Alley in Pisa, Italy

by Lars Kotthoff

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I am trying to get a shot of the subject that is a foot or two away from the background that I would like to have show as blurry.

This is what I am going after. Notice how the face of the dog is in focus and his rump is already very blurry. dog

This is what I get at max aperture of f/1.8 with my 35mm prime lens.

Note: The items behind my daughter are about 2 feet from her. My guess is the dog's tail is about the same distance from his face and is way more blurry. What am i missing? alt text

I thought with an aperture of f/1.8 I would get a razor thin depth of field.

EDIT: Here is an image I took inches from my daughter and the bokeh is nice at the distance the TV is. I am about 10 inches from the subject and seem to get nice bokeh at about ten feet behind her (distance from her to the tv). I would love to be able to get that same bokeh at 1 foot behind her. Is that possible and what hardware would be needed?

alt text

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Forget about the dog's rump... its ears and nose are even blurry :-). –  Tom Jan 10 '11 at 1:30
    
i suspect you use crop frame rather than a full frame? –  icelava Jan 10 '11 at 3:08
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By the way, the DOF still is rather thin - your background my not have the blur you want, but its still "clearly" out of focus. –  rfusca Jan 10 '11 at 3:47
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The photo of the dog is taken at 85mm, your photo was taken at 35mm. Get an 85mm lens :) –  Alex Black Jan 10 '11 at 16:29
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The DOF in the latest photo is indeed very thin. It looks like only the front part of her cup is in focus, and everything after that is OOF. The total depth of your scene still isn't very large, you appear to be working in a limited space. Try taking a shot outdoors, about 12-18 inches away, and see how the background bokeh looks. It should be a lot better. Also, compose your shot such that the background is usefully in contrast with your subject to help isolate and enhance, rather than fight for attention (the new photo is pretty "noisy" and compositionally lacking.) –  jrista Jan 12 '11 at 21:59
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5 Answers 5

up vote 10 down vote accepted

I think the key difference here is subject distance. Depth of field is a function of aperture, subject distance, and focal length. The closer you are to your subject, the thinner your depth of field will be. That said, longer focal length is also a way to get the effect you are looking for as well.

If you have the option, using a wider lens with a wide aperture very close to your subjects will produce that very nice, very thin DoF and produce that dreamy, creamy background blur. However, if you do not have the option of getting in real close, a longer focal length will also produce that dreamy bokeh.

You mentioned you were using a 35mm f/1.8 prime lens for the shot of your daughter. Try using a 50mm f/1.8, or even an 85mm f/1.8. Each one will successively narrow your field of view, but in doing so, they will also "compress" the background. That is, make it appear as though it is closer to the primary subject, and increase the amount of visible blur in it as well. You will want to shoot at the same distance as with the 35mm, however doing so will shrink the scope of the scene. With a 50mm, you might only capture the blue part of the ladder, and exclude that fantastically giant "rubber ducky". At 85mm, you would probably narrow the scene down to just your daughter, and maybe a little bit of the ladder.

You can, obviously, also reduce your DoF by getting closer with the 35mm, and keeping your aperture wide open. You might not get as much blur, however you would keep more of the background in the scene.

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LOL. I understand in theory your suggestions. But, in practice, the lens is about a foot away from my daughter. I am getting background blur but on the same note and close enough to her that she can reach out and get her greasy fingers on the lens. I can not, practically, get any closer. This leads me to believe I require the 50mm f/1.4 to get the shots I am looking for. –  kacalapy Jan 10 '11 at 2:24
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Yeah, sounds like you need a longer focal length. The 50mm f/1.4 on a cropped sensor is probably going to be ideal. Some of the best portraits I've ever seen were taken with the EF 85mm f/1.2 lens on a full-frame camera...the background blur is simply stunning at that focal length. A 50mm lens on APS-C is effectively about 81mm, so it should produce similar pictures. –  jrista Jan 10 '11 at 2:28
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@jrista - but the DOF of a 50mm lens on an APS-C body won't be the same as the DOF of a 85mm lens on a full-frame body I don't think. –  Alex Black Jan 10 '11 at 16:33
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To qualify my last comment...the difference in DOF between APS-C and FF is due to framing. If you use the same focal length and aperture on both cameras, and aim to achieve the same exact framing, you would have to get a lot closer with the FF camera. Since you are closer, your DOF is thinner than you can get with the APS-C for that particular framing. However, when comparing APS-C and FF with lenses that offer the same (or very similar) Field of View, you should be able to get the same framing at the same distance...and therefor get the same DOF. On APS-C, 50mm~=81mm, so DOF will differ some. –  jrista Jan 10 '11 at 20:10
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@rapscalli: Aye, you are correct. I was working the math, and the same 'absolute aperture' produces the same DOF, however at the same f#, the absolute aperture of a 50/1.4 is almost half that of an 85/1.4. The physical DOF with an 85/1.4 will be just under half that of the 50/1.4. The physical DOF with an 85/1.8 will be about 25% thinner than with a 50/1.4. You would need an 85/2.4 on FF to achieve the same DOF as a 50/1.4 on APS-C. –  jrista Jan 13 '11 at 21:54
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The subject distance is much closer in the dog picture. DOF decreases rapidly with closer subject distance.

Spend an afternoon on:

http://www.dofmaster.com/dofjs.html

Trying different settings and you'll soon get a feel for how to minimise DOF.

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This. I always wanted to spend an afternoon on this site (or copy the formulas into a spreadsheet) and make quick reference cards to help me learn what to expect w.r.t. DOF. –  Reid Jan 10 '11 at 0:15
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So I should get closer to my daughter to make the background more blurry? –  kacalapy Jan 10 '11 at 0:26
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No, you should spend some time on dofmaster.com/dofjs.html... –  Jay Lance Photography Jan 10 '11 at 0:29
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@kacalapy, that will help, but spend some time on dofmaster.com/dofjs.html. Really! :) –  Reid Jan 10 '11 at 0:33
    
@Reid when I have time I plan to rearrange the DOF equations so that you can enter what DOF you want and it will tell you what subject distance and what lens you need to pull it off! –  Matt Grum Jan 10 '11 at 0:53
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Comparing specifically, here's the parameters on the dog photo:

  1. f/1.4 vs. your f/1.8
  2. 85mm lens vs. 35mm
  3. full-frame sensor vs. crop sensor (maybe)
  4. subject may be closer (not 100% sure here)

All four of these things will lead to a shallower DOF if you have a crop sensor, and everything except #3 if you have a full-frame.

You can plug these numbers into the tool Matt suggests to see exactly how much; I suspect you'll be surprised. At 1 meter to subject, just going from f/1.8 & 35mm to f/1.4 & 85mm will take you from a 60 cm deep zone of focus to 10 cm.

In general, to get a really shallow DOF you want to use a long lens with a large aperture and get close to the subject.

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if the dog was closer to the lens than my daughter was then his breath would fog the lens. I was at max a little over a foot away from her. I have since got even closer to her as practice with a x-mas tree behind her and got a nice effect. but that close a distance is very impractical. I will not be able to get that close in public settings to people I barley know. Can you suggest a long Nikon lens with high aperture? –  kacalapy Jan 10 '11 at 2:28
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@kacalpy, having taking a lot of pictures of my daughters with a 40mm lens (on an APS-C camera), I think it's unlikely that you were quite as close as you remember in this particular shot. –  mattdm Jan 10 '11 at 5:12
    
"Can you suggest a long Nikon lens with a high aperture" - the Nikon 85mm f/1.8 –  Alex Black Jan 10 '11 at 16:48
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@kacalapy assuming the height of your daughters head is about 18cm, applying a bit of trigonometry reveals you were 90cm away, which is more like three feet! –  Matt Grum Jan 10 '11 at 18:45
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One thing you might also try is to position your subject further away from walls. As you can see from playing with the DoF calculator that others have mentioned, distance from your subject increases depth of field. Practically, what this means is that you should be aware of the relative distances between your camera, your subject, and your background.

In a nutshell, the further away you are from your subject, the further the background must be from them to be blurred.

By way of example, I take lots of photos of my daughters with a Canon 50mm ƒ1.8 Ⅱ lens on an APS-C Rebel XSi body. If I select ƒ4 and position my subject 10 feet away, the far limit of my depth of field is 11 feet. If I position my subject 20 feet away, the limit is 24.5 feet. An object at these positions will be equally blurry.

You say that you cannot get closer to your daughter. That’s fine — what you can do is recompose so the background is further away from her, which will have the same effect.

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Thanks for the simple and clear explanation. In addition to learning by way of these controlled shots where I can manipulate my daughter's distance from me and from the background, I am also trying to get a feel for what I would need to do to get shots of a subject if I was in an audience or crowd and wanted to get a shot of a particular person in a crowd of people (like on a dance floor) such that the individual is sharp and the rest of the composition is blurred forcing viewers eyes to focus on the subject. Seems like I am trying to overcome physics. –  kacalapy Jan 10 '11 at 4:55
    
This is good advice, I often recompose a shot of my child so that the background is further away (if the shallow depth of field is what I'm going for). –  rfusca Jan 10 '11 at 6:16
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You have a specific "effect" you're looking for and while a calculator like http://www.dofmaster.com/dofjs.html will tell you the DoF range given parameters, it won't tell you if the out of focus part is out of focus enough for you.

Before you lay down another chunk of change on a lens, I'd strongly recommend you borrow, rent, or use in store a lens you're considering and put it to a realistic test. (If you test one in store, realize the distance behind your subject may be much greater than normal and could vastly overstate your results).

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Actually, that is what i am doing. The 35mm f/1.8 is my neighbors who I just found out has a D90. –  kacalapy Jan 10 '11 at 15:26
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