Could someone explain why wide lenses have a high DOF?

Gel

Gel

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Chris
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I'm trying to understand my glass better so that I can apply it more correctly.

For example, I was at a wedding with my 85L and whilst the exposire was correct the focus was slightly out. (difficult to focus with nothing to focus on)

WIth the 35L though, even wide open the amount in focus was much higher, similar things can be said of the 16-35L too. Yet a telephoto has a narrower field of focus.

Why is this?
 
They have essentially the 'same' DOF (for any given aperture).

But the perspective, as the lens gets longer, compresses it.
 
The zoom length of a lens doesn't affect DoF. A 12mm lens has the same DoF as a 500mm lens and if they appear not to it's just an optical illusion caused by the size of the subject in the frame and size and amount of out of focus elements present.

Here's some interesting links and example shots...

http://www.film-and-video.com/dofmyth.htm

http://www.cybercollege.com/myths.htm

http://www.completedigitalphotograp...5cdContents/Chapter9/Focal Length and DOF.pdf

http://www.luminous-landscape.com/tutorials/dof2.shtml
 
Thanks guys, I'll look into it :D
 
That Luminous Landscape link is good. It shows how DoF is constant when the subject remains the same size, but also shows the big differences in field of view and perspective when the shooting distance is changed.

Edit: to be strictly correct, this does not apply absolutely is you compare extremes of focal length, like 10mm and 200mm. In that case, the 200mm lens will deliver noticeably less depth of field than the 10mm. But if you compare real life examples, like a 50mm lens vs 200mm perhaps, the difference is tiny. And anyway, with all the other big changes going on around perspective etc, a slight change of DoF is neither here nor there.

Check it out here with this on line calculator http://www.dofmaster.com/dofjs.html
 
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Ozei said:
As Voyager says, it's all to do with the size of the subect in the frame - i.e. a wideangle lens has the same DOF as a telephoto provided the subject is the same size in the frame and both lenses are on the same aperture.

Demonstration here http://www.luminous-landscape.com/tutorials/dof2.shtml


Edit: beaten to it by woof woof!
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i looked at that one, and while he has a valid point the longer the lens used the less in focus the tower in the background appears to be, or am i imagining that?
 
dunk said:
i looked at that one, and while he has a valid point the longer the lens used the less in focus the tower in the background appears to be, or am i imagining that?
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You are - kinda. Look at the image below where he's cropped the tower in the wide-angle shot. It's about as blurry as it was in the 400mm shot.

The reason far away objects look less blurry on a wide-angle is that because the objects are so much smaller, so is the degree of out-of-focus rings or bokeh. At least that's how I see it.
 
dunk said:
i looked at that one, and while he has a valid point the longer the lens used the less in focus the tower in the background appears to be, or am i imagining that?
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No, I think you're right. The LL writer has pushed the theory a bit too far by comparing 17mm with 400mm, where there is a noticeable difference - see my edit to post #6 above.
 
"The LL writer has pushed the theory a bit too far by comparing 17mm with 400mm, where there is a noticeable difference..."

I think you're falling for the optical illusion. I've just copied both the 17 and 400mm shots into CS2 and tried to balance the size of the tower in each image and although it's difficult to do with such low resolution shots I'm sure that the point is correct and that focal length does not affect DoF.
 
woof woof said:
"The LL writer has pushed the theory a bit too far by comparing 17mm with 400mm, where there is a noticeable difference..."

I think you're falling for the optical illusion. I've just copied both the 17 and 400mm shots into CS2 and tried to balance the size of the tower in each image and although it's difficult to do with such low resolution shots I'm sure that the point is correct and that focal length does not affect DoF.
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Why would I say that if it wasn't true? Check the dofmaster link I posted and do your own tests, instead of looking at a couple of pictures that by your own admission you can't see clearly.

If you shoot a sequence of pictures and compare them, the first thing that strikes you is that the field of view and perspective are so dramatically altered that any comparison of DoF doesn't seem that important. But if you do look carefully, you will see the DoF gets deeper at the wide angle end.

For example, if you fit a 160mm lens to your 20D, at a distance of 8m at f/8, DoF is 52.7cm. If the 'rule' that DoF doesn't change is to hold absolutely then if you switch to an 80mm lens and move forward to 4m to maintain exactly the same subject framing, then DoF would not alter to 52.9cm, which it does. A tiny change there, but as lens focal length decreases more, things start to happen. At 2m with a 40mm lens, DoF is 53.6cm; at 1m with a 20mm lens it is 56.6cm; and at 0.5m with a 10mm lens it jumps up to 72.9cm.

Another 20cm more DoF between those extremes, about one third more. That difference, between 160mm and 10mm, is equal to about one whole f/stop.
 
Hoppy, calm down. I'm not accusing you of lying. All I did was express an opinion. I'm pretty convinced that focal length doesn't affect DoF and if you want to believe otherwise please feel free.

I think that it's important to remember that whatever the true effect actually is it's also ok to ignore theory and go with what works in practice and in practice a wide lens can appear to give more DoF than a longer lens.

As for the small hard to see pictures...if that's what's stopping you from believing that focal length does not affect DoF you can search the net for better examples (like the ones in the links I posted above) or carry out the test yourself with your own lenses.
 
woof woof said:
Hoppy, calm down. I'm not accusing you of lying. All I did was express an opinion. I'm pretty convinced that focal length doesn't affect DoF and if you want to believe otherwise please feel free.

I think that it's important to remember that whatever the true effect actually is it's also ok to ignore theory and go with what works in practice and in practice a wide lens can appear to give more DoF than a longer lens.

As for the small hard to see pictures...if that's what's stopping you from believing that focal length does not affect DoF you can search the net for better examples (like the ones in the links I posted above) or carry out the test yourself with your own lenses.
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LOL I am calm bud :)

And I have carried out my own tests, with a host of about 20 red hot pokers that sprung up around the pond. Rather a good subject when it's actually quite hard to find something 'real world' that you might actually attempt with such a wide range of focal lengths.

As I said, when you look at the different images what strikes you most are the field of view and perspective changes, but the DoF change is also there - one stop difference is easy enough to spot.

It's not something that I worry about much in practise, and I actually agree with your way of looking at things (ie DoF stays the same) but I'm aware that when pushing things with a wide angle for example, there's maybe a bit more to be had.

On the other hand, maybe it's just knowledge for the sake of it ;)
 
Good, great stuff.

Anyway as it's something to talk about here's a little example. It's the same shot taken at 17mm and 47mm, f8, just seconds apart. The full shots are shown first and then the 17mm shot is shown next to the 47mm shot but this time the 17mm shot is cropped to give the same view. To me this shows that the DoF is the same.

combo.jpg
 
woof woof said:
Good, great stuff.

Anyway as it's something to talk about here's a little example. It's the same shot taken at 17mm and 47mm, f8, just seconds apart. The full shots are shown first and then the 17mm shot is shown next to the 47mm shot but this time the 17mm shot is cropped to give the same view. To me this shows that the DoF is the same.

<snip>
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Well as you say it's a talking point but in those two pictures the left hand image appears to have more detail in it, but that's a bit of a mystery as depth of field at that distance extends pretty much from front to back anyway :thinking:

On the other hand, the figures don't lie.
 
woof woof said:
I'm pretty convinced that focal length doesn't affect DoF
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Ask any photographer who uses a long lens and they will disagree with you on that.
 
I use a long lens.

See the links and make your own mind up, there's long lens shots in them. :) Or, post some shots that show that focal length does affect DoF. Now there's a challenge.
 
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fracster said:
Ask any photographer who uses a long lens and they will disagree with you on that.
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woof woof said:
I use a long lens.

See the links and make your own mind up, there's long lens shots in them. :) Or, post some shots that show that focal length does affect DoF. Now there's a challenge.
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This is where it gets difficult. The changes in depth of field, when you compare images with the same subject framing, only become apparent when you compare extremes of focal length from long to wide at quite close distance - see the figures I posted above.

When comparing long lernses, there is virtually no change in depth of field, but the field of view changes a lot and the bokeh is quite different. The impression of greater subject isolation is there, which often looks like a shallower depth of field effect, whereas in fact it's not.

I took these two pictures last week which illustrate this. They are intended to show the change in bokeh rather depth of field, but you can see both things happening. I've lifted them from this thread, post #13 http://www.talkphotography.co.uk/forums/showthread.php?t=257789

70mm f/4
IMG_3410.jpg


200mm f/4
IMG_3411.jpg
 
this thread is an eye opener, i thought it was based on not only distance to subject but focal lentgth as well. T
thanks for the examples. Just shows how little i know about the technical stuff.

a small FOV gives an impression of a small DOF
 
"but the field of view changes a lot and the bokeh is quite different"

Yes, the background appears to be relatively large with longer focal lengths and this exaggerates the out of focus areas and so gives the impression of shallower DoF.
 
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