depth of field question

Harvey_nikon

Harvey_nikon

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Hi all,

Noobie question:

I understand that the smaller the aperture the greater the DOF and the larger the A the smaller the DOP.

But how can I calculate out how much of an image will in focus?

For instance, I have an old 50mm prime lens that I have to operate completely manually. At f/1.8 and fairly close the DOP seems just enough to to get my lens cap completely in focus.

Also does DOF vary at different distances from the lens with the same aperture, ie. is the DOF the same at f/1.8 at a distance of 30cm as it would be I was focused on the same object from 6ft away?

Hope that makes sense?
 
You can use a calculator like Chaz says but you simply can't beat experience.
The more you use the lens the better you will become at judging the different aspects.

You don't want to be checking a website in the field when it matters do you?
 
Woodsie said:
You can use a calculator like Chaz says but you simply can't beat experience.
The more you use the lens the better you will become at judging the different aspects.

You don't want to be checking a website in the field when it matters do you?
Click to expand...

^What he said^

Remember that three things have a bearing on DOF

1) Focal length...longer = shallower

2) Aperture...as you already have it down

3) Distance between camera and subject matter...closer = shallower

Despite all of this, experience of your specific kit is still the best way to learn

You could also use your camera's DOF preview button if it has one but at very large f/numbers it could very nearly black out the viewfinder.
 
Woodsie said:
You can use a calculator like Chaz says but you simply can't beat experience.
The more you use the lens the better you will become at judging the different aspects.

You don't want to be checking a website in the field when it matters do you?
Click to expand...
But its a good place to start and learn.
Yes well all know it when we understand it.
 
Sorry to hijack but can someone confirm if I am correct:

18-70mm lens at aperture 3.5 at focal length 18 will have a larger DoF than 3.5 at say 50mm?

Also from where does the extra DoF come from when you adjust the aperture? i.e. does it start from the area in focus and extend to infinity, or does it extend towards the camera or is the are currently in focus in the middle of the DoF? The reason I ask is I don't have a DoF preview button.

Thanks

Daven
 
Daven

You are quite correct, there will be a greater DOF at a focal length of 18 than at 50mm if all other factors remain constant.

I have little understanding of the technical issues involved but rather some old school film days experience.

The zone of acceptible sharp focus (DOF) will be roughly 1/3 in front of the focal point (closer to the camera) and 2/3 behind.

As the lens aperture decreases the image is projected through the lens in more parrallel lines rather than the larger angles of image projection at larger apertures (not larger f/ numbers)

I am sure someone with greater technical optic explanations would hopefully be able to give a more complete scientific explanation but these are things I learned ages ago and simply accepted.

I think I posted in this thread earlier when I said that there are essentially 3 things that govern DOF...focal length, aperture size and distance between camera and subject matter.

Nowadays of course, the plethora of sensor sizes also play a role as the smaller the sensor, the more the depth of field...but let's not run away with the answer to your question.
 
chilliz said:
Using your depth of field preview button (if you have one) can also be a useful guide. :)
Click to expand...


yep, especially when you're getting used to a particularly lens and working distance, its a very quick and easy confirmation.

i'll echo experience though, once you've been using a particular range of focal lengths and apetures for a while you do start to think in "f4 on this group shot wont hack it for the chap behind, i better use f6.3" and so on.
 
Bear in mind with the DoF preview that the standard focus screen within the camera won't show much difference between f1.8 and f2.8 (but the sensor certainly will!).
 
daven1986 said:
Sorry to hijack but can someone confirm if I am correct:

18-70mm lens at aperture 3.5 at focal length 18 will have a larger DoF than 3.5 at say 50mm?

Also from where does the extra DoF come from when you adjust the aperture? i.e. does it start from the area in focus and extend to infinity, or does it extend towards the camera or is the are currently in focus in the middle of the DoF? The reason I ask is I don't have a DoF preview button.

Thanks

Daven
Click to expand...

Assuming the same f stop:
A wide angle lens gives a larger angle of view and also gives a larger DOF.
A telephoto lens has a smaller angle of view and also gives a smaller DOF.

Neglecting subject movement, camera shake etc whatever you have focused on should be in focus and sharp.
Things nearer than the subject will be focused to a point slighly in front of the sensor/film so by the time the light rays reach the sensor/film they will have spread out again into little circles.
Things further than the subject will be focused to a point slighly behind the sensor/film but as they reach sensor/film before they reach this focal point they will be little circles at the sensor/film.
These little circles (known as Circles of Confusion) cause those things nearer and further that the subject to be out of focus.

However the human eye cannot see the little circles if they are very small so when we look at a picture there is an acceptable range of things in front of, and behind, the subject which we see as being in focus. This range of acceptable focus is known as Depth of Field (DOF).
If you have a large aperture the circles are larger than if you have a small aperture which creates smaller circles.

I know that some people have said that they can judge DOF by experience but it must take a lot of experience to know the DOF for all your focal lengths, at all apertures and at all distances. You then need to know how far away your subject is.
 
Harvey,

The way I sorted out this issue was to use the link for the DoF calculator and make myself a table for the lens I am going to use.

So, for your 50mm prime lens it's rather easy. Just do a table with your subject at various distances and using various aperture sizes (say, distances of 2m, 5m, 10m, 20m, 40m, and apertures in one stop increments from f/1.8 all the way to f/22). Seeing things this way will make you visualise how it will turn out.

Of course, it gets a bit too cluttered when you have multiple lenses with zooms thrown in; but still that's how I do it ;)
 
Harvey_nikon said:
Hi all,

Noobie question:

I understand that the smaller the aperture the greater the DOF and the larger the A the smaller the DOP.

But how can I calculate out how much of an image will in focus?

For instance, I have an old 50mm prime lens that I have to operate completely manually. At f/1.8 and fairly close the DOP seems just enough to to get my lens cap completely in focus.

Also does DOF vary at different distances from the lens with the same aperture, ie. is the DOF the same at f/1.8 at a distance of 30cm as it would be I was focused on the same object from 6ft away?

Hope that makes sense?
Click to expand...

you got a slide rule handy...this was worked out long ago...first answer i think covers it
my nikon lenses did it for me...when i lined up the inf's with the apertures chosen on the lens aperture ring
it depends a lot on your eyesight whether the particular circle of confusion is the one for you
dof is really in the eye of the beholder
:D
 
A few more comments.

The link to DoF Master in post #2 is very good. It will tell you everything you need to know about depth of field - just tap in the focal length, f/number and focusing distance and there you have it. It gives total depth of field, amount of depth in front of and behind the subject, and hyperfocal distance etc.

From that site you can also download a pdf and print out custom DoF calculator dials for whatever lens, scale and size you want. V good :thumbs:

DoF depends on just two variables - f/number and image size. The comment you often hear that long lenses give less depth of field and wide angles give more, is not strictly true. What happens is that with a long lens for example, is that the image size is increased and this is what reduces DoF. But by the same token, if you then move back from the subject it gets smaller again, and DoF increases. These two factors cancel out, and if you maintain image size by changing both the focal length and the subject distance, DoF remains the same.

The other thing that changes DoF is the format size - that's physical area, not pixels. Smaller sensors deliver greater DoF. But you generally can't change that as it is fixed by the sensor.

I find it is virtually impossible to estimate DoF using the lens stop down button on the camera. It is much better to take a quick snap and preview it on the LCD, with the help of the magnification function. I think the A-DEP mode on Canons works pretty well.
 
Something I can't get my head round on this one. OLDER lenses had DOF marks on the lens barrel, either side of the aperture window, or on the barrel next to the focus range.
Some had different coloured lines for different apertures, and where the lines cross the focus distance on the lens barrel is the actual distances that are acceptable sharp - so it automatically adjusted for the aperture and the focussed distance, because the lines opened up as you focussed further away, and got closer as you focussed nearer.

have a look on your lens barrel and see if there are lines engraved in a kind of arcing pattern. Either that or engraved marks either side of the centre line, these are your DOF markings.

Any chance of a piccy of the lens so we can see if they are there. Cheap lenses didn't have them, but certainly anything from Vivitar upwards did.
 
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