Can someone simplify depth of field for me?

DoF is slowly driving me mad. I had the opportunity to take some photos with some models the other day. There were other photographers

on site so things were not entirely controlled. I was not very happy with the pictures I took. There seems to be something in common in

practically all portraits I take. A slight softness/lack of sharpness (if at all interested check my flickr:

http://www.flickr.com/photos/sanyflame/sets/72157607399371749/

 

I do know, however, that this has to do with how I focus (I'm practicing on that) and the depth of field. This is where I struggle. I understand

DoF and how it works. I know, for example that in my 50mm f1.8 lens if I shot at its widest aperture, 1 meter away I will have less than one

inch of DoF. My problem is all the mathematics required if I shot with different focal lengths at different distances. I'm lousy at measuring

distances or doing calculations in my head. Is there a simplified way I can approach DoF? Or should I just brush up on my math skills?

Many thanks for your patience and help, once more!

<a href="http://www.dofmaster.com/dofjs.html">DOF Master</a> or <a href="http://www.bobatkins.com/photography/technical/dofcalc.html">Bob Atkins</a>.<P>As an engineer, though, I would say brush up on your math.

Hi,

 

maybe it's just be me, but I can not really tell what you mean going by the small size of the photos posted on a flickr site and I don't mean the thumbnails either, the photos are just too small.

 

You said; "I do know, however, that this has to do with how I focus (I'm practicing on that)"

 

Does your viewfinder (eye piece) custom focus to match your own vision? That helps each individual see with their own vision what they are trying to focus the camera on better? A friend of mine who doesn't even wear glasses kept getting all softer looking, not so sharp photos and once they focused the view finder their camera focusing problem was solved and her photos were much sharper.

 

Wish I had more than that to offer.

Hi Susana

 

It's no surprise that you can not calculate the DOF in your head - the formula is far too complicated for a

normal human mind to calculate on the spot. There are DOF calculators for pocket computers and such, but even

these are not very easy to use.

 

There are DOF tables that you can print out, but these are also approximate - DOF is also relative to the

conditions you are looking at a picture (for instance print size, or magnification on the screen). It is also not

right to evaluate DOF looking at pictures zoomed at 100% on a computer, looking at paper prints is likely to give

a better evaluation of sharpness and depth of field.

 

In the end, the secret is to learn a bit by experimenting, and also trust what you see in the viewfinder and use

the lens step-down feature on your camera (also known as DOF preview on some cameras).

One of the beauties of older cameras and lenses is the DOF markings on the lens. If you focus on a particular subject and read the distance marked on the lens barrel, you can also see the range of focus at a given f/stop.

 

That's what I miss the most on my DSLR.

here is a link that might help explain things http://www.ephotozine.com/article/Depth-of-field-explained

In general, you'll learn to feel what you reed... f/2 is shallow almost all of the time... f 2.8 is usually passable for a single person if you want most of the face in focus, and if you want to be safe, f/4 helps... When shooting more than one person, f/4 is sometimes okay, but f/5.6 and f/8 are more likely to be what you want.

 

DOF varies more with distance to the lens, but you'll usually shoot people in the same ways... if you are taking macro shots of someone's eye all bets are off, but in general those numbers work.

 

I personally like shooting people at f/1.4, but you have to understand that you'll have one eye out of focus most of the time unless you really control the position...

 

hope that helps.

Depending on your camera, you may have a DOF preview function, which will help, though it's not very accurate

given the small scale through the viewfinder.

 

You can also get DOF calculators, even for smartphones if you use one of those. And there are printed tables, of

course.

 

But what exactly were you unhappy with? I looked at the album and they look pretty good to me. There's enough

DOF to get the models' face in focus while still blurring the background to keep it from distracting. I might

have wanted less DOF in a couple of shots, but that can easily be a preference item. Please expound. :-)

Here's my way of thinking of it. If you had ideal film or a sensor with an infinite number of pixels, and a perfect lens perfectly focused on a perfect point of light, the image would still be slightly blurred. The reason goes to optics with which I have worked for two decade. Imagine the ideal picture from the side view: The light comes from the lens and heads down to the film/sensor, sort of a triangle with the apex at the film/sensor. As it passes where the film/sensor lies (should you remove it) the light leaves the apex and forms an exiting triangle.

 

Unfortunately, the physics of light and imaging says otherwise. Yeah, you can worry about "learning all the math" but try reading a book such as Goodman's classic "Fourier Optics" and your head will spin. The fact is there is a fundamental "blur" size related to the light wavelength and aperture size and also the F number. And in the easiest, most simplistic manner (and I mean this as no put down, just taught for over a decade!), the apex of the triangle is blurred. At the longer F number, think of the triangle as much narrower, like your lens is "smaller". The blur begins (and ends) farther from the "ideal" apex and that's your DOF.

 

I repeat, I mean no put down of non scientists (and you may be one from another area), but this is the easiest way I can rationalize DOF. Don't waste you money on Goodman's book. With a PhD in mathematics, I have muddled thru it, but I don't need it to take photos!

 

DOF can give you great effects. I like my 21mm Leica lens at 1/250 and F/16 because nearly everything from the first several feet to infinity is in focus. But shooting at F/2 (a short DOF) I can have a bush in the near field perfectly in focus and the background blurred. Experiment with a single scene, varying the F number (and the exposure if a non-auto camera like my Leica M6) and study the results. Then throw away your math books and have fun!

On most cameras the lens aperture is wide open until you press the shutter; then it closes to the selected aperture opening. An old trick was to release the lens and turn it partially off. This released the aperture link and caused the aperture to close to the selected setting thus giving you depth of field preview. I don't know if that will work with your camera but give it a try.

Thanks so much to everyone for your explanations and links, they're much appreciated. Regarding my pictures I'm not entirely unhappy

with them but after asking for C&C for several of them the bulk of feedback was that images were too soft and not sharp and that led me to

wanting to try and master/ perfect the DoF. And I think that's why feedback on your images is good, it makes you want to perfect the

issues, or at least it does to me. Again, thanks for all your help!

Sure, no problem !! Here's the quick and dirty approach.....

Bracket the f-stop and shutter speed settings.

Take four or five bracketed shots, study 'em and see what's what.

With the price of film (insert chuckle here) for DSLR's, the learning curve goes ballistic!!

 

Bill P.

>>In general, you'll learn to feel what you reed... f/2 is shallow almost all of the time... f 2.8 is usually

passable for a single person if you want most of the face in focus, and if you want to be safe, f/4 helps...

When shooting more than one person, f/4 is sometimes okay, but f/5.6 and f/8 are more likely to be

what you want.

 

I think this is very good advice. (Even if you disagree on the numbers, you'll make up similar rules for

yourself based on your own preferences.)

 

There's lots of maths quoted about DOF but from a picture-lover's point of view it's not very helpful,

because there's no sharp line (no pun intended) between 'in focus' and 'out of focus', so calculating a

DOF to the nearest fraction of an inch gives a false sense of security. Instead perceived DOF depends

on a mixture of things including how large you're displaying the picture, from distance it's going to

viewed, whether that be on-screen or in print, and what your expectations of focus are.

 

There are time-honoured conventions about what's considered 'in-focus' (largely based on old film and

print characteristics) and they are the ones used to draw the lines on old lenses.

 

What will be more useful to you is to understand in a relative way how the DOF increases as you stop

down a lens, as Robert points out, so that when you have a group of 4 people and they all need to be

'sharp' then you have a good feel for why you start at f5.6 (for example).

Don't worry about the math. Not necessary to understand DOF. Heres a very simple test:

 

1. Camera on a tripod.

2. Subject on a table. No flash just natural light

3. Focus on the subject only. Disregard all else.

4. Make sure you get some forground and background in the photo. Don't make the crop too tight.

5. Start shooting with the widest aperture. Write each frame # and corresponding fstop down.

6. Progressively take a shot with all apertures one step at a time.

7. View the resusts with your notes.

 

Now look at the focus of the forground and the back ground. If you did the test correctly you'll see the foreground and

background coming into focus as you closed down the aperture.

 

The definition of DOF is the amount of area in front of and behind your subject thats in focus. Basic.

 

If you did the test correctly you'll clearly see these results.

 

Have fun.

 

John

There's no real way to measure the Dof by doing mental math calculations. At best, you can rely on the dof scale indicator which is on some lenses (usually 35mm slrs)

I sow your portraits. Some of its are really good. I understand that you work with some other photographers. Some of your

crop don't look best choice. But your question is about "depth of field", which means controlled sharp zone between two different characters or objects. (It makes with 3 stop shoot, and I don't know if that is your problem there)... or that it is what

are you asking for?

Look, it's pretty simple when you understand it. Depth of the field means when you take a picture how much depth is in the focusedfield. So, look at macro pictures, that are done with a lowfocal apperture, sometimes 2 or even 1. If the number in your focal aperture is minor, the things that will be focused will be less. This can be also seen in sports pictures, like car racing oreven soccer. The public is also unfocused so you can't watch details on the people.

Thanks again for all your help, I think I just need to keep on doing what I'm already doing. Experimenting!

 

Noel: thanks for your explanation but I do know what DoF is and how it's used. I justed needed a simplification on the maths as my original post explains!