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 Author Topic: Depth of field - Still much confusion on this subject  (Read 2409 times)
barryfitzgerald
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 « on: May 18, 2011, 10:55:32 AM » Reply

Not from me but it's often stated that focal length is not a factor with depth of field this of course is incorrect.
The formula for depth of field is:

http://www.dofmaster.com/equations.html

As we can see focal length is part of this calculation. It is also a "variable" as much as "focus distance" or hyperfocal distance and "aperture" is. As Michael's article's rightly pointed out you can get the same "overall" depth of field by adjusting your "distance to the subject" assuming you can match focus distances which vary from lens to lens.
I'd like to see the articles on DOF updated to more clearly state how the 3 important factors interact with each other and in some cases cancel each other out. I'd also like to see the information about distribution of front and rear DOF added to the "DOF 2 article" this is where a wider angle lens will have a bigger back distribution of DOF v a longer telephoto which has a more even front and back area in focus. Whilst the overall DOF may be the same the distribution may not.

I know you can get overly complicated on this but basic point is some folks are wandering around out there thinking focal length has nothing at all to do with depth of field! It does just as much as aperture and focus distance. It's how those 3 main variable interact that influences the DOF itself, and they can in some cases cancel each other out.

 « Last Edit: May 18, 2011, 10:58:20 AM by barryfitzgerald » Logged
Rob C
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I don't find it confusing at all: depth of field is theoretically the same with all lenghts of lenses at the same aperture as long as the subject size is kept the same at the image plane. Magnification is the key.

Rob C
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barryfitzgerald
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Problem is Rob it's not "exactly the same" that's the point the overall DOF might be very near to the same in some cases but the front and rear DOF is not depending on the focal length used. The article does not mention this at all. (DOF 2) The rule does not apply at all focal lengths as we can see here mainly based on hyper focal distance:

http://www.dofmaster.com/dof_imagesize.html

As you can see it's not a cast iron rule.

Shorter focal lengths will have a larger DOF behind the focus point than in front of it and longer focal lengths will have a more even distribution.

But more to the point I'm running into folks who say focal length has nothing to do with DOF and I'm not sure the articles on the site help in this regard.
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Christoph C. Feldhaim
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There is no rule! No - wait ...

If you shoot enough with the time passing you get a feeling how your images will look when chosing a specific aperture and distance ....
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stamper
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If you shoot enough with the time passing you get a feeling how your images will look when chosing a specific aperture and distance ....

Nicely put. Instead of getting anal about this subject judge for yourself what you find pleasing. Do a Google search for Colin Prior - the famous Scottish landscape photographer - who sells an enormous amount of images and who thinks that DOF is a waste of time. He did state it more coarsely than me.
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barryfitzgerald
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No problems with using experience for DOF but I feel any DOF article should at least be accurate this does not mean you have to bog folks down in formulae and tech talk.
I'm meeting more and more people who have read the articles on this site about DOF and I have to say I think it's confusing folks not clearing things up for them.

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Christoph C. Feldhaim
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There is no rule! No - wait ...

No problems with using experience for DOF but I feel any DOF article should at least be accurate this does not mean you have to bog folks down in formulae and tech talk.
I'm meeting more and more people who have read the articles on this site about DOF and I have to say I think it's confusing folks not clearing things up for them.

Once you know your tool, you simply don't need anymore articles.
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BartvanderWolf
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Once you know your tool, you simply don't need anymore articles.

Hi Christoph,

Until you shoot it under new circumstances. Only the non-creative stick to fixed patterns of use and stop to explore new possibilities. Trying to learn from avoidable mistakes never appealed to me.

Cheers,
Bart
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Christoph C. Feldhaim
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There is no rule! No - wait ...

Come on ... its not that hard to learn it.
Since the DOF scales on the lenses are only a number related to a coc you still have to learn just "how it looks".
I simply don't believe, that tables and calculation can give you any idea on the look later.
Even if you have a groundglass - once you stop down the image might be so dark you still have to know the look of lets say f11, f16 or f32 for your lens.
I'm not against theory - not at all - but especially chosing an F-Stop or a film type is where experience is crucial.
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barryfitzgerald
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Only point to add is I'm not the only one who reads articles on line many new users will as well so yes I fully agree hands on is a great way to learn about DOF.
I'd just say I'm disappointed to see articles which don't provide a balanced and accurate description of DOF.

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Rob C
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Come on ... its not that hard to learn it.
Since the DOF scales on the lenses are only a number related to a coc you still have to learn just "how it looks".
I simply don't believe, that tables and calculation can give you any idea on the look later.
Even if you have a groundglass - once you stop down the image might be so dark you still have to know the look of lets say f11, f16 or f32 for your lens.
I'm not against theory - not at all - is where experience is crucial. but especially chosing an F-Stop or a film type

Chris, absolutely right; nothing more needs be said.

Rob C
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Rob C
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Problem is Rob it's not "exactly the same" that's the point the overall DOF might be very near to the same in some cases but the front and rear DOF is not depending on the focal length used. The article does not mention this at all. (DOF 2) The rule does not apply at all focal lengths as we can see here mainly based on hyper focal distance:

http://www.dofmaster.com/dof_imagesize.html

As you can see it's not a cast iron rule.

Shorter focal lengths will have a larger DOF behind the focus point than in front of it and longer focal lengths will have a more even distribution.

But more to the point I'm running into folks who say focal length has nothing to do with DOF and I'm not sure the articles on the site help in this regard.

“Problem is Rob it's not "exactly the same" that's the point the overall DOF might be very near to the same in some cases but the front and rear DOF is not depending on the focal length used. The article does not mention this at all. (DOF 2) The rule does not apply at all focal lengths as we can see here mainly based on hyper focal distance:”

Look, I’m not good enough with the non-photographic bits of PS to draw this for you, but let’s see if I can do it in words.

Imagine a diagram that shows a shot taken with the widest lens (cone) you have. Then, change the acceptance angles inwards to show the different space covered by longer optics (narrower cones). If you do not move the camera, all that using longer lenses does is narrow the view and area covered and increase the image size.

But the relationship between background and subject has not been changed, you are just showing less of that background. Keep the image size the same as in the original shot and nothing changes other than you now have a greater camera/subject distance.

In other words, depth of filed is the result of image magnification, aperture being kept constant.

Sorry if this doesn’t explain it, but it’s as good as I can write it!

Rob C
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barryfitzgerald
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Magnification or focus distance is just one variable.
So it's not correct to say that the DOF is "ALWAYS" the same regardless of focal length if the subject is always kept the same size. In some cases it is but as the DOF master article says quite correctly this does not apply at all focus distances.

This is why focal length is part of the calculation for the depth of field, and it is also why the front and back areas in focus are not the same with various focal length lenses. At certain points they are very close but not in every case. I've no problems with de-bunking the frequently stated in photography books line "wide angle has big DOF v telephoto" because obviously that ignore one key area focus distance or distance to subject. A WA lens will have a shallow DOF for a close up shot..conversely you can take a distant landscape with a 200mm lens and if you subject or point of focus is a long way away the DOF will be big..take a head shot of someone 20 feet away and it will be very small.

If you wanted to be smart you could easily say your "fast aperture lens" has a big DOF even wide open if you focus at a subject a long way away. For macro you could argue even stopped down quite a lot at a close distance will still have a pretty shallow DOF.

Half the problems I've seen is that it's just one variable mentioned. WA = big DOF, or fast aperture has narrow DOF. A lot of books I have read bar possibly one have a lot to answer for because they say things that are simply not correct.

But you have to tell both sides of the tale. Or rather 3 sides..focal length, aperture, and focus distance
 « Last Edit: May 24, 2011, 10:40:33 AM by barryfitzgerald » Logged
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