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07/22/2003 10:01:41 PM · #1 |
I was asked a question tonight and I didn't get a chance to answer, so I thought I would post here where some others could assist me with the answer...
The question: What is the difference between focus and depth of field?
My answer:
Focus and depth of field are related. Focus is obviously the state of sharpness and visible detail in a photograph. Parts of a photo that are out of focus are softer with less detail or simply blurry.
The Depth of Field is the part of your composition in a three dimensional area that is in focus. Consider your photographic 'scene' as a three dimensional 'box' in space with length, width, and depth. Within this box, you will have an area where your focus is sharp and clear. The 'thickness' of this particular area of focus is your 'depth of field'. You may have a very shallow depth of field where only a small sliver of this 3d box is sharp and in focus. This is called a 'shallow' depth of field because objects immediately in front of and immediately behind this area are out of focus. Shallow depth of field begins with a LARGE aperture (small F number). Decreasing the size of your aperture (increasing the size of the F number) will increase the depth of field in your composition where a thicker chunk of your 'box' is in focus. This is called a 'deeper' depth of field.
Aperture size is not the only element that increases or decreases your depth of field. The two other items that affect depth of field are 'focal length' and 'distance to subject'.
Focal length is your 'zoom setting'. Longer focal lengths (zooming out) naturally decrease your depth of field and make it more shallow. Shorter focal lengths (wide angle views) naturally make your depth of field deeper.
The 'distance to subject' is the physical distance between your lens and the subject your are focusing on in your photograph. I am not 100% sure of the relationship between this element and depth of field, so maybe someone else can help me out here by trying to describe it in layman's terms. When your subject is closer to the camera, your depth of field will naturally be more shallow. When your subject is off in the distance, your depth of field will be greater.
I hope this helps and I hope someone will jump in here and help with this question :)
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07/22/2003 10:29:05 PM · #2 |
Interesting question. To me, depth of field is simply the region of sharpness around the plane of focus. A lot of factors influence depth of field (sensor size, focal length, aperture) as you noted.
A good reference (with diagrams and samples) for those wanting to know more about DOF can be found here. |
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07/22/2003 11:12:08 PM · #3 |
Focus I would say is the overall sharpness of the picture or subject. DOF would be the area of a scene in acceptable focus.
DOF is dependent on the plane of focus (focusing distance). That is the only plane/point that can be in critical focus. In front and behind that is acceptable focus. One rule of thumb that I know of is DOF is 1/3 in front of the POF and 2/3 behind it, for any POF from 20x the focal lenght to infinity. DOF is also directly proportional to focusing distance. 1/3 of 30 ft. is greater than 1/3 of 3 feet. But as Eddy pointed out, there are quite a few factors contributing to DOF. I would say DOF is measured by the Circle Of Confusion.
Thank you for posting this. Good to have on topic discussion once in a while ;) I believe having to express or explain of something helps us understand it more. I'm sure most know what DOF and focus mean and maybe even how to use them effectively. But it gives you so much more when you have to put it into words.
Message edited by author 2003-07-22 23:23:52. |
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07/23/2003 01:12:33 AM · #4 |
| Apart from this being a very interesting article, the first part of it deals with DoF. |
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07/23/2003 01:19:47 AM · #5 |
| I should also mention the way-cool online depth-of-field calculator. The site also has free Palm and Windows DOF software to download. |
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07/23/2003 07:56:18 AM · #6 |
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07/23/2003 08:42:53 AM · #7 |
Mag9999 that is a great tutorial.. really good work..
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07/31/2003 10:47:12 PM · #8 |
| My understanding of the distinction between DOF and focus is that DOF deals with the acceptable amount of what's in focus front to back. Focus, on the other hand, I believe to be the clarity of the picture in the side-to-side and up and down planes. Focus has to do with how well the different points of light that come from the subject come together on the focal plane...I think... :) |
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08/01/2003 01:05:13 AM · #9 |
| Where the circles of confusion reach a point... |
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09/22/2006 06:16:09 PM · #10 |
| I have a DSLR. I manage to take portraits of one person fine, but when I get three people and (sometimes two) that are not standing in line with each other, the one that is furtherest away is out of focus. I guess I am not getting the depth of field right. Any advise on what settings to try would be really appreciated. |
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09/22/2006 06:19:05 PM · #11 |
Originally posted by laytardel:
I have a DSLR. I manage to take portraits of one person fine, but when I get three people and (sometimes two) that are not standing in line with each other, the one that is furtherest away is out of focus. I guess I am not getting the depth of field right. Any advise on what settings to try would be really appreciated. |
Choose a higher numbered aperture. The higher the number the greater the depth of field. If you have DOF preview on your camera, use it to check the area in focus. |
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09/22/2006 06:25:15 PM · #12 |
| What colette said. But you have to watch your shutter speed, the smaller the aperture (The bigger the F-number) the less light gets in and the longer your exposure time. |
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09/22/2006 08:03:54 PM · #13 |
| Thanks, will give that ago and let you know the results. Have a great day. |
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09/22/2006 08:12:46 PM · #14 |
| /me brushes the cobwebs off the thread... ;-) |
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09/22/2006 09:43:45 PM · #15 |
Originally posted by jhonan:
/me brushes the cobwebs off the thread... ;-) |
Hey, you should congratulate laytardel for actually SEARCHING and reviving instead of starting a new thread! |
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09/22/2006 10:50:58 PM · #16 |
You can simplify the answer by saying that depth of field = depth of focus. The area of any given photograph that is 'in focus' is three-dimensional... it has 'depth'.
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09/23/2006 12:13:55 AM · #17 |
Another aspect of DOF, one that is not touched on here, is that it depends on image reproduction size and viewing distance. Not sure what I mean? Think how often you've noticed thumbnails in challenges that look pretty cool, but when you pull up the 640-pixel image they don't look so nice.
DOF is a very subjective measurement, essentially used to define the range of acceptable sharpness, and this will vary dramatically depending on the size of the image being viewed and the distance at which it is viewed.
Also (and this runs counter to what we all think we've been taught) DOF is solely dependent upon the actual physical size of the aperture and the distance being focused upon, and entirely independent of the focal length of the lens. The reason we get confused is because an "f/stop" is a ratio between the physical size of the aperture and the focal length of the lens. So a 25mm aperture on a 50mm lens is f/2, while a 25mm aperture on a 200mm lens is f/8.
If you focus on an object 6 feet away with a 50mm lens set to f/2 and a 200mm lens set to f/8, then blow up the central portion of the wider (50mm) view to match the framing of the 200mm view, you'll find the DOF of the resultant images is the same in a side-by-side comparison, disregarding any softening effects from more noticeable graininess in the cropped image.
Between two lenses set to the same f/stop and focused to the same distance, of course, the longer the focal length the shallower the DOF, because the physical aperture on the longer lens must be larger to attain the same f/stop ratio.
R.
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