# What is the depth of focus for a given F stop?

Discussion in 'Digital Photography' started by n, Dec 1, 2003.

1. ### nGuest

Most of my shots are taken at about 3 meters. My effective focal
lenght is 100-300 on my usual lens.

Roughly speaking, what depth of field will be in focus at f2.8, 5.6
and so on?

How do I do the calculation in advance of seeing the subject through
the view finder?

Thanks

n, Dec 1, 2003

2. ### RobertwgrossGuest

It is for reasons like this that many cameras have a Depth of Field Preview
button.

---Bob Gross---

Robertwgross, Dec 1, 2003

3. ### David J. LittleboyGuest

There are lots of DOF calculators out there. One that I like is:

http://www.nikonians.org/html/resources/guides/dof/hyperfocal2.html

Note that it doesn't do digital cameras.

If you are using a 1.5/1.6x dSLR, use the 35mm-equivalent focal length and
the next smaller f-stop. So to figure out the DOF at f/2.8, use f/4.

If you are using a 2/3" prosumer camera (A1, F717, 5700), use the
35mm-equivalent focal length and the f-stop four f-stops smaller. So to
figure out the DOF at f/4, use f/16.

If you are using a consumer P&S camera (S85, C-5050, G5), use the
35mm-equivalent focal length and the f-stop four f-stops smaller. So to
figure out the DOF at f/4, use f/22.

These are all approximations, but quite reasonable ones. For example, with
the Minolta A1 at 10 ft, f/4.0 at 150mm equiv. focal length, your DOF will
be +/- 0.5 feet, and your background will be nicely out of focus for
portraits, even if dSLR snots such as myself tell you otherwise.

David J. Littleboy
Tokyo, Japan

David J. Littleboy, Dec 1, 2003
4. ### Robert E. WilliamsGuest

The answer is somewhat complex e.g.,
The Equation for depth of field is:
DOF = 2NC(D)^2 / F ^2 where:
N = f-number
C = circle of confusion (about 0.03 mm)
D = distance to the subject
F = focal length of the lens
^2 means the quantity is squared
This gives the entire depth of field, but DOF is not symmetrically
distributed around D. It is larger behind than in front.
You can do the math but I have seen a table with these values
calculated. Try Google with "Depth of Field Table"
Bob Williams

Robert E. Williams, Dec 1, 2003
5. ### ** Pee Cee **Guest

There isn't one on my digital camera ?
This is rec.photo.digital is it not?
Paul.

** Pee Cee **, Dec 1, 2003

7. ### Andy HewittGuest

Doesn't the view on the LCD display show real time preview? It certainly
does on my Minolta Z1, and I'm pretty sure my Olympus C960 did too,
although that didn't have any manual over-rides at all.

Andy Hewitt, Dec 1, 2003
8. ### RobertwgrossGuest

Many digital cameras do not have any preview on the rear display. On a D-SLR,
the optical viewfinder must be used to line up the shot. On a good D-SLR, the
Depth of Field Preview button will show how much is present. Then only after
the shot does the rear display light up to show the image captured.

---Bob Gross---

Robertwgross, Dec 1, 2003
9. ### Andy HewittGuest

Blimey, is that really so?

I tested this on my Z1, and in manual mode you can play around with all
the settings and immediately see the effect in the viewer before taking
the picture.

Actually I just went off and rechecked, and I can see the effect on
pressing the shutter release halfway.

Andy Hewitt, Dec 1, 2003
10. ### RobertwgrossGuest

Blimey does not post here. Yes, it is true. On a D-SLR camera, there is a
mirror in the way of the image sensor, so the subject view is seen on the
optical viewfinder and nothing on the rear display. When the shutter button is
pressed, the mirror flies up, the shutter opens, and the image is captured on
the sensor. Then maybe one second later, the image shows up on the rear
display.
That is why there are many grades of digital cameras out there. Some users like
some features, and some users like other features.

---Bob Gross---

Robertwgross, Dec 1, 2003
11. ### Andy HewittGuest

Haha, OK.
I see, so they aren't really an SLR then. At least in the respect of a
true SLR anyway, where the image you see in the viewfinder is from the
lens, and therefore what will reach the film once the shutter is
released.

traditional SLR works, the only difference being a CCD is used instead
of a film. What passes the mirror, and reaches the viewfinder, should be
the same image that reaches the CCD/film.

Indeed, the viewfinder on the digital should be more accurate, an SLR
can obviously offer a DOF previewm and at the same time give a visual
indication of the apature setting too. However it cannot allow for the
shutter speed. A digital camera could do this, and could give a truer
preview of the image.

I appreciate that the circuitry, and journey of the light would differ
slightly, but this could be corrected within the software. I would
certainly expect it to with the cost of a reasonable DSLR being what it
is compared to a tradtional SLR (even in their peak times).
Personally I think they have made it far too complicated now. When I
bought my first SLR back in the 80's, I only had to worry about whether
to have a manual, or auto, and what shutter speed range I needed. Your
budget dictated the build quality. It's certainly not so easily defined
now.

Andy Hewitt, Dec 1, 2003
12. ### RobertwgrossGuest

The D-SLR is very similar to the film SLR, so I think you are getting it
confused. To a certain extent, if you take a film SLR, take out the film
transport mechanism, and then insert a digital sensor (and associated image
processing), then you've got the D-SLR. With the SLR or D-SLR, the image in the
optical viewfinder is what is coming through the lens.

In contrast, the non-SLR digital camera handles lots of things differently, and
often there is a digital display preview of the subject.

---Bob Gross---

Robertwgross, Dec 2, 2003
13. ### MikeWhyGuest

The lens typically doesn't stop down to the selected aperture until you snap
the shutter or push the DoF preview button. All SLRs I've ever seen,
including the cheapest of the cheap digital ones, have a depth of field
preview button. (Push the button, and see.)

MikeWhy, Dec 2, 2003
14. ### Jules BabyGuest

DOF relies on 3 factors:-
1) Aperture or f-stop: wider = less DOF eg.f2
2) Lens focal length: longer = less DOF eg.300mm
3) Camera to subject distance: closer = less DOF

My advice...do lots of tests to get a feel for it.

Jules Baby, Dec 2, 2003
15. ### Andy HewittGuest

Yes I see that. So of course you can't see what the exposure will do for
you. Understood now.
Yes.

Cheers.

Andy Hewitt, Dec 2, 2003
16. ### Andy HewittGuest

Yes, I have that on my SLR too, I just thought there'd be more to the
digital version.

Andy Hewitt, Dec 2, 2003
17. ### Bob SalomonGuest

Do you really need to know what the depth of focus is? That is the range
of sharpness BEHIND the lens where the film or imaging sensor must lie
to have an in focus picture. The longer the lens the LARGER the depth of
focus.

Or do you want to know what the depth of field is? That is the area in
FRONT of the lens that ranged from the front of the point the lens is
focused on to behind the point the lens is focused on. The shorter the
lens the GREATER the depth of field. The smaller the aperture the
greater the depth of field. The larger the circle of confusion the
greater the depth of field.

However judging depth of field on an SLR is deceptive because you are
seeing it only at the magnification that the viewfinder uses. If you
make a print that has more magnification then the viewfinder then you
will have LESS depth of field then you saw in the finder.

Bob Salomon, Dec 2, 2003
18. ### Andy HewittGuest

Who mentioned that?
Yes, I know that much.
But not the otherway around I would suspect. You can't make something in

Andy Hewitt, Dec 2, 2003
19. ### Bob SalomonGuest

It is the subject line of this thread.

Bob Salomon, Dec 2, 2003
20. ### Andy HewittGuest

So it is, I hadn't even considered a difference. I think we all just
assumed depth of field was meant.

Question is, are we all wrong, or are you being a pedant?

Andy Hewitt, Dec 3, 2003