Depth of field.

Paul pminteriors

Paul pminteriors

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Evening all. I have a question about depth. F1.8 at 6m distance gives a dof of say 15cm. F1.8 at 60m gives a dof of say 4m. What about f1.8 at 60m but zoomed in with a telephoto lens. Would this dof be 15cm or 4m?

(15cm and 4m are guessed)
 
Focal Length (mm) Focus Distance (m) Depth of Field (m)
10 0.5 0.482
20 1.0 0.421
50 2.5 0.406
100 5.0 0.404
200 10 0.404
400 20 0.404
 
Paul pminteriors said:
Do you know what the calculation is or would be?
Click to expand...

If you want to calculate it, you have to start with the hyperfocal distance. This is given by

(Focal length squared) divided by (lens aperture multiplied by the size of the circle of confusion)

I'll come back to this circle shortly.

Now define X as (distance focused on minus the focal length of the lens) - this will make the next part easier to type.
The near limit of the depth of field is given by

(hyperfocal distance multiplied by distance focused on) divided by (hyperfocal distance plus X)

and the far limit is
(hyperfocal distance multiplied by distance focused on) divided by (hyperfocal distance minus X)

The circle of confusion is the size of the largest cirlce that you'll accept as a point in the final print, so depends on the degree of enlargement, the lighting, your eyesight and how picky you are. Plus how much sharpening you've applied.
 
Paul pminteriors said:
Depth over distance stays the same using zoom?
Click to expand...

Yes if you move away.
Hoppy UK will be along in a moment to tell you it is all about magnification :)
You have doubled the focal length and at the same time doubled the distance so the DOF stays the same.
 
Not quite. The depth of field will be different in magnitude and also in positioning. As you focus closer, a greater percentage of the distance in focus will be in front of the plane of focus, and less behind it.
 
I'm not trying to be too complicated and mathematically accurate but just in basics. Rough notes for field. Besides, the batteries in my 300m tape measure are in the telly remote....
 
Well, clearly in most cases the focal length of the lens is very small compared to the distance focused on, so we can simplify X to say that it's D. But that isn't going to help you calculate it without a calculator. Your best bet is to carry some depth of field tables, or if your camera allows, preview the depth of field (by magnifying the image if necessary).

In the old days, lenses had depth of field markings to that you didn't have to do anything. That's progress.
 
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Well, the question wasn't "how can I easily calculate it in the field" but "how do I calculate it" and that complex pair of equations is the answer.

The are a few simple lessons we can learn. Every time we double the focal length, the hyperfocal distance is quadrupled.

The other interesting lesson (if you work through the maths) is that there is a simple set of ratios that determine where the depth of field lies when you focus on a whole fraction of hyperfocal distance.

Focus on H (hyperfocal distance) and everything is sharp between H/2 and H/0 (h/0 is infinity). And H = H/1. This principle applies across the range:

Focus on H/3 and the near and far points are H/5 and H/4. At H/7, it's H/8 and H/6. And so on.

So if you have a few values of H written down, you can estimate reasonably.

All you have to fit in is that every time you double the aperture (which means stop down two stops) you halve the hyperfocal distance.

So there's a simplified rule of thumb.
 
Paul pminteriors said:
I'm not trying to be too complicated and mathematically accurate but just in basics. Rough notes for field. Besides, the batteries in my 300m tape measure are in the telly remote....
Click to expand...

Get a smartphone app, like the one on the DoFmaster link in post #2.
 
Seriously though... in real life, who calculates it? I mean, once you know the formulas to do so, it's easy enough, but who actually gets a calculator out when shooting? All anyone really needs to know is that the closer you focus, the less the DOF is, and the further away the focused object is, the greater it is. That it also reduced with focal length Calm down Hoppy, before your head explodes :).. I know that's not actually true, but that's the EFFECT, and that's all people really need to know. Also, making the aperture smaller increases it. and making it larger decreases it.

None of that is scientific, but knowing those things will allow you to work. You will know how to use DOF, and you will not be a worse photographer for not knowing the maths that allow you to calculate it precisely.

Can I do the maths? Yes. Do I bother when out shooting? Of course not! :) Who does?

I wouldn't teach this on a degree: I'd explain that there are ways to calculate these things precisely, and recommend a book or two, then they can if they want... but most people (myself included) would just rather understand the practical application of these principles, and go shooting instead :)

I can roughly guess that for instance... A 50mm (on full frame) lens focused at 1 meter wide open (1.8) would give me less than 10cm DOF.... and the same lens wide open focused at something 10 metres away would give me around 5 metres DOF.... and at infinity, anything closer than around 10 metres would start to get soft. I did no calculations there... that's from experience. I'm not even going to bother working out whether I'm right or not... it's 7a.m.. and I've not had coffee yet :) I bet anyone on here though £10 that I'm close enough with those guesstimates to work effectively on the field... plus.... I'd just use my DOF preview anyway.

Go out there.... find out PRACTICALLY and get used to how your equipment works instead of sitting there solving maths problems!! :)

Just my advice of course. You may like solving maths problems more than shooting.. in which case.. carry on :)
 
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Pookeyhead said:
Seriously though... in real life, who calculates it? I mean, once you know the formulas to do so, it's easy enough, but who actually gets a calculator out when shooting? All anyone really needs to know is that the closer you focus, the less the DOF is, and the further away the focused object is, the greater it is. That it also increased with focal length Calm down Hoppy, before your head explodes :).. I know that's not actually true, but that's the EFFECT, and that's all people really need to know. Also, making the aperture smaller increases it. and making it larger decreases it.

None of that is scientific, but knowing those things will allow you to work. You will know how to use DOF, and you will not be a worse photographer for not knowing the maths that allow you to calculate it precisely.

Can I do the maths? Yes. Do I bother when out shooting? Of course not! :) Who does?

I wouldn't teach this on a degree: I'd explain that there are ways to calculate these things precisely, and recommend a book or two, then they can if they want... but most people (myself included) would just rather understand the practical application of these principles, and go shooting instead :)

I can roughly guess that for instance... A 50mm (on full frame) lens focused at 1 meter wide open (1.8) would give me less than 10cm DOF.... and the same lens wide open focused at something 10 metres away would give me around 5 metres DOF.... and at infinity, anything closer than around 10 metres would start to get soft. I did no calculations there... that's from experience. I'm not even going to bother working out whether I'm right or not... it's 7a.m.. and I've not had coffee yet :) I bet anyone on here though £10 that I'm close enough with those guesstimates to work effectively on the field... plus.... I'd just use my DOF preview anyway.

Go out there.... find out PRACTICALLY and get used to how your equipment works instead of sitting there solving maths problems!! :)

Just my advice of course. You may like solving maths problems more than shooting.. in which case.. carry on :)
Click to expand...
If only this could be made into a single concise statement.

Depth of field is the simplest topic largely made complicated that we have.

Of course the reality is that it's straightforward in practice.
 
Pookeyhead said:
That it also increased with focal length...

I know that's not actually true, but that's the EFFECT, and that's all people really need to know. Also, making the aperture smaller increases it. and making it larger decreases it.
Click to expand...

Amen to that.

The theory is all very well, nice, good and interesting but what also matters is what it all looks like.
 
What we end up with is loads of people reading the theory on the internet, seeing experts disagreeing about the best way to show the calculations and then give up because it's all so complicated.

What there should be instead is a simple explanation that inspires them to go out and experiment. Because we all know it's not complicated in real life: ie how to make it work, whereas calculating it and describing it is the root of thousands of hours of internet quarrelling.
 
Pookeyhead said:
Seriously though... in real life, who calculates it? I mean, once you know the formulas to do so, it's easy enough, but who actually gets a calculator out when shooting? All anyone really needs to know is that the closer you focus, the less the DOF is, and the further away the focused object is, the greater it is. That it also increases decreases with focal length Calm down Hoppy, before your head explodes :).. I know that's not actually true, but that's the EFFECT, and that's all people really need to know. Also, making the aperture smaller increases it. and making it larger decreases it.

None of that is scientific, but knowing those things will allow you to work. You will know how to use DOF, and you will not be a worse photographer for not knowing the maths that allow you to calculate it precisely.

Can I do the maths? Yes. Do I bother when out shooting? Of course not! :) Who does?

I wouldn't teach this on a degree: I'd explain that there are ways to calculate these things precisely, and recommend a book or two, then they can if they want... but most people (myself included) would just rather understand the practical application of these principles, and go shooting instead :)

I can roughly guess that for instance... A 50mm (on full frame) lens focused at 1 meter wide open (1.8) would give me less than 10cm DOF.... and the same lens wide open focused at something 10 metres away would give me around 5 metres DOF.... and at infinity, anything closer than around 10 metres would start to get soft. I did no calculations there... that's from experience. I'm not even going to bother working out whether I'm right or not... it's 7a.m.. and I've not had coffee yet :) I bet anyone on here though £10 that I'm close enough with those guesstimates to work effectively on the field... plus.... I'd just use my DOF preview anyway.

Go out there.... find out PRACTICALLY and get used to how your equipment works instead of sitting there solving maths problems!! :)

Just my advice of course. You may like solving maths problems more than shooting.. in which case.. carry on :)
Click to expand...

LOL David. Couldn't agree more, though there's a typo re focal length in the first para (you mean decreases).

I don't carry a calculator either, though I do have a few hyperfocal distances noted inside the lens cap of my super-wide. With a bit of experience, you just get an instinctive idea of how much DoF there's likely to be. When in doubt, an easy way to check is to take a snap and zoom in on the LCD to see what it looks like (y)
 
I was readint this thread and actually shaking my head..... I was wondering should I mention that I havent a clue what half of you are on about.. or is that too embarresing from someone who makes a living from this stuff.


then...


Pookeyhead said:
None of that is scientific, but knowing those things will allow you to work. You will know how to use DOF, and you will not be a worse photographer for not knowing the maths that allow you to calculate it precisely.
Click to expand...


Thats me... I can confidently say I know what I am doing.. I know how it works.. I know how to set my camera up for best results in just about any situation..I know what manual setting for best exposure as soon as I walk into a room (99%).. I know what settings for best effect at anything I am photogrpahing.... But put me on a quiz show about photogrpahy and I wouldnt get a question right..




Pookeyhead said:
Can I do the maths? Yes. Do I bother when out shooting? Of course not! :) Who does?
Click to expand...

Can I do the maths? No . Do I bother? No.. ....Who does? Yeagh i was wondering that :)


Pookeyhead said:
Go out there.... find out PRACTICALLY and get used to how your equipment works instead of sitting there solving maths problems!! :)

Just my advice of course. You may like solving maths problems more than shooting.. in which case.. carry on :)
Click to expand...

Couldnt agree more :)
 
I have been reading up on this recently and I think I am correct in saying there are pretty much only two factors that affect DoF - Aperture and Subject Magnification. Notice that focal length is NOT one of those factors. A headshot taken at 50mm f2 will have the same DoF as a headshot taken at 135mm f2.

This is all slightly academic though as we are not (usually) aiming for a short depth of field; we are aiming for a blurred background. This is where focal length does come in to play - a longer lens will "magnify" the background blur more giving the look we are after (the other option is to move the subject further from the background but this is not always practical). A final example would be to take the headshot again using a 50mm f1.4 and 135mm f2.8. The 50mm will have a lot less DoF (5cm vs 11cm) due to the larger aperture but assuming the background is a decent distance away the 135mm shot would have the more diffusely blurred background - the perfect portrait; whole face in focus, lovely soft background.
 
KIPAX said:
I was readint this thread and actually shaking my head..... I was wondering should I mention that I havent a clue what half of you are on about.. or is that too embarresing from someone who makes a living from this stuff.

Thats me... I can confidently say I know what I am doing.. I know how it works.. I know how to set my camera up for best results in just about any situation..I know what manual setting for best exposure as soon as I walk into a room (99%).. I know what settings for best effect at anything I am photogrpahing.... But put me on a quiz show about photogrpahy and I wouldnt get a question right.

Can I do the maths? No . Do I bother? No.. ....Who does? :)
Click to expand...

My sort of photographer ;)
 
Mr Will said:
I have been reading up on this recently and I think I am correct in saying there are pretty much only two factors that affect DoF - Aperture and Subject Magnification. Notice that focal length is NOT one of those factors. A headshot taken at 50mm f2 will have the same DoF as a headshot taken at 135mm f2.
Click to expand...

I'm not convinced, both from the theory and my own experience. But it may depend on exactly what you mean by "subject magnification" since I can see at least two different meanings that the words could bear.
 
KIPAX said:
I was readint this thread and actually shaking my head..... I was wondering should I mention that I havent a clue what half of you are on about.. or is that too embarresing from someone who makes a living from this stuff.

then...

Thats me... I can confidently say I know what I am doing.. I know how it works.. I know how to set my camera up for best results in just about any situation..I know what manual setting for best exposure as soon as I walk into a room (99%).. I know what settings for best effect at anything I am photogrpahing.... But put me on a quiz show about photogrpahy and I wouldnt get a question right..

Can I do the maths? No . Do I bother? No.. ....Who does? Yeagh i was wondering that :)

Couldnt agree more :)
Click to expand...

Its a very nice refreshing view to hear a from competent professional. That you're not machines, churning out figures of exposure, ISO, aperture and DOF every second... :)
 
StephenM said:
I'm not convinced, both from the theory and my own experience. But it may depend on exactly what you mean by "subject magnification" since I can see at least two different meanings that the words could bear.
Click to expand...

By subject magnification I mean the size of the image projected on the sensor compared to life size - i.e. in my example the subject fills the same amount of the frame. I'm aware that this is a slightly difficult sell but that's because of what I cover in the second part of my post - the confusion between amount of blur and depth of field.

Try it yourself - A 135mm lens has a 15.2° angle of view which means that to take a picture of a subject 0.75m wide you'd need to stand 2.8m away from it. A 50mm lens has a 39.6° angle of view which means that to take a picture of the same 0.75m wide subject you'd need to be 1.04m away from it.

DoF@1.04m, 50mm f/2 = 5cm
DoF@2.8m, 135mm f/2 = 5cm

Lets be silly and add a (theoretical) 14mm in to the mix - that needs to be just 29cm from the subject: DoF@0.29m 14mm f/2 = 5cm

Run these numbers through any DoF calculator and you should get pretty much the same results. Don't believe the calculators? Go and take some pictures of a horizontal ruler or piece of newspaper and you'll see its true - the area of the picture that is sharp remains constant. The difference will be the background - The 135mm version will appear much more blurred but that is NOT (by definition) of depth of field.
 
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There are a number of interesting points raised - and I'll accept that on that definition of subject magnification the calculators bare you out (assuming that your figures on subject distances are correct, but I'm more than happy to accept that they are). I'm not quite so convinced that the strict definition of depth of field is what most people would acually understand by the concept, but as the original query has been fully answered I'll let that pass.
 
StephenM said:
There are a number of interesting points raised - and I'll accept that on that definition of subject magnification the calculators bare you out (assuming that your figures on subject distances are correct, but I'm more than happy to accept that they are). I'm not quite so convinced that the strict definition of depth of field is what most people would acually understand by the concept, but as the original query has been fully answered I'll let that pass.
Click to expand...

Put simply, depth of field is how much of the image is in focus; It says nothing about the out of focus parts.

I agree that this strict definition differs from a lot of people's intention but it is the strict definition that the calculators all work to, trying to use them for anything else just results in confusion.

If anyone feels like getting all mathematical, what most people asking this question want to know is the size of the airy-disc circle of confusion for a point on the background. howmuchblur.com provides a good approximation but I'm unaware of anywhere that easily spits out the raw numbers.

(edited to replace airy-disc with circle of confusion)
 
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Mr Will said:
Put simply, depth of field is how much of the image is in focus; It says nothing about the out of focus parts.

I agree that this strict definition differs from a lot of people's intention but it is the strict definition that the calculators all work to, trying to use them for anything else just results in confusion.

If anyone feels like getting all mathematical, what most people asking this question want to know is the size of the airy-disc for a point on the background. howmuchblur.com provides a good approximation but I'm unaware of anywhere that easily spits out the raw numbers.
Click to expand...

I don't think that any of the calculations take the Airy Disc into account.
This is caused by diffraction and limits the minimum size of the in-focus points, as well as causing the oof circles of confusion to be larger.
 
Suvv said:
I don't think that any of the calculations take the Airy Disc into account.
This is caused by diffraction and limits the minimum size of the in-focus points, as well as causing the oof circles of confusion to be larger.
Click to expand...
My mistake, I think it was circle of confusion I meant! I'll edit my original post for the benefit of any later readers.
 
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Iy's relatively easy to calculate the size of the circloe of confusion - the difficulty for most people is knowing what is acceptable in the print (takes account of the degree of enlargement and your eyesight, viewing distance, light levels). I can easily demonstrate the variation of DOF with print size - and so can anyone else for that matter.

Provided that the value specified for the C of C is greater than that of the Airy disc, I don't think it matters. Calculating the size of the Airy disc is simple if you only want to take account of the primary ring size; the problem is knowing where you want to stop.

But all this is way beyond what was asked.

Just back tracking - if most people's intention when they speak of depth of field differs from the strict definition used by calculators, doesn't this imply that advising people who are working to a different idea to use a calculator is actually misleading?
 
I think Stephen makes a good point about background blur. A lot of people believe it's just a question around depth-of-field, but there's quite a bit more to it. If you want to emphasis subject isolation, with the subject sharp against a soft blur behind (many people would call that 'good bokeh' and for sake of argument I'll leave it at that) then the methods to employ are these:

- use long focal length
- move the subject as far from the background as possible
- make the main subject fill the frame
- use a low f/number

To illustrate, these two images are both taken at f/4, the first one at 70mm and the second one at 200mm from a greater distance. Depth of field is the same in both, but shooting from greater distance with a longer lens narrows the field of view and the result is the background is enlarged and appears more blurred.

70mm f/4


200mm f/4. Depth of field is unchanged, but the background is made larger and appears more blurred
 
StephenM said:
Just back tracking - if most people's intention when they speak of depth of field differs from the strict definition used by calculators, doesn't this imply that advising people who are working to a different idea to use a calculator is actually misleading?
Click to expand...

I think most people understand and use the correct definition, being able to get things in focus is more important than being able to blur them!

I don't think anyone has advised using calculators though - I've only brought them up in the context of proving that although the DoF is constant the amount of blur can vary, which I think is the key to understanding this topic.
 
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