How come telephoto part of a zoom lens has shallower DOF?


wizz747

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#1
I have the Kit I lens 18-55mm f/3.5-5.6. From what I have read, I can shoot at f/3.5 at 18mm and if I zoom in at 55mm, I get a narrower aperture of f/5.6.

I have also read that the DOF at the telephoto end of a zoom lens is shallower than the DOF at the wide-angle focal length of a lens.

However, according to me, I should be having a shallower DOF at the wide-angle part of my lens (18mm), since I am shooting at an aperture of f/3.5 whereas at 55mm the aperture is only f/5.6.

Am I missing something? I'll be grateful if somebody can enlighten me on this. Thank you.
 

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ZerocoolAstra

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Mar 13, 2008
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#3
I have the Kit I lens 18-55mm f/3.5-5.6. From what I have read, I can shoot at f/3.5 at 18mm and if I zoom in at 55mm, I get a narrower aperture of f/5.6.

I have also read that the DOF at the telephoto end of a zoom lens is shallower than the DOF at the wide-angle focal length of a lens.

However, according to me, I should be having a shallower DOF at the wide-angle part of my lens (18mm), since I am shooting at an aperture of f/3.5 whereas at 55mm the aperture is only f/5.6.

Am I missing something? I'll be grateful if somebody can enlighten me on this. Thank you.


errrr... because DOF is also a function of focal length?
 

giantcanopy

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#4

wizz747

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#5

noraa80

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#7
DOF will be the same but the "distance" from object will be different.
 

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Jan 16, 2010
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#8
DOF.. the depth of field depends on so many factors.

Read here http://en.wikipedia.org/wiki/Depth_of_field

Yes, it is complicated but as long as you understand the rule of thumbs, you will be able to handle it. Some key factors are f number, focal length, background (how far?) and how far you are from the subject and so on. and ofcourse natural light, strong Flash will mess up the bookeh.
 

GRbenji

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#9
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GRbenji

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#10
Do you mean like if I am using a 24-70mm F2.8L lens for example, the DOF will be the same at 24mm and 70mm?
No. Longer focal length shallower DOF.
 

wizz747

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#11
No. Longer focal length shallower DOF.
Are you sure about this? Somebody posted a link above in this thread and it's said there that the DOF will be the same.

I want to know how/why the DOF is shallower at longer focal lengths (if it really is) and not just blindly accept it.
 

meekahsoh

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#12
There's also the issue of perspective. Taking a photo of someone at 1m, using the same aperture, using a longer focal length will cause the dof to be less as you are still standing at 1m. To get the same image at double the focal length, you will need to be at 2m away, and then the depth of field will be the same.

Using dofcalculator,

At 1m, f/2.8, DOF at 40mm = 6.46 cm / 80mm == 1.55 cm

However, at 2m, DOF at 80mm = 6.45 cm, essentially the same.
 

GRbenji

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May 24, 2010
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#13
Are you sure about this? Somebody posted a link above in this thread and it's said there that the DOF will be the same.

I want to know how/why the DOF is shallower at longer focal lengths (if it really is) and not just blindly accept it.
DOF Calculator

Use the above DOF calculator and varies the parameters 1 at a time and see for yourself.

DOF is shallower with longer focal length, larger aperture, and nearer subject distance.
 

android17

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Sep 27, 2009
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#14
focal length divided by aperture = f-number

the bigger the F number, the shallower the DOF.


Thus for a 70-200mm f2.8 lens, @ 200mm the DOF will be much shallower than it is at 70mm.

200mm (focal length) / f2.8 (aperture) = 71.4
vs 70mm / f2.8 = 25

Distance also plays apart in determinding the DOF, basically the closer it is, the shallower the DOF.
 

android17

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Sep 27, 2009
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#15
I have the Kit I lens 18-55mm f/3.5-5.6. From what I have read, I can shoot at f/3.5 at 18mm and if I zoom in at 55mm, I get a narrower aperture of f/5.6.

I have also read that the DOF at the telephoto end of a zoom lens is shallower than the DOF at the wide-angle focal length of a lens.

However, according to me, I should be having a shallower DOF at the wide-angle part of my lens (18mm), since I am shooting at an aperture of f/3.5 whereas at 55mm the aperture is only f/5.6.

Am I missing something? I'll be grateful if somebody can enlighten me on this. Thank you.
18mm / f3.5 = 5.1

55mm / f5.6 = 9.82

got it?

This is also the reason why ur 50mm f1.8 will hv shallower DOF than ur kit lens, cos 50/1.8 is 27 f-number
 

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SamTac

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Jul 19, 2007
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#16
focal length divided by aperture = f-number

the bigger the F number, the shallower the DOF.


Thus for a 70-200mm f2.8 lens, @ 200mm the DOF will be much shallower than it is at 70mm.

200mm (focal length) / f2.8 (aperture) = 71.4
vs 70mm / f2.8 = 25

Distance also plays apart in determinding the DOF, basically the closer it is, the shallower the DOF.

Bigger F number = deeper DOF?

Smaller F number = shallower DOF?
 

sf_kang

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#17
the DOF is shallower at longer focal lengths (if it really is) and not just blindly accept it.
[UNQUOTE]
a) A lens is a combination of various glass elements arranged in groups to achieve a certain angle of view.
b) Prime (fixed) lens have one specific focal length, e.g. 24mm, 35mm or 50mm, or 100mm, each offering a specific angle of view.
c) The longer the focal length, the narrower the angle of view, e.g. 24mm offers a 'wide angel view', a 100mm offers a 'narrower telephoto view'.
d) Now, one more factor, is the aperture; the bigger the aperture opening, e.g. f1.8 the brighter is the image and hence allows faster shutter speeds. If the lens is e.g. f4.0, the image will be dimmer, so a longer shutter time will be needed for correct exposure assuming same ISO setting. The f-number is a mathematical relation between diameter of aperture opening to the focal length of the len, e.g a 50mm/f2.0 lens means the diameter of the aperture blades is 25mm while the lens focal length is 50mm.
e) Finally, the DOF is also a mathematical relation (law of optics) and are influenced by 1) aperture size 2) focal length 3) subject-to-lens distance:-
f) f1.8 will produce shallower DOF than f5.6. f5.6 will be shallower than f11.
g) a zoom lens shot at 200mm will have shallower DOF than at 70mm.
h) DOF would be shallower if the subject is shot very near the lens as compared with greater subject-to-lens distance.

IT'S JUST LAW OF OPTICS. Lens designers use various knowhow to try and make the lens easier to use, but they all have to work within the physical laws of optics.

I try to be as 'untechnical' as possible but this is the best I can do to help understand DOF.
Fred
 

wizz747

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#19
the DOF is shallower at longer focal lengths (if it really is) and not just blindly accept it.
[UNQUOTE]
a) A lens is a combination of various glass elements arranged in groups to achieve a certain angle of view.
b) Prime (fixed) lens have one specific focal length, e.g. 24mm, 35mm or 50mm, or 100mm, each offering a specific angle of view.
c) The longer the focal length, the narrower the angle of view, e.g. 24mm offers a 'wide angel view', a 100mm offers a 'narrower telephoto view'.
d) Now, one more factor, is the aperture; the bigger the aperture opening, e.g. f1.8 the brighter is the image and hence allows faster shutter speeds. If the lens is e.g. f4.0, the image will be dimmer, so a longer shutter time will be needed for correct exposure assuming same ISO setting. The f-number is a mathematical relation between diameter of aperture opening to the focal length of the len, e.g a 50mm/f2.0 lens means the diameter of the aperture blades is 25mm while the lens focal length is 50mm.
e) Finally, the DOF is also a mathematical relation (law of optics) and are influenced by 1) aperture size 2) focal length 3) subject-to-lens distance:-
f) f1.8 will produce shallower DOF than f5.6. f5.6 will be shallower than f11.
g) a zoom lens shot at 200mm will have shallower DOF than at 70mm.
h) DOF would be shallower if the subject is shot very near the lens as compared with greater subject-to-lens distance.

IT'S JUST LAW OF OPTICS. Lens designers use various knowhow to try and make the lens easier to use, but they all have to work within the physical laws of optics.

I try to be as 'untechnical' as possible but this is the best I can do to help understand DOF.
Fred


Thanks for the nice explanation mate. Can you explain in technical terms also?
 

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