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Thread started 10 Dec 2011 (Saturday) 15:56
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Assistance with understand DOF differences on short vs long lenses

 
Gel
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Dec 10, 2011 15:56 |  #1

One of those things I've never thought about until now.

Up until recently the longest lens I've had is a 200mm.

Now I've a 100-400L and a 2x Extender. With the 100-400L I'm finding that the amount of bokeh present at 5.6 is very similar to an 85L prime at 2.8

Is this because by zooming to 400mm I'm pulling the subject matter to me, thus increasing the distance between foreground and background?


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xarqi
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Dec 10, 2011 17:21 |  #2

Gel wrote in post #13523883 (external link)
One of those things I've never thought about until now.

Up until recently the longest lens I've had is a 200mm.

Now I've a 100-400L and a 2x Extender. With the 100-400L I'm finding that the amount of bokeh present at 5.6 is very similar to an 85L prime at 2.8

Bokeh isn't a quantity, it is a quality.

Do you mean that you are seeing a shallower depth of field?
If so, there are three factors of relevance here: focal length, distance to subject, and f ratio (typically called "aperture").

Search for "dofmaster" and you'll find a utility where you can see what the DoF will be for various combinations of parameters. You'll need to supply a fourth parameter though, and that will be determined by the size of the sensor in your camera.

If you mean something beyond that, the degree of background blur, that is something I don't have a good handle on myself, but I have seen an excellent analysis done here on POTN, so maybe a search will reveal it. The key factors I think are focal length and the relative distance of the background compared to the subject. Aperture could enter into it also, and doubtless crop factor as a way of standardising across different formats that require different rendering magnifications to reach the same scale.

Is this because by zooming to 400mm I'm pulling the subject matter to me, thus increasing the distance between foreground and background?

That is physically impossible, so no.




  
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Gel
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Dec 10, 2011 17:39 |  #3

Thanks!

With this:

'Is this because by zooming to 400mm I'm pulling the subject matter to me, thus increasing the distance between foreground and background?'

It wasn't meant in the literal sense. But as with a wide angle lens, foreground objects appear bigger some effect may come into play a long telephoto lens?


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Dec 10, 2011 17:55 |  #4

You get blurrier backgrounds with a longer lens, yet still have the same DOF. That's because with longer lenses, you have less of the background in the photo, so that background is essentially "Stretched" across the whole frame, making it seem more diffuse, even though DOF (the amount of the image that is perceived to be in focus) is constant.

If you maintain same framing:
Focal Length does not affect DOF if you maintain the same framing
With the same framing, a longer focal length will give you the same DOF, but a blurrier background

Example: If you take a group photo at 35mm, and then you switch to a 200mm lens and take the same group photo with same framing, whilst holding aperture constant. Your perspective and background blur will be different, but your DOF will be the same.

If you do not maintain same framing:
Focal Length does affect DOF if you do not maintain the same framing
When the same framing is not maintained, a longer focal length will give you shallower DOF and a blurrier background.

Example: If you are photographing a bird from a bird blind, and you first take a photo with a 200mm lens, then the same photo with a 800mm lens. Holding aperture constant, you will get shallower DOF and a blurrier background with the 800mm lens.


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JeffreyG
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Dec 10, 2011 18:40 |  #5

The depth of field on the subject (DOF) and the degree to which the background is blurred (the size of the blur disks as a fraction of the format) are not the same thing.

As an example, suppose you use both an 85mm lens and a 200mm lens at f/2.8 to take a portrait with equal framing. You will necessarily be further back with the 200mm lens. In this example the DOF on the subject will be identical with each lens, but the 200mm lens will render the background much more blurred. This is because it will render the background much larger relative to the subject (the blurry stuff is magnified, essentially).

So how to use this knowledge? Essentially what this means is that when you have absolute freedom of composition and your only goal relative to the background is to make it a smear, then a longer slow lens is more effective than a shorter fast lens. This is because the longer lens will give you both more DOF and more blur, both items together tend to make the subject 'pop'.

A slow lens like the 400/5.6L will give incredible subject isolation if you have the room to use it, for instance.

Far too many people think DOF and blur at 1:1 correlated. They use DOF calculators not to get a rought idea of the blur, but as absolute value. This is incorrect.


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toxic
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Dec 10, 2011 22:46 |  #6

^ that. Depth of field is an estimation of how far away an object can be from the focus plane and still appear in-focus. Nothing else.




  
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Dec 11, 2011 00:47 |  #7

I'm still getting used to how it all works, so don't kill me if I'm wrong :D

30mm f/1.4 at 10ft to subject - DOF is 1.82ft

To keep the same framing using a 50mm lens
30mm/50mm =.6
10ft/.6 = 16.66666666666667ft

50mm f1.4 at 16.66ft to subject - DOF is 1.82ft

So as others have said, framing and DOF is the same but the background blur will be more smeared with the 50.


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kawi_200
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Dec 11, 2011 02:43 |  #8

A few years ago I saw a video explaining your question, but I can't seem to find it now. It basically explains that a picture taken with a wide lens and a tele lens both will have the same amount of blur in the picture, but since telephotos magnify, they are just magnifying the blur that a wide angle sees. See the sample images provided in the link below with Bryan's explaination on how it works. About 2/3 of the way down there is a picture of a purple flower posted 3 times side by side. If you do a word search for "compressed" it will take you to the paragraph above the picture.

http://www.the-digital-picture.com …SM-Macro-Lens-Review.aspx (external link)


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Gel
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Dec 11, 2011 04:12 |  #9

JeffreyG wrote in post #13524423 (external link)
The depth of field on the subject (DOF) and the degree to which the background is blurred (the size of the blur disks as a fraction of the format) are not the same thing.

As an example, suppose you use both an 85mm lens and a 200mm lens at f/2.8 to take a portrait with equal framing. You will necessarily be further back with the 200mm lens. In this example the DOF on the subject will be identical with each lens, but the 200mm lens will render the background much more blurred. This is because it will render the background much larger relative to the subject (the blurry stuff is magnified, essentially).

So how to use this knowledge? Essentially what this means is that when you have absolute freedom of composition and your only goal relative to the background is to make it a smear, then a longer slow lens is more effective than a shorter fast lens. This is because the longer lens will give you both more DOF and more blur, both items together tend to make the subject 'pop'.

A slow lens like the 400/5.6L will give incredible subject isolation if you have the room to use it, for instance.

Far too many people think DOF and blur at 1:1 correlated. They use DOF calculators not to get a rought idea of the blur, but as absolute value. This is incorrect.

Thank you, this makes perfect sense. I was thinking portraits, where subject isolation is important to me and a long lens is flattering. I always wondered why some pros use zooms in the studio when there are all these lovely primes like the 85L about.

Would this also mean though, that two images, one shot with an 85L @ 1.8 and one at 400mm @ F8 would mean the latter has the potential to have more of the subject in focus yet blow the background more? (As in theory, I understand the figures may be out).


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JeffreyG
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Dec 11, 2011 07:52 |  #10

Gel wrote in post #13526059 (external link)
Would this also mean though, that two images, one shot with an 85L @ 1.8 and one at 400mm @ F8 would mean the latter has the potential to have more of the subject in focus yet blow the background more? (As in theory, I understand the figures may be out).

In theory yes, although the actual distance from the subject to the background will matter as well. If the subject is tight to the background, the short, fast lens will blur the background more. For most situations though, the longer slower lens will result in more blur.

And since the long, slow lens will give more DOF, it will probably make the subject 'pop' out better. Partially blurry subjects melt into blurry backgrounds.

Check out this example I shot a while ago (ingnore the other technical aspects, I shot this strictly as a DOF / blur example). The first shot is 50mm and f/1.2. The second shot is 200mm and f/2.8.

Despite being more than two stops slower, the longer lens gives about the same blur and the larger DOF brings out the subject separation.

If you look close, you will see a white triangle shape in the background. This is a shade tent on my neighbor's deck. Note how much larger this tent is (relative to the subject) when using the long lens. This relative difference in sizes between the subject and background is why long lenses can create more blur at larger apertures.


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Dec 11, 2011 09:42 |  #11

Read these two:

http://www.cambridgein​colour.com/tutorials/d​epth-of-field.htm (external link)

http://toothwalker.org​/optics/dof.html (external link)

A lot of detail, but they are the best explanations I have found.


Check out my photos at http://dkoretz.smugmug​.com (external link)

  
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stover98074
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Dec 11, 2011 09:57 |  #12

I think you can get bokeh from shorter lenses and agree a longer lens aids in the process. I also think shorter lenses are easier to work with than longer lenses.

If I am shooting with a shorter lens and want to increase the out of focus area I focus as closely as possible to the subject (or min. focus distance).

This was taken with a Nikkor 50 1.2 shot at f2.0 at min. focus distance. The background is a glass door and wall with a picture frame and light switch. It was about 3 feet away from the camera.

IMAGE NOT FOUND
IMAGE IS A REDIRECT OR MISSING!
HTTP response: 404 | MIME changed to 'text/html' | Byte size: ZERO

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Gel
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Dec 11, 2011 11:35 |  #13

Thanks guys, it's helped a lot :D


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Dec 11, 2011 13:01 |  #14

stover98074 wrote in post #13526734 (external link)
I think you can get bokeh from shorter lenses and agree a longer lens aids in the process. I also think shorter lenses are easier to work with than longer lenses.

If I am shooting with a shorter lens and want to increase the out of focus area I focus as closely as possible to the subject (or min. focus distance).

This was taken with a Nikkor 50 1.2 shot at f2.0 at min. focus distance. The background is a glass door and wall with a picture frame and light switch. It was about 3 feet away from the camera.

IMAGE NOT FOUND
HTTP response: 404 | MIME changed to 'text/html' | Byte size: ZERO

That's great for that picture, with the lens carrier nice and sharp and the camera body also well out of focus. The lens carrier is also pretty flat which also helps the composition. I think a portrait taken under those conditions would look rather odd, the DoF is so thin that if the eyes were on the focal plane then both the tip of the very large looking nose and the very small looking ears will look fairly well defocused. There is a reason that 85-135mm focal lengths (on full frame) are so popular, they allow you to get back far enough so that the perspective looks good while allowing a frame filling head shot. Some will even use longer focal lenghts than I have suggested.

Alan


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Assistance with understand DOF differences on short vs long lenses
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