Photography question

[rhythmworx]

Here is a question I would like to ask, it is not a trick question and there is nothing within the question trying to catch you out.

Scenario 1 - A camera with a 50mm lens is placed on a tripod and a subject 5 metres away is focused on and a photo is taken @ f2.8.

Scenario 2 - The same camera on the same tripod in the same place now has a 150mm lens on and is shooting the same subject that hasn't moved. The same area is focused on as scenario 1 and a photo is taken again @ f2,8

Which do you believe to be true?

[MJP]

It's why I have a vast collection of M42 mount 135mm bokeh monsters.

[Tywais]

For those who wish to cheat a bit here are two DOF calculators. Also DOF calculators can be downloaded for iPhone and Android devices.

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

http://www.cambridgeincolour.com/tutorials/dof-calculator.htm

I wish I could find an article I read where someone went out to prove empirically that they are the same. His tests looked convincing but defies the law of physics (optics).

[NeverSure]

The field of view (angle of view or width of view) will change with focal length, but the depth of field won't at a given aperture setting.

There may be a perceived difference, especially with a shallow depth of field because the wider the field, the more difference you can see. But it's only an illusion. Crop the pic taken with the wide angle lens and the fact they are the same will be apparent.

[Tywais]

The field of view (angle of view or width of view) will change with focal length, but the depth of field won't at a given aperture setting.

Both f/2.8 and 5 Meters but 50 and 150mm

[NeverSure]

The field of view (angle of view or width of view) will change with focal length, but the depth of field won't at a given aperture setting.

Both f/2.8 and 5 Meters but 50 and 150mm

dof50.JPG

dof150.JPG

I still believe that those are perceived (seen) due to the wider field of view with the 50mm. As both were taken at the same distance, you have a much wider field to see and measure with the 50mm.

Now crop the 50mm to show only exactly what the 150mm saw, and resize the pic so they are both the same size, and I'm pretty sure the differences in DOF disappear.

[fimgirl]

The field of view (angle of view or width of view) will change with focal length, but the depth of field won't at a given aperture setting.

Both f/2.8 and 5 Meters but 50 and 150mm

dof50.JPG

dof150.JPG

I still believe that those are perceived (seen) due to the wider field of view with the 50mm. As both were taken at the same distance, you have a much wider field to see and measure with the 50mm.

Now crop the 50mm to show only exactly what the 150mm saw, and resize the pic so they are both the same size, and I'm pretty sure the differences in DOF disappear.

Think about a shot down the scale of a ruler. The 50mm will show much more of the scale (DOF) than the 150mm, given placements and apertures being equal. Now, cropping the shot to see a narrower angle of view will still show the greater scale. DOF is the amount in focus. Cropping does not affect the amount in focus.

The conclusion is that the DOF does reduce as the focal length increases.

[Tywais]

A video showing the basics of DOF and what controls it. Focal length demonstration is around the 9:00 minute mark. It is a very good visualization of what is going on optically.

[Tywais]

NeverSure must have been watching this video, Focal Length Myth. The funny thing is looking at his example photos he actually verified that focal length effects DOF despite his argument (which goes against physics and the math)

[vaultdweller0013]

The field of view (angle of view or width of view) will change with focal length, but the depth of field won't at a given aperture setting.

Both f/2.8 and 5 Meters but 50 and 150mm

dof50.JPG

dof150.JPG

I still believe that those are perceived (seen) due to the wider field of view with the 50mm. As both were taken at the same distance, you have a much wider field to see and measure with the 50mm.

Now crop the 50mm to show only exactly what the 150mm saw, and resize the pic so they are both the same size, and I'm pretty sure the differences in DOF disappear.

Happen to catch me with a little free time -- waiting for the Monaco GP to start. So, here you go. Scene as taken with the 50mm f4 and crops of identical sections for 50mm, 135mm, and 200mm all at f4. Note that the 50mm is just about a 1:1 crop.

Still believe that DOF will be the same for all shots at a given f-stop regardless of focal length?

[rhythmworx]

I messed up here.

Within the question I meant to add if you fill the frame with the subject. Tywais link to the cambridge site confirmed the theory that the depth of field is "virtually the same if the subject fills the frame".

The reason I asked it is after a lengthy conversation I had with a Lens distributor, within the conversation a woman in Lycra leggings walked past and I lost the ability to absorb what was being said, as at the time I was admiring the light reflecting off the curves of her bum.

[Guest]

I messed up here.

Within the question I meant to add if you fill the frame with the subject. Tywais link to the cambridge site confirmed the theory that the depth of field is "virtually the same if the subject fills the frame".

The reason I asked it is after a lengthy conversation I had with a Lens distributor, within the conversation a woman in Lycra leggings walked past and I lost the ability to absorb what was being said, as at the time I was admiring the light reflecting off the curves of her bum.

Maybe we think the same, but in different ways. I think that the DOF is absolute value and independent what is projected to the final image. In case an image has DOF of 20cm, it will be the same 20cm whether the image is cropped to fill the frame or not. In cropped image, larger proportion of the frame is in focus compared to the not cropped image. Still the absolute DOF is the same.

I think an lense as an pipe, quite like on Tywais's first video. Longer pipe means that the angle of the light coming from the subject to the sensor will have less angle. Thus the DOF would be Longer in this single point of light source presentation. The effect would still be miniscule. I think that part follows the physics as well.

But.. when talking about the DOF, we should not be talking about the single points where the light is coming focused from the source to the sensor. Instead we should think of all the other light sources, the ones which causes blurring on top of the focused light.

Longer lens allows light emitted from the source to take various different paths to reach the sensor. These different paths are the ones which cause final image to be out of focus, except in the DOF. On longer lens the light from the lens to the sensor comes with less angle, which causes that there is more out of focus light closer the the center of the image. Therefore the image has shorter DOF.

Uh, oh.. That's still a RAW though, which needs processing.. and definitely more coffee.

[Tywais]

For those who wish to cheat a bit here are two DOF calculators. Also DOF calculators can be downloaded for iPhone and Android devices.

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

http://www.cambridgeincolour.com/tutorials/dof-calculator.htm

I wish I could find an article I read where someone went out to prove empirically that they are the same. His tests looked convincing but defies the law of physics (optics).

I found that article now.

Do Wide Angle Lenses Really Have Greater Depth of Field Than Telephotos?

Testing The Theory

Most photographers accept the common belief that short focal length lenses have greater depth of field than do long lenses. A wide angle lens, in other words, will give greater depth of field than will a telephoto. Right?

Sounds about right, but it's not the case. In fact, if the subject image size remains the same, then at any given aperture all lenses will give the same depth of field.

Because this is such a controversial statement, and because it initially flies in the face of common experience, I have created the following set of example images.

The set-up for it was as you see below. Camera on a tripod and two light stands holding test subjects — a gremlin, which is the main subject, and a hand puppet the other. A tall tower at infinity is the third.

Full article here with demo shots - Luminous-Landscapes

[fimgirl]

I might be missing something but all he's proven here is that in every shot the tower is out of focus, which is inevitable due to the selected focus point and aperture employed not being capable of reaching infinity. He even states that f5.6 was chosen to acheive this effect. Surely a more accurate test would have been to employ the hyperfocal distance per length and fixed aperture to see the real effect and variation in DOF.

Or have I missed something?

[Tywais]

Digging deeper and found some interesting articles that suggest that focal length affects DOF is a myth. However, the way they are presented doesn't necessarily defy the physics as it takes other parameters such as FOV and perspective into the equations. I will post these as separate posts

We've been told that depth of field and the spatial relationships between objects in a scene are directly related to lens focal length.

We've always believed that because many textbooks said so and our experience seemed to confirm it.

But when you investigate the facts you find that neither depth of field or perspective is dependent upon lens focal length.

...all things being equal, focal length does NOT affect depth of field. (caps in original)

-Jay Holben, Videography, January 2009.

Focal Length and Depth of Field

Although focal length appears to affect depth of field (the area in focus along the lens axis), this appearance is actually based only on differences in camera-to-subject distance and target image size.

This has been confirmed by tests including the one illustrated in Popular Photography ("Depth of Field 101," July, 1994.)

For a specific lens-to-subject distance and comparable image size on the target, all lenses of comparable optical design will (regardless of focal length) have the same depth of field when used at the same f-stop.

More here - Cybercollege

[Tywais]

Myth #9: Tele-photo lenses have a shallow DOF

In this short form, this statement is wrong. The accurate version would be “Tele-photo lenses used at short distances and at wide apertures have a shallow DOF”. Maybe too inconvenient, but with enough omissions even a correct statement can turn into an incorrect one.

Here're the facts: DOF is a function of aperture and magnification (on film or sensor), and magnification is a function of focal length and distance. When you shoot a longer lens from a greater distance you can get the same magnification as when shooting a shorter lens from a closer distance. When you also use the same aperture, you will get the exact same DOF. So DOF with a longer lens will only be shallow when you also shoot from a short distance.

Many super-tele lenses can't focus particularly close. You often don't get magnifications greater than 1:6 or 1:8. For example, a 600/4 shot at its closest distance of 6 m will have a deeper DOF than a 100/2.8 shot at 80 cm (and at f/4), even though it's six times as long.

Often you get a smoother out-of-focus background with a longer lens. But that's not because DOF is shallower. The longer lens with its narrower angle of view just sees a smaller section of the background, and it's easier to find a smooth section of the background when it's small rather than large. For example, you have to turn a 600 mm lens by only 4° to get a completely new background. To do the same with a 100 mm lens you have to turn it by 24°. So if you're after a smooth background, using a longer lens may be a good idea. But if you're actually after a shallow DOF, using a longer lens may not be enough.

Mhohner

[Tywais]

And a converse argument:

Depth of Field Hell—The Sequel

By Ctein

I don't know why this myth about the depth of field being utterly independent of focal length (for constant magnification) keeps on going, because it's easy to prove that it's wrong and sometimes it really matters that it's wrong.

Nonetheless, it is a myth and it is wrong.

Here's the real skinny. At close working distances, depth of field is nearly independent of focal length. In fact here's a handy approximate equation for depth of field that depends solely on circle of confusion ©, aperture (f), and on-film (or sensor) magnification (M):

DoF (on either side of the subject) = c*f*(M+1)/(M*M)

As you decrease the magnification, lens focal length starts to become more important. As you get close to the hyperfocal distance, to maintain comparable total depth of field, the magnification becomes inversely proportional to the focal length of the lens rather than being independent of it!

A good rule of thumb is that for just about any photography you do indoors, depth of field isn't going to be affected by focal length, just on-film magnification. But as you start photographic more distant subjects, the effect of focal length increases and eventually becomes as important as aperture.

More here - theonlinephotographer

[Tywais]

An interesting paper disproving that focal length affects DOF. I'm getting a headache.

Sorry, but have to put part of the article up as a snapshot due to the pdf file being protected.

Focal Length and DOF (pdf)