Does changing your aperture value affect what you've previously focused on?

ie, when using manual focus, if I have set my focus point but then dial my aperture value up or down, do I need to refocus again? I assume that part of the lens actually moves when the aperture value is changed. Does that affect what has been focused on?

The points are not influenced in aperture dialing

paul_b said:

ie, when using manual focus, if I have set my focus point but then dial my aperture value up or down, do I need to refocus again? I assume that part of the lens actually moves when the aperture value is changed. Does that affect what has been focused on?

Click to expand...

No, the aperture does not affect focus. In fact, the DSLR lens is wide open at maximum aperture for the viewfinder, and for focusing, and for all of the time until the shutter button is actually fully pressed. Everything is brighter that way, viewfinder, and easier focusing. Then, only at shutter time, the lens actually stops down to your selected f/stop value for the exposure, and then it opens to maximum again. As you rotate your aperture wheel for different f/stops, you are just changing this wheel, not the lens... not until next time you press the shutter button, when then it stops down to the selected aperture.

You can watch into the front of the lens to see this as you click the shutter (a slower shutter speed will help to see it). It is very normal, just the way it works.

In general changing aperture does not change focus, that is how lens are designed. But I read that in some cases there is a slight focus shift with change in aperture value. This is mostly evident with fast lenses where with a narrow DOF it would be discernible.

https://photographylife.com/what-is-focus-shift
https://www.lensrentals.com/blog/2010/10/how-to-shoot-with-wide-aperture-lenses/
photo technique magazine » The Secret Life of Lenses

Great. Thanks for the replies

Maybe i should of been more accurate with my description, so here it is:

Recently I photographed my child's running event for the 1st time. I pre focused on the finish line using manual focus (using a standard kit lens 18-55mm f3.5 to 5.6 and a tripod). I chose to use aperture mode (f8), set the ISO to 400, which allowed the camera to adjust the shutter speed for each shot. However, as expected the shutter speed fluctuated as the available light changed and some of my shots had shutter speeds that were a bit too low/slow and thus a few of the runners were a bit blurred (a bit embarrassing).

So next time I’m thinking about using shutter speed mode instead and setting that to the ideal speed that I need to capture the runners, which allows the camera to automatically adjust the aperture and ISO for each shot. Which brings me to my question:

Using shutter speed mode, if I’ve pre-focused on the finish line using manual focus (and it will stay there), and the camera starts automatically adjusting aperture values for each shot, does a changing aperture value affect what has been pre-focused on.

Many thanks.

Depends. When you focused were you in a Live View situation where you had DoF preview? If so then any aperture bigger (i.e. smaller Dof) than the one used to set focus will result in something that used to be in focus no longer being in focus.

If you are focussing on a running child why would you be using manual focus? It seems to me it would be difficult to time the shot just as the finish line is crossed.

Ive set the camera up on a tripod to face the finish line straight on, on the right side of the image, ie the runners move from the left side of the image to the finish line on the right side of the image (they are funneled). Once focus is set then every runner should be in focus no matter where they are in the shot. The line they follow is parallel to the face of the camera.

Ive tried AF tracking with continuous auto focus, but each runner is in the shot for no longer than a second. I find it too hard to do.

Ps Nikon D3100, and Samsung nx300m, both with kit lens, 18-55mm

Here's a sample shot:

A few things here. First, focus point seems to be beyond the cones, which would make anyone between them OOF. Second, while you might think 1/500s would be fast enough to freeze movement, looking at the foot and hand of the child you can see it definitely is not. I would recommend shooting in Manual Mode at 1/1000 and f8, set your ISO accordingly if the D3100 doesn't support Auto ISO in manual mode (not sure if it does) and then stand someone in the spot the kids actually run to set the focus.

I did actually pre-focus on the floor in between the cones, but the D3100 (to the best of my knowledge) doesn't have manual focus assist where it temporary magnifies the image to help you focus (which is probably one of the reasons i'm starting to like compact system cameras more and more).

I actually wish i was better at using auto focus on moving subjects, but i just can't seem to get it to work well for me.

If speed and blur are you primary concerns, then use shutter priority. Just make sure your ISO is high enough to allow an aperture with enough depth of field since the aperture is the value that will get changed when metering.

Just check if Auto ISO is applicable when you set the camera in Aperture Priority and select the minimum ans maximum shutter speeds.

In D3300 you
. Shooting Menu -> ISO Sensitivity Control -> Auto ISO sensitivity ON, then select Maximum sensitivity (ISO), Minimum Shutter speed

The camera
. set the Mode in "A"
. set ISO = 100
. set Aperture to F/8 if I want a lot of DOF, else F/4-F/5.6 for normal DOF

In auto ISO
. Maximum ISO = 1600
. Minimum shutter speed 1/500

The camera will then
. As light decreases : Increase the ISO till maximum is reached, then reduse the aperture and then reduse the shutter speed
. As light increases : Decrease the ISO till the ISO set is reacheed, then increase the shutter speed if minimum ISO is reached, increase the shutter speed once maximum shutter speed is reached

aroy said:

Just check if Auto ISO is applicable when you set the camera in Aperture Priority and select the minimum ans maximum shutter speeds.

In D3300 you
. Shooting Menu -> ISO Sensitivity Control -> Auto ISO sensitivity ON, then select Maximum sensitivity (ISO), Minimum Shutter speed

The camera
. set the Mode in "A"
. set ISO = 100
. set Aperture to F/8 if I want a lot of DOF, else F/4-F/5.6 for normal DOF

In auto ISO
. Maximum ISO = 1600
. Minimum shutter speed 1/500

The camera will then
. As light decreases : Increase the ISO till maximum is reached, then reduse the aperture and then reduse the shutter speed
. As light increases : Decrease the ISO till the ISO set is reacheed, then increase the shutter speed if minimum ISO is reached, increase the shutter speed once maximum shutter speed is reached

Click to expand...

I would never of thought of that. Great advice. Thank you