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06-15-2011, 10:06 PM   #1
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Depth of Field Question... (probably a dumb one)

For a given FL, aperture and distance to the object, is it fixed or does it also depend on the optics involved, e.g. for some 85/1.4 lenses it is greater at the same aperture and distance than others?

(sorry if this is a dumb question - my understanding of optics is as shallow as the noctilux DoF at f/0.95)

06-15-2011, 11:29 PM   #2
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As you suspected, DOF is determined purely by FL, aperture and subject distance. Interestingly, if you're interested in getting an image of an object of a certain size to occupy a given area of frame (e.g. in portrait or macro photography), DOF is dependent on aperture only (though you'll need to vary subject distance as you vary FL).
06-16-2011, 12:15 AM   #3
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This Spreadsheet can help you get a grasp on how the various optical & physical elements come together.
06-16-2011, 12:45 AM   #4
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QuoteOriginally posted by DanielT74 Quote
For a given FL, aperture and distance to the object, is it fixed or does it also depend on the optics involved, e.g. for some 85/1.4 lenses it is greater at the same aperture and distance than others?

(sorry if this is a dumb question - my understanding of optics is as shallow as the noctilux DoF at f/0.95)
Certainly not a dumb question, as many people consider that depth of field is an inherent property of a particular lens. See how many well-regarded reviewers state in their writings that such 50 mm lens has a much deeper depth of field than others, or that the depth of field of a particular 85 mm lens is shifted to the back compared to another one...

Cheers!

Abbazz

06-16-2011, 02:11 AM   #5
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QuoteOriginally posted by Abbazz Quote
Certainly not a dumb question, as many people consider that depth of field is an inherent property of a particular lens. See how many well-regarded reviewers state in their writings that such 50 mm lens has a much deeper depth of field than others, or that the depth of field of a particular 85 mm lens is shifted to the back compared to another one...

Cheers!

Abbazz
Yes, I've read some reviews which state that such-and-such a lens seems to have greater depth of field (not so sure about "much deeper" though). One which springs to mind involved a test of 28mm Nikon, Olympus and Zeiss lenses, the extra DOF applying to the Zeiss. I was somewhat puzzled at the time, so I think it would be interesting to get to the bottom of this.

For an explanation of DOF (in a traditional sense), try these:

Understanding Depth of Field in Photography
A Flexible Depth of Field Calculator

But what about this:

http://www.gbosecurity.com/downloads/GBO_PressRelease_090917_USLetter.pdf

Is there really some (perhaps unusual) optical design which can give enhanced DOF? Even if there is, would there be any differences in DOF for the "standard" lens configurations?

If not, I suppose it comes down to the subjective effects of MTF characteristics and bokeh.
06-16-2011, 04:40 AM   #6
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The important difference to me between lenses of different maximum apertures (given the same aperture, subject distance and focal length) is the OOF rendering, particularly in the highlights.

DoF should theoretically (and intuitively) be the same for instances where the variables that affect DoF are being kept constant.
06-16-2011, 04:59 AM   #7
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You should get this book, everything is well explain

Understanding Exposure, Bryan Peterson - Shop Online for Books in Australia - Fishpond.com.au
06-16-2011, 06:17 AM   #8
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QuoteOriginally posted by Ash Quote
The important difference to me between lenses of different maximum apertures (given the same aperture, subject distance and focal length) is the OOF rendering, particularly in the highlights.

DoF should theoretically (and intuitively) be the same for instances where the variables that affect DoF are being kept constant.
I think ash has hit the point as clearly as any one.

I have, for example, 3 different 135mm lenses in M42 mount:
- S-M-C Takumar 135/3.5
- Tele-Lentar 135/2.8 Preset
- XR Rikenon 135/2.8

They all perform differently, even when set to the same aperture, the Rikenon has a color cast and is at any aperture 1 stop slower to yeild the same exposure,

I find that the Tele-Lentar has a much different Bokeh, or out of focus rendering, which I largely attribute to the 14 blade aperture that collapses to a circle at all f-stops, as opposed to the 6 blade aperture of the other 2 lenses, AND what I think is more important, is the difference in location of the aperture blades within the lenses.

The impact of this difference in Bokeh is that the tele lentar seems to give much better separation between foreground (in focus) and back ground (out of focus) areas. The impression is that it has "less depth of field" when in fact, I think in looking closely at the images produced by all 3, the in focus area is exactly the same.

06-16-2011, 02:06 PM   #9
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QuoteOriginally posted by m42man Quote
As you suspected, DOF is determined purely by FL, aperture and subject distance. Interestingly, if you're interested in getting an image of an object of a certain size to occupy a given area of frame (e.g. in portrait or macro photography), DOF is dependent on aperture only (though you'll need to vary subject distance as you vary FL).
How will format effect it? i.e. micro 4/3, aps-c, 36x24 . . .

http://www.dofmaster.com/dofjs.html
06-16-2011, 03:48 PM   #10
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There are 3 other factors the need to be adding to DOF. Crop, display/viewing distends (i.e. print size) and your individual tolerates for what is in focus. If you make crop of a photo and display it the same size as the un-cropped it will look like it has less DOF. If you print and display it small (like on a PC monitor) it will look like it has more DOF but if you print it larger (like at 24x36 at 3 feet) it will look like it has less DOF. If you have something that has no easily defined focus (like grass) or you are not look to close it can look like more DOF.

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06-16-2011, 06:07 PM   #11
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DOF is a complex product of photography, presentation, and perception.

1) PHOTOGRAPHY: Frame size (film or sensor format); lens focal length and aperture; subject-to-lens distance; lighting; fore- and back-grounds; these are all (theoretically) controllable by the photographer.

2) PRESENTATION: Display size, framing, brightness / contrast, position; these may or may not be controllable by the photographer.

3) PERCEPTION: How an audience mentally 'sees' an image is totally beyond the photographer's control, unless you are adept with mind-control rays and ocular surgery.

With ALL ELSE HELD CONSTANT, different lenses of the same focal length and aperture, on the same camera, should produce the same DOF. This assumes that those lenses are calibrated equally. But just because two lenses are CLAIMED to both be 50/2 or whatever, doesn't mean that they ARE. Those differences, and varied light transmission due to different coatings, may prompt the photographer to make adjustments so that DOF is NOT the same.

Same with frame size: The same lens on cameras with 645, 135/FF, APS-C, and m4/3 frames will produce the same DOF -- WITH ALL ELSE HELD CONSTANT. But all else is usually NOT held constant. The photographer may change position to fill the visible frame... and that changes the DOF.

Read the Wikipedia entry on DOF if you dare. Then, despair. It is horribly complex.
06-16-2011, 10:37 PM   #12
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QuoteOriginally posted by Blue Quote
How will format effect it? i.e. micro 4/3, aps-c, 36x24 . . .

Online Depth of Field Calculator
I was assuming the OP was wanting to compare lenses on the same camera, but of course the sensor size is a major factor. For a given FOV, subject distance and aperture (f-number), DOF increases inversely with sensor (linear) size. So, the smaller the format, the greater the DOF. (I think one way of explaining this is that, for a given FOV you need a smaller FL, so the subject distance becomes larger relative to the lens's dimensions.)

However, if you were to keep the actual aperture dimensions constant, DOF would remain constant. You would find yourself having to expose with different f-numbers though, so you would need different shutter speeds and/or ISOs.
06-16-2011, 11:00 PM   #13
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Yes, DOF is the same between lenses for a particularly focal length at a particular f/stop assuming that all other factors (subject distance, capture frame size, final image size, and final viewing distance) are kept constant.

OOF characteristics (aka as bokeh) is different from DOF and many of the comments regarding the "quality" of DOF are actually talking about bokeh.


Steve


BTW...DOF does not really exist, particularly if you pixel peep. It is a subjective concept related to acceptable sharpness in front of and behind the actual frame of focus with emphasis on the word "acceptable".
06-16-2011, 11:05 PM   #14
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QuoteOriginally posted by m42man Quote
...if you were to keep the actual aperture dimensions constant...
Physical aperture (diameter in mm) rather than relative aperture (f/stop). I was going to mention that...
06-16-2011, 11:35 PM   #15
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QuoteOriginally posted by stevebrot Quote
Physical aperture (diameter in mm) rather than relative aperture (f/stop). I was going to mention that...
Yes, you've made it rather clearer than I did...
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