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DPChallenge Forums >> Tips, Tricks, and Q&A >> Aperture vs shutter speed and DOF
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12/22/2004 01:45:21 PM · #1
Hi
I'm a newbie to photography and am just beginning to understand the technical aspects of it. I understand (more of less) how shutter speed affects subject movent and how aperture affects DOF.
One thing I've been wondering about, at a constant aperture does the time then control DOF? Let's say I fix my lenst at it's sweet spot of, say, F/8 but I can control the amount of light with ND filters. Let's say at F/8 the correct exposure is at 1/125 so if I screw on a 2xND and adjust the time to 1/30 will I get a deeper DOF?
Thanks doctornick in advance for your answer - I bet you'll be the first to reply :)
12/22/2004 01:50:44 PM · #2
No the apeture sets the dof and then the shutter speed will determine how much light you let in. On "Program" mode of most cameras the camera can select a "perfect" shutter with any given aperture, and then on manual you can override that to over/underexpose the shot or create the exact amount of light you want.
12/22/2004 02:03:27 PM · #3
theh DOF depends opon the focal length which really governs the total lens opening for a fixed aperture, the larger the opening, the shallower the DOF.
e.g. F8 at 50mm will have a deeper DOF as compared to F8 at 200mm.

Also the DOF will depend upon the distance of the subject from your lens. subject at 1feet will have a shallower DOF than subject at 4feet.
12/22/2004 02:04:04 PM · #4
For more information that you need to know see: Depth Of Field.

I highly recommend getting out a tripod and setting up a scene, probably outdoors with a tree or something in the foreground and something else, house or fence in the background. Then take the same photo at every aperture you can. With the D70 you can do this on the "P" mode - by rotating one of the dials (can't immediately recall which one) it will cycle through all the same equvalent exposure settings. Then you can see how they all are relative to each other on your camera and your lens.

(Edit: Miss-saw your camera - thought you were a fellow Nikonian - don't know squat about the 10D, you might have to do it all on M?)

Message edited by author 2004-12-22 14:06:04.
12/22/2004 02:07:55 PM · #5
Shutter speed has absolutely NO bearing on DOF, DOF is solely determined by your aperture and the focal length of your lens. For example if you have a 15mm wide agnle, you'd be very hard-pressed to get a blurry background with f/2.8, on the other hand a 200mm at f/2.8 will have a very shallow DOF in the range of a few centimetre.

LOL and no I was not the first to answer that...I slipped...LOL, must be getting old. ;)

Message edited by author 2004-12-22 14:08:26.
12/22/2004 02:09:03 PM · #6
ok. to clarify the question:
Is DOF a function of, and ONLY of, aperture? or can other factors (like speed etc) come into play as well? (I mean on any given lens, at constant zoom)
12/22/2004 02:10:04 PM · #7
Originally posted by doctornick:

Shutter speed has absolutely NO bearing on DOF, DOF is solely determined by your aperture and the focal length of your lens. For example if you have a 15mm wide agnle, you'd be very hard-pressed to get a blurry background with f/2.8, on the other hand a 200mm at f/2.8 will have a very shallow DOF in the range of a few centimetre.

LOL and no I was not the first to answer that...I slipped...LOL, must be getting old. ;)

ok thank you. you answered the clarified Q before I posted it so you're still quick enough :)
12/22/2004 02:10:15 PM · #8
The third element of depth of field is distance to subject. If you do a photo at f/8 @ 50mm and the subject is 20 feet away, the depth of field will be deeper than if the subject was 10 feet away at the same settings.
12/22/2004 02:11:16 PM · #9
Originally posted by doctornick:

Shutter speed has absolutely NO bearing on DOF, DOF is solely determined by your aperture and the focal length of your lens...


I was just reading up on DOF and ran into a claim that DOF is NOT a factor of the focal length but of aperture only - this seems convincing to me...?
12/22/2004 02:11:31 PM · #10
You can use this Depth of Field Calculator to figure the DOF you need out. It doesn't depend on the shutter speed though, as people have pointed above, only on focal distance of your lens and aperture.
12/22/2004 02:13:07 PM · #11
Originally posted by joebok:

I was just reading up on DOF and ran into a claim that DOF is NOT a factor of the focal length but of aperture only - this seems convincing to me...?


A 20mm lens would have much bigger DOF at a set aperture than a 200mm lens at the same aperture.
12/22/2004 02:13:10 PM · #12
Originally posted by yurasocolov:

You can use this Depth of Field Calculator to figure the DOF you need out. It doesn't depend on the shutter speed though, as people have pointed above, only on focal distance of your lens and aperture.

thank you. interesting link.
12/22/2004 02:15:22 PM · #13
as mentioned by Gaurawa, the distance to your subject (from the lens) will also affect DOF. But shutter speed will not.

3 main ingrediants of DOF: Focal length, aperture, and distance to subject.
12/22/2004 02:15:28 PM · #14
Originally posted by yurasocolov:

Originally posted by joebok:

I was just reading up on DOF and ran into a claim that DOF is NOT a factor of the focal length but of aperture only - this seems convincing to me...?


A 20mm lens would have much bigger DOF at a set aperture than a 200mm lens at the same aperture.


That is what is often said, but I don't believe it - why?

Edit - I am beginning to understand - aperture AND distance to subject - in the arguement. If you change lenses and adjust your distance to the subject so that the subject is the same size, then the DOF will be the same in each photo.

Message edited by author 2004-12-22 14:20:40.
12/22/2004 02:18:24 PM · #15
Well I type too slow. If I had seen the few posts that beat me by a couple minutes, I would have realized my post was unnecessary.
12/22/2004 02:19:07 PM · #16
Originally posted by joebok:

Originally posted by yurasocolov:

A 20mm lens would have much bigger DOF at a set aperture than a 200mm lens at the same aperture.


That is what is often said, but I don't believe it - why?


It's not about beliefs, it's about pure math. Depth of field is inversely proportional to the focal length of your lens squared. Believe it or not.

Here is a little more info on the subject.

edit: fixed the quote

Message edited by author 2004-12-22 14:20:02.
12/22/2004 02:19:25 PM · #17
Originally posted by yurasocolov:

A 20mm lens would have much bigger DOF at a set aperture than a 200mm lens at the same aperture.

This isn't quite true and depends on the conditions of the comparison. It is a common mistake to judge the depth of field by the degree of background blur. Yes, a telephoto lens may have a large amount of background blur, but this is not a matter of DOF. Based on the definition of depth of field ("the range of object distances within which objects are imaged with acceptable sharpness"), it should be judged by the in-focus parts of the image, no matter how "blurry" the out-of-focus parts may be.

Edit to add: even the link you reference in your second post says "Depth of field is also NOT directly related to background blur."

Message edited by author 2004-12-22 14:20:18.
12/22/2004 02:22:50 PM · #18
thank you all for very swift replies and interesting links.
I was gonna ask why, but as I type Yura has come up with the relevant link. now I just need some coffee and a bit of hard thinking. (well, this can't be more difficult than Kant or McDowell).
12/22/2004 02:24:04 PM · #19
I was only talking about DOF vs focal length, not the blur.
12/22/2004 02:30:45 PM · #20
Originally posted by yurasocolov:

Originally posted by joebok:

Originally posted by yurasocolov:

A 20mm lens would have much bigger DOF at a set aperture than a 200mm lens at the same aperture.


That is what is often said, but I don't believe it - why?


It's not about beliefs, it's about pure math. Depth of field is inversely proportional to the focal length of your lens squared. Believe it or not.

Here is a little more info on the subject.

edit: fixed the quote


I think the confusion is that there are two variables - aperture and distance to subject. If you keep aperture constant but allow the distance to subject to vary so as to keep your subject the same size in the image, then DOF is constant. Changing to the correct distance to keep the subject the same size with different focal lengths is the balancing item - so if you DON'T change position, then your effective DOF is different with different lenses.
12/22/2004 02:33:42 PM · #21
Originally posted by yurasocolov:

I was only talking about DOF vs focal length, not the blur.

And that is what I'm talking about. Saying the DOF of a 20mm lens at /8 is "much bigger" than a 200mm lens at /8 is not necessarily true. DOF is defined as "the range of object distances within which objects are imaged with acceptable sharpness". And the actual range of distance (which determines DOF) may actually be identical, depending on object distance.

If the object distance is kept the same, the 20mm image will indeed show a greater DOF, but also a larger field of view, and, consequently, a smaller subject. On the other hand, if the subject is framed the same, it is an entirely different situation. Now the DOF between the two lenses may be comparable or even identical. However, in both cases the photographs are not the same and the comparison is one between two different pictures. The former is a comparison with preservation of the perspective (viewpoint) and different magnifications, the latter preserves the magnification (the size of the imaged subject) but not the perspective.

Message edited by author 2004-12-22 14:35:23.
12/22/2004 02:55:19 PM · #22
Originally posted by EddyG:

Originally posted by yurasocolov:

A 20mm lens would have much bigger DOF at a set aperture than a 200mm lens at the same aperture.

This isn't quite true and depends on the conditions of the comparison. It is a common mistake to judge the depth of field by the degree of background blur. Yes, a telephoto lens may have a large amount of background blur, but this is not a matter of DOF. Based on the definition of depth of field ("the range of object distances within which objects are imaged with acceptable sharpness"), it should be judged by the in-focus parts of the image, no matter how "blurry" the out-of-focus parts may be.

Edit to add: even the link you reference in your second post says "Depth of field is also NOT directly related to background blur."


Hrm, this could partly explain something I've been noticing but still not something I can work out. Looking at professional images of birds I've seen some images shot at f/8 or so and yet the background is completely blurred with that almost complete color blur (I'm sure you know what I mean). So here I am thinking that I dont really need the 2.8 lens if I'm just going to be setting the aperture to f/8. Yet I've not been able to get that completely blurry background with my 75-300.

So my question is... what creates that blurred background? DOF? Lens length? Focal range? Distance to subject? Bokeh?
12/22/2004 02:57:38 PM · #23
Originally posted by EddyG:

Originally posted by yurasocolov:

I was only talking about DOF vs focal length, not the blur.

And that is what I'm talking about. Saying the DOF of a 20mm lens at /8 is "much bigger" than a 200mm lens at /8 is not necessarily true. DOF is defined as "the range of object distances within which objects are imaged with acceptable sharpness". And the actual range of distance (which determines DOF) may actually be identical, depending on object distance.

If the object distance is kept the same, the 20mm image will indeed show a greater DOF, but also a larger field of view, and, consequently, a smaller subject. On the other hand, if the subject is framed the same, it is an entirely different situation. Now the DOF between the two lenses may be comparable or even identical. However, in both cases the photographs are not the same and the comparison is one between two different pictures. The former is a comparison with preservation of the perspective (viewpoint) and different magnifications, the latter preserves the magnification (the size of the imaged subject) but not the perspective.


Yes, we are talking about the same thing. I said that in the context of a claim that DOF does not depend on the focal length of a lens. Just said that it does. It was implied that other variables were the same in comparison, but perhaps i didn't voice that fully.

Message edited by author 2004-12-22 15:14:02.
12/22/2004 02:59:10 PM · #24
photoshop?

Originally posted by moodville:

So my question is... what creates that blurred background? DOF? Lens length? Focal range? Distance to subject? Bokeh?
12/22/2004 03:00:26 PM · #25
Originally posted by hopper:

photoshop?

Originally posted by moodville:

So my question is... what creates that blurred background? DOF? Lens length? Focal range? Distance to subject? Bokeh?


I'm sure some people do it that way! I was kinda wanting to do it in camera. Call me a purist.... for a change :P
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