Depth of field Question I can't find an answer to

[Terry_Hill]

I’ve searched the threads and other sites around the net and I can’t find an answer to my question as most comments on depth of field seem to relate to the effect it has on the effect of aperture on the same shot in a consistent light.

I’m thinking of getting a fast prime and was wondering if using a large aperture in a dark area with a longish shutter speed still give a shallow depth of field, or due to the longer shutter opening, does it increase the depth of field?

Thanks for any help on this, I'm sure it has a simple answer but for some reason I just don't get it!!!

[gonzogolf]

I’ve searched the threads and other sites around the net and I can’t find an answer to my question as most comments on depth of field seem to relate to the effect it has on the effect of aperture on the same shot in a consistent light.

I’m thinking of getting a fast prime and was wondering if using a large aperture in a dark area with a longish shutter speed still give a shallow depth of field, or due to the longer shutter opening, does it increase the depth of field?

Thanks for any help on this, I'm sure it has a simple answer but for some reason I just don't get it!!!


Shutter speed has no effect on depth of field.

[krb]

I've heard any claims that using a longer exposure has an affect on DOF.

I'm a little confused by part of the question. The point of a larger aperture is that it lets you shoot in low light without needing such a long shutter. And if you are on a tripod so you can do a long shutter then there's no reason not to just use a small aperture and use a really long shutter.

[EOS_JD]

Dof is affected by Aperture, Distance to subject and magnification

[tzalman]

I'm a little confused by part of the question. The point of a larger aperture is that it lets you shoot in low light without needing such a long shutter.
Sort of depends on just how low the light is, doesn't it?

[PhotosGuy]

I've heard any claims that using a longer exposure has an affect on DOF. It might increase the blur on a background if the background is moving, like trees in the wind, but the actual DOF is the same.

[EOS_JD]

It might increase the blur on a background if the background is moving, like trees in the wind, but the actual DOF is the same.

If the foreground is moving too it could be blurred with longer shutter speeds :)

[EOS_JD]

Sort of depends on just how low the light is, doesn't it?

But the DoF wouldn't be affected :)

[gcogger]

Wow - we're good at going off on tangents here :) It was a simple question, with a very simple and unambiguous answer that was given in the first post. What more needs to be said?

Of course, I've just been guilty of doing the same thing...

:oops:

[Terry_Hill]

Thanks guys I thought that might be the case.

So I take it that the depth of field is the same for a given subject if under strong light and a 1/500 sec shutter as it is for low light and 1/50 sec speed so other than lighting creatively flash would have no effect on depth of field. I'm ignoring any possible blur from motion caused by the difference in speed.

[gonzogolf]

Correct.

[DrPablo]

Dof is affected by Aperture, Distance to subject and magnification
And focal length.

http://www.dofmaster.com/equations.html

[Tom Reichner]

And focal length.

http://www.dofmaster.com/equations.html

Ummmmm . . . as used here, magnification means focal length.

[Madweasel]

So I take it that the depth of field is the same for a given subject if under strong light and a 1/500 sec shutter as it is for low light and 1/50 sec speed so other than lighting creatively flash would have no effect on depth of field. I'm ignoring any possible blur from motion caused by the difference in speed.
Well the (usual) point about flash in low light is that it enables you to use a smaller aperture and therefore get more depth of field, but as you're talking about your desire for a fast prime I guess you're thinking more about keeping the DOF shallow.

[gonzogolf]

If you are using shallow depth of field as means of isolating the subject you can do a similar isolation by controlling the light so that only the subject is illuminated with the background (and foreground if you wish) remains dark.

[Madweasel]

Ummmmm . . . as used here, magnification means focal length.
Not the same thing, though they can have similar effects. Here magnification means the enlargement of the image from its physical size on the sensor to the viewing size, either on print or screen. If you enlarge an image more, the slightly out of focus areas look a little more out of focus, making the depth of acceptable sharpness (the definition of DOF) shallower. That is why DOF calculators give a different answer for the same lens on different-sized camera sensors. The image from a smaller sensor is magnified more in the viewing.

[number six]

Well, magnification is a function of focal length and distance, so can't we say that DOF is a function of aperture and magnification? That would seem to cover all the variables...

-js

[Madweasel]

Mmm, I sense a simple answer getting more complicated. I'm going to resist the temptation!

[EOS_JD]

And focal length.

http://www.dofmaster.com/equations.html

No just magnification - but for what we are describing here you can substitute one for the other.

[EOS_JD]

Well, magnification is a function of focal length and distance, so can't we say that DOF is a function of aperture and magnification? That would seem to cover all the variables...

-js

Right but then focal distance is also a factor (distance to subject)

[EOS_JD]

Mmm, I sense a simple answer getting more complicated. I'm going to resist the temptation!
It can get more complicated but for what we use, dof is affected by distance to subject, magnification and aperture.

Online calculators are very good and once you play around with them you'll see the effect. As noted Magnification is used because different cameras have different magnification. Crop Cameras like Canon/Nikon using same focal length would have different dof for any given subject because of the small difference in sensor sizes (that's why I used that).

Cheers
JD

[number six]

Right but then focal distance is also a factor (distance to subject)

Of course distance figures into the calculations, but my point is that distance is already included in the magnification factor - magnification is a function of focal length and distance.

So you can say DOF depends on aperture, focal length and distance. Or you can say DOF depends on aperture and magnification.

Both accurately describe the relationship. There's no conflict here.

-js

[stsva]

I think the problem in this last series of posts is that there may be three different meanings for magnification - the "magnification" resulting from the lens focal length, the "magnification" resulting from the sensor size, and the "magnification" resulting from the image viewing size (which is directly related to the second meaning and is taken into account in the circle of confusion and other assumptions built into the depth of field calculations with regard to what constitutes "acceptable focus"). I really think it would clearer if each component affecting depth of field had a clear and separate name, rather than use what might be confusing terms. "Focal length" is pretty clear, "magnification" may not be.

[number six]

I think the problem in this last series of posts is that there may be three different meanings for magnification

Really? I thought the term was well understood in photography.

Magnification: the ratio between the size of the subject and the size of the image of that subject on the sensor (or film).

If you use a macro lens at 1X magnification (also called 1:1) to shoot a bug that's 10 mm long, the image on the sensor will be 10 mm long (about half the sensor size).

-js

[stsva]

Really? I thought the term was well understood in photography.

Magnification: the ratio between the size of the subject and the size of the image of that subject on the sensor (or film).

If you use a macro lens at 1X magnification (also called 1:1) to shoot a bug that's 10 mm long, the image on the sensor will be 10 mm long (about half the sensor size).

-js

It's well understood in photography; however, I don't think its use in this thread is equivalent to that well understood photographic meaning.

[number six]

It's well understood in photography; however, I don't think its use in this thread is equivalent to that well understood photographic meaning.

Until now. :p

[stsva]

Until now. :p

You've left me way behind with this one. Bye for now. :D

[number six]

Oh, don't go away! No offense intended!

[gonzogolf]

This reminds me of two codgers arguing about the best directions to give a traveler after his car has pulled away and gone several miles down the road :)

[number six]

:lol::lol::lol:

[stsva]

:lol::lol::lol:

At least I'm housebroken. ;):D

EDIT: Hope the TF isn't watching; I remember when number six got that title.

[DrPablo]

No just magnification - but for what we are describing here you can substitute one for the other.DOF equations use FL as a variable. The issue of magnification has to do with how you define the acceptable circle of confusion, which in itself is a variable in DOF equations.

[EOS_JD]

Of course distance figures into the calculations, but my point is that distance is already included in the magnification factor - magnification is a function of focal length and distance.

So you can say DOF depends on aperture, focal length and distance. Or you can say DOF depends on aperture and magnification.

Both accurately describe the relationship. There's no conflict here.

-js

No.... Sensor size also affects magnification. So to simply say that at x distance and y focal length you get z dof, that is not the case using a crop camera and a ff camera.

Focal length whilst it matters is not the whole story so magnification is what matters. Only one is accutate

[EOS_JD]

DOF equations use FL as a variable. The issue of magnification has to do with how you define the acceptable circle of confusion, which in itself is a variable in DOF equations.

No wrong.
Whilst the DOF calculators use focal length, they also use camera type and that works out the sensor size which affects the magnification.

DoF affects the CoF not the other way round. DoF is only affected by Distance to subject, Aperture and Magnification

[stsva]

No wrong.
Whilst the DOF calculators use focal length, they also use camera type and that works out the sensor size which affects the magnification.

DoF affects the CoF not the other way round. DoF is only affected by Distance to subject, Aperture and Magnification

Depth of field is an artificial construct that depends on certain assumptions; the circle of confusion is one of those assumptions, which in turn is based on assumptions about image size and viewing distance.

[DrPablo]

Depth of field is an artificial construct that depends on certain assumptions; the circle of confusion is one of those assumptions, which in turn is based on assumptions about image size and viewing distance.
Yes, exactly, and for this reason magnification / final output size / viewing distance / focal length are all completely inseparable factors when quantitatively expressing DOF. It so happens that focal length is a variable when calculating DoF and magnification factor is not -- though when you're talking about resolution (and therefore to some degree sharpness) magnification is the single most important variable of all.

[20droger]

Jeez, Guys! Why not take a subject that so many already have trouble understanding and make it even more complicated?!!

Depth of field is a function of four factors: the aperture (a); the focal length of the lens (f); the focus distance (P); and the diameter of the circle of confusion (c). This is shown in the standard DoF equation:

DoF = P * f / { f ± a * c * [( P / f ) - 1 ]}

The "±" is "+" for the near limit and "–" for the far limit. If the expression in braces "{...}" is equal to or less than zero, the far limit is infinity.

Depth of field is not a function of magnification. Magnification is an entirely separate thing that is itself a function of focal length and focus distance.

Magnification is not a factor in determining the CoC diameter. The CoC diameter is traditionally derived from the mean visual acuity of the human eye. This is realized in practical terms as the print size and viewing distance. For a given print size, the enlargement (not magnification) of the image is a function of the sensor size. The CoC diameter is therefore a function of the sensor size.

The CoC diameter is somewhat arbitrary, with several assumptions in its determination. Most commonly, the CoC diameter is approximately the diagonal of the film/sensor image divided by 1400. For a 36×24mm (35mm full-frame) image, this gives a CoC diameter of 0.031mm. However, different manufacturers disagree: Leica uses 0.025mm, while Canon uses 0.035mm.

For a 1.6× APS-C sensor, the most commonly used CoC diameter is 0.019mm.

[DrPablo]

Magnification is not a factor in determining the CoC diameter. Ehh, I wouldn't go this far. The desired CoC is determined based on assumptions about viewing distance for a given output size, and how you get to that output size is determined by magnifications from capture size. Thus, an 8x10 contact print from an 8x10 view camera has a FAR more lenient CoC requirement than an 8x10 enlargement from a point and shoot with an 8x15mm sensor, because you're magnifying what like 25x to achieve the same output.

[20droger]

Ehh, I wouldn't go this far. The desired CoC is determined based on assumptions about viewing distance for a given output size, and how you get to that output size is determined by magnifications from capture size. Thus, an 8x10 contact print from an 8x10 view camera has a FAR more lenient CoC requirement than an 8x10 enlargement from a point and shoot with an 8x15mm sensor, because you're magnifying what like 25x to achieve the same output.
What you're speaking of here is enlargement, not magnification. Magnification is a camera thing, not a print thing.

Using the same standards as the 35mm community, the CoC diameter for an 8×10" view camera would be about 0.23mm. It remains a function of visual acuity and image size. That the image size and print size are the same is irrelevant.

Of course, no 8×10 view camera photographer would apply 35mm standards to his images.

[DrPablo]

Yes, I am, but that's because I assumed that the original mention of magnification here actually meant to mean enlargement. And yeah, I know about the visual acuity issue, I have a great article written for View Camera magazine by a physicist about this.

[Terry_Hill]

Ok my feeble mind is destroyed!!!!!

I think my original question was trying to work out if shutter speed effected depth of field so by using a longer shutter speed more was in focus..... looks like from the formula (my algebra is a little rusty though) that it has no effect and so my question is answered.