Focus, aperture and depth of field

[KreGg]

Hey, so I was reading Understanding Exposure and he says sometimes something like "i set the aperture to f/22 and then focus did not matter". Not exactly those words but this is what I got from reading what he said sometimes.

The smallest the aperture, say f/22 or f/32, for even more depth of field, the more the background will also be in focus. Right?
So if I put on the smallest aperture and focused on something in front of the subject, shouldn't it render everything in focus? From the focused foreground to the subject all the way to the furthest background?

Anyway, I picked up my camera and did some tests, with my aperture set at f/22 and f/32.

My subject was about 2 meters in front of the camera and the only way to get it in focus was to set the focus precisely on it. If I manually focused in front of it, or behind it, the subject would be out of focus, even though I had my aperture as small as possible...

What's the matter here?

 

[SCraig]

Did you take photographs with it or just look through it? If you just looked through it then the aperture was set to the widest opening unless you used a depth of field preview control on your camera.

 

[KreGg]

I did take the pictures and looked at them on my LCD screen in the back of the camera.

 

[Trever1t]

You don't say what lens or how zoomed. Widest view 18mm or less, closest subject possible, smallest Fstop and you'll have everything behind it in relative focus. This is for and effect and not to be done for everything. Re-read or watch what he does, again.

 

[Kerbouchard]

Yep, what SCraig said. Your lens doesn't actually stop down the aperture until the shot is taken. This is to let as much light in as possible for focusing. You can use the DOF preview button to actually see the DOF or if your camera lacks the DOF preview option, you can just take the shot and review it.

As far as everything being in focus, that depends on more than just your aperture. It has to do with your distance to your subject and the focal length of the lens you are using as well.

Here is a link to a DOF calculator so you can know exactly what you should expect.

Online Depth of Field Calculator


Edited to add: just saw your reply. Plug in your numbers to the calcuator linked above. If it doesn't match your results, you might be doing something wrong or your lens may be malfunctioning.

 

[480sparky]

Download this DOF calculator.

 

[Ron Evers]

Gota love old glass - the DOF scale is right on the lens.

 

[tirediron]

Did you take photographs with it or just look through it? If you just looked through it then the aperture was set to the widest opening unless you used a depth of field preview control on your camera.

^^ This.

 

[Rephargotohp]

No matter what your DOF or aperture is, There is only ONE point of focus and then everything else is just within the field of acceptable focus. a Deep depth of field is NO excuse for not focusing on your subject

Plus shooting at f22 will give a soft image due to diffraction

You shouldn't shoot much above f/11 on a crop camera or f/16 on full frame

If you truly want to know about how to obtain the deepest depth of field learn about Hyperfocal distance instead of the f/22 nonsense you read

 

[KreGg]

You don't say what lens or how zoomed. Widest view 18mm or less, closest subject possible, smallest Fstop and you'll have everything behind it in relative focus. This is for and effect and not to be done for everything. Re-read or watch what he does, again.

I think that's it.
I actually took my previous test pictures on the 75mm end of my 28-75mm lens.
So I guess the 75mm focal length killed the depth of field here, since the subject was close.

Anyway, I did the test again, now using the 28mm end, and mostly everything was in good focus.

Yep, not something that I wanna be doing all the time, just some new knowledge, and something that struck me when I was reading it.

Thanks all

 

[Trever1t]

The wider the better for this effect. He demonstrates it in a field of sunflowers if I recall correctly. Obviously using F22 or more is not going to give you the sharpest image

 

[KreGg]

Hmm I had no idea of this diffraction thing.
I did some small sharpness test of my Tamron 28-75 a while back and found out that till f/28 it was pretty sharp. At f/32 the sharpness went down a bit though.

 

[Rephargotohp]

Not focusing on the subject and making them the Point of focus (the sharpest point) would do more harm than f/22, But I still don't use f/22 unless I have no choice or am Trying to make stars out of point source lights

 

[ClearBlueDaze]

Yep, what SCraig said. Your lens doesn't actually stop down the aperture until the shot is taken. This is to let as much light in as possible for focusing. You can use the DOF preview button to actually see the DOF or if your camera lacks the DOF preview option, you can just take the shot and review it.

As far as everything being in focus, that depends on more than just your aperture. It has to do with your distance to your subject and the focal length of the lens you are using as well.

Here is a link to a DOF calculator so you can know exactly what you should expect.

Online Depth of Field Calculator


Edited to add: just saw your reply. Plug in your numbers to the calcuator linked above. If it doesn't match your results, you might be doing something wrong or your lens may be malfunctioning.

Thank you sooooo much!!!