A question about DOF.

Ady Williams

Ady Williams

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Having been on this forum for a while, am I right in thinking that there is a difference between the DOF on a FX and a DX camera?

If anyone has some examples, it may help my decision as to what camera to buy next?

I'm torn between a D300 and a D700, if anyone is interested.

Many thanks.
 
wouldn't have thought so as DOF is related to Aperture diameter. Wide open (F low number) = shallow depth of field. Closed down (F high number) = Longer depth of field.

Not being a Nikon owner, not sure what you mean between difference between Fx and Dx - I assume model numbers.

Interestingly back to DOF - only leart today that 2/3 of the DOF is behind the focal point. Don't ask me why (as I don't know)
 
DoF is not only controlled by aperture but also by focal length and the distance between the camera and the subject and the distance between the subject and the background. FX and DX refer to Full Frame ad crop sensors respectively and I do believe that there may be a difference in the DoF between the two formats but I can't be sure as I have never used full frame.
 
Thanks for the replies, and I've read a few comments about a more pronounced DOF effect on a FX ( Full Frame ) camera. I was always under the impression that it was down to the lens, but I'm old school, and bought up on 35mm.
 
fabs said:
DoF is not only controlled by aperture but also by focal length and the distance between the camera and the subject and the distance between the subject and the background. FX and DX refer to Full Frame ad crop sensors respectively and I do believe that there may be a difference in the DoF between the two formats but I can't be sure as I have never used full frame.
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To add to that, the depth of field is smaller with FX over DX but as a proportion of the picture rather than size. For example if the depth of field is 1cm on a DX camera, that same 1cm will be in focus on FX except there is more bokeh either side of it because of the larger frame. You really start to notice this if you use medium format cameras after using 35mm SLR's
 
oh, didn't realise so many factors..... thanks for stepping in Fabs.... I shall have a serious word with the Adult Education "tutor" who runs the year long photography course I am enrolled on, as thats what I was told......:'(

As I was only taught half (well more like a quarter), could you please explain how focal length affects DOF - as currently I am sitting here scratching my head on that one..........

Ady - sorry for potentially duff info!!! :) I've always been told aperture controls DOF..........
 
Lynton said:
oh, didn't realise so many factors..... thanks for stepping in Fabs.... I shall have a serious word with the Adult Education "tutor" who runs the year long photography course I am enrolled on, as thats what I was told......:'(

As I was only taught half (well more like a quarter), could you please explain how focal length affects DOF - as currently I am sitting here scratching my head on that one..........

Ady - sorry for potentially duff info!!! :) I've always been told aperture controls DOF..........
Click to expand...

this will help alot.
http://www.cambridgeincolour.com/tutorials/depth-of-field.htm
 
Lynton said:
oh, didn't realise so many factors..... thanks for stepping in Fabs.... I shall have a serious word with the Adult Education "tutor" who runs the year long photography course I am enrolled on, as thats what I was told......:'(

As I was only taught half (well more like a quarter), could you please explain how focal length affects DOF - as currently I am sitting here scratching my head on that one..........

Ady - sorry for potentially duff info!!! :) I've always been told aperture controls DOF..........
Click to expand...

Basically, the longer the focal length, the shallower the depth of field (in conjunction with the other contributing factors of course). One of the reasons you get such a deep DoF with UWA lenses, even at wide apertures.
 
I think it's important to remember that whilst the depth of field you get with wide angle lenses may appear to create the effect of greater dof in reality they don't (other than in some specific instances, as far as I remember this is to do with hyperfocal and high zoom, Google for more...) If you look at the same scene taken with a wide angle and a zoom and adjust for size the depth of field will usually be the same.
 
Rather than use the word focal length then, use the word magnification. This will have a much more pronounced effect on dof than aperture. Get up close with a long lens and even at small apertures your background will be out of focus due to the small dof.

Use a wide lens and even at larger apertures much of your image will be in focus.
 
Ady Williams said:
Having been on this forum for a while, am I right in thinking that there is a difference between the DOF on a FX and a DX camera?
If anyone has some examples, it may help my decision as to what camera to buy next?

I'm torn between a D300 and a D700, if anyone is interested.

Many thanks.
Click to expand...

Yes, is the answer to your question. With Nikon Fx and Dx, the smaller Dx format has 1.2 stops more depth of field, when the subject is framed the same. The formula is very simple - f/number x crop factor, ie x1.5 for Nikon. In terms of whether to choose a D300 or D700, roughly one stop of depth of field difference between the two isn't a huge deal, but of course it's one of the factors.

There are actually only two things that affect depth of field that you need to worry about in practise, that is magnification (size of the subject image on the sensor) and f/number. The lower the magnification, ie the smaller the image, the more depth of field. With Dx format, the image is naturally smaller to start with, so automatically more depth of field. For the same reason, the yet smaller 4/3rds format has 2 stops more depth of field than full frame Fx, and the tiny sensors of compacts have loads of depth of field at similar f/numbers.

Magnification is in turn affected by two things, focal length and subject distance. So if you fit a longer lens, the subject gets bigger and depth of field is reduced, and vice versa. Likewise, if you move closer the subject gets bigger and the same applies. However, if you fit a longer lens, but then move back so that the subject remains the same size in the frame, depth of field stays the same. So in that sense, it is untrue to say that, for example, long lenses automaticaly deliver less depth of field. (Although the more distant viewpoint in this case will give a reduced field of view, but that's quite different.)

Input a few figures here and you'll see how it all pans out http://www.dofmaster.com/dofjs.html
 
I think you can actually go mad trying to rationalise this.

Every fibre of my body wants to say that wide angle lenses gave greater depth of field but I know that it's an illusion and that if you take a shot with a 20mm lens and then take the shot with a 300mm lens as long as the subject is the same size in the frame the dof will be the same, unless high magnification or hyperfocal distance intervene.

I've never tested this but I believe that subject size within the frame and format size as well as aperture decide the dof.
 
woof woof said:
I think you can actually go mad trying to rationalise this.

Every fibre of my body wants to say that wide angle lenses gave greater depth of field but I know that it's an illusion and that if you take a shot with a 20mm lens and then take the shot with a 300mm lens as long as the subject is the same size in the frame the dof will be the same, unless high magnification or hyperfocal distance intervene.

I've never tested this but I believe that subject size within the frame and format size as well as aperture decide the dof.
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Haha yes! If you want to be strictly accurate about it, if you take the two very extreme examples you have given, a 20mm lens and 300mm lens, there is actually a fractional difference. But it is very slight, like a fraction of a stop, and given how innaccurate depth of field can be in practice it is best to ignore it (eg if you crop an image all the calcs are changed quite a lot, as you are effectively changing format). If you were to do the same thing with a less extreme example, say a 50mm lens and a 200mm lens, the difference would be barely measurable.

However, I mentioned field of view earlier and while it is completely different to depth of field, the effect can be similar. That is, with a long lens you have to move back to maintain the same framing. The field of view is therefore reduced and there is now much less background visible behind the subject, often resulting in a less cluttered, more simple and more plain background. This makes the main subject stand out more, which is visually similar to shallower depth of field.
 
I think that there are some things that we really shouldn't talk about, things like Canon v Nikon and what is dof. Otherwise I'll never be able to sleep.

Maybe these things should be referred to as "The Scottish Play."
:lol:
 
woof woof said:
I think you can actually go mad trying to rationalise this.

Every fibre of my body wants to say that wide angle lenses gave greater depth of field but I know that it's an illusion and that if you take a shot with a 20mm lens and then take the shot with a 300mm lens as long as the subject is the same size in the frame the dof will be the same, unless high magnification or hyperfocal distance intervene.

I've never tested this but I believe that subject size within the frame and format size as well as aperture decide the dof.
Click to expand...

Who do you know that has tried to take the same shot with a 20mm and a 300mm lens? In the real world that would likely never happen.
 
Yes and no...
DOF is controlled by the lens - it's Focal-Length, the angle of view that focal-length produces and the aperture selected...

Using the same lens on a DX body and an FX body will produce images with different DOF because the change in format alters the effective angle of view.

However if you use a 35mm lens on a DX camera and a 50mm lens on an FX camera (which should produce almost the same angle of view) then at the same aperture setting, the DOF should correspondingly also be the same.
 
woof woof said:
I think it's important to remember that whilst the depth of field you get with wide angle lenses may appear to create the effect of greater dof in reality they don't (other than in some specific instances, as far as I remember this is to do with hyperfocal and high zoom, Google for more...) If you look at the same scene taken with a wide angle and a zoom and adjust for size the depth of field will usually be the same.
Click to expand...

Just to clarify, I was referring to the wide focal length, just using a UWA lens as an example. I didn't mean that a UWA lens at 16mm would produce a different DoF than a zoom lens at 16mm. ;)
 
woof woof said:
I think that there are some things that we really shouldn't talk about, things like Canon v Nikon and what is dof. Otherwise I'll never be able to sleep.
Click to expand...



Lol I think you got it right so far, give or take. :thumbs: Its one of those subjects that’s not easy to put into words, especially if your old school and learnt it all before calculators where invented.

Oddly I find we never discuss what exactly dof is caused by, people just don’t seem to care, just the method to control it via the tools, nothing on why it occurs which personally I think could eliminate all the confusions..

...now that would be a revelation, and far too geeky I suspect, although I think I know of few like me would enjoy it. :lol:
 
Forbiddenbiker said:
Oddly I find we never discuss what exactly dof is caused by, people just don’t seem to care, just the method to control it via the tools, nothing on why it occurs which personally I think could eliminate all the confusions..

...now that would be a revelation, and far too geeky I suspect, although I think I know of few like me would enjoy it. :lol:
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Did you read the Cambridge In Colour article referred to by scottthehat? Here.

It contains a simple diagram which answers your question (though you really need to read the accompanying text to understand it fully):

tut_DOF_lensdiagram.png
 
Not this time Steward and yes it does doesn't it . Although I think the beginner might struggle to understand it. I'm not meaning to shun any excellent links given on this thread.

I'm just saying we at TP seldom discuss stuff like light rays when people try and understand DOF; Like the angles of reflected rays, aperture controls wide, narrow and parallel ray proportions, how this explains exposure correction and dof, and what exactly is CoC at the focal plane etc.

We used to discuss all this, but now we just get text book answers and links. (excellent answers nonetheless) Im not sure doing away with the old chats for what now seems more like a quiz nowadays, is the only way, or even the best way.
 
We used to discuss all this, but now we just get text book answers and links. (excellent answers nonetheless) Im not sure doing away with the old chats for what now seems more like a quiz nowadays, is the only way, or even the best way.
Click to expand...

Photography is not so much the mix of art and physics it used to be and more a blend of art and electronics.
 
Given how much the electronic bits all seem to hate me, I really hope so too. :lol:
 
Forbiddenbiker said:
Not this time Steward and yes it does doesn't it . Although I think the beginner might struggle to understand it. I'm not meaning to shun any excellent links given on this thread.

I'm just saying we at TP seldom discuss stuff like light rays when people try and understand DOF; Like the angles of reflected rays, aperture controls wide, narrow and parallel ray proportions, how this explains exposure correction and dof, and what exactly is CoC at the focal plane etc.

We used to discuss all this, but now we just get text book answers and links. (excellent answers nonetheless) Im not sure doing away with the old chats for what now seems more like a quiz nowadays, is the only way, or even the best way.
Click to expand...


As a youngster to photography I loved all the physics, including baffling (most) people with the fact that we don't actually know the colour of anything and may well live in a totally B&W world :D

As an older chap - I don't give a flying frig for the mechanics only the application & its results matters :)

A bit like me & driving. I love driving too, and I'm good at it, but I only ever lift the bonnet to fill the washer bottle and if it didn't start on turn of the key I wouldn't have a clue (other than calling the AA)

You nerds can keep your technie stuff :p - the rest of us will simply concentrate on talking/taking pics :thumbs:

DD
 
DiddyDave said:
As a youngster to photography I loved all the physics, including baffling (most) people with the fact that we don't actually know the colour of anything and may well live in a totally B&W world

As an older chap - I don't give a flying frig for the mechanics only the application & its results matters

A bit like me & driving. I love driving too, and I'm good at it, but I only ever lift the bonnet to fill the washer bottle and if it didn't start on turn of the key I wouldn't have a clue (other than calling the AA)

You nerds can keep your technie stuff - the rest of us will simply concentrate on talking/taking pics :thumbs:

DD
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:lol: I'll play.

In order:

Don't be daft!

And you call yourself a photographer too...

Without actually know what your doing with the light....right okay you'll need that concentration then!

;) :D
 
Forbiddenbiker said:
:lol: I'll play.

In order:

Don't be daft!

And you call yourself a photographer too...

Without actually know what your doing with the light....right okay you'll need that concentration then!

;) :D
Click to expand...


In reply ;)

Not being daft - it's true - as a photographer (and nerdy type :D) you should know this, even WB settings tell you it's true

Yep - so :razz:

I don't need to know what I'm doing with petrol/diesel to know how to drive and get the effect I want from them do I

DD
 
Quite right too. :thumbs:

I was only playing Dave.... :( :suspect: :p I think your a very good photographer.

As it goes, I don't think I know of that particular Black and white argument (haha @ pun) ... I've been trying not to highjack threads (done it a few times recently, by accident) so I'll not suggest you go into it ....you can pm me if you like though. :geek:
 
Arkady said:
...However if you use a 35mm lens on a DX camera and a 50mm lens on an FX camera (which should produce almost the same angle of view) then at the same aperture setting, the DOF should correspondingly also be the same.
Click to expand...

No! That's the OP's point. If you frame things the same, the Dx image will inevitably be smaller on the sensor than Fx, so DoF is increased. The difference is f/number x crop factor, ie 1.2 stops for Nikon, 1.3 for Canon, and 2.0 for 4/3rds format.
 
Although not a Nikon owner I have the simular in the Canon D series (50D) & 1DS MkII ie Full Frame sensor (35mm) & a 1.6 Sensor. In your case the DOF if you purchased the D700 would be what you are used to on 35mm however due to the focal lenght change given by the smaller sensor D300 tends to give greater DOF. If find the big difference when doing macro photography rather than my norma work of landscapes
 
DiddyDave said:
I don't need to know what I'm doing with petrol/diesel to know how to drive and get the effect I want from them do I

DD
Click to expand...

Just pondering this some more...

The thing is though Dave your not getting the driving effect from the fuel are you, that’s all automated for your use. If you had to balance the air and fuel mixture yourself then it would be a reasonable comparison I think.

Ask a racing driver if knowing the technical stuff is useful and the only valid answer I think you'll find is yes. Like ask a racing team if the winning results can be achieved with just a good driver and the answer will be no.
 
Forbiddenbiker said:
Just pondering this some more...

The thing is though Dave your not getting the driving effect from the fuel are you, that’s all automated for your use. If you had to balance the air and fuel mixture yourself then it would be a reasonable comparison I think.

Ask a racing driver if knowing the technical stuff is useful and the only valid answer I think you'll find is yes. Like ask a racing team if the winning results can be achieved with just a good driver and the answer will be no.
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What happened to not hijacking this thread ??? :D

DD
 
EOS JD - "Who do you know that has tried to take the same shot with a 20mm and a 300mm lens? In the real world that would likely never happen."

But as amateur (or even professional) photographers it's an interesting point and it's worthwhile not spreading too much disinformation in response to a question. I just thought that it was worthwhile mentioning that dof isn't as simple as wide angle = a lot and long zoom = not much.

In the real world I mostly use 20mm and 30mm primes and a 12-24mm zoom, admittedly on an APS-C camera, and I'm well aware that these relatively wide lenses can give great scope for control of dof. Without using and understanding wider lenses it's possible to believe that they'd just give you huge amounts of dof but in reality of course that simply isn't the case. Usually. A lot of dof is just one thing that a wide angle lens can offer you should that be what you want.
 
HoppyUK said:
No! That's the OP's point. If you frame things the same, the Dx image will inevitably be smaller on the sensor than Fx, so DoF is increased. The difference is f/number x crop factor, ie 1.2 stops for Nikon, 1.3 for Canon, and 2.0 for 4/3rds format.
Click to expand...


Read it again - NOT if you change the focal length of the lens to take account for the crop factor.
 
woof woof said:
EOS JD - "Who do you know that has tried to take the same shot with a 20mm and a 300mm lens? In the real world that would likely never happen."

But as amateur (or even professional) photographers it's an interesting point and it's worthwhile not spreading too much disinformation in response to a question. I just thought that it was worthwhile mentioning that dof isn't as simple as wide angle = a lot and long zoom = not much.

In the real world I mostly use 20mm and 30mm primes and a 12-24mm zoom, admittedly on an APS-C camera, and I'm well aware that these relatively wide lenses can give great scope for control of dof. Without using and understanding wider lenses it's possible to believe that they'd just give you huge amounts of dof but in reality of course that simply isn't the case. Usually. A lot of dof is just one thing that a wide angle lens can offer you should that be what you want.
Click to expand...

The factual answer to the OP's question, is a simple yes.

Now here we are, well over 30 posts later, some from professionals, and how many of those posts are either wrong, have misunderstood the question, are partly wrong or at best only partly right?

I think that's why depth of field comes up so often, but also that actually, in practise all you really need to know is that higher f/numbers give you more DoF, and vice versa. Generally speaking, all the other parameters are usually fixed - camera format, focal length, distance - you either don't want to change them, or can't. So just change the f/number until you get the result you want, knowing that there's not much else you can do about it anyway. Simples :D
 
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