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Thread: Focal Length and DOF Blurring

  1. #1

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    Focal Length and DOF Blurring

    I am considering getting a portrait lens - maybe the Nikon 85mm or 105mm - for my D5100 mostly to get headshots (portrait mode) with as creamy a background as possible.

    I saw the CiC tutorial on DOF and it appears to suggest that the focal length has no affect on blurry backgrounds other than the fact that in wide lenses, the sharpness is skewed to the rear and blurriness is exaggerated in longer lenses because of magnification. On the face of it, there is no appreciable difference when comparing 85 and 105.

    So to my question, assuming that there exists a f/1.8 105mm Nikon and I compare that with a f/1.8 85mm Nikon, would there be any consideration for blurry background keeping aside all other differences like quality of resolution, price, etc (and that I would have to increase distance with the 105 to get the same composition) - using max aperture and fixed distance when comparing.

    I am confused with the following in the tutorial - not sure what is meant by "pictures will show something entirely different" - appears to contradict what was said before in the tutorial:

    On the other hand, when standing in the same place and focusing on a subject at the same distance, a longer focal length lens will have a shallower depth of field (even though the pictures will show something entirely different). This is more representative of everyday use, but is an effect due to higher magnification, not focal length.

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    Re: Focal Length and DOF Blurring

    Achieving a "creamy" background is dependent upon the "bokeh" that the lens produces. Bokeh (or Boke) is a Japanese term describing the aesthetic quality of the out of focus areas of the image, usually referring to the smoothness of the area. A very smooth bokeh is often described as "creamy".

    Of course, a lens that can produce a narrow depth of field will accentuate the bokeh produced by that lens because more of the image area can be out of focus. This is especially true if the lens has a relatively wide aperture at longer focal lengths.

    The aesthetic quality of the OOF area (smoothness thereof) is mostly controlled by the circle formed by the iris blades of the lens. Usually a lens with more blades (such as 8 or 9) will form a more perfect circle than a lens with fewer blades (such as the Canon 50mm f/1.8 II which has only five blades). The more perfect circle will usually produce the smoothest looking out of focus areas.

    There are at least two lenses (there could very well be more across the product line of the several lens manufacturers) that go one further step in enabling the formation of an aperture that is a perfect circle. Both the Canon 70-200mm f/2.8L IS II and 70-200mm f/4L IS lenses utilize rounded iris blades that can form a more perfect circle producing a creamy and smooth OOF area.

    Canon EF 70-200mm f/4L IS lens
    Focal Length and DOF Blurring

    Smooth creamy bokeh is usually preferred by most of us because it doesn't conflict for the viewers interest with the main subject. It can sometimes even enhance the subject as in this shot done in Bejing's Forbidden City...

    Focal Length and DOF Blurring

    I think that the smooth OOF rendition of the Chinese characters in the background add to the interest of this image of a Phoenix statue. If the look of the OOF areas (bokeh) was ragged or choppy, I think that it would distract from the attention given to the Phoenix statue...

    The OOF areas don't have to be completely blurred to center interest on the main subject or subjects.

    Focal Length and DOF Blurring

    Just the slight blurring of the above background tends to isolate the subjects and center interest on them. Normally, viewer interest is centered on faces (usually the eyes) of people, statues or animals; bright colors or areas; printed words and then sharp areas in an otherwise blurry image...

    DOF is gererally controlled by subject distance, focal length and aperture. However, the subject distance will often be predicated by the focal length you are using. When shooting with a shorter focal length, you will most often shoot from a closer distance to the subject.

    I don't want to confuse the issue but DOF is actually determined by the image ratio and aperture. This is usually considered in close-up and or macro photography when say, shooting with any number of macro lenses. Using 50mm, 60mm, 65mm, 90mm, 100mm, 105mm, 150mm and 180mm lenses, all at a 1:1 image ratio, will achieve the same DOF which will then only be dependent on the f/stop you are using. (This is true for both crop and full frame cameras.) You will, of course be shooting from different distances to achieve the 1:1 image ratio with different focal lengths but, the same distance when using both crop and full frame cameras.

    The same thing is true for non close-up or non macro photography but, we usually don't consider the image ratio when we are shooting a portrait of a person or a shot of a building...

    So, generally; the longer focal lengths and wider apertures can be considered to produce the most narrow DOF enabling selective focus to place more emphasis on your subject.
    Last edited by rpcrowe; 25th August 2013 at 04:00 PM.

  3. #3

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    Re: Focal Length and DOF Blurring

    Denis: a couple of things or more, as your camera is a DX or crop your realize that the 85mm and 105mm are for FX full frame camera and so you would up by 1.5. Another thing if you put the same lens say the 85mm f/1.8 on the DX camera the DoF will be greater than if put on the FX camera so the FX camera has a shallower DoF compared to the DX camera with the same lens. If you are looking for that "as creamy a background as possible" that also depends on how far the background is from the subject the farther away the creamer. To get that creamy background you subject needs to be separated from it by up to 10ft that will make a greater difference than using f/1.8 rather than f/2.8 or 1.3 stops on a 85mm mounted on a DX camera the difference if focused on the subject 10ft away is about 3" increase in DoF. I feel that the best bang for your money would be the 105mm f/2.8 at this time looking a the equipment you have at this time, if you wanted to go wild than I would suggest the Zeiss 135/f2 however it is about 3 times the cost of you complete kit at this time.

    Cheers:

    Allan

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    Moderator GrumpyDiver's Avatar
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    Re: Focal Length and DOF Blurring

    Shooting a f/2 lens wide open on a DX body will get you a similar DoF as a f/2.8 lens would get you on an FX body. A lens that is shot wide open is going to be a touch softer than when you stop it down, but for portraits, that's not necessarily a bad thing.

    Nikon makes two fantastic portrait lenses; the f/2 105mm DC and the f/2 135mm DC. The DC stands for Defocus Control, and it lets you throw the background (or foreground) out of focus. Both are designed for the creamy bokeh and this can be adjusted via a control on the lens. I own the 105mm DC and it is my favourite portrait lens, bar none. The main issue for you is that your camera does not have a built in focus motor, so you would have to focus it manually. Neither lens is stabilized and both are often not stocked by the camera store and need to be special ordered. Needless to say, they are not cheap either.

    Nikkor f/2 105mm DC at f/5.6 at ISO 100 at 1/125 s using a Nikon D800.

    Focal Length and DOF Blurring

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    Re: Focal Length and DOF Blurring

    Quote Originally Posted by DennisS View Post
    I am confused with the following in the tutorial - not sure what is meant by "pictures will show something entirely different" - appears to contradict what was said before in the tutorial:

    On the other hand, when standing in the same place and focusing on a subject at the same distance, a longer focal length lens will have a shallower depth of field (even though the pictures will show something entirely different). This is more representative of everyday use, but is an effect due to higher magnification, not focal length.
    I'm not 100% sure but I think what they mean is that as they're standing in the same spot with the same camera to subject distance the longer lens will have shallower DoF but... the framing will be different.

    For head shots with an APS-C camera I think I'd go for the 85mm because for me personally 105mm (on APS'C) may be starting to get a bit... long plus of course if you get an 85mm you can get a f1.4 or f1.8 but you can't as far as I know get a 105mm with that f value.

    Apart from the actual DoF the character / rendering and what the shot ends uplooking like could be the deciding factor. If it helps at all I went for the Sigma 85mm f1.4 rather than the Canon alternatives I was faced with and I find it to be a very nice lens.

    PS. Excellent example shots!

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    Re: Focal Length and DOF Blurring

    Quote Originally Posted by Polar01 View Post
    as your camera is a DX or crop your realize that the 85mm and 105mm are for FX full frame camera and so you would up by 1.5.
    Good point Allan - I did not consider that, so yes, the 105 would be a bit long and I would have more options for a wider aperture.

    Thank you Richard for discussing the type of blades. I had completely forgotten about that.

    Good to hear about the DC control from Manfred.

    So this feedback, now gives me a little more direction - assuming I want to stay with a DX lens (to avoid manual focus) between 85-105 with around f/2.0 or f/1.8, what are good Nikon choices at this time in the market - my budget would be around US$ 500-600? I do own the 55-200 but max aperture is f/4.0. Should I consider a telezoom rather than prime?

    Additionally does the OOF area get more blurry the further out from the rear DOF or is it all the same once you get past the limits of DOF?

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    Moderator GrumpyDiver's Avatar
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    Re: Focal Length and DOF Blurring

    If you want to stick with Nikon, you really have only two choices for the focal length and price range; the f/1.8 50mmG and the f/1.8 85mmG. These are the only two lenses that are fast enough or fall into the price range you are looking at. Any of the fast zooms are outside on the money you are willing to spend (and they are f/2.8 max aperture (which is fast enough, by the way)) are going to be around 3x more than your budget,

    FX and DX have nothing to do with manual or autofocus; these designate full-frame and crop frame sensors. What you are looking for are the G lenses which have the built-in focus motor in the lens, so any of these will autofocus on your camera body.

    The 50mm lens is a bit short (75mm full frame equivalent) for my liking and the 85mm (127.5mm) is a bit longer, and more the way I would shoot head shots. As an aside, I also use the f/2.8 70-200mm for portraiture, but this is a large and expensive lens.

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    Re: Focal Length and DOF Blurring

    Quote Originally Posted by GrumpyDiver View Post
    FX and DX have nothing to do with manual or autofocus; these designate full-frame and crop frame sensors. What you are looking for are the G lenses which have the built-in focus motor in the lens, so any of these will autofocus on your camera body.
    Another good point Manfred - so I could stick with an FX lens if needed, as long as it was G - would have a built-in focus motor. I see this on Adorama for $496 Nikon 85mm f/1.8G AF-S FX Nikkor (http://www.adorama.com/NK8518U.html). I guess that should meet my needs but could be a bit long at 127mm DX (my home studio restricts me from stepping too far back - 15 feet would be the most I could and would prefer an 85% fill to allow for different print formats). I cannot locate a Nikon DX G f/1.8 lens between 85-105 as yet. I guess I may have to compromise on the focal length.

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    Moderator GrumpyDiver's Avatar
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    Re: Focal Length and DOF Blurring

    Quote Originally Posted by DennisS View Post
    I cannot locate a Nikon DX G f/1.8 lens between 85-105 as yet.
    I'm not quite sure why you are hung up on the f1/8 maximum aperture, especially if you are shooting in a home studio. The shot I posted abober was done in around the same space, but I used a cheap white seamless paper backdrop. You can get it in any other colour you might want to use. A roll of seamless is quite inexpensive and you can either get a commercial stand to hold it or cobble something together with PVC piping.

    That way you could use your existing 55-200mm lens. This is an early portrait I did with that lens.

    Focal Length and DOF Blurring

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    Re: Focal Length and DOF Blurring

    Yep - you are correct in that the f/1.8 may not be a factor in a home studio but I was hoping for it to do double duty outside too. I have used the 55-200 quite effectively with plain white/black backgrounds inside (but had to blow out the background with a flash at a higher power ratio) but it does not work too well outside where the backgrounds are more busy. Even indoors I would like to have more control on the DOF when I have a not so plain background. I have to keep some distance between the subject and the background and that is always not possible. In my home studio, I can get about 7-8 feet between subject and background that allows me about 10-13 feet (I could stretch to 15 feet) between camera and subject. Things get more complicated if I attempt to move the subject further away from the background (to keep the distance to the focal plane as small as possible - for more DOF - but yet not have an impact on perspective) and leaves me fewer options on where the camera can be placed.

    I guess I would also like to add some more capability to the lenses I already own and not duplicate any functionality that they already have. The 35mm f/1.8 I have is just too short for headshots. I believe what I really needed at the time, was a longer lens with around f/1.8, but I made that decision with inexperience and before I joined CiC and had access to all this awesome knowledge.

    BTW: Excellent portrait shot...!!

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    Re: Focal Length and DOF Blurring

    Hi Dennis,

    In a studio setup, long focal length fast glass in the F1.8 to F2.0 range for head and shoulder portraits are a waste of money for a couple of reasons

    (a) If you're using studio heads then you won't be able to get the output low enough at those apertures to get within the sync range, and

    (b) Your DoF will be razor thin, and you'll end up with issues like an eye being in focus but the tip of the nose being out of focus.

    You don't need wide apertures to throw the background out of focus - you only need to set a generous camera to subject -v- subject to background ratio. In most cases I'm struggling more to get sufficient DoF over the face then shooting with longer focal lengths -- in practice I'm usually shooting at F11 and sometimes as far down as F22.

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    Re: Focal Length and DOF Blurring

    The first 'assignment' I was given on starting at photo school was to make a background... this involved getting a large sheet of paper and painting various tones on it. Since we were confined to the studios for the first term and worked on little tables doing still life subjects the sheet of paper probably wouldn't have been big enough for a portrait but the principle remains the same ... DIY rather than BUY.
    My background from those days c1952
    Focal Length and DOF Blurring

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    Re: Focal Length and DOF Blurring

    Quote Originally Posted by Colin Southern View Post
    Hi Dennis,

    In a studio setup, long focal length fast glass in the F1.8 to F2.0 range for head and shoulder portraits are a waste of money for a couple of reasons

    (a) If you're using studio heads then you won't be able to get the output low enough at those apertures to get within the sync range, and.
    Maybe not PC in Colin's studio but I would simply tape a facial tissue or two over the flash head. I did it to soften the light becuase I put backgrounds OOF in editing ... though this can be a problem with straggly unruley hair styles. Fortunately I don't do portraits these days

    In all this talk of OOF backgrounds I have for long been of the opinion that it is not something one really achieves 'in-camera' unless the background is 'way back' compared with camera to subject distance

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    Moderator GrumpyDiver's Avatar
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    Re: Focal Length and DOF Blurring

    Quote Originally Posted by DennisS View Post
    Yep - you are correct in that the f/1.8 may not be a factor in a home studio but I was hoping for it to do double duty outside too. I have used the 55-200 quite effectively with plain white/black backgrounds inside (but had to blow out the background with a flash at a higher power ratio) but it does not work too well outside where the backgrounds are more busy. Even indoors I would like to have more control on the DOF when I have a not so plain background. I have to keep some distance between the subject and the background and that is always not possible. In my home studio, I can get about 7-8 feet between subject and background that allows me about 10-13 feet (I could stretch to 15 feet) between camera and subject. Things get more complicated if I attempt to move the subject further away from the background (to keep the distance to the focal plane as small as possible - for more DOF - but yet not have an impact on perspective) and leaves me fewer options on where the camera can be placed.

    I guess I would also like to add some more capability to the lenses I already own and not duplicate any functionality that they already have. The 35mm f/1.8 I have is just too short for headshots. I believe what I really needed at the time, was a longer lens with around f/1.8, but I made that decision with inexperience and before I joined CiC and had access to all this awesome knowledge.

    BTW: Excellent portrait shot...!!

    Dennis - I find that when I shoot outdoor portraits, I tend to use a longer lens than for indoor shoots; i.e. when I use the 70-200mm lens, it's usually pushed up to darn close to 200mm to help throw the background out of focus. One pro I know uses a prime 300mm lens for his most of outdoor portraits.

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    Focal Length and DOF Blurring

    Quote Originally Posted by jcuknz View Post
    Maybe not PC in Colin's studio but I would simply tape a facial tissue or two over the flash head. I did it to soften the light becuase I put backgrounds OOF in editing ... though this can be a problem with straggly unruley hair styles. Fortunately I don't do portraits these days
    Um, it won't soften the light because the effective size isn't changing significantly.

    The other problem is it also becomes an increasing portion of ambient, which means you need to black out the studio more -- and then AF doesn't work and you can't see unless you use modelling lights - but then they set fire to the tissue paper, and also become part of the exposure ...

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    Re: Focal Length and DOF Blurring

    So to my question, assuming that there exists a f/1.8 105mm Nikon and I compare that with a f/1.8 85mm Nikon, would there be any consideration for blurry background
    Dennis,

    You have a couple of different issues here.

    First, as you saw in the tutorial here, focal length has very little effect on depth of field if you change position so that the subject stays the same proportion of the frame. The main reason focal length will matter for portraits is perspective. Shorter lengths make people look bulbous; longer lengths flatten perspective (because you stand farther away). That is a matter of taste, but in the 35mm days, many people liked 90-100mm, which translates to around 60-65 on your camera.


    Second, background blur is not the same as DOF. Longer focal lengths will give you more background blur. For an explanation and some examples, check out this page: http://toothwalker.org/optics/dof.html.


    Third, there are other characteristics of lenses that affect the quality of the out-of-focus area--the bokeh that Richard referred to. Unfortunately, while one can point to some features that tend to improve bokeh, like rounded aperture blades and a larger number of blades, it is very hard to judge the bokeh of a lens without trying it under different circumstances. So for that, you have to ask about specific lenses. (Since I don't shoot Nikon, I can't speak to those lenses.)

    Dan

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    Re: Focal Length and DOF Blurring

    Quote Originally Posted by GrumpyDiver View Post
    ... I find that when I shoot outdoor portraits, I tend to use a longer lens than for indoor shoots.
    Agree - there is more potential outside - more space so even with a prime you can step back with a longer lens or increase the distance of subject to background.

    Quote Originally Posted by Colin Southern View Post
    ...you only need to set a generous camera to subject -v- subject to background ratio
    Looks like that is one of the challenges I have - not much room to play with.


    Quote Originally Posted by DanK View Post
    ... which translates to around 60-65 on your camera.
    and this is what I believe I would like to have but appears Nikon does not have that at present - I would have to settle for 50mm DX (a bit short) or 85mm FX (a bit long).

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    Re: Focal Length and DOF Blurring

    Quote Originally Posted by DennisS View Post
    Agree - there is more potential outside - more space so even with a prime you can step back with a longer lens or increase the distance of subject to background.
    Outside - for individuals - F2.8 rules

    Focal Length and DOF Blurring

    Focal Length and DOF Blurring



    Looks like that is one of the challenges I have - not much room to play with.
    I know what you mean, but generally, faster glass in a studio creates many more problems than they solve in terms of DoF & ambient light. Personally, I find studio portraits quite sterile; yes, one can get nice lighting, but with a bit of effort, it's not hard to get good lighting outside also -- and infinitely less problems with backgrounds. Also - if you can't get good subject to background separation then you end up with cross contamination of your separate light zones (key, fill, and hair lights give uneven backdrop lighting, and backdrop lighting can cause unintended rim lighting, and even flare issues if the lights aren't flagged off).

    and this is what I believe I would like to have but appears Nikon does not have that at present - I would have to settle for 50mm DX (a bit short) or 85mm FX (a bit long).
    Mostly in the studio for head and shoulders, I'm in the 120 - 170mm region (full frame).

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    Re: Focal Length and DOF Blurring

    Quote Originally Posted by DennisS View Post
    . . . and this is what I believe I would like to have but appears Nikon does not have that at present - I would have to settle for 50mm DX (a bit short) or 85mm FX (a bit long).
    Dennis, you might want to look at the Sigma f/2.8 70mm EX DG Macro lens which I believe can be obtained with the Nikon mount, It's about the size you mentioned above, i.e. shorter than 85mm. "DG" is Sigma-speak for "FX". And it has a 9-blade aperture - just the job for your 'bokeh'.

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    Re: Focal Length and DOF Blurring

    Quote Originally Posted by Colin Southern View Post
    Um, it won't soften the light because the effective size isn't changing significantly.
    ...
    It did in my case because the tube was small and the tissue at the end of the snoot was considerably bigger ... it may not work for your pro gear but it did with my home stuff ... sorry BUT. Your second para is best ignored.

    Pity Cantab was misled.
    Last edited by jcuknz; 26th August 2013 at 06:32 AM.

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