Apature Priority and Lens Selection - newbie question

crotonmark · Nov 10, 2012

Hi All

Brand new T4i owner (1st DSLR) and I am trying to learn digital photography by reading Peterson's book on exposure.
One thing that has me confused is if I should use a zoom lens or a fixed lens when trying to learn depth of field.
Is it better to use a wide angle lens or a telephoto lens?

Thanks - my next question is why my attempts at DOF don't match what Peterson says should happen
(but I guess that will be for another post)

TIA
Mark

Ysarex · Nov 10, 2012

Congrats on the camera -- enjoy.

It doesn't matter whether the lens you use is a zoom or not. DOF is the same either way. As for Perterson's book, you'll want to double check some other sources particularly as regards DOF as Peterson's book contains obvious errors concerning this subject (at least it did the last time I checked) -- he clearly doesn't understand DOF and the book is wrong. It's wrong about quite a few other things as well so read carefully.

DOF is basically a function of f/stop and magnification with magnification being the dominant factor. Lens focal length plays a critical role to the extent that it helps determine magnification. With a wide angle lens magnification is reduced and DOF then increases. With a telephoto lens magnification is increased and DOF then decreases. The rate of change in DOF relative to f/stop changes is approx. 50% per stop. The rate of change in DOF for changes in magnification conforms to the law of inverse square: alter magnification by a factor or 2 and DOF will change by a factor of 4.

A little simplistic but accurate enough in this context:
Magnification is a function of focal length and subject distance.
DOF is a function of f/stop and magnification.

Joe

Tuffythepug · Nov 11, 2012

Ysarex said:
Congrats on the camera -- enjoy.

It doesn't matter whether the lens you use is a zoom or not. DOF is the same either way. As for Perterson's book, you'll want to double check some other sources particularly as regards DOF as Peterson's book contains obvious errors concerning this subject (at least it did the last time I checked) -- he clearly doesn't understand DOF and the book is wrong. It's wrong about quite a few other things as well so read carefully.

DOF is basically a function of f/stop and magnification with magnification being the dominant factor. Lens focal length plays a critical role to the extent that it helps determine magnification. With a wide angle lens magnification is reduced and DOF then increases. With a telephoto lens magnification is increased and DOF then decreases. The rate of change in DOF relative to f/stop changes is approx. 50% per stop. The rate of change in DOF for changes in magnification conforms to the law of inverse square: alter magnification by a factor or 2 and DOF will change by a factor of 4.

A little simplistic but accurate enough in this context:
Magnification is a function of focal length and subject distance.
DOF is a function of f/stop and magnification.

Joe

Well that all sounds right to me. Maybe a little complicated for a novice or beginner but the simplistic final summation is plain enough. DOF is a complicated thing to describe but fairly easy to achieve if you understand the basics. I would also add to the above tutorial by saying that distance from camera to the closest focused point is a factor as well. Wide angle lenses and telephoto lenses exploit depth of field in opposite ways. Wide angle with a small aperture is great for landscape shots where you may want everything in the frame to be in focus; from the tree in the foreground, to a person in the middle ground to the mountains in the background; everything can be in sharp focus if you choose. In the same scenario with a telephoto lens using a larger aperture you could zoom in on just the people and make the mountains be pleasantly out of focus to make the person stand out.
You can experience all of these concepts easily with your dslr. Try the different combinations of all the factors and see the results right away.
As far as zoom vs fixed the dof would be the same at any given focal length and aperture setting. But if you are tryng to choose between wide angle and telephoto you might benefit from a zoom lens that covers both ends. My 28mm-200mm zoom works for the majority of my shots

TCampbell · Nov 11, 2012

Experience is a great teacher. Give yourself some exercises in this. Grab a bunch of cans, bottles, or whatever you've got. Line them up somewhere with a about 1' between each can or bottle. Go stand at one extreme end of the row. Carefully focus the camera on a can or bottle about half-way down the row. Use the widest f-stop your lens can manage and take the shot. Change to a much higher f-stop and shoot again. If you've got a zoom lens, do this at your widest zoom and again at your narrowest zoom. You can also keep the zoom the same, keep the f-stop the same, but double your distance from the subject (walk farther back to take the same shot) and notice what that does to DoF as well.

You should notice the following trends:

1) Whatever the DoF is (let's say the DoF turns out to be about 6' thick) about 1/3rd of that distance will be in FRONT of your focused-subject (nearer to the camera) and the remaining 2/3rds will be BEHIND the focused subject.

2) Wider angles have more DoF (I have a 14mm f/2.8 lens. I can pretty much focus this thing manually to about 3' and shoot without focusing and nearly everything will be in-focus because at 14mm the DoF is huge.)

3) Low focal ratios (e.g. f/2, f/2.8, f/4) have much shallower DoF than the higher focal ratios... the higher the f-stop, the broader the DoF. That means f/16, f/22, etc. are great for landscapes because everything will tend to be acceptably focused.

4) DoF is also affected by your focused distance to the subject. The closer you are, the shallower the DoF. If you use a macro lens to shoot close-up shots, the DoF at low focal ratios is paper-thin. I have a photo of a penny somewhere that I use to show that at a very low f-stop, the surface of the penny is focused... the desk surface that the penny is resting on is already out of focus (in merely the thickness of a penny.)

When we're talking about depth-of-field, we're not talking about 'tack sharp' focus... we're talking about 'acceptable focus'.

crotonmark · Nov 11, 2012

TCampbell said:
Experience is a great teacher. Give yourself some exercises in this. Grab a bunch of cans, bottles, or whatever you've got. Line them up somewhere with a about 1' between each can or bottle. Go stand at one extreme end of the row. Carefully focus the camera on a can or bottle about half-way down the row. Use the widest f-stop your lens can manage and take the shot. Change to a much higher f-stop and shoot again. If you've got a zoom lens, do this at your widest zoom and again at your narrowest zoom. You can also keep the zoom the same, keep the f-stop the same, but double your distance from the subject (walk farther back to take the same shot) and notice what that does to DoF as well.

You should notice the following trends:

1) Whatever the DoF is (let's say the DoF turns out to be about 6' thick) about 1/3rd of that distance will be in FRONT of your focused-subject (nearer to the camera) and the remaining 2/3rds will be BEHIND the focused subject.

2) Wider angles have more DoF (I have a 14mm f/2.8 lens. I can pretty much focus this thing manually to about 3' and shoot without focusing and nearly everything will be in-focus because at 14mm the DoF is huge.)

3) Low focal ratios (e.g. f/2, f/2.8, f/4) have much shallower DoF than the higher focal ratios... the higher the f-stop, the broader the DoF. That means f/16, f/22, etc. are great for landscapes because everything will tend to be acceptably focused.

4) DoF is also affected by your focused distance to the subject. The closer you are, the shallower the DoF. If you use a macro lens to shoot close-up shots, the DoF at low focal ratios is paper-thin. I have a photo of a penny somewhere that I use to show that at a very low f-stop, the surface of the penny is focused... the desk surface that the penny is resting on is already out of focus (in merely the thickness of a penny.)

When we're talking about depth-of-field, we're not talking about 'tack sharp' focus... we're talking about 'acceptable focus'.

Thanks so much all. I really appreciate it. My initial tries have failed but the above exercises sound great.

crotonmark · Nov 11, 2012

Also I own a 50mm 1.8 lens and a 35-105 3.5/4.5 lens

Any suggestions if these are acceptable to work with?

Ysarex · Nov 11, 2012

TCampbell said:
1) Whatever the DoF is (let's say the DoF turns out to be about 6' thick) about 1/3rd of that distance will be in FRONT of your focused-subject (nearer to the camera) and the remaining 2/3rds will be BEHIND the focused subject.

This is incorrect, but commonly repeated. Petersen makes the same mistake in his book. DOF does distribute unevenly around the focus plane but it does not tend to a 33/66% break. It's a seamless progression from 49% front, 51% back all the way to 1% front, and infinity back. 33% front, 66% back will occur along the way.

On a 35mm camera:

35mm lens focused at 10 feet f/11, DOF is 7% front 94% back.
85mm lens focused at 6 feet f/2.8, DOF is 48% front 52% back.
28mm lens focused at 20 feet f/8, DOF is 1% front ? -- in this case DOF reaches infinity in the back.

Joe