Myth busted: telephoto lenses have thinner DoF

[Daniel Browning]

Thank you all for your responses. There are two basic elements:

1. What the in-focus part looks like.
2. What the out-of-focus part looks like.

Number one is almost wholly defined by DoF.

Number two is affected by many factors, only *one* of which is DoF. The mistake is thinking or saying DoF affected the OOF blur (i.e. background blur) when in reality it was another factor, such as magnifying the OOF with a telephoto.

There is a real difference between OOF blur caused by DoF and OOF blur caused by other factors. Opening an f-stop blurs the OOF differently than a telephoto magnification. Sure, they both look more blurry, but they are not the same. One is the same blur, magnified. The other is a totally different type of blur.

As sander kamp said above, "getting a face at a certain image size sharp, changing the focal length is no use." If a 300mm f/2.8 shows an ECU with the eyes in focus but the ears and nose out of focus, one might mistakenly believe that a shorter focal length will bring them in focus, but they'll be surprised when their 200, 135, and 85 all have the same problem at f/2.8. The OOF blur, perspective, and background will change with each lens, but not DoF.

If the focal plane already lies within the hyperfocal distance, there wont be *any* increase in background blur from a change in focal length (for the same framing). The foreground blur, if there is any, would be the only change. On the other hand, if there had been a *real* change in DoF would have increased blur.

Sometimes there is an emphasis on telephoto lenses for achieving background blur with deep DoF. If the difference is understood, one might consider a shorter focal length with a wider aperture to achieve a similar blur, when the background and perspective warrant.

To me, there's mathematical DOF and artistic DOF. On a given camera, the only thing that really changes DOF(m) is aperture.

Aperture and focal distance. What you call "artistic DOF" is not really DoF at all. I call it the OOF area. "There can be only one".

different T/stop loss [in various lenses and circumstances]

Your post highights the importance of knowing the true f/stop of a lens in addition to its t-stop. I'm sure some focus pullers know what lenses have the greatest light loss (and therefore diffence between actual aperture and theoretical aperture for light transmission).

 

[sander kamp]

That's because you change the magnification factor. You don't have the same picture framing at focus point.

I am not sure what you mean by that.

But if I look at my second example the lens at 70mm has let's say a depth of field of 10cm while the 300mm lens has a depth of field of a little more than 1cm.

They where both taken at the same image distance and f-stop so for me the longer lens has less depth of field. The wider image was enlarged and cropped to get the same field of view but when scaled down it would be even more obvious.

 

[Adrian T.]

I am not sure what you mean by that.

But if I look at my second example the lens at 70mm has let's say a depth of field of 10cm while the 300mm lens has a depth of field of a little more than 1cm.

They where both taken at the same image distance and f-stop so for me the longer lens has less depth of field. The wider image was enlarged and cropped to get the same field of view but when scaled down it would be even more obvious.

If you don't crop and enlarge the shot, you have a completely different field of view. And if you do, you change the circle of confusion factor.

You're absolutely right in saying that a 300 mm lens at a certain f stop and focus has a smaller DOF than a 70 mm lens at the same f stop and focus. But it's a completely different shot. :wink:

 

[David Mullen ASC]

The issue is using a shorter focal length to shoot a subject to increase DOF or the reverse, using a longer focal length to shoot the subject to decrease it -- but you'd be changing the distance to compensate to maintain the same size on the subject, not keeping the distance the same.

If you crop the wide-angle shot and enlarge it, then yes, you're seeing the DOF increase of a shorter focal length set at the same distance. At f/1.4, a 50mm at 10' has a DOF range of 9'7" -10'5" (10") whereas a 100mm has a range of 9'11" - 10'1" (2").

But we're talking about backing up and putting on a longer lens to get less depth of field compared to being close with a wide-angle lens. As I said on the previous page:

If you're not cropping and enlarging, and thus using a different CoC figure, then just look at a depth of field chart to see what happens when you double the focal length and back up by twice to maintain the same view:

For example, compare DOF (according to the ASC Manual charts):
25mm at f/1.4 at 5' = 4'7" - 5'6" = 11" DOF
50mm at f/1.4 at 10' = 9'7" - 10'5" = 10" DOF
100mm at f/1.4 at 20' = 19'7" - 20'5" = 10" DOF

Now the difference between the 25mm and 50mm may be due to rounding up or down from fractions of inches.

But more or less, there is no change in DOF when you change focal lengths and compensate by changing distance, just a change in apparent depth of field because of how the longer lenses magnify the out of focus areas.

 

[sander kamp]

Yes, yes. Everything clear to me now

 

[Lauri Kettunen]

The depth of field is a function of the following four parameters:
- f stop
- focal length
- focus
- circle of confusion

Without recognizing the assumptions behind some conceptual idea the underlying discussion gets easily fuzzy.

Sander does have a point. He keeps the framing/composition fixed and this is what makes all the difference. Of course, if the framing is fixed, one has to change the distance when changing a lens. Consequently, under this assumption the DoF also changes. If one fixes the distance as big lebowski seems to suggest, then yes, it's a different game. And all what is said is just ok.

However, in my eyes Sander's approach is more intuitive. For, a photographer typically has a composition in mind. I mean, rather seldom one first sets the distance and then starts to seek for a framing. Typically it goes the other way around. One has first some idea of composition and only then the lens is chosen and the distance is set.

After thought: Well ... in press photo the distance is often given. So, "rather seldom" is a too strong expression.

 

[Dan Hudgins]

Head shots...

Head shots...

For example, compare DOF (according to the ASC Manual charts):
25mm at f/1.4 at 5' = 4'7" - 5'6" = 11" DOF
50mm at f/1.4 at 10' = 9'7" - 10'5" = 10" DOF
100mm at f/1.4 at 20' = 19'7" - 20'5" = 10" DOF

When you shoot a head shot with a 25mm, 50mm, and 100mm so that the distance between the eyes is the same in the frame and the subject is looking forward you will notice something,

1) In the 25mm shot the actors nose is bigger and his ears smaller than in the 50mm shot. Typically the nose will look softer than the ears(?).

2) In the 100mm shot the ears and nose will look about the same softness. In the 100mm shot the nose is smaller than in the 25mm shot, and the ears are larger than in the 25mm shot.

Because the nose is closer in relation to the focal length for the 25mm shot the asymmetry of the out of focus of the foreground and backbround should be greater than for the 100mm?

This can be tested by using three flashlight bulbs one at nose one at eye one at ear distance, then shoot the three images, then measure the diameter of the circle of confusions to see what asymmetry there is.

Some lenses make a ring circle of confusion for the focus on side and a point with fuzz around for the other side. This changes with the stop used, so the tables do not correspond to the results since the circle of confusion is not evenly illuminated.

The Cooke Deep Field Panchro was one lens that the distribution of light in the circle of confusion was optimised. Some lenses like the Astro Berlin have strong rings in their circle of confusion and were prized for their odd shallow DOF, with things going extra fuzzy out of the plane of "best focus".

Did the DOF tables take into account that all the FL are longer for near subjects, like a 25 becomes a 27, a 50 becomes a 55, and a 100 becomes a 125 when focused on a close subject? Because the longer lens needs to have its iris opened up more to keep the same T/stop for close subjects its f/stop (setting) and FL both increase.

 

[Davide B.]

But more or less, there is no change in DOF when you change focal lengths and compensate by changing distance, just a change in apparent depth of field because of how the longer lenses magnify the out of focus areas.

Agreed, here's a nice visual example of this: http://www.luminous-landscape.com/tutorials/dof2.shtml

 

[Stephen Williams]

I thought the point was NOT to move the camera. That would give a different distance between foreground and background and different focus.

Hi,

The image size must remain the same after changing the camera, then the DOF will be identical regardless of lens.

DOF is controlled by 2 things Image size on the sensor & F stop, thats it.

Stephen

 

[Stephen Williams]

On another point, the T/stop loss is greather for long lenses than short lenses when the subject is close, so when applying the T/stop compensation for bellows extension for the longer focal length lenses the iris is OPENED by as much as 2 stops and opening the iris would decrease the DOF limit points distance from the plane of best focus.

.

Hi,

T stop is irrelevant in DOF discussions, only F stop should be used IMO.

Past 1:1 you can get a far greater bellows extension than 2 stops.

Stephen

 

[Stephen Williams]

sander,

The depth of field is a function of the following four parameters:
- f stop
- focal length
- focus
- circle of confusion

[edit] fixed a typo.

Hi,

DOF is calculated by F stop measured from the front nodel point of the lens & not the film plane.

Most DOF charts are calculated from the filom plane & lens specific charts will be calculated for the actual T stop of that lens.

I don't understand what you mean ba focus? DOF is a distance either side of the focus distance.

I simplify DOF to be due to image magnification & F stop only.

Stephen

 

[Adrian T.]

I don't understand what you mean ba focus? DOF is a distance either side of the focus distance.

The DOF depends on the focus distance you set on the lens.

DOF_near = H * s / (H + s)
DOF_far = H * s / (H - s)
DOF=DOF_far - DOF_near
where H is the hyperfocal distance (based on focal length, f stop and CoC) and s is the focus distance.

I simplify DOF to be due to image magnification & F stop only.

Image magnification is a function of focal length and focus distance. So you're still missing the circle of confusion factor then. :wink:

 

[Stephen Williams]

Image magnification is a function of focal length and focus distance. So you're still missing the circle of confusion factor then. :wink:

Hi,

The COC is just how much out of focus you accept as being sharp. It's more of a personal choice & format related than anything else.

Stephen

 

[Adrian T.]

Hi,

The COC is just how much out of focus you accept as being sharp. It's more of a personal choice & format related than anything else.

Stephen

I know. But it's nevertheless an essential factor for the DOF.
Just because you originally said "I simplify DOF to be due to image magnification & F stop only." :wink:

 

[Stephen Williams]

I know. But it's nevertheless an essential factor for the DOF.
Just because you originally said "I simplify DOF to be due to image magnification & F stop only." :wink:

Hi,

As I never chance the COC, I always assume worst case on a cinema screen. Problem is finding enough good focus pullers in CH.

Stephen