Focus questions

mylanternhill

Hi! I'm trying to figure out how to achieve two focus settings on my D3100 that I had on my film SLR camera that I really miss.

The first one is a setting where the entire frame is in focus. I understand setting depth of field using various f-stops but that doesn't always help. For example, when I'm in a situation where the lighting is low and I can't use a flash, but I want an entire group of people in focus, I switch to Auto-area AF. Sometimes, only one or two of the focus points will lock and not everyone's face is in focus. Is there a way to make all 11 points lock so the entire frame is in focus?

Secondly, I also miss the "infinite" focus option I had on my old SLR. I was trying to take photos of lightening a few weeks ago. Since it was too dark for AF, I switched to manual. I tried turning the focus ring all the way to the right so everything in the distance would be in focus. They looked good on my screen, but on the computer they were blurry. How do I manually focus the camera in such a situation?

Thank you for any help and feedback!

bruto

Getting the entire frame in focus will depend on depth of field and what point you choose for your center of focus. It also depends greatly on distance from the camera and on lens focal length.

Depth of field increases with a smaller F stop. It increases with a shorter focal length, and it increases with distance from the camera.

Distance and focal length do not entirely cancel each other out. Even if you keep framing the same by varying distance, the shorter focal length will give more depth of field. Getting too wide will give you distortion, but if you can, go a little shorter. Remember too that you can crop a little. If you can get better DOF by going both wider and a bit further, it may pay to throw away some pixels.

You may recall getting more in focus in film, but some of that will be due to the limited acuity of the film, and also perhaps to a 35mm format having less diffraction loss at very small apertures.

You will be compromising some between depth of field and diffraction here. The DX format will begin getting a bit less sharp at about f/11, but depth of field will improve beyond that, so for group portraits where super pixel peeping is not required, I'd go down to f/16 or even lower. In the dark where you do not use flash, you'll need to ramp up the ISO and this also will lose some acuity, so diffraction loss may be a non issue anyway.

As a general rule, depth of field is greater behind than in front of your focus point, and a useful vague rule of thumb for medium distance groups is to divide your desired depth of field into thirds, and put your focus point so that about 1/3 is on this side, and about 2/3 behind it. If possible, it may be easier to get the right placement with a single focus point than with auto area. If you set the servo to S you can pick a focal point and recompose. Or if you get brave, try back button focus, which makes recomposing a cinch.

Unfortunately, there is no convenient DOF preview on these cameras, but if you first set your aperture, then switch to Live View, the image on the screen will be shown with the aperture you set.

Most of the lenses we see nowadays actually focus past infinity, to allow for zoom variation and thermal expansion. So your best bet for infinite focus is to find something very far away and focus manually on it. You might do better by switching to Live View briefly, where you can magnify the image a little.

If you can't see anything in the viewfinder to focus on, one other option, rather primitive, is to take the same picture at different focus points. Start at full infinity, pull it in a little and shoot again, and do this a few times. Save the sharpest one. Remember, though, that the AF lenses have very short focus travel. A little goes a long way.