DOF rule of thumb?

[steve george]

Does anyone have a rule-of-thumb for working out DOF depending on

focal length and aperture?

 

I'm looking for a way of calculating where in a scene from points X

to Y will be in focus for any given combination of the above. e.g.

if the point of focus is 5 metres away and I'm using a 50mm lens at

f4 is there a quick and easy way formula that can be done

mentally "on the fly" to workout how much foreground / background

will be in focus too? Likewise for a 90mm lens, 35mm etc etc etc.

[jonathan_reynolds]

Steve, check your email for a small Excel calculator.

[steve george]

Thanks Jonathan - that's great, and unbelievably quick!

[john sypal]

Don't know if you have a Mac or not, but there is a <a href="http://www.apple.com/

downloads/dashboard/calculate_convert/dofcdepthoffieldcalculator.html">DOF widget</a>

out there too.

[svante.johansson]

The DOF does not depend on the focal length. It depends on the degree of magnification (size of the image capture device in relation to the size of the object), and on the aperture. This means that you can use the same rule of thumb for any focal length. <br>

<br>

Here is one rule of thumb I accidentally discovered while playing around with one online calculator, you can easily verify it for yourself:<br>

<br>

<b>If the degree of magnification for 35mm film is such that the short side (24mm) covers 30cm in reality (as could be the case in a head-shot for instance) then the dept of field is approximately equal to the aperture number!</b><br>

<br>

This means that if you are shooting a tight portrait at f1.4 you have about 1.5cm of DOF, at f8 you have 8cm of DOF etc - independent of the focal length. Using the rules of similar triangles this can be extended to other degrees of magnification.<br>

[Troll]

That's what the DOF marks on the lenses are for. D'oh!

[albert_smith]

<I>That's what the DOF marks on the lenses are for. D'oh!</I><P>

 

Exactly! We are using one of the only camera systems that still feels the need for comprehensive DOF marks on its lenses, so use those marks. My problem with external calculators is that they may tell you to focus to a distance that is not marked on the lens.<P>

 

The DOF marks on the actual lens makes this a non-issue. Focus on the near subject and check that distance, focus on the far subject and check its distance. Find the aperture that those two distances will fall between on the scale. Need higher enlargement? Go one more f-stop, such as f/11 dialed in when the DOF scale is read for f/8.

[steve george]

Yes Bill, thanks for that. There's not always time to look at the markings though that's why I asked the question...well...for myself at least I don't like taking my eyes off what I'm photographing too much anwyay and if I can avoid that then that's preferable.

[erudolph]

Being new to Leica, I've wondered if folks tend to trust the DOF scale or do they assume

it's either ober- or under-stating the DOF?

 

Ed

[timarmes]

Hi,

<p>

I used to use DOF markings. I no longer find them useful.

<br>

I used to use tables. I found them long, complicated and ultimately not useful.

<p>

I understood the concepts of DOF, of the circle of confusion, of the effect or film/sensor size, etc. It all makes sense, it's logical, and yet in reality it never seems to work all that well.

<p>

What I expected to be sharp, wasn't really as sharp as I wanted, and the results weren't what I expected.

<p>

Fortunately I wasn't the only one to notice this. Someone much cleverer than me noticed it too, and about 15 years ago he wrote various articles and even a book on a new way of thinking about depth of field, based on first principles. His method is quick and easy (although not as quick as DOF marks, it has to be admitted), and more importantly it really does put the photographer in control of level of sharpness he or she needs in a particular situation.

<p>

I now understand than when I use DOF marks, I'm setting a minimum level of acceptable focus that is actually based on some really very bad assumptions. I now understand that focusing at the Hyperfocal distance will give me backgrounds that aren't very sharp at all - acceptably sharp? Not to my eyes.

<p>

Now, when I stand in front of a scene and DOF is important, I considered the DOF that I want, and the level of sharpness that I need. Do I need to resolve pebbles? Hair? Distant window frames? Do I want to ensure than certain objects are thrown well out of focus, or do I want everything to be as sharp as possible? I consider the physical size of the objects in question, and then I choose the appropriate f-stop and focus point.

<p>

Taking your example, I would think like about it like this:

<p>

A 50mm lens at f4 gives me about an aperture of about 12mm (actually 12.5).

<br>

I want to resolve object at least .5mm - freckles. .5mm is 1/24 of of aperture diameter.

<br>

This means that the DOF for my chosen minimum resolution size will be 1/24 of the distance to the object, each side of the object.

<br>

5m * 1/24 is appox 20cm. My DOF is from 4.8m to 5.2m. Focus on the eyes and I'll resolve freckles that are 20cm away (and they're much nearer than than, obviously).

<p>

All that, and the format size isn't even taken into account. How can that be?

<p>

Here's a very good overview. It's worth reading it carefully a few times to let it sink it. You may never turn back - I haven't.

<p>

<a href="http://www.trenholm.org/hmmerk/DOFR.html">http://www.trenholm.org/hmmerk/DOFR.html</a>

<p>

Here's are 4 articles covering the subject in more depth:

<p>

<a href="http://www.trenholm.org/hmmerk/HMbook14.html">http://www.trenholm.org/hmmerk/HMbook14.html</a>

 

<p>

Here's the book itself, probably too in depth for most, as a PDF:

<p>

<a href="http://www.trenholm.org/hmmerk/index.html#TIAOOF">http://www.trenholm.org/hmmerk/index.html#TIAOOF</a>

 

<p>

Enjoy,

<p>

Tim

[skeeter]

tim, i'm not sure i understand how the size of the freckle translates to determining the calculation of the depth of field. a freckle thst is 1cm and one that is .01 cm, will both be resolved sharply (assuming the lens and film are of suffecient quality) if they are at the same distance from the lens and fall within the depth of field for that particular aperture. can you enlighten me?

[david j.lee]

that is a good one, svante. thanks!

[billsr]

I occasionally refer to the DOF scale on the lens, but I rarely concern myself that much about DOF. If I want the background blurred out, I simply use the largest aperture that my required shutter speed will allow and focus critically on the most important part of the subject. If I want lots of DOF, I focus on the most important part of the subject and use the smallest aperture my shutter speed will allow. Some of you people have dealt with this subject so deeply and obsessively that it makes my eyes glaze over to read this thread! ;<)) Best regards, Bill (Doh!)

[steve george]

Thanks all for taking the time to give such interesting and useful answers - much appreciated.

[peterblaise]

.

 

Earlier on this thread: "...Here's a very good overview. It's worth reading it carefully a few times to let it sink it. You may never turn back [from "traditional" DOF depth of firld scales and calculations] - I haven't.

 

http://www.trenholm.org/hmmerk/DOFR.html

 

Here's are 4 articles covering the subject in more depth:

 

http://www.trenholm.org/hmmerk/HMbook14.html

 

Here's the book itself, probably too in depth for most, as a PDF:

 

http://www.trenholm.org/hmmerk/index.html#TIAOOF

 

Enjoy,.."

 

Peter Blaise responds: Giving credit where credit is due, and allowing Google to associate the author's name with the links above, the articles are from

 

Harold M. Merkliner

 

of Halifax, Nova Scotia, Canada.

 

I find his work to be quite challenging of supposedly age-old "truisms" - such as the presumption that we want everything close and far to be the same out of focus blur based on an arbitrary circle of confusion size that is unrelated to the as-photographed object size - rather than have objects in the image rendered as clearly as possible, near and far, dependent on their as-photographed size compared to the physical aperture size!

 

I found his book "The INs and OUTs of Focus" to be well documented and full of experience- and logic-based research and reporting -- available FREE as a PDF but I have the hard cover and re-read it yearly - ISBN 0-9695025-0-8 - worth searching for used, but who would let it go?!? Great stuff!

 

Thank you, Tim. I see you are a "photo fallacy hunter" too! ;-)

 

Click!

 

Love and hugs,

 

Peter Blaise peterblaise@yahoo.com http://www.peterblaisephotography.com/

[peterblaise]

.

 

Oh, and the "DOF rule of thumb", so to speak, as asked is:

 

*** Focus on the farthest thing in the subject scene you'd like to be accurately rendered, and shoot away! ***

 

Things closer usually also appear larger and can withstand a little more blur than things farther which are usually smaller and cannot withstand the "equivalent blur" that traditional DOF scales and calculations tell you to use!

 

However, Steve George, original poster of the initial inquiry here, when mentioning DOF in your opening post, I think you are really after NOT having things near and far in focus. Rather you are trying to exclude near and far things and render them out of focus, right?

 

Steve, is there more you can share about what you are trying to do? Can you share some example pictures?

 

Click!

 

Love and hugs,

 

Peter Blaise peterblaise@yahoo.com http://www.peterblaisephotography.com/

[timarmes]

Hi Skeeta,

 

I was being purposely vague so that people would read the referenced sites - Mister Merklinger explains it all so well.

 

Essentially, he turns DOF on its head. He noticed that the concept of the circle of confusion isn't always very useful. What the circle of confusion tells us about the use of the hyperfocal distance, for example, doesn't actually translate very well in practice. Indeed, the circle of confusion disassociates the scene being protographed from the impression of sharpness - he explains his reservation on this approach.

 

Harold noticed that focusing at infinity produced far better results than focusing at half the hyperfocal distance. He also discovered that photos containing distinct images that looked truly sharp were displaying circles of confusion of arount 1/200mm - much smaller than the 1/30mm that we use today. (He explains why he thinks this value is non-sensicle)

 

However, using a 1/200mm circle of confusion and pluging that into the DOF equations suggests that the DOF would be practically zero, and that clearly wasn't the case. The circle of confusion is simple not useful in estimating image sharpness.

 

In turning DOF around and considering the actual size of the smallest objects that you want to resolve, he demonstrates why the tradional approach to DOF is poor, and presents a method that's truly useful.

 

I'll let you read the links above.

 

Tim

[jimdesu]

Read "Ins and Outs of Focus" (linked above, and stop worrying about trying to compute image-plane depth-of-field. You'll be really glad you did. Or, if you want a really "thumby" rule of thumb:

 

As you move from the plane of focus towards your lens, items smaller than your lens's aperture will be rendered out of focus. Items will defocus by the same amount as you move away from the plane of focus away from you, and, after you've covered the same distance away from the object as you are from it, you'll keep getting even more out of focus at the same rate.

 

But of course Merklinger says all this much better than can I.