Depth of field, one more question
Hello all, I learned from you how to better control depth of field under difficult lighting conditions (in an earlier thread 30921). The setup (AF-C, AE-L to AF-on, preview button to AE-hold and Back-button focus) works great for me in my early picture taking tests. But I have three additional questions. The first is why AF-C? It seems that by disconnecting the exposure and focusing functions from the shutter any of the three settings (AF-C, AF-S and AF-A) should be able to achieve focus and not change it when the shutter is used to take the picture. The second question is why doesn't the focus icon in the viewfinder stay on after I release the back button? It probably is because I am using AF-C but, if so, it doesn't matter as the focusing, though still active will not change when the shutter is used to take the picture. The last question is whether spot metering can be used in this setup. I am using matrix metering now and use it to improve exposure of the subject by focusing on the subject even though matrix metering covers the entire image. Spot metering may be a more accurate way to meter the subject though. Hope I an clear in my questions. Thanks in advance. Stephen
Visit my Nikonians gallery.
#1. "RE: Depth of field, one more question" | In response to Reply # 0aolander Nikonian since 15th Sep 2006Sat 28-Dec-13 02:42 PM
By using AF-C with AF-ON, you can press and hold to use continuous focusing or press and release to select a focus spot (like you'd get with AF-S). You have the ability to do both by setting it up this way without changing any focus modes.
Not sure why the focus spot doesn't stay on. I wondered about this, too.
Using spot metering can throw your exposure off unless you realize what this type of metering is doing. If you spot meter a white area in a scene, your general exposure will most likely be underexposed. If you meter a dark/black area, the general exposure will likely be overexposed. You need to meter a mid-tone or adjust exposure if reading lighter or darker toned areas. The meter wants to make everything a mid-tone. Using what the meter says when spot metering will make a black cat gray or a snow man gray.
#2. "RE: Depth of field, one more question" | In response to Reply # 0briantilley Nikonian since 26th Jan 2003Sat 28-Dec-13 03:05 PM
The main reason for recommending AF-C with "back-button" focusing is that it puts the photographer in control. The camera will continue focusing on whatever the chosen AF point is on for as long as you keep the button pressed, but it will stop focusing when you remove your finger from the button, and will not re-focus when you press the shutter release. If you chose AF-S (or AF-A), the camera could stop focusing once focus is achieved, so if your subject then moves and you didn't notice, it would be unsharp.
#3. "RE: Depth of field, one more question" | In response to Reply # 2Sat 28-Dec-13 03:26 PM
Thanks to you both. So, the answer to my first question is that AF-C provides me greater control over the picture -taking process. That makes me very happy and in control of things. The answer to the third question seems to be not to use spot metering as it will cause the camera to overreact to light or dark subjects and change black and white to gray. By using matrix metering to deal with a little too dark or a little too bright subjects is the way to go, along with exp comp, I guess. S
Visit my Nikonians gallery.
#5. "RE: Depth of field, one more question" | In response to Reply # 3km6xz Nikonian since 22nd Jan 2009Sat 28-Dec-13 08:49 PM | edited Sat 28-Dec-13 09:07 PM by km6xz
Matrix uses the whole metering sensor to evaluate the tone range of the scene, but still assumes the object under the active focus point to be the most important part of the scene when deciding the exposure values. For most common situations it works very well. If the focus target is darker than the rest of the scene, parts of the scene will probably be overexposed in order to boost the part of the scene that was under the FP. The difference between Spot and Matrix is how dominate the area under the focus point is in determining the correct exposure. Use SPOT when you can ignore the exposure of the areas away from the focus target. I use spot a lot, mostly for faces there the surrounding areas are not essential that it be exposed properly but the subject is very important to get right. That happens in portraiture a lot.
Think of spot as being only concerned with the small area where the focus point is set and Matrix as concerned with the whole scene but biased towards the area under focus point.
AF-On in AF-C mode is very flexible but has one downside, minor for most people, in that the AF Assist light does not function in AF-C mode. AF Assist is annoying to people when used with the popup flash but is very effective and not noticed when using a shoe mounted SB series Nikon Flash. If you are shooting in dark conditions and need to use AF, switching to AF-S will allow focusing in total darkness. AF-S works with the back button and each press emits a burst of patterned near infrared light from the external flash that greatly aids the AF system obtain focus in very low light. That is particularly important when using slow lenses or variable maximum f/stop such as the kit zooms where it might be fine focusing at 18mm because the wide open aperture is 3.5 or so but 5.6 at 105 or 200 and might have a hard time focusing in the same low light. AF focusing is done at maximum aperture and then when the shutter is pressed fully, the lens is stopped down to the correct value for the exposure.
St Petersburg Russia
Visit my Nikonians gallery.
#6. "RE: Depth of field, one more question" | In response to Reply # 5Sun 29-Dec-13 10:01 AM
Thanks Stan, I wish I could go back to the museum where I took the picture shown in the Noise topic discussed in an earlier thread to try spot metering there. You helped me understand in your comments about the noise in that picture that by underexposing that photo more would have helped brighten the subjects while darkening the surrounding area, thus increasing dramatic effect while helping with the noise. It sounds like spot metering would have accomplished the same thing. (While there I would have used a lower f-stop as well! And a tripod) it looks like the biggest thing for me now is learning how to keep whites white and blacks black when determining exposure. So, of the two approaches mentioned above, do you think underexposing with matrix metering or spot metering would be better in this regard? Stephen
#4. "RE: Depth of field, one more question" | In response to Reply # 0
As Alan and Brian indicated, if you are using back button focus you CAN use the other focus modes AF S and AF A, but there isn't any reason to. By picking your focus spot and deciding whether to continue focusing or not by continuing to press or release your finger, you can achieve the same results of either of those other focus modes without having to switch back and forth between them. AF A starts in single servo focus mode and switches to AF-C if the camera detects subject movement. Since you can do the same thing with your finger AF-A mode is not necessary any more than AF-S is.
That is the beauty of the back button focus system and why so many experienced photographers use it. They know they can adapt to any autofocus situation without having to switch between autofocus modes.
Visit my gallery.