DOF for Dummies

crmitche

Is the DOF on my Canon 70-200 f4L the same as it would be on a Canon 70-200 f2.8L at f4? Or is DOF non-existent at maximum aperature regardless of the value?

Cheers
Craig

pyry

crmitche wrote:

Is the DOF on my Canon 70-200 f4L the same as it would be on a Canon 70-200 f2.8L at f4? Or is DOF non-existent at maximum aperature regardless of the value?

Cheers
Craig

If you've got 150mm at f/4, you've got the dof of 150mm at f/4. It doesn't matter what it says on the lens, the incoming light doesn't stop to read it on the way in

DOF depends on the actual aperture more than anything else I think (in addition to focus distance). Hence the 70-200/2.8 is capable of producing a shallower dof because it opens up wider. It isn't non-existent at max apertures on all lenses either, my 10mm f/4 ultrawide has a huuuge dof whatever the setting - because it's effective aperture is about 2-3mm, tops.

Here, play with this. It should give you some idea of how dof works.

Scott_Quier

DOF is a function of:

• Aperture - larger aperture --> less DOF if all else held constant
• Focus distance - the distance between camera sensor plane and the point of critical focus - less distance --> less DOF if all else held constant
• The focal length - longer focal length --> less DOF if all else held constant
• Size of the sensor/film - larger sensor --> less DOF if all else held constant

So, to answer OP question, f/4 at any given focal length, on the same camera, at the same subject to camera distance will yield the same DOF.

Another DOF calculator that I like can be found here

pathfinder

Good answer, Scott.

Lots of folks know the significance of focal length and aperture, but are not as clear about the effects of lens to subject distance ( which has a profound effect for macro shooters) and the effect of sensor size. P&Ss have vastly greater DOF for a given focal length and aperture than a 35mm camera.

crmitche

You guys ROCK!

Cheers
Craig

Scott_Quier

pathfinder wrote:

Good answer, Scott.

Thanks, but I learned at the knees of the masters

Nikolai

Scott_Quier wrote:

Size of the sensor/film - larger sensor --> less DOF if all else held constant

Sorry Scott, this is not accurate. Size of a sensor is akin to crop, it has no effect on optical behaviour of the lens.

pyry

Nikolai wrote:

Sorry Scott, this is not accurate. Size of a sensor is akin to crop, it has no effect on optical behaviour of the lens.

Format size will have an effect on dof, if the field of view is not allowed to change. Larger formats would then require longer lenses that go on to produce images with shallower dofs - at the same aperture ratio and therefore exposure.

Scott_Quier

Nikolai wrote:

Sorry Scott, this is not accurate. Size of a sensor is akin to crop, it has no effect on optical behaviour of the lens.

It has no effect on the behavior of the lens - but that's not the discussion at hand.

We are talking about DOF. Consider the DOF characteristics differences between, for example, a 30D and a 5D. With the same lens, the 5D has a much shallower DOF. Cogitate on that and get back to me D

pathfinder

DOF gets confusing because focal length does not affect DOF in the same format size, IF ( and its a big if ) the subject size is held constant on the film plane. If you shoot a portrait with a 24mm lens or a 300mm lens, and the portrait is the same exact size on the film plane, then the DOF of both lenses is the same - not intutitve but mathematically accurate. But in the real world, one rarely uses a 24mm lens and a 300mm lens to capture a portait the same size on the image plane. The lens to subject differences are usually a lot different with these differences in focal length.

The sensor size contributes to a greater DOF for a P&S camera, because the focal lengths are so much shorter for them - typically 7-44 mm goes from wide angle ( sort of ) to mild tele. On a FF DSLR, 7mm is extremely wide angle and 44mm is just approaching normal. On a medium format camera 75 to 110mm is more of a normal lens if my memory serves.

I quote directly from Wikipedia - http://en.wikipedia.org/wiki/Depth_of_field#DOF_vs._format_size

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To a first approximation, DOF is inversely proportional to format size. More precisely, if photographs with the same final-image size are taken in two different camera formats at the same subject distance with the same field of view and f-number, the DOF is, to a first approximation, inversely proportional to the format size. Strictly speaking, this is true only when the subject distance is large in comparison with the focal length and small in comparison with the hyperfocal distance, for both formats, but it nonetheless is generally useful for comparing results obtained from different formats.
To maintain the same field of view, the lens focal lengths must be in proportion to the format sizes. Assuming, for purposes of comparison, that the 4×5 format is four times the size of 35 mm format, if a 4×5 camera used a 300 mm lens, a 35 mm camera would need a 75 mm lens for the same field of view. For the same f-number, the image made with the 35 mm camera would have four times the DOF of the image made with the 4×5 camera.
In many cases, the DOF is fixed by the requirements of the desired image. For a given DOF and field of view, the required f-number is proportional to the format size. For example, if a 35 mm camera required f/11, a 4×5 camera would require f/45 to give the same DOF. For the same ISO speed, the exposure time on the 4×5 would be sixteen times as long; if the 35 camera required 1/250 second, the 4×5 camera would require 1/15 second. In windy conditions, the exposure time with the larger camera might allow motion blur. Adjusting the f-number to the camera format is equivalent to maintaining the same absolute aperture diameter.
The greater DOF with the smaller format can be either an advantage or a disadvantage, depending on the desired effect. For the same amount of foreground and background blur, a small-format camera requires a smaller f-number and allows a shorter exposure time than a large-format camera; however, many point-and-shoot digital cameras cannot provide a very shallow DOF. For example, a point-and-shoot digital camera with a 1/1.8″ sensor (7.18 mm × 5.32 mm) at a normal focal length and f/2.8 has the same DOF as a 35 mm camera with a normal lens at f/13.

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Nikolai

Scott_Quier wrote:

It has no effect on the behavior of the lens - but that's not the discussion at hand.

We are talking about DOF. Consider the DOF characteristics differences between, for example, a 30D and a 5D. With the same lens, the 5D has a much shallower DOF. Cogitate on that and get back to me D

Since you one can't change the body at will this issue is fairly academical, but just be clear: DOF (Depth of field) does not depend on the *size* of the sensor, but rather on the distance between the the sensor (aka film plane) and the lens. I agree that this parameter is different on different bodies (IIRC FF sensors are a bit further from the lens than APC ones, and P&S are much closer), which in fact does affect the DOF, but again it's the distance, not the size.

jeffreaux2

Here is an example of the sensor size effect....

http://www.bhphotovideo.com/find/newsLetter/digi_photo_voodooDOFapr06.jsp

Nikolai

jeffreaux2 wrote:

Here is an example of the sensor size effect....

http://www.bhphotovideo.com/find/newsLetter/digi_photo_voodooDOFapr06.jsp

Nice article!
Yet again, "it's not a size, it's how you use it", i.e. "how close is your sensor to your lens". And while there is a correlation between the sensor size and the distance wetween the sensor and the lens in most cameras, the theory behind it as not linked to the sensor size per se.
Post hoc is not always propter hoc.

pyry

Nikolai wrote:

Since you one can't change the body at will this issue is fairly academical, but just be clear: DOF (Depth of field) does not depend on the *size* of the sensor, but rather on the distance between the the sensor (aka film plane) and the lens. I agree that this parameter is different on different bodies (IIRC FF sensors are a bit further from the lens than APC ones, and P&S are much closer), which in fact does affect the DOF, but again it's the distance, not the size.

Nope. For example: EF lenses have exactly the same flange to focal plane distance regardless of what sensor happens to be back there. Otherwise the lenses would not even focus correctly.

What's happening between format sizes is actually mostly aperture (that's aperture size, not ratio). Take a normal lens for 35mm. It's a 50mm, right? At f/2 it has a certain dof. APS normal comes out to about 30mm. At f/2 - same field of view, same exposure, it has a wider dof - because the size of the aperture is smaller: 50/2=25mm versus 30/2=15mm.

Another example: APS body with 135/2 against 35mm body with 200/2.8 - same fov, same dof, different exposure (ballpark accuracy here).

Nikolai

Good griefs, guys, try to increase the canvas size in PS and see if it miraculously change your image structure - while viewed at 100%...

jeffreaux2

Nikolai wrote:

Good griefs, guys, try to increase the canvas size in PS and see if it miraculously change your image structure - while viewed at 100%...

Im sure everyone who has posted understands where you are coming from Nik. What you have said makes perfect sense.

Still....a Canon 50mm F1.4 lens(for instance) will produce a noticably shallower DOF on a full-framed sensor body than on an APS-C sensor body when shot at the same aperture.

pyry

jeffreaux2 wrote:

Im sure everyone who has posted understands where you are coming from Nik. What you have said makes perfect sense.

Still....a Canon 50mm F1.4 lens(for instance) will produce a noticably shallower DOF on a full-framed sensor body than on an APS-C sensor body when shot at the same aperture.

Again, nope. This is the example Nik was after I think. The same lens at the same setting will produce the same depth. It'll look shallower on a larger format because there's a bunch more of the out of focus stuff around the sharp band - there's a change in fov. But the sharp bit covers the same distance.

Format size only affects dof indirectly, if you maintain field of view (and exposure).

Nikolai

jeffreaux2 wrote:

Still....a Canon 50mm F1.4 lens(for instance) will produce a noticably shallower DOF on a full-framed sensor body than on an APS-C sensor body when shot at the same aperture.

Oh I would never object to that. Different bodies, different distances to sensor. From what I understand, the said distance varies even along the APC line (10D..50D), if ever so slightly.

jeffreaux2

Nikolai wrote:

Oh I would never object to that. Different bodies, different distances to sensor. From what I understand, the said distance varies even along the APC line (10D..50D), if ever so slightly.

Gotcha.

I am nearing completion of the most intense training of my adult life. 6 weeks so far full of chemistry, physics, and computer interfacing. I told my trainer that he should use the bucket of water and dime technique with me. That is....if you keep pouring buckets of water on a dime ...SOME...of the water will remain on the dime.

I'm a tad slower at thinking these days. To be honest I am frazzled!!!

Just keep pouring the water on us Nik.....some of it will remain on the dime!

Scott_Quier

Admitting that I might be wrong (it's happened once or twice before:D), all this discussion is getting us nowhere. What we really need to see is an example to either prove or disprove the assumption(s).

Suggest someone with both an AP-C and FF camera shoot a subject keeping all factors the same ... including appearant size of the subject in the frame ... and see what the results are.

I would be more than willing to do this if someone would like to send me (I'll even cover the cost of shipping and insurance) a Canon FF camera for the duration of the experiment - should have the camera back to you in 3 or 4 weeks:) Just PM me for shipping information.

pathfinder

As I said, initially, focal length does not have any effect on the depth of field IF - AND ONLY IF - the image of the subject is exactly the same size on the sensor plane - Nik is correct that the focal length does not alter DOF -- whether 24mm or 200 mm - so certainly, a 50mm lens does not change its DOF characteristics when moved from a point and shoots 2/3s sensor, to a full frame sensor. But the lens is used very differently because it is a wide angle FOV on an 8x10 view camera, a normal lens on a 35mm camera, and a telephoto on a point and shoot - all the same focal length. The distance to the subject will change dramatically to keep the image size the same in the film plane if you change focal lengths.

But due to the size of the sensor, the crop factor as Nik called it, the lens is used differently and the images are NOT THE SAME SIZE at the sensor plane for most of these discussions.


Put a 24mm lens on your camera on a tripod and shoot a doll's head and look at the surrounding DOF. Do the same thing, with a 200mm lens on the same body ( but from a vastly different lens to subject distane, in order to have the doll's head the same size on film as the first shot, the DOF in BOTH images will be the same. I know it is hard to believe, because we are all certain that wide angle have more DOF than telephotos, but that is only because tend to use we use wides for subjects close up, and teles much farther away. As Nik said, FOV.

There was an article demonstrating exactly this in Popular Photography some years ago - in the film era.

Nikolai

Scott_Quier wrote:

Admitting that I might be wrong (it's happened once or twice before:D), all this discussion is getting us nowhere. What we really need to see is an example to either prove or disprove the assumption(s).

Suggest someone with both an AP-C and FF camera shoot a subject keeping all factors the same ... including appearant size of the subject in the frame ... and see what the results are.

I would be more than willing to do this if someone would like to send me (I'll even cover the cost of shipping and insurance) a Canon FF camera for the duration of the experiment - should have the camera back to you in 3 or 4 weeks:) Just PM me for shipping information.

Scott, don't sweat it, buddy!
For all intents and purposes, there *is* a very strong direct correlation between the sensor size and the sensor distance IRL, so if you choose any actual full frame body over APC one, the distance inevitably will be larger, hence no experimental proof this way.
Let's have a and go out and shoot, I know I'm going to: four young, beautiful and very adventurous ;-) ladies are meeting me at one of the secluded Malibu beaches in an hour...

jeffreaux2

Scott_Quier wrote:

Admitting that I might be wrong (it's happened once or twice before:D), all this discussion is getting us nowhere. What we really need to see is an example to either prove or disprove the assumption(s).

Suggest someone with both an AP-C and FF camera shoot a subject keeping all factors the same ... including appearant size of the subject in the frame ... and see what the results are.

I would be more than willing to do this if someone would like to send me (I'll even cover the cost of shipping and insurance) a Canon FF camera for the duration of the experiment - should have the camera back to you in 3 or 4 weeks:) Just PM me for shipping information.

.....wow Scott....so generous of you to offer your time to conduct such a test!!! Maybe you should mail me a 50D so that part of your test isn't biased?

I thought about this more last night...when I ought to have been asleep.

Hypothetical........just follow along...
You are at a school play....shooting a 50mm F1.4 on a Canon 40D(1.6x crop sensor) at F2.8 due to crappy lighting. You are perfectly positioned to take frame filling full length shots of the main characters. Then......you swap to a 5D.....mount the lens and plug in your settings including the F2.8 aperture. Raise the camera to your eye and....uh oh......now you are too far away to get those frame filling shots. You have two choices. You can move closer to the action.....that is somewhere in the neighborhood of 35% closer...to get the same feild of view......OR you can dig through your bag and find a longer focal length lens to make up the difference without physically moving yourself. An 80mm lens would do it (hypothetical remember)....yet the light is still crappy so you must still operate at the same F2.8. Both solutions will provide the same feild of view....both will also have a dramatic effect on depth of focus. This has nothing to do with sensor distance to lens, but simply distance to subject more than anything.

That is why.....a photographer using a full frame camera taking a head and shoulder shot of a model can produce much shallower depth of focus that I can with my crop sensor camera given an identical composition with the same lens and F stop..

Right?


OK hypothetically again....the full frame camera at the school play is "in effect" slower!!!

I will have to stop down the aperture to get the same DOF....that will cause me to either up the ISO or lower my shutter speed. IE....slower.