Rangefinder Producing More Bokeh?

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BNLee

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I've been confused with this question for a while now, and since I am on the forum, might as well just ask it.

We all know since the rangefinder lens is much closer to the film plane, it allows for the construction of small lenses, whereas SLRs contain a mirror, where the lenses have to be built bigger. So the question is, does the amount of bokeh increase as the lens is mounted closer to the film plain? Or is it simply the same as the amount of bokeh produced by a SLR lens (For the same focal length and aperture of course...)?

For example, does a 50mm lens at f/1.4 on a rangefinder produce more bokeh compared to a 50mm lens at f/1.4 mounted on a SLR?

This actually came into my mind since the closer you get to a subject you are photographing, the more bokeh you get (Thinner DOF).

Just for general knowledge, not that it changes the photographic world anyway. Have A Nice Day!
 
There is no "More" or "Less" bokeh ... It is just the description for pleasant or unpleasant rendering of out-of-focus areas. That said, lenses for SLR cameras might follow a little different design approach compared to RF lenses, a more abrupt change from "in-focus" to "out-of-focus" is desirable to facilitate focusing on the ground glass.
 
It's an interesting question but difficult to answer without diagrams, but here goes. You have to visualise what is happening to the rays passing through the lens. A simple one element lens captures the (parallel) rays from a distant on-axis object and focusses them onto the film plane. The cylinder of rays intersected by the lens is bent into a cone by the lens and the apex of the cone is the 'in focus' point, as I said in the film plane when the lens is focussed correctly. The cone angle of the rays determines the f number and the bokeh. When you stop down, the lens captures a smaller cylinder of parallel rays and the cone angle is less, and so on. A real lens has length i.e. is more like a cylinder with many lens elements along the axis. Some of these elements can be beyond the point where the lens can be considered to have formed the 'cone' which is why the lens back focal length (the lens-to-film plane distance) is always less than the focal length. For example, for a 50mm SLR lens, if it were a simple single element lens, the back focal length would be 50mm; in fact it's about 35mm, just enough for an SLR mirror. For a sonnar it's even less, not enough for a mirror, so there is no 50mm sonnar SLR lens.
According to this visualisation, at a given f-number, a 50mm sonnar would have the same bokeh as a 50mm SLR lens (i.e. same 'cone angle') even though the back focal distance is less.
But there's more to it than that. For example, in any image the foreground is in focus behind the film plane and the background is in focus before the film plane: what intersects the film plane is part of the respective cone = bokeh. Off axis rays (i.e. from the edges od the image) pass along the lens at an angle to the lens axis and can get clipped by the walls of the lens. This makes the background/foreground 'cone' flattened and orientated about the centre of the image (i.e. giving rise to the oft mentioned 'swirly bokeh').
Because the cone is 'clipped' in this way, the foreground and background tend to be more in focus (less 'bokeh'). I have 50mm f1.8 and a 50mm f1.4 SLR lenses. The exit diameter for the f1.8 lens is about 23mm and for the f1.4 lens is about 33mm. The f1.4 lens bokeh at say f5.6 is about the same as the f1.8 lens bokeh at f4 and so on. It's all due to how 'cleanly' the rays pass through the lens.
So I think the OD of the lens elements together with the length of the lens are the most important factors determining the 'extent' of the bokeh at a given f-number.
Hope this helps!
 
All bokehs found on the rangefinder forum are better, rounder, more beautiful bokehs. It doesn't matter how far from the rangefinder forums plane of existence either. There's no real explanation. RFF produces better bokeh.
Even if you shoot an slr. Post the image here and suddenly the bokeh takes on more significance and presence in the world. Try it. You'll see with your own eyes. 🙂
 
Jack Conrad said:
All bokehs found on the rangefinder forum are better, rounder, more beautiful bokehs. It doesn't matter how far from the rangefinder forums plane of existence either. There's no real explanation. RFF produces better bokeh.
Even if you shoot an slr. Post the image here and suddenly the bokeh takes on more significance and presence in the world. Try it. You'll see with your own eyes. 🙂
Click to expand...

Yes ... I've noticed that too
 
Gabor, yeah, I meant shallower DOF, but you guys get the meaning.

Oh hey Nico, I saw some of your videos regarding Canon lenses back then!

Wow, that was a true physics class breakdown, jcb4718, I for one haven't gone that far in this respect, I just shoot my cameras! Haha... Thanks for the explanation though, I have been trying to understand about what you were explaining, and I must say, I only understand half at the most, as I still remember some bit of my physics! Thanks again!

Hey Jack, haha... Would try that out soon!
 
Well, you can get 'more bokeh' with the SLR 50mm f1.4 lens because it usually focuses closer than its RF counterpart. You actually can get tons of bokeh with one of that digital small sensor P&S focusing down to 1cm or so.
 
Well BNLee I confess I'm a physicist by trade! You are right shooting cameras is best. You can analyse things too much! As I said I was amazed at the difference in bokeh between my 50mm f1.8 and f1.4 lenses and decised to try and understand why this was so.
Reading my reply after a few hours I can see text is no substitute for diagrams but I think I'll stop here before this begins to sound like a physics lecture!
 
I am not a physicist but I think the following is more or less an workable representation without getting into the technicalities.

At the outset let me say I should have thought that a rangefinder lens might have more DOF than an SLR lens which is designed as a retrofocus lens. (Note I said "might" - as it all depends...........).

Leaving aside other variables (like the distance between the camera and the subject when in focus) DOF becomes largely a function of the absolute diameter of the aperture ie not the f number per se.

For example, F8 in a lens designed for a full frame (35mm) camera is physically smaller than f8 on a lens of equivalent focal length designed for a medium format camera (and f8 on an m4/3 camera for example, is smaller still than on the 35mm camera lens).

It needs to be so, as on the medium format camera the lens sits further away from the film plane so to deliver the same amount of light to the film plane (which is what the f number measures) then the aperture has to be physically of a larger size. (The inverse square law sees to that!)

The same applies to longer and shorter focal length lenses on the same camera - the longer focal length lens has has the aperture / iris sitting further out from the film plane so in use at any given f stop, DOF is shorter than a shorter focal length lens at the same f stop on the same camera.

(As an aside, for the same reason you sometimes see large format cameras use what apprear to be very very large f numbers - like f64. I often wonderered how they could do this without refraction degrading the image till I realised that f64 on a large format camera might be physically no smaller than perhaps f11 on a 35mm camera lens. )

So, if DOF is a function of the absolute size of the aperture, f4 (say) in a non- retrofocus rangefinder lens might be smaller than f4 on a retrofocus lens that is further away from the film plane - designed this way so the SLR mirror box can sit in between the two.

But it comes down to lens design..........................

And of course all of the above has nothing to do with bokeh which is a subjective statement about the quality of the OOF areas of the image. That comes down to a range of other factors like the shape of the aperture leaves and the nature of the uncorrected abberations in the lens.

I believe the above is approximately correct, although scientists (picky fellows that they are) will no doubt pull me up on some points. At this explanation works for me and helps me undetrstand how it all hangs together.
 
peterm1 said:
This is a tautology. The aperture diameter and the focal length determine how much "light goes in there" - measured at the film plane.
Click to expand...

No, they limit it. There are additional losses in the glass and coatings, not accounted for in f-stops, which are calculated from focal length and aperture diameter. If you want stops as a figure of light at the film plane, use speed in t-stops (which are measured and hence include all transparency factors) like the motion picture industry.
 
kanzlr said:
the f number does not measure how much light goes in there.
It is just a factor of the aperture diameter relative to the focal length. . .
Click to expand...
Ummmm.... Actually, it does. That's why a 200/2 puts the same amount of light on the film as a 24/2. Subject of course to Sevo's caveats.

OK, you could say that it's a question of how much light goes in there versus how much light hits the film but this strikes me as a less than useful distinction.

Cheers,

R.
 
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