shallow dof and focus points

86JR · 19 Oct 2010 at 20:32

Ok im trying to get to the bottom of my focus issue.

When i take a picture at say, F8 for my lens to be sharp, people tell me i should do it at F1.4 to bokeh out the background, but if i do this, and focus on the eyes of the model, the rest of the model will be blurred out.

But here in one of raymonds pictures we see that the background is blurred as it should be however the suit, and the camera and the strap are all sharp, even though the background on all heights of the picture are blurred.

On that picture we know the focus point is they eyes and that the aperture was F1.6.

On this picture, the aperture was F1.4 and yet the smaller object is blurred out, the focus point was on the middle of the air freshner.

Now I do not understand why my subject is blurred literally less than an inch away from the focus point, whereas raymonds isnt... almost a foot away from his focus point.

The setting for my camera is 51 point AF +3D however its set to the same setting as this picture :
http://www.kenrockwell.com/nikon/d300/images/d300-back-up.jpg

So it should focus on what I focus on using the red square in my viewfinder, and not the outer focus points.

I have read the manual and its not really helping the situation.

robertgilbert86 · 19 Oct 2010 at 20:35

The closer your subject is, and the longer the focal length, the shallower the DOF.

Raikiri · 19 Oct 2010 at 20:39

Because the DoF will change based on the distance to your subject and the focal length as well.

At 10 feet with a FF frame sensor, 50mm lens and f1.6 the DoF is 1.15 feet.

At 3 feet (like you appear to be), with a crop sensor, 50mm (75mm) at f1.6 it is 0.07 feet. 13.8 inches vs. 0.84 inches!

86JR · 19 Oct 2010 at 20:39

Ok i get that, but why would my focus be off ?

JKD · 19 Oct 2010 at 20:40

This link should help. Check out the calculator at the bottom to work out how far away from the subject you should be to get all their face in sharp focus at a certain aperture/distance/focal length.

http://www.bobatkins.com/photography/technical/dofcalc.html

robertgilbert86 · 19 Oct 2010 at 20:40

You're confusing focus with depth of field.

EDIT: Just google depth of god damn field and read the first few the pages entirely.

Raymond Lin · 19 Oct 2010 at 20:41

First time a picture of ME has been used as an example. lol

Think of a focal plane, like a sandwich board, standing 90 degrees to your lens.

smaller the number, thinner that board.

Raikiri · 19 Oct 2010 at 20:41

twoblacklines said:
Ok i get that, but why would my focus be off ?

Arrrgh, your focus isn't off. Only 1cm in front of your point of focus is actually within the DoF, the car freshener is at an amgle which means the edges are OOF.

86JR · 19 Oct 2010 at 20:43

So why is the right of the freshner in focus when the left isnt, yet the distance between left of it to focal point and right of it to focal point is the same ? surely both sides would be OOF or both sides would be in focus ?

Raikiri · 19 Oct 2010 at 20:44

The right edge is slightly out of focus.

86JR · 19 Oct 2010 at 20:48

Raikiri said:
Arrrgh, your focus isn't off. Only 1cm in front of your point of focus is actually within the DoF, the car freshener is at an amgle which means the edges are OOF.

If my focus isnt off, why are 4 people telling me it is in my "50mm prime shots" thread (which is why i have created this)

http://forums.overclockers.co.uk/showpost.php?p=17595976&postcount=7

http://forums.overclockers.co.uk/showpost.php?p=17596952&postcount=8

http://forums.overclockers.co.uk/showpost.php?p=17597267&postcount=9

http://forums.overclockers.co.uk/showpost.php?p=17598773&postcount=10

Everytime i post up im trying to learn, so when people say all my shots are OOF then people say "argh, they arent OOF" i cannot learn and hence cannot progress.

Raymond Lin · 19 Oct 2010 at 20:49

it is "slightly" off, the text should be sharp.

Raikiri · 19 Oct 2010 at 20:50

The focus is fine in that first one (centre point, by the looks of it), however some bits are out of focus due to the shallow DoF.

It may be due to the lens not being really sharp wide open. A little USM (74% @ 0.4) and it looks fine.

D.P. · 19 Oct 2010 at 20:52

It doesn't look like the air-freshner is perfectly horizontal to the focal plane.

And that raises a second question. The focal plane is not always a plane, but a manifold that bends in 3D like a bowl. The edges of the focal "plane" maybe further away than the centre. Normally the effect is minimal but some lenses at wide open apertures at minimal focusing distance will not be within the zone of focus.

Although I doubt you have that effect in your photo. You have to remember that 1mm makes a big difference when your DoF is so small.

Raymond Lin · 19 Oct 2010 at 20:55

I think the best and worst thing about this thread is that (at least to me) lol

1 - It is scary to see my own face when i click on the thread

2 - A 12 year old girl who has never held a SLR before took that photo after 10 seconds of holding it, and the only instruction I told her was that make the Red square land on my eye.

D.P. · 19 Oct 2010 at 20:56

twoblacklines said:
If my focus isnt off, why are 4 people telling me it is in my "50mm prime shots" thread (which is why i have created this)

http://forums.overclockers.co.uk/showpost.php?p=17595976&postcount=7

http://forums.overclockers.co.uk/showpost.php?p=17596952&postcount=8

http://forums.overclockers.co.uk/showpost.php?p=17597267&postcount=9

http://forums.overclockers.co.uk/showpost.php?p=17598773&postcount=10

Everytime i post up im trying to learn, so when people say all my shots are OOF then people say "argh, they arent OOF" i cannot learn and hence cannot progress.

It is a little confusing because in general when people talk about focus they mean some general function of percieved sharpness, focus point or distance and the depth of focus of he photo.

You could have the photo well focused but since the depth of focus is too shallow for the scene then you have a soft image and some people might clal that a focus problem although that is incorrect terminology.

Raymond Lin · 19 Oct 2010 at 20:56

D.P. said:
It doesn't look like the air-freshner is perfectly horizontal to the focal plane.

And that raises a second question. The focal plane is not always a plane, but a manifold that bends in 3D like a bowl. The edges of the focal "plane" maybe further away than the centre. Normally the effect is minimal but some lenses at wide open apertures at minimal focusing distance will not be within the zone of focus.

Although I doubt you have that effect in your photo. You have to remember that 1mm makes a big difference when your DoF is so small.

Better than i could explain, i am so tired at the mo lol

86JR · 19 Oct 2010 at 21:10

Im sorry for being an annoying **** with this thread but if i dont get through this ill have a lifetime of taking soft pictures even when i finally figure out composition

I wish you had the red square for focusing and could simply press buttons to make the square larger in the viewfinder, then just put everything you want sharp and in focus inside that square/rectangle and everything outside it would be nice and blurred.

Raymond Lin · 19 Oct 2010 at 21:12

It is quite simple - Make the red square land on what you want in focus.

(without going into ISO, shutter speed, camera shake etc)

mrthingyx · 19 Oct 2010 at 21:26

Raymond Lin said:
It is quite simple - Make the red square land on what you want in focus.

(without going into ISO, shutter speed, camera shake etc)

Also, there is no legal requirement to shoot everything at F1.6 or whatever. You could hive a smaller aperture (F2.8 possibly) and still have a nice diffuse background...