Depth of field vs dynamic range

<p>I've recently acquired a point and shoot (powershot A800) and while testing it out and checking out the metering, pondered about how to reduce depth of field on a 7mm lens. Upping the exposure and lightly blowing out the highlights seems to provide at least an illusion of lower depth of field. In fact if I underexpose slightly, the depth of field apparently seems to increase. Has anyone else observed it? I do regularly see wedding photographers use a blown out background to keep attention on the subject.</p>

<p>After pondering some more, it seems depth of field is a property of the print (and viewing distance), not the lens. If the print could not resolve finer gradations of gray (I've never seen DOF measurement scales in anything but black and white), then is it not possible that the depth of field would increase if finer gradations of grey could be resolved?</p>

<p>It is common knowledge that the response curves for film and digital are vastly different. The gaussian of the film response would also make it difficult to resolve gradations near extreme highlights or shadows, probably thus leading to a reduced perceived depth of field. With digital having a mostly linear response, we do not gain from these benefits, particularly at the highlights, till they start to blow out. So does depth of field really do differ in the real world, between digital and film (or a large high dynamic range sensor with enough "knee")?</p>

<p>Print size does have some effect on perceived DOF. DOF is really just the perception of sharpness, in that the circle of confusion is small enough to give the perception of sharpness. As print size increases, those circles get larger, until the image stops appearing sharp at the same viewing distance. Smaller apertures give this effect to a greater part of the image, but in reality, only a plane parallel to the sensor is truly in focus. From that plane, everything in the image gradually becomes less sharp. The rate at which this happens is dependent on aperture and print size.</p>

<p>Since blown highlights or clipped shadows remove detail, it is logical that these areas do not appear to be sharply focused, since the detail is gone. However, it doesn't have anything to do with DOF. Try photographing a light subject against a dark background, and overexpose your subject. You'll lose detail in your subject, even at the plane of focus.</p>

<p>Hi Indraneel. If you want to produce a shallow depth of field on a compact camera - or at least, if you want to blur the background - can I suggest taking two shots, one focussed (possibly manually) much closer than the subject. This should give you a blurred background, which you can then blend digitally with the in-focus subject. I believe someone (possibly Nikon?) recently produced a patent trying to automate this process.<br />

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Or you could just blur the background digitally with a lens-blur filter, of course, but by using the camera you retain some depth information in generating the out-of-focus area, which might make it look more realistic. That said, I can't imagine the primary design criterion for that camera's lens was the quality of its bokeh. :-)</p>

<p>For reasons already suggested, this is a reason there are single lens reflex and larger format "rangefinder" cameras. Point and shoot cameras are made the way they are so that depth of field is maximized to reduce focus errors. Look up the concept of "hyperfocal distance", for example.<br>

With larger and more controllable cameras, you set the aperture to the widest you can, to make a shallow depth of field. Then you focus on the psychological 'center' -- the eyes. If they are sharp, people tend not to notice if other parts of the face are not so sharp.</p>

 

<blockquote>Point and shoot cameras are made the way they are so that depth of field is maximized to reduce focus errors.</blockquote>

 

<p>I think that's an effect rather than a cause. The tiny sensor in most compact digital cameras means that the lens focal length for a common field of view (wide-ish to mildly telephoto, for the small compacts) is correspondingly short - 6.6-21.6mm, for the 37-122mm-(full-frame-)equivalent zoom on the A800. It's listed as f/3-5.8, but it's got the depth of field as if the full-frame lens were f/16.8-32.8. To get the depth of field of a full-frame f/3-5.8 37-122mm lens (which isn't particularly exotic) it would have to be f/0.54-1.0, which would both make it big and probably produce horrible optical quality - it's very difficult to make a good lens anything like this fast. However, compact cameras are often perfectly capable of coping with a shallow depth of field at macro distances, so it's not like they fundamentally can't cope with a shallow depth of field - though I admit that it helps to compensate for the speed of contrast-detect autofocus if the subject is in the hyperfocal range.<br />

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Indraneel: The response to varying light levels (at least bloom, leakage and noise management) certainly affects the softness of different portions of the image, but the depth of field, as such, is a function of the aperture, enlargement, and geometry of the scene. Search for about half the threads with my postings in them in photo.net for a longer discourse. :-)</p>

<p>Several years ago I had a Canon A80 digicam. It had something called a Portrait Mode, and messing with that left me with the impression that the camera would focus such that the principal subject was at the far end of whatever was the depth of field at the working distance and the set focal length and aperture. Deliberate front focussing, in other words.</p>

<p>Mukul - good point, and I've heard of that being done in an attempt to blow the background further out of focus. I suspect it's quite difficult to get right under user control, especially with "depth of field" being a term that's so dependent on usage, but it might be worth a try.</p>

<p>Andrew, the 'strategy' of eliminating focus problems through shorter lenses with depth of field goes way back before digital cameras were even dreamt of. Of course, it's not the only reason for the way these cameras are.<br>

It was one reason zone focus could work for most people.</p>

<p>JDM - true, but the result of that (in 35mm) was to rely on a moderately wide-angle lens and nobody using a cheap camera wanting to use it to produce large enough prints to make the depth of field shallow. Modern digital compacts have perfectly functioning autofocus (usually, except those with a very small aperture and sensor - like some cell phones) and zoom lenses that include the telephoto end. People view the result at maximum magnification (1:1 pixels) on screen. In other words, I don't think you'd get away with relying on focal length to give you a deep depth of field with a modern full-frame digital sensor - camera uses and expectations have changed. With a small sensor and small physical aperture on the lens, however, the deep depth of field is a convenient bonus. Unless, of course, that's not what you want!</p>

<p>Andrew, as far as I can tell, you're arguing with yourself. That may be all true, but has little to do with my simple historic observation.</p>

<p>It wouldn't be the first time I've argued with myself. :-) Historically, I don't dispute you - cheap 35mm cameras were given short focal length lenses to increase the depth of field. I just suggest that, for a modern compact digital camera, the primary design goal was to use a small, cheap sensor, which happens to result in a short, cheap, portable lens and (at reasonable f-stops) a deep depth of field. I don't think the intent was to produce a camera with a large depth of field, so the manufacturers chose a small sensor. People <i>like</i> the option of a shallow depth of field (you can [almost] always stop down), so once you've got autofocus on your camera, there would be a lot more large-sensor compacts on the market if the economics of sensor size and portability weren't driving sales. Chronology is not the same as cause and effect.</p>

<p>eek! forgot to follow up!</p>

<p>Thank you everyone for your responses. True, light subject on a dark background does not benefit from over exposure. Here the recommended exposure is better and the dark background begins to lack detail. I feel the A800 has about +1 to -1 stop dynamic range (default settings, without pulling up blacks using curves or levels in post)</p>

<p>Closer focusing does help for most things except portraits where the nose becomes sharper than the eyes. By some judicious mix of close focus and exposure compensation, the "bokeh" does not look all that bad either. The only diaphragm that I can see is perfectly circular. At smaller apertures it becomes somewhat semi circular, and appears to have at least 6 blades. The shutter is just behind it.</p>

<p>Portrait mode on the A800 does not have any benefit with regards to depth of field. The focus point is the sharpest.</p>

<p>From all my experiments, at least on this camera, the lens appears to be better than the sensor... which is not bad either, but they could have reduced the megapixels more and improved the dynamic range -- more megapixels should have cost more, right? that's what I paid less for... ah, but I need the less dynamic range to decrease depth of field... choices, choices...</p>