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Old Jan 23, 2003, 4:48 PM   #1
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Default Depth of Field in Digital cameras

I've got a question about F-stops and Depth of field. I have a 7i, but this is really a general digital camera question. I understand that since the "imaging plane" in a digital camera (the CCD, I think it is called?) is a smaller area than 35mm film, that the "real" focal length on my digital camera lens equates to a longer focal length on a 35mm. Minolta conveniently indicates the focal length "adjusted" to 35 mm equivalents on the zoom lens barrel.

What about F-stops and depth of field ? Since an F-stop is the ratio of lens diameter to focal length, I assume that F4 is F4 whether on a digital or 35mm film camera. True ?

But what about depth of field ? Will lenses of equal adjusted focal length produce greater or less depth of field at a given F-stop on my digital vs on a 35MM film camera ?

The reason I ask is that my 7I's lens stops down to only F9.5. My typical film SLR lenses stop down to F11, 16 and some to F22. Does the lens on my digital camera at 50 mm indicated produce greater depth of field at F8 than my film camera's 50 mm lens at F8 ?

Thanks. Mike T.
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Old Jan 24, 2003, 9:18 PM   #2
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Miket, depth-of-field has been discussed on this forum before so I you want you can look back for the previous discussions. Briefly, the CCD sensor is smaller than 35 mm frame size. This means that for the 7 series cameras a shorter focal length lens than would be used for a 35 mm camera will give the same field of coverage. Since 35 mm cameras are pretty much considered a standard Minolta has done us a favor by stating the focal length for the camera in 35 mm camera equivelents. The true focal length of the lens is 7.2 - 50.8 mm (page 160 of the manual), which is equivalent to a 28 - 200 mm 35 mm camera lens.

The depth-of-field for a lens increases as the focal length decreases - BUT ONLY FOR A FIXED IMAGE SIZE. Since the image of the 7 is smaller than that of a 35 mm camera it turns out that the depth of field is the same as it would be for a 35 mm camera with the equivalent focal lenth lens. What this means is that for all practical purposes just pretend that you are shooting a 35 mm camera and you will do fine on depth-of-field.
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Old Jan 24, 2003, 11:05 PM   #3
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Quote:
Since an F-stop is the ratio of lens diameter to focal length, I assume that F4 is F4 whether on a digital or 35mm film camera. True ?
True. You can transpose the same light/flash meter reading to the cameras wether it's a film or digital!
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Old Jan 25, 2003, 10:40 AM   #4
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Don, thanks for the note. Also, I did see a post in another forum regarding DoF charts for the 7i. They are posted at

http://www.alldimage.com/modules.php...download&cid=4.

These charts show the DoF at different F-stops and focal lengths. My question is there seems to be two sets of charts that show different data, but I can't figure out what causes the difference? The only fields that differ in the settings are " cc:1/1000 FL " and "cc:1/2000 FL". Does anyone know what these terms refer to ? Should not be shutter speed because that does not impact DoF. :?:
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Old Jan 25, 2003, 6:17 PM   #5
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Quote:
Originally Posted by Miket
Don, thanks for the note. Also, I did see a post in another forum regarding DoF charts for the 7i. They are posted at

http://www.alldimage.com/modules.php...download&cid=4.

These charts show the DoF at different F-stops and focal lengths. My question is there seems to be two sets of charts that show different data, but I can't figure out what causes the difference? The only fields that differ in the settings are " cc:1/1000 FL " and "cc:1/2000 FL". Does anyone know what these terms refer to ? Should not be shutter speed because that does not impact DoF. :?:
These refer to the size of the 'Circle of Confusion'. Key to calculating depth of field is how large an image circle from a point source can be and still be considered to be 'in focus'. This is called the Circle of Confusion.
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Old Jan 25, 2003, 8:46 PM   #6
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Default Question on autofocus when using flash

Just shot a bunch of pictures with my 7i using the Minolta 3600 HD external shoe-mounted flash. MANY of them were slightly out of focus. Exposures look ok.

In reading the manual I spotted the following:

P33 of the 7I manual states: `When flash is in use, the focus is set between 3.0m and 3.8m.` I don`t understand this statement.

The Minolta 3600H(D) allows me to shoot subjects that are farther than 3.8m. Will the camera properly autofocus on subjects further than 3.8m when I use my external flash ?

Does the 3.8m limitation apply only to the internal flash due to its lower guide number ?

Anybody know what this means ?
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Old Jan 25, 2003, 9:32 PM   #7
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Default Re: Question on autofocus when using flash

See here

http://www.stevesforums.com/phpBB2/v...?p=29253#29253
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Old Jan 26, 2003, 4:30 AM   #8
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See URL - http://www.binbooks.com/books/photo/i/l/53536AFB7F

Quote...

Your digital camera's very short focal length expands the DOF well
beyond what you've come to expect from a 35 mm camera, which has a much
larger image receiver and operates at ~5 times longer focal lengths.

A good rule of thumb with current consumer-grade CCD sizes is that
digital DOFs are roughly equivalent to DOF on a 35 mm camera with an
aperture 5 stops smaller. Thus a digital aperture of f/4 gives the same
DOF as a 35 mm f/22 (f/4 -> f/5.6 -> f/8 -> f/11 -> f/16 -> f/22).

Unquote...

Hope this helps, for more info, do a Google search under "DOF and Digital" :-)

- Ivan
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Old Jan 26, 2003, 9:12 AM   #9
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The above link doesn't seem to work for me! There's a good link over here as well:

http://www.stevesforums.com/phpBB2/v...ht=depht+field
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Old Jan 27, 2003, 6:27 PM   #10
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Default DoF

Thanks everyone, I had no idea that the DoF was so much greater on my 7i than my 35MM SLR.

Now if they just added a DOF preview button ....
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