Calculating physical depth of field by lens/f-stop

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Calculating physical depth of field by lens/f-stop

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Started 8 months ago by Postman
Latest reply from Darkhost
Related Topics:
1. What is the best f stop?
2. Opinion - Use the D800 as a Camcorder replacement

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Postman

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I've done a quick search and couldn't really find anything related, so here comes a new thread.

A comment someone made in the PAD thread got me thinking. The comment in question stated that a shot they'd taken at f2.8 will of had an in focus area about 1.25" deep (assume unit is feet rather than inches).

Is there a constant guide that you can use to calculate what area of the shot is going to be in focus in this way? I.e. if I'm trying to get a shot of a couple of chaps talking to each other, but I'm not level with both, one being nearer and one being farther away - but for example, say they're both within a 3 feet band of each other. Is there some calculation I need to be aware of and mentally making to adjust the f-stop accordingly?

This relationship will change I'm sure dependant on sensor size (DX vs FX) but what about lens? Is it a constant calculation for DX across all lenses for example?

I hope that makes sense, because I'm at home recovering from knee surgery at the moment and still very much feeling the affects of the general anaesthetic, so forgive me if this is nonsense!

Posted 8 months ago #
Darkhost

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Postman, great post. I'm looking forward to an educated response as I assume you are refering to my post in the PAD. I pulled that reference from the website below, but they don't go into detail with concerns to calculations.

http://digital-photography-school.com/advanced-tips-for-tack-sharp-images

"If your subject is 10 feet away and you’re shooting f/2.8 at 200mm, your depth of field is only 1 1/2 inches deep! That means that your subject will come into focus at 9.94 feet away, and drop out of focus at 10.06 feet away. Do you want to leave a 1.5 inch depth of field up to your camera? What if it focuses on the tip of the nose? Now your subjects eyes are out of focus."

Posted 8 months ago #
Postman

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Yep, it was indeed your post.

This sounds like its going to be very complicated - I hadn't factored in distance to subject in my thinking!

I wonder too, if I have my focus mode set to single point, and I focus that single point on a the tip of a chap's nose, does the focal plane or area in focus fall behind that point, in front of it, or with that point in the centre?

Posted 8 months ago #
Correlli

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There are some formulas that you can use for those calculations:

near = (d * f^2) / (f^2 + u * k * (d - f))
far = (d * f^2) / (f^2 - u * k * (d - f))

With:
near: the closest distance that is still in focus
far: the far distance that is still in focus
d: distance to subject
f: focal length of the lens
k: f-stop value
u: diameter of circle of confusion (diameter of a point that is still regarded as in focus).

The circle of confusion in film was set to 1/1000 to 1/1500 of the film diagonal. For 35 mm film this was app. 0.03 mm. This value was selected by assuming that you would enlarge the negative and view the final photo from a certain distance. Most DOF calculators I know still use this value, so it might not be ok for pixel peepers... :)

Some additional formulas:

If you know the near and far distance you need to have in focus you can calculate the distance you need to set the lens to:
d = 2 * (near * far) / (near + far)
And the required f-stop:
k = f^2 * (far - near) / ((near + far) * (d - f) * u)

This is just theory, but it might give you a starting point for your own adjustments.

There are some DOF calculators in the App store for the iPhone/iPod and I am sure you can find similar applications for Android. I sometimes use SimpleDOF or Lens-Lab.

Hope this helps.

Posted 8 months ago #
Yetibuddha

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Postman, there is a depth of field calculator for android phones, its free. One thing nice about this depth of calculator (search for that name on the google play site) is that it also includes diagrams which show both the depth of field for a given camera, lens and f-stop, but illustrates the hyperfocal distance as well. I use it a lot.

Posted 8 months ago #
Geoff_K

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I also DL an app for my android phone. Makes it easy to tap tap tap to see what the DOF will be

Posted 8 months ago #
Eric

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I use an iPhone DoF calculator called "Focal". Focal gave me the exact same numbers that Darkhost posted above. The software also provides a Hyperfocal Distance.

Posted 8 months ago #
bowen

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I ran across an excellent site a while ago that has an online configurator. I built my own tables for my lenses that I used to carry around with me until I got the basics down:

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

You can choose your camera, focal length, f-stop and distance, and it will tell you the near & far depth of field. (how close items will be in focus and how far behind your focus point items will still be in focus).

With any of these calculators, you can take them as approximations - because, although they give you an exact number, realistically, if you have something 'slightly' out of focus it may not need to be exact. For example - with my D5000 - focusing on the head of my cat at 15-20 feet away with my 300mm f/4, it gives me about 2.5 inches of DOF (1.2 in front of my cat's eyes, and 1.2 inches behind my cats eyes) if I get it slightly off, it's not going to matter.

If you are dealing with extreme closeups, or even portrait work (especially candid moments where your subjects are moving) it's good to have an idea of the DOF and how forgiving it can really be. For example - same camera - with my 35mm 1.8 - I know if my cat is 4 feet away, I have about 3.3 inches of DOF even at f/1.8, so as long as I'm focusing on it's head, I'm getting the eyes in focus. However - same camera and lens - 35mm 1.8 - with my newborn girl, and 12 inches away, I only have 0.8 inches DOF total. So I could possibly focus on her ear and get the eyes out of focus, or focus on the end of the nose, and have the eyes mouth etc out of focus.

The important thing for me was realizing how much of an f-stop I had to change to make a significant difference. for example - the last shot of my little girl - changing to f/4 gives me 1.8 inches total DOF, focusing on the eye gives a good amount of the rest of the face in focus, but still gives a nice out of focus background.

Posted 8 months ago #
TaoTeJared

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I use Bowen's link as well.

On the Iphone I use Simple DoF, TrueDof, DOFMaster and Field Tools. All have a slightly different interface and some show more detail than others.

ExpoImiging has a set of (cardboard/plastic) DOF calculators as well.

If everything is lined up on your camera and lenses, you should have 1/3 in focus in-front of the point and 2/3rds behind it as the focus zone.

Posted 8 months ago #
Darkhost

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Thanks for sharing the great links and iPhone apps. I'm on the right track now.

Posted 8 months ago #

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Calculating physical depth of field by lens/f-stop

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