Some observations on nikkor lense A/M focus

[vedran]

So, here's the deal...i wanted to make some tests to see how A/M focus function on nikkor 18-105 mm lense behaves. I have 4 images (for sakes of comparison, not made to be pretty) . You are free to critique my assumptions and help me figure this one out.

Images 1. and 2. are 18mm f/3.5, 3. and 4. are 50mm f/5 ( i know...i would use f/1.4 if i had one ).

1.) First image was shot in Auto focus mode, with red focus dot aligned like on the picture. The first thing i notice...the camera won't let me shoot in A mode with little dot on the dark area. (???)
When i move the focus to the green dot it works, but nothing else did change in the picture...it's still exposed the way it was.

2.) This is the second image, made in manual focus mode. Not much difference, except I've noticed the images differ in exposure, could this happen and why ?

Ok...now something really frustrating for me. I also noticed having an object out of focus in A and then swithching to M, the object stays out of focus no matter what i do, like in the image below. Also is there any way to lock the focus, so that for example when i use the half-pressed button to focus an object, and then release it, pressing the button again won't start the af motor again and for some reason go out of focus (frustrating!).

3.) This is the image where all 3 objects are out of focus in Auto mode (on purpose), and switched to Manual just to prove you can't focus what should usually be in focus.

4.) The final image is made as the image before, except one object is focused in A, and it stays that way when switching to M.

Oh and i've emptied the bottle of rum before, for the sake of experiment

 

[qleak]

2) what's creating the light outside the window? Unless you have absolute control of it, that could explain exposure difference.

If you're additionally in aperture priority mode, unless you do an auto exposure lock expect changes.

 

[vedran]

2) what's creating the light outside the window? Unless you have absolute control of it, that could explain exposure difference.

If you're additionally in aperture priority mode, unless you do an auto exposure lock expect changes.

It's just a lamp behind a curtain, to soften the light. The camera was in Manual mode. How do i do an auto exposure lock?

 

[Dave442]

The Nikon metering can take the focus point and give that some priority in setting the exposure. In Manual exposure also see that you are not in Auto ISO. The dark spot used as the focus point in the first shot is probably too dark for the camera to achieve focus and you are probably set to focus priority for the shutter release in AF-S (in AF-A mode the camera will see this everything is still in the image and automatically select the AF-S mode).
To keep from refocusing can use the AE-L/AF-L button (configured for AF-L), or set the button as your focus button and have the shutter button only act as the shutter button (rear button focusing).

 

[qleak]

It also sounds like you may want to try something like:

back button focus

This is a technique to separate your shutter controls from your autofocus Controls. Google it to see if it's possible and how to set it up on your camera.

 

[vedran]

Ok looks like the AE-L/AF-L button on nikon d5100 is already set for exposure and focus hold so that solves it...it's very helpful.

 

[wfooshee]

But..... by default it does both, and if you slip off of the button or let go without thinking, then you have to start over, as the shutter press will again meter and focus from scratch.

qleak's suggestion about back button focusing is something you should try. It removes auto-focus from the shutter press. Set the AE-L/AF-L button to the last option, AF-ON. In that mode, the shutter button meters but does not focus. You press the AE-L/AF-L button to focus, and no other focus changes happen after you release the button.

Now the problem is that you don't have an AE-lock button if you need one. If you still want an exposure lock button, you'll need to set that on the Fn button. Now you have the functions on separate buttons, and the shutter button won't restart your focus.

 

[bc_steve]

It also sounds like you may want to try something like:

back button focus

This is a technique to separate your shutter controls from your autofocus Controls. Google it to see if it's possible and how to set it up on your camera.

Highly recommended