[normhead]

I'm gearing up to head out.

I have a choice Sigma 70 2.8 Macro to the same lens with the 1.7 AF adapter on it. The question,

Will I have the same DOF. My narrowest possible DoF should be narrower because it's a longer lens. My DoF should be wider because I'm shooting ƒ4 instead of ƒ 2.8. Which set up will give me the creamiest bokeh?

And corollary question would be is there a relation size between the size of the front element and background blur in a complex lens. IN a simple lens, the bigger the lens the bigger the circles of confusion. In constructing a complex lens are you able to create large circles of confusion from a smaller front element? Or am I just totally out to lunch thinking this?

I guess the real question, is could we theoretically alter the light path enough to create large 2.8 type circles of confusion from an F8 lens?

Just wondering.

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Last edited by normhead; 05-22-2017 at 07:22 AM.

[bigoak]

Why not just do some comparison shots?

[normhead]

Why not just do some comparison shots?

I,m headed out... but I think this morning's trial is going to be using my 50 macro on a Q-S1 for macro flower images. There's only so much a guy can do in a day.

And I have so many tests i want to run, I guess I could add it to the list.

But alternatively, someone might know.

[bigoak]

fair enough, happy shooting!

[normhead]

fair enough, happy shooting!

Thanks.

[photoptimist]

Interesting (3-part) question!

1. Depth-of-field in terms of sharpness over some range of distance: probably not much difference especially if the subject isn't far away. The focal length effect on the in-focus-range is only strong for really wide angles at longer distances (there's probably not much difference between a 70 and 119). And at magnification of 0.1 (i.e., a subject 10 inches wide in an APS-C frame), all focal lengths have a similar DoF.

2. Size of OOF features in pixels: probably not much difference assuming your goal is to have the in-focus parts of the scene be the same size whether you use the TC or not (e.g., you want a picture of a pinecone or flower that fills the frame in both pictures).

3. Creamy bokeh: Tricky! Although the blur circles for the two lens configurations will be the same size, the image of the background scene will be magnified by the TC so there will be less total detail in the background (e.g., maybe only two background branches behind the pinecone instead of thee or four). With less total detail in the background, it may look creamier. Note that any dust, coating variations, or glass polishing irregularities in the teleconvertor are going add to the granularity/busyness of the bokeh.


It's not so much the physical size of the front element that affects blur issue as the visual size of the aperture pupil. If you hold the lens at arms length and look into the lens from the front, you'll see how wide the aperture is in terms of gathering light. For telephoto lenses, the visual pupil size and physical front element size do tend to be pretty close. But in wide angle lenses, the pupil is tiny compared to the front element especially if the lens is a complex , highly corrected, retrofocus design.


P.S. One issue with teleconverters (and speed boosters) is they may fail to capture the full aperture of larger aperture lenses. If the front element of the teleconverter/booster is smaller than the back element of the main lens, there a chance of this issue. If you mount the TC on the lens and look down the front of the lens you can see whether the limits of the light path are defined by the lens aperture (good) or the TC's narrow throat (bad). It's probably not an issue with f/2.8 lenses but it does mean that an f/1.2 on a 2X doesn't work wide-open the way one would expect.

[bobbotron]

I'm gearing up to head out.

I have a choice Sigma 70 2.8 Macro to the same lens with the 1.7 AF adapter on it. The question,

Will I have the same DOF. My narrowest possible DoF should be narrower because it's a longer lens. My DoF should be wider because I'm shooting ƒ4 instead of ƒ 2.8. Which set up will give me the creamiest bokeh?

And corollary question would be is there a relation size between the size of the front element and background blur in a complex lens. IN a simple lens, the bigger the lens the bigger the circles of confusion. In constructing a complex lens are you able to create large circles of confusion from a smaller front element? Or am I just totally out to lunch thinking this?

I guess the real question, is could we theoretically alter the light path enough to create large 2.8 type circles of confusion from an F8 lens?

Just wondering.

To TC's change the focal length of a lens, or do they do something weird like a speed booster? If it's just a FL adjustment... you could use a DOF calculator to figure out how the DOF would change.

I've found if you *really* want a shallow DOF, jump straight to 100mm FL more more, and use extension tubes if you're trying to get close.

This was with a 135mm + extension tube.

[normhead]

Interesting.... inspired by complete lack of appreciation shooting with my K-1 and FA 50 2.8 macro yesterday, today I ended up with the Tamron 90 and HD DA 1.4 on the K-3 after a diffraction disaster shooting macro with the Q at f/8.

[leekil]

Doing a DOF comparison here:

Online Depth of Field Calculator

APS-C Pentax with 70mm f/2.8, focused at 40 feet away gives you 11.4 feet of DOF
APS-C Pentax with 100 mm 4/4 focused at 40 feet away gives you 7.81 feet of DOF

Circle of confusion is said to be the same, 0.02 mm.

The creaminess is a wildcard, as the TC could affect it, as photoptimist says above. Plus there could be additional factors given that it is a complex and not a simple lens.

If you could alter the light path to make a 2.8 COC with a f/8 aperture, someone would have already done it. You could also potentially alter the shape of the aperture in order to change the character of the bokeh, but probably not toward the creamy end.

There are apodization lenses that affect it, but I'm not sure it's exactly what you want:

The True King of Bokeh – the Minolta/Sony 135mm f/2.8 Smooth Transition Focus (T/4.5) | ilovehatephotography

[BrianR]

There is a nifty background blur calculator here that will approximate the blur disk sizes:

How Much Blur? - A Bokeh Calculator - Asklens

It will let you compare different lens configurations and formats. It is obviously not taking the complex lens designs into account, but the approximations are generally pretty decent. I've set it to compare a FF & 50/2.8 with an APS-C & 90/2.8 + 1.4TC, photographing a 20cm wide subject.

From my own lens stable, I've found my 300/4 can really mashify a background that my 100/2.8 makes just creamy by comparison. The wider apparent aperture is a factor, but as photoptomist said, a tighter field of view can be a big deal for background mashiness. It can make getting a solid colour much easier, and help you avoid icky background areas. Depends on what's behind the subject. Just another reason those 200mm macros are desireable. If you haven't already tried it, give your big lenses a try on flowers (assuming they have enough magnification for a given flower).

[angerdan]

There is a nifty background blur calculator here that will approximate the blur disk sizes:
How Much Blur? - A Bokeh Calculator - Asklens

Anyone knows anything about what happened to asklens.com ?

At least the archived version still works:
How Much Blur? - A Bokeh Calculator - Asklens

And there's a new one
How Much Blur - Compare background blur by Crop Factor and Lens

[swanlefitte]

angerdan,
Take the framing of something 300inches wide on apsc with a 70mm vs 35 mm. One stands about 440 inches away with a 35mm and 900 with the 70. Dof for f1.2 is about 1000 vs 550at the far end or 100-110 inches past the subject(1000-900 and 550-440) so it ends up basically the same, probably closer with more precision. However at f6.4 the 35mm reaches hyperfocal and its all in focus so the key is knowing where hyperfocal begins or it doesn't matter since the dof is so close. I just used a isosceles triangle calculator to figure out distance equivalence between framing a 35 and 70. It confirmed what i already believed but maybe its not consistent across all mm.

[angerdan]

To archive a low depth of field and separate the subject from the background, longer focal length with faster aperture is useful. Hyperfocal distance isn't a big deal then.
Knowing the difference between lenses for the desired purpose with a tool lich "Hoch Much Blur" is a great help.

[swanlefitte]

so if you dont care whats in the background but care about where you stand then you get much more oof with a 70 at 2.8 than a 35 at 2.8 I cut a strip through the bottom right and the 35 is clearly sharper.


Now if you need the same background you have to stand much farther away with the 70. Here you can see the blur is about he same. look at the blue wheel in back and the yellow bike lock.


So if composition is first priorty as it should be I think. then you never choose lens for dof, only f stop. You can see how much the composition changes. This is what I was getting at.

[swanlefitte]

I could not NOT do this after that. On the op's original question years ago.
@normhead
sigma 70mm without and with 1.4x tc. Those are dril bits behind ranging on the right about 3mms behind to the left at 25mms behind.