achieve the same look as an example image when they shot it at the same exposure setting listed. The answer is because of the lens used. Think of it this way, in the days of film SLR’s and today’s full frame DSLR’s, a 50mm lens is considered a “normal” lens. Basically what this means is that looking through the viewfinder with a 50mm lens on, you will see a FIELD OF VIEW of about 45 degrees which is similar to what our eyes normally see. Objects will appear correct in size and distance. Now, put on a wide angle lens and suddenly the FIELD OF VIEW is expanded and not only are we taking more in through the viewfinder but objects appear smaller and more distant. Put on a telephoto lens and just the opposite happens. FIELD OF VIEW is compressed and objects appear closer and larger. You’re probably asking yourself what all the “FIELD OF VIEW” talk is about and what does it have to do with aperture and DOF which is what we are supposed to be learning about here, right? The answer is this, with “expanded” field of view (wide angle lens) comes “expanded” DOF, and with “compressed” field of view (telephoto lens) comes ‘compressed” DOF. It’s that simple! Well….. kind of. Now to make it a little more complicated, let me add the following. DOF can be different even at the same focal length and aperture! What? How is that possible? I know I just explained how field of view, or basically the focal length of the lens, is equally important as aperture in determining DOF so how can two lenses with the same field of view and aperture have different DOF? The answer is lens type and lens construction. If I take a 28-70mm lens and a 70-200mm lens and set them both to 70mm, f8, which one would have greater DOF? Hmmmm….. from what we learned above, it would make sense that the 28-70mm (wide) lens would have more DOF at the same aperture as the 70-200mm (telephoto) lens would and it does if we focus on an object from the same distance (same field of view!) with each lens. However, every lens has a “minimum focusing distance”, that is, the closest the front element can be to a subject and achieve sharp focus. DOF is shallower the closer we are to the minimum focusing distance of any lens and becomes greater as we focus out toward infinity. Since wide angle lenses have much closer minimum focusing distances than telephoto lenses do, we can get much closer to the subject with the 28-70mm set at 70mm than with the 70- 200mm set at 70mm. This “compresses” the field of view and therefore shrinks the DOF! Now, if we take and add an extension tube to the 70-200mm lens and make the minimum focusing distance equal, the DOF then becomes greater on the 28-70mm lens! Confused yet? We’re not quite done…… Macro lenses have extremely close minimum focusing distances and therefore less DOF at a given aperture than a regular lens. Adding extension tubes to a regular lens will allow closer focusing just like a macro lens but you will lose DOF as well as the ability to focus out to infinity. The closer you get, the shallower the DOF so when approaching 1:1 or life size magnification, DOF can be reduced to several millimeters even when stopped down all the way. Aperture choice and focus point are critical in this situation so keep this in mind! One last point before we move on. Believe it or not, sensor size can affect DOF! It’s not as drastic an effect as what we have covered so far but it is important to know. Earlier I mentioned that FOV was referenced to 35mm film and full frame digital cameras. Most of today’s DSLR’s have sensors that are smaller than a piece of 35mm film. We usually see this expressed as a “crop factor” or the dreaded “magnification factor”. Most cameras have a 1.5 (Nikon) or 1.6 (Canon) crop. Basically what that means is that the sensor is roughly ¾ the size of a piece of 35mm film so you get a narrower field of view from a lens as you would on a full frame or film camera. In other words, if you were to set up a full frame camera next to a crop frame camera and put 50mm lenses on both of them, the images from the crop frame camera would appear as if they were shot with a 75mm (Nikon) focal length or an 80mm (Canon) focal length lens. Now, we all know that the DOF will be greater at a given aperture on a 50mm lens than on an 80mm lens, right? So, which image will have greater DOF? Are you sure about that? If you said the one from the camera with the smaller sensor (80mm FOV) you’re right! Take a bow as you’ve managed to follow all of this correctly so far! But wait a minute…. We’ve just spent all this time learning how narrower field of view affects DOF so the image with the 80mm field of view should have shallower DOF not GREATER! That is correct, but think about it carefully. We shot both images with a 50mm lens. We are only seeing the equivalent field of view to an 80mm lens on the cropped sensor camera. To get the same size image with the same field of view we would have to put a LONGER lens on the full frame camera which would then give us less DOF for the same field of view! So, the smaller the sensor, the greater the DOF at a given focal length and aperture. I know this was a lot to take in, but trust me, it will all make sense as we move through the rest of this lesson. By fully understanding how all of these factors affect DOF, you will truly be able to make the most out of your camera and lenses using Av! One of our workshop participants right after I explained DOF to him! f/3.2, 1/800sec, 120-300mm lens @300mm Av As stated earlier, Aperture Priority allows you the freedom to focus all of your creativity on your subject without having to worry about changes in exposure values.