Bokeh: How much background blur will I get? Tom Axford 2023

Not only at max. aperture but also at smaller apertures you can count the size of the bokeh shapes dividing focal length by aperture number.
When stopped down those shapes are no more round "balls" but polygons with as many edges as your aperture has blades.
Look at this thread: dprevived.com/t/adapted-lenses-with-interesting-bokeh-shapes/3094/2/#post-38567

I was just looking at some new shots in there. Very clever work with those results.

Danny.

Thanks.
Tom's fantasy apertures in his post is something I consider as very clever!
And I think it should be posted in the thread about interesting bokeh shapes.

It would be nice to have a large-aperture lens where a slot was instead of the mechanical iris to drop disks or cards into, but the only systems I've see for doing this are with small lenses, with relatively small apertures that result in smaller bokeh balls, like the LensBaby option. Putting cut-outs in front of the lens can be fun, but getting the aperture where it belongs is the purest route. Too far from the correct location and then you start to get parallax issues with close things.

Yes, putting a cut-out in front of the lens can lead to severe vignetting for some lenses. It is usually worst for wide-angle lenses and is rarely a problem for lenses with a large entrance pupil and a narrow angle of view.

I haven't heard of a lens with a slot for an alternative iris, but some very large lenses are made with a slot for an internal filter. I have no idea how well a home-made iris would work in the filter slot.

Has anyone here tried such a thing?

I know a couple of lenses with such a slot for inserting filters, however most of those are enlarging- or repro-lenses and thus not 'fast' by taking lens standards. This one might work fairly well:

Rodenstock Rogonar-SC 50 mm f/2.8
deltalenses.com/product/rodenstock-rogonar-sc-50-2-8/

It's also not the fastest lens of course. I own one of those, so perhaps I'll have a chance to try it. What material would you suggest for making such an insert?

shop.lomography.com/uk/lenses/petzval
They have a slot for Waterhouse stops.

A while back, I made an Open Office spreadsheet for blur size at the sensor ...

Lots of blur references here:

www.trenholm.org/hmmerk/TIAOOFe.pdf

That's very interesting though a little bit too complicated for me... Would you be able to answer me a question though?

How big would the lens diameter of a 50 mm f/2.8 lens with full frame coverage need to be? I have a very small lens which I suspect to have those specs, but because it's unmarked I'm curious if it's even possible. The lens with the smallest outer diameters that I know of is probably the Meyer Domiplan 50 mm f/2.8... so I guess that's probably close to the limit.

I'm usually more the type to just try stuff when it comes to bokeh:

You think bokeh isn't science? by simple.joy, on Flickr

That being said I can absolutely appreciate the skill needed for a more technical approach and the possibilities it offers. For example it would be a great benefit in order to know which experiments might even be possible or worthwhile!

I wouldn't call this a serious, but certainly a first attempt: 😂

Created with the Rodenstock Rogonar-SC lens mentioned before plus an insert into the filter slot. Cut out an 'M' (for mystery, because the origin and application of the lens shown certainly is still), but it's not fully shown. Would need to make it smaller...

Some models of Lensbaby had interchangeable irises that could be dropped in to the front of the lens and would magnetically secure in place. If I recall correctly, they shipped with standard f/stop sizes but also had creative kits with apertures shaped like hearts, stars, and other designs.

I've seen a lot over the years and you are simply the best I've seen. I use 40 year old tele lenses and get it now and again, but what you do is very clever 😉 A real pleasure to see.

Danny.

Took me a while to understand the question, sorry. In basic lens theory, the required diameter of your aperture is 50mm/2.8 = about 18mm but there's actuation stuff inside the lens body so quite a bit bigger than that, eh?

Well from the above calculation, "the limit" is 18mm ... ignoring obfuscation due to talk of thick lenses, pupils, and other lenticular stuff.

Thank you so much - you give me too much credit though... I feel like bokeh & light are still mostly playing with me, instead of the other way around! 😅

Many thanks for that! Seems like it's indeed possible. The whole lens is around 25 mm (1 inch) in diameter and there's almost no fluff around or inside the glass, which is 19-20 mm. The lens has an aperture ring, but no real body, and given that it seems to be a kind of enlarger lens, it doesn't need a focusing mechanism. It's really just the essentials.

As shown in the thread about Adapted lenses with interesting bokeh shapes I've tried a couple of times to manipulate the bokeh by using objects outside of the lens.

Trying to bokeh hole in my idea? by simple.joy, on Flickr

Overwhelming pressure by simple.joy, on Flickr

Choc-full of sweetness by simple.joy, on Flickr

Time to get in shape for Christmas! by simple.joy, on Flickr

Optical essence on fire by simple.joy, on Flickr

While doing that I realized that this type of maniuplation usually seems to work better with big diameter lenses: For example it felt easier to do that with a 150 mm f/2.8 lens than with a 50 mm f/2.8 lens. I would be very grateful if some of the more knowledgeable people here would be able to tell me, if there is some scientific reasons for that, or if it was just my subjective feeling?

Also image quality degradation can be quite severe, particularly if you cover some of the central area, so that's another thing to keep in mind. In cases like these maximizing image quality is not high on my priority list, but there's also a limit to what I find okay. I'm sure there's a couple of superfast scientific lense which could provide excellent playing fields for experiments like that, but it's not easy to find some with decent correction for CAs and/or visible light in the first place.

The size and shape of the bokeh balls you see in the image are determined by the size and shape of the entrance pupil of the lens.

Take the lens off the camera and look into the front of the lens with a brightly lit wall some distance behind the lens. The hole that you see inside the lens is the entrance pupil.

By putting a cut-out in front of the lens, you are trying to change the shape of the entrance pupil. You cannot make it bigger, you can only make it smaller. So, the cut-out must be smaller than the entrance pupil to have any effect.

The diameter of the entrance pupil is equal to the focal length divided by f-number. Your 150mm f/2.8 lens has an entrance pupil of 150/2.8 = 54mm, while your 50mm f/2.8 lens has an entrance pupil of 50/2.8 = 18mm.

Put your cut-out on the front of the lens and look into the front of your lens + cut-out to see the new entrance pupil. In general, the shape of the entrance pupil depends on the position you view it from, both the distance away from the lens and the angle.

I think the entrance pupil and aperture are different things. They are seldom at the same plane in the lens and also their diameters are different.
If you for example look at Rogonar-S 50mm f/2.8 in bill Claff's Optical bench:
www.photonstophotos.net//GeneralTopics/Lenses/OpticalBench/OpticalBench.htm#Data/DE01089991_Example01P.txt,figureOpacity=0.25,AxisO,OffAxis,Pupils,AxisR

You'll see that aperture (A) and entrance pupil (P) are in different places and their diameters are different. PD=17.86mm, AD=15.352mm.

Quite true. It is the entrance pupil that matters in determining the size and shape of the bokeh. It is also the entrance pupil that determines the light-gathering power of the lens. It is also the entrance pupil that determines the depth of field.

The entrance pupil is the image of the aperture seen through the front of the lens.

Thank you both for the explanations and examples! It would be interesting comparing the Rogonar-S and Rogonar-SC in Bill Claffs Optical bench... there must be significant difference in the spacings because of the big slot for inserts.

He had not Rogonar-SC in his bench. There is also Rogonar 50mm.

My experience is that aperture blades make the shape of the bokeh balls. Look at this thread: dprevived.com/t/adapted-lenses-with-interesting-bokeh-shapes/3094/

Bokeh is fun, romantic, fantasy-like, a cool effect, artistic- or is it? I like it but lately, if you scan this and other photo forums you may notice it has become a controversial subject as to its definition and production and fols wat to reduce it to a precise scientific phenomenon!

I like the physics of light and mathematics but, back in college, my brain exploded after integral calculus and differential equations! Wahtever science I ascertained and retained, I really don't take it ALL out in the field when shooting.

What's even crazier is folks going out and purchasing an expensive so-called "Bokeh" lens. True enough, cerin optical formulas, fast apertures, and various aperture blade configurations make for some exotic Bokeh but the effect can be produced with most lenses if one knows what they do and understands the basic techniques.

I have to explain to various folks on numerous occasions that there are some basic components-aperture, selective focus, lighting and specularity, transillumination, depth of field control, and dist between the camera and the subject and the subjection of the background, Whenever I start to explain theses elements I always get an argument, "NO someone admonishingly exclaims, THAT has nothing to do with this or that and then I am presented with a complex equation or formula. One person posted a thesis on the spectral composition of circles of confusion!

My suggestion is that if you want to learn how to produce, manipulate and modify the effect, siml take your camer out and conduct some simple tests and observe the results. Select an appropriate background, and shootit in focus. Then select a subject and usg a wide aperture and a norm to longh focal lenght and observe the varios effect of changing focal olengh distances and aperaires. Tyo background with frontal and backlighting to transilluminate foliage.

Once you get the "feel" of it, you will be able to produce the smooth or more detailed effect you prefer. If you know how to establish the
exposure ratio between the subject and the backgornd you can darken or lighten the background to achieve the effect ot key that you prefer. If you don't want to make aperture cutouts, you can use just about anything to interfere with a light path by placing it right in front of the lens. I sho thrg grass, twigs, foliage, or simply stick a few strip od blac pin-stripe tape on an old skylight filter and these patterns will so up in the bokeh.

After you get the hang of it, at least, you will know if yo need to purchase a new lens.

Attached, is a test sequence.

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I did not save that article- it was a post on another forum. If I can locate it, I will post a link for you.