The green focus confirmation dot

batmura

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I have a few questions regarding focus confirmation (the green dot that appears on the lower left of the screen). I do a lot of long exposure photos during daylight and, based on a video by Scott Kelby as well as a number of articles I have read, this is how I used to do it: Without the 10-stop ND filter attached, I auto-focus using the 11 focus points and compose the frame. Then, I switch to manual focus and put the filter on before using a shutter cable to shoot. Of course, if I don't like the shot and I have to reframe I have to take off the filter, switch back to auto-focus and then back to manual and filter attached.

I was shooting a long exposure in NYC last week following my usual workflow when I was approached by a professional photographer who told me switching from auto to manual focus was unnecessary; I could stay on manual and as long as I saw the green dot on the left bottom, I should not worry about sharpness as the dot indicated everything was in focus already and taking off and attaching the filter was unnecessary. When I checked the photos on the computer, they were sharp indeed, but I never picked a focus point and why do many photographers like Kelby recommend the other method?

Or did I miss the guy's point entirely? Do you consider the green dot at all or do you prefer switching filters and focus points?
 

Big Mike

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The focus confirmation doesn't not mean that "everything was in focus"...it just means that the focus point(s) that is active, have come to a point of being in focus. I'm not a big fan of use 'Auto point selection' because you never really know what the camera will choose to focus on. I much prefer to manually choose the point.

But yes, when shooting in manual focus, you can use the focus confirmation light, rather than switching to auto and back. But, you also need to realize that the AF system (including the confirmation light) need a certain amount of light to function. So with the filter on, it may not be enough.
 
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batmura

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when shooting in manual focus, you can use the focus confirmation light, rather than switching to auto and back. But, you also need to realize that the AF system (including the confirmation light) need a certain amount of light to function. So with the filter on, it may not be enough.
Thanks! But when I am on auto focus with the filter on, the camera keeps hunting forever, and it won't focus, which is why I thought Kelby's suggestion was the most practical and easiest method. However, when I am on manual focus with the filter on, the camera somehow finds a focus point and the green dot is illuminated. Also, the photo is not blurry at all. My question is how does the camera manually focus with the filter on while it keeps hunting on auto? And what exactly is my focus point sinve I do not tell the camera using the 11 points like I do when on auto before switching to manual? I am trying to understand why this method is less common compared to Kelby's and many websites' more cumbersome task.
 

Derrel

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Most AF systems need a certain minimum amount of light to focus; common smallest-sized apertures are f/5.6. On some of the very newest, higher-end Nikon AF modules, the minimum aperture value is f/8. If there is less light than f/5.6, or f/11, then the AF module will almost always be impaired, or even helpless at automatically focusing. Obviously, on slower lenses, like the common f/3.5~5.6 consumer lenses, anything that cuts the light coming in to a point that is lower than f/5.6 (or f/8, on only some cameras), and the AF system will not work reliably, if at all.

There is also a minimum light level for successful,dependable autofocus, usually stated in EV (Exposure Value); this varies with the price and year of the camera. Some of the newer, highest-end professional cameras like the Canon 5D-II or Nikon D4, can focus in light so,so dim that you can not read a magazine by that light...cheaper cameras, not quite as good. Sooo, when you slap on that 10-stop ND filter, well...there is NOT enough light for the AF system, and it can just go back and forth... "Dzzt---dzttt-dzzt-dzzttt-dzzt!" (focus hunting sound effects--all for free!)

The electronic rangefinder will usually show a green dot when a good AF lock is obtained. It's being powered by you. If it will work well for you, and YOUR camera and lenses and YOUR shooting situations with the ND filter on, as in bright light conditions for example, then I say, do it that way.

I myself usually do it the Scott Kelby way....focus and meter first, then screw the filter on, and shoot.
 

480sparky

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............I myself usually do it the Scott Kelby way....focus and meter first, then screw the filter on, and shoot.


That's how I roll. But since I've never read his book, it's not his method.... it's the 480sparky method. ;)
 

TCampbell

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Manually focus cameras used to have split-prism focusing aids that made it really obvious when you had correctly focused. Even with many DSLRs you can buy optional focusing screens which make it easier to manually focus (they have a textured finish which causes anything just slightly out of focus to appear even more strongly out of focus -- making out of focus areas stand out.) Not all cameras allow for optional focusing screens. However... the focus-confirmation feature that many cameras have allows you to manually focus the camera while watching for the confirmation.

I don't have a Nikon, so I'm not familiar with the green dot. Does this dot work whether or not you manually chose the focus point vs. allowing the camera to auto-select the point? My "guess" is that it would work in either auto-point selection or manually point selection mode, but one of the Nikon guys will need to confirm that.

Regardless of whether you use the "Scott Kelby" way vs. the way this other photographer told you, you STILL have to remove the 10 stop ND to compose/frame & focus the shot, then put it on to take the shot. He is ONLY suggesting that you don't have to flick the switch from auto focus to manual focus because the camera will tell you when it think you have locked focus even if you are manually focusing the lens.

On my Canon bodies, when I'm in manual focus, I get a beep _and_ the specific focus point that locked focus will be indicated (how it's indicated varies by which camera model I'm using but it's always obvious which point got the lock. I can disable the beep and still have the AF point flash if I want). So I don't just know that "some" point locked focus... I know "which" point locked focus (even in auto-point selection mode... although I generally always pick my focus points. (Canon also adds focus selection data to the image. Utilities which support it can show you which AF point was used when you took that particular shot.)

Auto-point selection has limited use since you're not usually interested in focusing on a random subject... you want focus on YOUR subject. The reason auto-point selection works most of the time is because it's generally tuned to lock focus on the "closest" subject and due to the way most people tend to frame up the "average" shot, there wont be anything nearer than the intended subject (obviously there are LOTS AND LOTS of exceptions to this. That is definitely NOT a rule -- just something which they implement because it's probably true for the majority of shooters in the majority of shots.)

If your camera will let you pick a specific AF point, and the green dot will appear when it gets a lock at that point, THEN you can use the green dot as your focus confirmation and not bother to switch between auto and manual. If you are using automatic AF point selection and the camera doesn't tell you WHICH point got the focus lock, then I would not use the mode (not unless everything in your frame is "roughly" the same distance and safely inside your depth of field.)
 

bratkinson

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Although I've never used/played/experimented with a ND filter, I suspect the same methods I've used in unusually low-light situations and taking shots of decorated Christmas trees in low ambient light would work.

I let the camera auto focus on whaterver focus point I've chosen (center, usually), I'll take one shot, switch to MF if I don't have an L mounted, and then take a couple of 'bracket shots' adjusting the focus a bit each way. Then I just pick the one I like the best in post.
 

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