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Depth of Field

 
 
ASAAR
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      09-18-2006
On 18 Sep 2006 10:46:28 -0700, Scott W wrote:

> f 32 vs 1.8 is a ratio of 17.8 to 1, a much larger range then the F828.
>
> So I was not trying to say the 20D has 16 time more range but that the
> 20D has an f stop range that is greater then 16, sorry for the
> confusion.


Oh, ok. But simply dividing the actual max f/stop number by the
min f/stop number isn't providing a very useful measure of anything,
is it, other than making greater/lesser comparisons? The number of
f/stops is useful for those wanting to adjust shutter speeds and
apertures to get equivalent exposures without having to do any math.
If you want to compare the amount of light collected by using
different apertures, the ratio of the squares of the f/stops is
useful. But your 17.8 to 1 ratio is simply the ratio of the
diameters of the f/1.8 and f/32 apertures, and not often spoken of
photographically, although it might be used as an intermediate value
in some calculations. Most people would simply say that f/1.8
through f/32 encompasses a range of 9 f/stops. Saying that it also
represents a ratio or range of 17.8 to 1, while accurate, is bound
to cause some head scratching for those more familiar with the
traditional way of comparing f/stops. At least it did for me.

 
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Scott W
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      09-18-2006

ASAAR wrote:
> On 18 Sep 2006 10:46:28 -0700, Scott W wrote:
>
> > f 32 vs 1.8 is a ratio of 17.8 to 1, a much larger range then the F828.
> >
> > So I was not trying to say the 20D has 16 time more range but that the
> > 20D has an f stop range that is greater then 16, sorry for the
> > confusion.

>
> Oh, ok. But simply dividing the actual max f/stop number by the
> min f/stop number isn't providing a very useful measure of anything,
> is it, other than making greater/lesser comparisons? The number of
> f/stops is useful for those wanting to adjust shutter speeds and
> apertures to get equivalent exposures without having to do any math.
> If you want to compare the amount of light collected by using
> different apertures, the ratio of the squares of the f/stops is
> useful. But your 17.8 to 1 ratio is simply the ratio of the
> diameters of the f/1.8 and f/32 apertures, and not often spoken of
> photographically, although it might be used as an intermediate value
> in some calculations. Most people would simply say that f/1.8
> through f/32 encompasses a range of 9 f/stops. Saying that it also
> represents a ratio or range of 17.8 to 1, while accurate, is bound
> to cause some head scratching for those more familiar with the
> traditional way of comparing f/stops. At least it did for me.


It shows that for a small sensor camera this is very limited control of
DOF compared to a larger area sensor.

Scott

 
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acl
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      09-18-2006
Scott W wrote:
> ASAAR wrote:
>>On 18 Sep 2006 10:46:28 -0700, Scott W wrote:
>>
>>>f 32 vs 1.8 is a ratio of 17.8 to 1, a much larger range then the F828.
>>>
>>>So I was not trying to say the 20D has 16 time more range but that the
>>>20D has an f stop range that is greater then 16, sorry for the
>>>confusion.

>> Oh, ok. But simply dividing the actual max f/stop number by the
>>min f/stop number isn't providing a very useful measure of anything,
>>is it, other than making greater/lesser comparisons? The number of
>>f/stops is useful for those wanting to adjust shutter speeds and
>>apertures to get equivalent exposures without having to do any math.
>>If you want to compare the amount of light collected by using
>>different apertures, the ratio of the squares of the f/stops is
>>useful. But your 17.8 to 1 ratio is simply the ratio of the
>>diameters of the f/1.8 and f/32 apertures, and not often spoken of
>>photographically, although it might be used as an intermediate value
>>in some calculations. Most people would simply say that f/1.8
>>through f/32 encompasses a range of 9 f/stops. Saying that it also
>>represents a ratio or range of 17.8 to 1, while accurate, is bound
>>to cause some head scratching for those more familiar with the
>>traditional way of comparing f/stops. At least it did for me.

>
> It shows that for a small sensor camera this is very limited control of
> DOF compared to a larger area sensor.
>
> Scott
>


In fact, all else (focal length, print size etc) being kept constant,
the DOF (ie x_max-x_min, where x_{max/min} the max/min distance of
acceptable sharpness) is directly proportional to the (numerical) value
of the f/stop (well, to a good approximation, at least). So the number
you quoted is the ratio of the largest attainable DOF to the smallest.

This is irrespective of the sensor size, by the way.
 
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John Turco
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      09-19-2006
Dave Martindale wrote:
>
> John Turco <(E-Mail Removed)> writes:
>
> >Exceedingly. (Maximum telephoto does the trick.)

>
> Maximum telephoto gives you the shallowest DOF available with that
> camera and lens.
>
> But a lens that gave the *same* field of view on a DSLR would have
> considerably shallower DOF at the *same* f/number. So the shallowest
> DOF on the DSLR is shallower than you can achieve on the small-sensor
> camera.
>
> There are circumstances where you can't use maximum telephoto because
> you need to cover a wider FOV. The larger sensor DSLR maintains the
> same advantage at all FOVs: with FOV and f/number constant, the DSLR
> DOF is going to be shallower.
>
> And with a field of view that would qualify as "normal" or moderate
> telephoto, the DSLR will be able to use fixed focal length lenses that
> are a stop or two faster than the zoom in your camera, further reducing
> the minimum DOF available.
>
> So if you want the shallowest DOF for a given FOV, the larger the sensor
> the better.
>
> Dave



Hello, Dave:

Is it safe to assume that this also holds true for film cameras, then?
I have two Pentax SLR's (ZX-60 & Auto110), although I've never shot
anything with them, yet.

Would the ZX-60 (35mm) provide a greater range of DOF, than the Auto110
(110mm)?


Cordially,
John Turco <(E-Mail Removed)>
 
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Dave Martindale
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      09-19-2006
http://www.velocityreviews.com/forums/(E-Mail Removed) writes:

>Indeed, the same FOV and the same aperture on a camera with a sensor A
>times smaller would imply a DOF A times larger (for both images printed
>to the same size).


Yes, as long as we agree that "A times smaller" means that both
dimensions of the sensor are scaled by a factor of 1/A. In other words,
if the sensor is 1/3 as large in both dimensions, it is 1/3 the size,
not 1/9 the size. Some people here use area, not scale factor, as a
measure of "size".

Dave
 
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Dave Martindale
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      09-19-2006
John Turco <(E-Mail Removed)> writes:


>Hello, Dave:


>Is it safe to assume that this also holds true for film cameras, then?
>I have two Pentax SLR's (ZX-60 & Auto110), although I've never shot
>anything with them, yet.


>Would the ZX-60 (35mm) provide a greater range of DOF, than the Auto110
>(110mm)?


That's a slightly different question than "which can achieve minimum
DOF?".

The general rule applies to film as well as digital, so *at any one
specific aperture*, any 35 mm camera will have less depth of field than
any 110 camera covering the same field of view and set to the same
aperture.

Now, the 35 camera is likely to have a maximum aperture comparable to
that of the 110, so when both are wide open, the 35 will have the
shallower DOF. But the minimum apertures are likely to be somewhat
different (and depend on the lens chosen), so either one could have the
greatest DOF.

Dave
 
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