DOF Preview & Focusing Screens
For those of you with SLR cameras most of you have the option of a depth of field preview button. When you look through the viewfinder the aperture is completely open so that you can see the brightest, easiest view so you can manually focus. But if you're shooting a picture at f/ it's gonna be completely different when that aperture stops down and you get a whole bunch more depth of field. So a long time ago professional cameras put on a depth of field preview button over on the side. It is now migrated all the way down to some of the entry levels, not all of the entry levels but some of the entry level cameras will have this little hidden button down on the side. And when you press it normally in the viewfinder you get this shallow depth of field look. Because that's what you're gonna see with the lens wide open. When you press it in it closes the aperture down and when you look through the viewfinder it's gonna give you a really dark image. Now most people who see this for the first ...
time, when you're explaining it to them, they always say the same thing, I can't see anything, it got dark. Let your eyes adjust, take a moment for your eyes, to pupils adjust and then you'll see that you're actually getting more depth of field. Do you see how much is in focus? Oh, I see it now, and so you press the button in, you wait for a second or two for your eyes to adjust and then you can see how much depth of field you're gonna get. Now back in the days of film this was really important 'cause you didn't know how much depth of field you were getting until you got your film back from the processor. Now with digital cameras this is kind of useless because if you wanna see how much you get in focus just shoot a picture and look at it. The fact of the matter is though, if you're using an SLR, it's hard to see the back of the camera in bright sunlight. And so for a landscape photographer who's got a lot of light that they're working with, it's nice to hit that depth of field preview to see if those flowers in the foreground are in focus with the mountain in the background. And so it's a handy little device. Now those of you with mirrorless cameras, you may want to do a little experiment. Don't take a picture of yourself but hold the camera pointed at yourself and press halfway down. When it presses halfway down some of the cameras will do an automatic depth of field preview for you. I believe Fujis will do this. So when you press halfway down, you hear a little noise, the aperture stops down. Other cameras don't stop down until the actual picture is taken and this may even be a feature that you can turn on and off on your camera. But you will get this as an option on some mirrorless cameras if you look for it in that manner. Now for those of you with SLRs and any of you that have very fast lenses, there is an important little secret change that has happened in cameras over the years when we went in to auto focus. The focusing screen in your camera, in past cameras, actually used to be replaceable and you used to be able to interchange it and you could exchange it for one with grid lines or had a special focusing mark on it. And there was also some ones that were brighter than normal. And you could change these but now there are no current model of cameras where you can change the focusing screen anymore, they've all disappeared from the market. What you get in the camera is it. And when we went to auto focus they had to change or they chose to change the type of focusing screens that are in the cameras. And it's because there was a new demand for these small, lightweight, slow lenses. And in order for them to work they needed to change the way these focusing screens were in the camera. And they made them more bright, less accurate. And so we traded accuracy so that we could actually see our subjects. I don't know if anybody remembers the old film cameras but if you remember film cameras with a slow lens, at that point it was like an f/4 lens, and you didn't get your eye right in the middle everything got really dark. The thing was, it was accurate but it was dark. And so now we're choosing brightness over accuracy. And what that means for the person who uses an SLR camera is that if you use a fast lens, anything faster than 2.8, the camera in the viewfinder will not show you the proper shallow depth of field that you are likely to get. So if you have a 1.4 lens you look through the viewfinder and you go, okay, yeah I see how much depth of field I'm gonna get and then you shoot it at 1. and you're gonna get even shallower depth of field. And so this focusing screen changes what you are seeing in the amount that you're gonna get by the depth of field. And so for anyone with a very, very wide open aperture it's gonna look different in the final picture than it does through the viewfinder. Take a look, do your own tests. It's true so check it out.