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Thread: Depth of field vs focal length

  1. #1

    Default Depth of field vs focal length

    When I first picked up a camera there was no magnification factor (actually cropping factor), and a 20 mm lens is really wide, with tremendous dof. Today my kit lens 18-70 is in mm count wider than my 20 mm, but in actual fact delivers only 27 mm wide equivalent.

    Now comes the question - I seem to have read somewhere that the dof is dependent on the focal length. If this is correct, then the dof from my kit lens at 18 mm would be comparable to that of the 20 mm, more so than that of a 28 mm.

    Is this correct?

    That's with the DSLR, the question gets further complicated when we moved to PnS, tiny sensors. My S60 at the tele end is 20.7 mm (35 mm equivalent of 100 mm). Does that mean I get more dof using the PnS?

    And I can only stop down to f8 on the PnS, as opposed to f22 on my DSLR.

    Or put it another way, if I try to get macro shots on my PnS, with accessories lens, stopped down to f8 (sigh, back in those days there was a saying, f5.6 and be there), do I have enough depth of field? Like if I use my 100 mm equivalent at f8, do I get more dof than 105 micro Nikkor at f8?

    20 mm on film is very wide, on DSLR is just starting to be wide, but a tele with PnS. What an animal of a term.

    Any thoughts are much appreciated.

  2. #2
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    Default Re: Depth of field vs focal length

    Hi diediealsomustdive,

    I am not sure when you say enough DOF, you mean enough depth or enough shallow? But the basic principles for DOF are:

    1. Shorter focal length (wider angle) gives more depth
    2. slower aperture gives more depth

    Quote Originally Posted by diediealsomustdive
    When I first picked up a camera there was no magnification factor (actually cropping factor), and a 20 mm lens is really wide, with tremendous dof. Today my kit lens 18-70 is in mm count wider than my 20 mm, but in actual fact delivers only 27 mm wide equivalent.
    I am not very sure, but if your DSLR is not full frame (like the Canon D5), then there will be a magnification factor, so 18 translated to 27mm (I duno how you get the 27mm) seems reasonable to me.

    Now comes the question - I seem to have read somewhere that the dof is dependent on the focal length. If this is correct, then the dof from my kit lens at 18 mm would be comparable to that of the 20 mm, more so than that of a 28 mm.

    Is this correct?
    You can calculate from you mag factor. Assuming your 27mm is correct, your mag factor is 27/18=1.5. So just punch the calculate to find out, to get 20mm (35mm equivalent) you need at least 13mm focal length on this cam...and shorter focal length means deeper DOF.

    That's with the DSLR, the question gets further complicated when we moved to PnS, tiny sensors. My S60 at the tele end is 20.7 mm (35 mm equivalent of 100 mm). Does that mean I get more dof using the PnS?
    Think you should simply convert all camera model's specs into 35mm equivalent, then u can compare them apple to apple. Then it's very easy.

    And I can only stop down to f8 on the PnS, as opposed to f22 on my DSLR.

    Or put it another way, if I try to get macro shots on my PnS, with accessories lens, stopped down to f8 (sigh, back in those days there was a saying, f5.6 and be there), do I have enough depth of field?
    Enough DOF or not depends on how you want the pic to be. For landscape or macro? Very diff requirements.

    Like if I use my 100 mm equivalent at f8, do I get more dof than 105 micro Nikkor at f8?
    You need to calculate your 35mm equivalent on your DSLR Mag factor, I think.

    20 mm on film is very wide, on DSLR is just starting to be wide, but a tele with PnS. What an animal of a term.
    This is the effect of the Mag factor, or so call the CCD/CMOS sizes' effect.

    I am not a DSLR owner and I am also new to photography. Hope I am right and can someone point out if I am wrong.

    Thank you.

  3. #3
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    Default Re: Depth of field vs focal length

    dude i dont understand what you're talking about.
    sigh.

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    Default Re: Depth of field vs focal length

    Hi, which dude you talking about? Me or the TS?

    Quote Originally Posted by Clown
    dude i dont understand what you're talking about.

  5. #5

    Default Re: Depth of field vs focal length

    Think you are missing the point. If you have a DSLR and fixed focal length lenses or zoom with DoF market (big IFs given today's community), set up your camera with a tripod, select an aperture (like f/8), focus on hyperfocal point for that aperture, carefully note where the nearesting DoF point is on the lens marking. Then take a measuring tape and put in markers at the near DoF point, and a few more points further. Take the photo and see if the points that should be sharp by the DoF markers are indeed sharp. You may be in for a surprise.

    And DoF means areas that are in acceptable sharpness.

    PnS do not have that DoF markers. I hazard to guess that if I shoot at f/8 on my PnS at 105 equivalent as compared to a film SLR 105 micro I will get more DoF on the PnS.

    Indeed there are talks that equivalent focal lengths, like 75 mm on a film SLR and a 50 mm DSLR (Nikons, crop factor 1.5), you get more DoF on the DSLR simply because the actual focal length is 50 mm (and hence more DoF). And you get less DoF than marked on the lens due to the crop factor (i.e. 50mm lens on DSLR vs film camera).

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    Default Re: Depth of field vs focal length

    Quote Originally Posted by lastboltnut
    Hi, which dude you talking about? Me or the TS?
    my bad.. referring to the TS
    sigh.

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    Default Re: Depth of field vs focal length

    bolt.. i think you're confusing me even more. I know you're trying to be helpful but I think maybe you should recheck your information a bit first.

    18mm on a (i assume nikon/KM/Sony/Pentax body with 1.5x crop) will give you 27mm equivalent field of view. On a Canon will give you 28.8 (just round off to 29mm)
    But the DOF remains at 18mm.
    This means that you are actually getting a 18mm DOF with 28mm FOV. Compare this to the full frame (or film) body using a 28mm lens, you would have a deeper DOF on the cropped sensor with the same FOV.

    Apply the same threory to the P&S tiny sensor, you would be getting a 20.7mm DOF on a 100mm FOV which translate to very very deep DOF when compared to the same 100mm on a full frame body.
    “How fortunate for leaders that men do not think.” - Adolf Hitler

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    Default Re: Depth of field vs focal length

    Hey! Guys, may be your guys are correct! As I say, I am a newbie, so just trying to discuss and see. Now that you guys mention, it sounds logical that only FOV changed and not DOF. Got it! Thanks!

    But after a second thought, if a compact has diff DOF from SLR due to sensor size diff, then DSLR (other than the full frame model) should have a diff DOF from 35mm SLR loh.
    Last edited by lastboltnut; 1st August 2006 at 11:39 PM.

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    Default Re: Depth of field vs focal length

    Quote Originally Posted by lastboltnut
    Hey! Guys, may be your guys are correct! As I say, I am a newbie, so just trying to discuss and see. Now that you guys mention, it sounds logical that only FOV changed and not DOF. Got it! Thanks!
    It's good to discuss but the culture here is to do research before posting. Being a "newbie" or not is irrelavant.

    Quote Originally Posted by lastboltnut
    But after a second thought, if a compact has diff DOF from SLR due to sensor size diff, then DSLR (other than the full frame model) should have a diff DOF from 35mm SLR loh.
    Correct.
    “How fortunate for leaders that men do not think.” - Adolf Hitler

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    Default Re: Depth of field vs focal length

    Hi yanyewkay, it is definitely good to be sure of what one say, but I did mentioned that I may be wrong. I may misled others only if I did not say I am not sure right?

    Quote Originally Posted by yanyewkay
    It's good to discuss but the culture here is to do research before posting. Being a "newbie" or not is irrelavant.

  11. #11

    Default Re: Depth of field vs focal length

    Quote Originally Posted by yanyewkay
    Paraphrased: 18mm on DSLR gives a 27mm or 28.8 mm FOV but the DOF remains at 18mm. Compare this to the full frame (or film) body using a 28mm lens, you would have a deeper DOF on the cropped sensor with the same FOV.

    Apply the same threory to the P&S tiny sensor, you would be getting a 20.7mm DOF on a 100mm FOV which translate to very very deep DOF when compared to the same 100mm on a full frame body.
    I thank you for explaining it in simple terms.

    So at f/8 on a tiny P&S sensor I might (just might) be getting equivalent DOF of maybe f/16 on a full frame 35 mm body. This I will experiment when I next attempt macro (underwater).

    Mind if I ask you about the DOF markings on a fixed lens meant for full frame bodies?

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    Default Re: Depth of field vs focal length

    yes it's meant for FF bodies but it is also applicable to cropped sensors.

    eg: for a certain lens XXmm at f/2.8 you might get DOF marking from 3m to 5m for a film (or FF) body, mounting it on the DSLR you get 1.5XXmm but the DOF still remains at 3m to 5m. The FOV has just changed that's all.
    “How fortunate for leaders that men do not think.” - Adolf Hitler

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    Default Re: Depth of field vs focal length

    Yah. Now I know why I calculated the DOF for my compact (based on 35mm equivalent) is totally out...I tot I have 400+mm tele and got 10mm DOF, but when I shoot, it doesn't tell the same ting....

    At least this thread is good, it teaches something! Thanx to yanyewkay.

    Quote Originally Posted by yanyewkay
    yes it's meant for FF bodies but it is also applicable to cropped sensors.

    eg: for a certain lens XXmm at f/2.8 you might get DOF marking from 3m to 5m for a film (or FF) body, mounting it on the DSLR you get 1.5XXmm but the DOF still remains at 3m to 5m. The FOV has just changed that's all.

  14. #14

    Default Re: Depth of field vs focal length

    Quote Originally Posted by yanyewkay
    yes it's meant for FF bodies but it is also applicable to cropped sensors.

    eg: for a certain lens XXmm at f/2.8 you might get DOF marking from 3m to 5m for a film (or FF) body, mounting it on the DSLR you get 1.5XXmm but the DOF still remains at 3m to 5m. The FOV has just changed that's all.
    First of all thanks for your information, appreciate you coming forward to clarify some of my confusion.

    But that's not what I was told on some online resources. But then the information is only as accurate as the knowledge of the provider.

    I will experiment, and report on it if I can have it figured out.

    I hope you are right, it makes life so much simpler.

    Cheers.
    Last edited by diediealsomustdive; 2nd August 2006 at 11:59 AM.

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    Default Re: Depth of field vs focal length

    Well, you should think of PnS cameras being similar to a dSLR.
    While your nikon dSLR has got a 1.5x crop factor, a typical small PnS has a crop factor of 4X to 10X.

    Remember that
    1) a 20mm lens will have a DOF characteristic to a 20mm lens. even when used on a 1.5x crop dSLR. As said, the FOV is different.
    2) smaller the focal length, the deeper/larger the DOF. the 35mm equivilant on a PnS is usually some lens that's like 6mm or 7mm... your DOF is gonna be huge..
    3) your S60 has a crop factor of 100/20.7 = 4.83X

    Thus if you use a 20mm on a dSLR and your S60 at 100mm (which is really 20.7mm) equivilant, you're going to get the same DOF, just different FOV

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