At What Focal Length? (85mm f/1.8 vs 70-200mm f/4L)

[TakahiroW4047]

I was wondering, at what focal length with the 70-200mm f/4L will I'll be able to recreate the 'light blur' size of the 85mm f/1.8? (Both wide open so bokeh should be rather similar?).I came across this equation online that I wanted to confirm.


It said that if I wanted to recreate the same size 'light blur' I would have to have an equivalent 'blur disk' size which is calculated by Focal Length / Aperture.


So if I wanted to mimic the 85mm @f/1.8 - 85/1.8=47.22


I would have to have roughly 189mm @f/4 - 189/4=47.25


The camera to subject distance was obvious, but does this calculation assume that the subject to background distance remains the same?

[HDNitehawk]

I will be interested in seeing some technacal explanation of this. Because I do not see how they could be the same at equal distances since the DOF for the aperature you specified is not the same at equal distances. Because of that the OOF areas will be diffrent.


Also I doubt they would render the same Bokeh. It might be similar but two diffrent lenses will produce Bokeh diffrently due to other factors.

[neuroanatomist]

I was wondering, at what focal length with the 70-200mm f/4L will I'll be able to recreate the 'light blur' size of the 85mm f/1.8?

What do you mean by 'light blur'? The size of the disc resulting from an out of focus point source of light? That's going to be dependent on how far outside of the DoF that light source is positioned.


From a DoF standpoint, 127mm f/4 is equivalent to 85mm f/1.8 for the same subject distance - but in that case, the subject framing is obviously going to be different. If you don't care about the framing, the 70-200mm f/4 will result in a thinner DoF than the 85mm f/1.8 (same subject distance) at any focal length longer than 127mm. At a given subject distance, DoF with at 85mm f/1.8 is ~2.5 times deeper than 200mm f/4.


If you want the same subject framing (i.e. you have to back up further from the subject with the 70-200 so the field of view is the same) you cannot achieve the same thin DoF that you get with the 85mm f/1.8 - you'd need a 70-200mm f/1.8 lens for that. Three elements control DoF - aperture, focal length, and subject distance. Since field of view is determined by focal length and subject distance, if you keep the FOV the same as you zoom in, you are changing both focal length (longer meaning shallower DoF) and subject distance (longer meaning deeper DoF) - the two cancel other out, so the only factor influencing DoF is aperture. In other words, you will never be able to recreate the OOF blur of the 85mm f/1.8 with the 70-200mm f/4 if you need to keep the framing the same.


But if tighter framing is acceptable, you can get the same or thinner DoF with the f/4 zoom. In practical terms, that means if you take a head+torso portrait with the 85/1.8, then from the same distance take a head shot with the 70-200mm at 200mm f/4, you'll get more OOF blur with the zoom lens. But of course, if you put the 85mm f/1.8 back on and moved in closer for that head shot, you'd geteven moreOOF blur (which to achieve at 200mm f/4 you'd need to take a portrait of the subject's nose, or something like that).


Note that all of the above refers to the quantity of OOF blur. Strictly speaking, "bokeh" refers to the quality of OOF blur, not how much of it there is (although many people use the term to mean both aspects). Bokeh is determined by the number and shape of the aperture blades, the optical corrections in the lens design, etc.


Hope that makes sense, if not I can try again...


--John

[HDNitehawk]

John


I thought his questions was along this line. Bokeh if created perfectly would be cone shaped. As you move further from DOF in to the out of focus area the circle or cross section of the cone would be become larger. For example a wall that is OOF 10

[HDNitehawk]

The more I think about this. Aperture only affects the volume of light that is allowed in the lens, Shutter speed the duration the light is let in.


The only thing that could change the size of the cones and circles would be the distance to the OOF background, compared to the focus point, distance to the camera and size of the lens. Wouldn

[TakahiroW4047]

What do you mean by 'light blur'? The size of the disc resulting from an out of focus point source of light?

Yes I was referring to the OOF blur size, of say christmas lights or shimmer of light passing through tree leaves, etc. I wanted to avoid saying 'Bokeh', because as you said it describes the quality. I'm trying to achieve the OOF blur size of a 85mm f/1.8 by moving farther back and maintaining the same framing of the subject. I'm hoping to find that sweet spot where the compression of field of view from the longer focal lengths will yield the maximum OOF blur size.

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Three elements control DoF - aperture, focal length, and subject distance. Since field of view is determined by focal length and subject distance, if you keep the FOV the same as you zoom in, you are changing both focal length (longer meaning shallower DoF) and subject distance (longer meaning deeper DoF) - the two cancel other out, so the only factor influencing DoF is aperture. In other words, you will never be able to recreate the OOF blur of the 85mm f/1.8 with the 70-200mm f/4 if you need to keep the framing the same.

I guess this applies the best for what I'm trying to figure out.Actually this may simplify things even more. You're saying that since I'm maintaining the same frame of view, the +focal length and +subject distance will cancel each other out. Therefore, you still get the same OOF blur that's maximally possible at f/4. But that still leaves the compression of field of view right? Pardon my elementary drawing [:P] I'm not too worried about the quantity of OOF blur. Please correct me if I have this concept incorrectly!


EDIT: Oops I forgot to add the picture of the subject in the 'final images' haha


[img]/resized-image.ashx/__size/700x0/__key/CommunityServer-Discussions-Components-Files/12/3513.Bokeh-copy.jpg[/img]

[wickerprints]

Let me label the diagram you posted with the terminology I am using. We call the distance from the point "Camera" to the point "Subject" the subject distance, which we might label S. The distance from "Subject" to "Lights" is the subject-to-background distance, or B. We will use the traditional F for for focal length and N for f-number. Finally, not directly shown in your diagram is the subject magnification M, which is the ratio of the size of the subject on the image plane, to the size of the subject in the object plane. In other words, M is simply the size of the subject in focus on the sensor divided by the size of the same subject in real life.


When you say that you want to maintain the same "frame of view," what you are actually meaning is that you wish for M to remain the same. For a pair of different focal lengths as you have drawn in your diagram, what this requires is a change in S, which you have also drawn. You have also shown that because F relates to angle of view, changing S also changes the relative size of the blur circle at S+B.


But as you have correctly surmised, by stopping down in the lower diagram, you are able to decrease the size of the blur circle to match those in the upper diagram. Alternatively, you have also correctly inferred that, if the f-numbers of the upper and lower diagrams are fixed at, say, f/1.8 and f/4, then there is a relationship between the focal lengths that would result in the same degree of background blur.


However, what my previous posts have explained is that this relationship would only hold true for a SPECIFIC subject distance S and a SPECIFIC subject-to-background distance B. If you changed either of these, then the focal length required in the lower diagram to match the degree of blur in the upper diagram will also change. Even worse, there are certain combinations of S and B for which NO focal length in the lower diagram will give you the same degree of blur.

[wickerprints]

Fortunately, I have created this nifty tool to illustrate the relationship between focal length, f-number, magnification, subject-to-background distance, and blur circle diameter. Below, we assume a fixed magnification M = 1/30. For the focal lengths being compared, the subject distance is automatically calculated so as to achieve this value of M. Then the value of B is logarithmically plotted on the horizontal axis, and the blur circle diameter in the image plane is plotted on the vertical. We have two hypothetical lenses, an 85/1.8 and a 189/4. This chart demonstrates the principle we previously discussed, in that for a background at infinity, the blur circles for both lenses are the same size (about 1.56mm). But as you can also clearly see, when a background object is not infinitely far away, the blur circle it projects on the image plane will not be the same in both cases--the longer lens' blur circle is smaller at each value of B. it should also be noted that, for any lens at f/4, 188.88mm is the shortest focal length that will yield this behavior; any shorter, and the blur circle will ALWAYS be smaller than 85/1.8, even at infinity, no matter the magnification chosen.


[img]/resized-image.ashx/__size/716x0/__key/CommunityServer-Discussions-Components-Files/12/5381.coc06.gif[/img]


Next, we show another comparison, the same 85/1.8 against a 300/4, for the same subject magnification (M = 1/30). Note the red curve is unchanged (only the vertical scale has changed), but now the 300mm lens shows that at a subject-to-background separation of about 8.5 meters, the two lenses will have the same blur circle diameter. Beyond that, the 300/4 renders objects much more blurry.


[img]/resized-image.ashx/__size/716x0/__key/CommunityServer-Discussions-Components-Files/12/5700.coc07.gif[/img]


Finally, we see how changing the subject magnification also affects the degree of blur. Here, we have chosen a relatively large value of M = 1/5, again between the 85/1.8 and 300/4. Notice that the shape of the curves appears to have shifted to the left, but also the vertical scale has increased greatly, so that the blur circles are MUCH larger than in the M = 1/30 case. In this scenario, the crossover occurs around 1.6 meters.


[img]/resized-image.ashx/__size/716x0/__key/CommunityServer-Discussions-Components-Files/12/3007.coc08.gif[/img]


I hope these diagrams help. It is my intent to make this interactive tool freely available for use by the general public in the near future.

[HDNitehawk]

Fortunately, I have created this nifty tool to illustrate the relationship between focal length, f-number, magnification, subject-to-background distance, and blur circle diameter.

Now I have to ask this, and this questions is for the OP as well: Why? What would a scenario be that aperson would need to know the blur circle of two diffrent lens and try and match them to the same size?

[neuroanatomist]

It is my intent to make this interactive tool freely available for use by the general public in the near future.

Great work! I'd love a copy, when it's ready for sharing. Looks like it'll run on my Mac. [H]