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I am trying to compose a detail shot where I want to get as much background blur as possible. I am using Sony a6500 camera (APSC 1.5x crop factor body). I am considering two lenses:

  • Sigma 30mm f/1.4 (native APSC lens)

  • vintage Minolta 50mm f/1.8 full frame lens (with adapter for Sony E mount)

There are plenty of threads that discuss the math and such behind using full frame lens on crop body(examples here, here, here ).

But no one seems to be addressing the effect that this might have on bokeh and other artistic values of the images.

I can get some pretty decent bokeh with my Sigma lens, especially on detail shots, but the bokeh I can get with the Minolta lens is also just as good, sometimes better, when I use that Minolta lens on a "full frame" 35mm film camera.

So I am trying to figure out:

  • which lens is going to give the most bokeh on a 1.5x crop sensor body?

Also, recognizing that the two lenses are different full frame equivalent focal lengths for this body (the 50mm becomes a 75mm equilvalent on the Sony a6500), if I swapped the 30mm Sigma f/1.4 for the 56mm Sigma f/1.4;

  • would the 56mm f/1.4 Sigma or the 50mm f/1.8 Minolta give more bokeh on the Sony a6500 body?
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    A 30mm lens is 30mm whether or not it is APSC or full frame. – Eric S Jan 9 at 15:31
  • Yep. Although acceptable circle of confusion is a slightly mitigating factor here... but generally accepted as irrelevant compared to a drastic focal length difference. – rackandboneman Jan 9 at 15:59
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    "A 30mm lens is 30mm whether or not it is APSC or full frame." that is not what I am asking here, thanks – user5359531 Jan 9 at 16:23
  • @user5359531, Re, "A 30mm lens is a 30mm lens..." Yeah, you're asking about 30mm vs. 50mm, but I think that what Eric was trying to tell you is that it doesn't matter whether you use a lens that is able to cover a full-frame sensor or a lens designed to cover your camera's smaller sensor. – Solomon Slow Jan 9 at 17:49
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    You were the one that specified APSC vs. full frame on the lenses. All I was trying to communicate is that on a APSC camera, that distinction is irrelevant. – Eric S Jan 10 at 23:16
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Depth of Field is computed from focal length, aperture, focus distance, and also sensor size. The distance limit of the depth of field is where the blur circle diameter is computed to exceed the maximum acceptable Circle of Confusion, which is computed from sensor diagonal size.

With all other factors equal:

A shorter focal length has greater depth of field than a longer focal length.

A stopped down aperture has greater depth of field than a more open aperture.

A greater focus distance has greater depth of field than a shorter distance.

The CoC computed for a smaller APS sensor has LESS DOF than a larger sensor, because (like tiny film) a smaller sensor is a smaller image which must be enlarged more to the final viewing size, and enlargement is detrimental to apparent depth of field. We recognize blur much easier when enlarged more. Sensor size is definitely a factor of Depth of Field.

However, a large offsetting factor is that the smaller sensor ordinarily must use a shorter lens to see the same normal field of view as the larger sensor, which becomes a greater factor (computing DOF squares focal length, but the other terms are not squared). So typically in practice, small senors with their necessary shorter lens see more DOF than larger sensors with their longer lens.

Any Depth of Field calculator will give the DOF numbers. My site offers one at https://www.scantips.com/lights/dof.html that will also compute the blur circle diameter at some specified background distance. It promotes the standard idea that a longer lens standing back with perhaps a f/4 lens can offer greater background blurring than a shorter lens standing closer with a f/1.8 lens (yet with greater DOF AT THE SUBJECT, and doesn't otherwise suffer the f/1.8 aberrations).

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    OP appears to be asking about background blur, not depth of field. – xiota Jan 9 at 22:53
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    Agreed, and if OP's "the most bokeh" means diameter, my last paragraph is about computing an answer. – WayneF Jan 9 at 23:11
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The size of the eye you are looking out is f/a where a is the aperture number. That's independent from crop factor. So you have 30mm/1.4, 50mm/1.8, 56mm/1.4 and the latter clearly is the largest. So at equal distance, the background will be most separated with the last lens. Of course, you would not use those lenses at equal distance if you aim for the same framing.

At equal framing, your object distance will be proportional to the focal length, so the geometry of the cone from entrance pupil to a point in the subject plane, given equal framing, only depends on the aperture number. The cone sections of this cone determine the size of bokeh circles in the world. However, the world makes it into the camera by perspective, and with a shorter distance, objects grow faster in the foreground and shrink faster in the background. So for best background blur, you want the bokeh circles to appear larger and would choose a focal length that does not let the background shrink as much.

In other words: 56mm/1.4 clearly is the bokeh winner for background bokeh, even though at equal framing the 30mm/1.4 lens will be better at dissolving foreground objects (like when shooting through a thicket or fence that is still in reach, given your desired subject framing).

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The Sigma 56mm f/1.4 would have the most background blur of the three, but the Sigma 30mm f/1.4 is very close because you need to move much closer for the same framing. The Minolta 50mm f/1.8 will have more depth of field, and not as much background blur as the other two.

The type of lens or "native" mount does not matter. All that matters is the camera format used, the aperture used, the actual focal length of the lens, and the shooting distance. Find yourself an online Magnification calculator, and Depth of Field calculator, and you can run some numbers yourself.

In general "Faster" lenses have less depth of field than slower lenses provided you are shooting them wide open.

Longer lenses tend to have less depth of field, but in order to get the same framing for each photo, you will have to move closer with the shorter lenses. Because you need to move closer with shorter lenses, the differences tend to be cancelled out.

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I'm going to chime in with a far less precise response (credit to the previous responders). You asked which setup would provide "more" bokeh. As I'm sure you know, bokeh in all its facets - the 'amount', the shape of the background objects (circular, angular etc) - is one of the most subject aspects of photography. It is so much more than blur. That definitely applies to being able to quantify it; there is no real way to do so.

What I can say is this: assuming correct framing, composition and focus, you're going to have 'a lot' of bokeh either at f-1.4 or f-1.8 for focal lengths ranging from appx 35mm-200mm. The deciding factor has to be which one do you prefer the look of. There are some lenses which have been engineered specifically to generate "beautiful" bokeh and in certain cases this is true. The soon-to-be-delivered Nikon Nocte 58m f-0.95 will likely be one of them, the Zeiss Otus 55mm f-1.4 is another although the latter is also supernaturally sharp (both are full frame). However these are both multi thousand dollar lenses and thus not available to most of us. If you have access to lens rental that's an obvious approach but I'm guessing that may not be the case. To be honest, the greatest importance of having a f-1.4 vs f-1.8 is the extra light. If I had to make your specific decision, I'd opt for the f-1.8 simply because that's what I use and the bokeh is lovely.

Of course if one wants to really go for it, you can take the approach Stanley Kubrick took when filming "Barry Lyndon". He had a scene lit by only three candles and did not want any artificial light. He therefore procured a f-0.7 from NASA although it was a bummer for the actors who had to move in a perfectly lateral manner given how razor thin the DOF was. But I digress now...

(More of a qualitative answer, I appreciate but I hope the perspective can be helpful).

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  • thanks but this does not take into consideration the effects of using the full frame lens on crop sensor body. The driving factor behind this is that you can get something like the 50mm f/1.8 or even a f/1.4 full frame vintage lens for cheap, some go as low as $50-60, add another $20 for an adapter and you still beat the price of modern 'fast' lenses by a wide margin. The problem is trying to preserve that advantage when using the lens with crop sensor bodies, where it seems like the full frame f/1.8 is actually worse than e.g. a crop lens f/1.8 for DoF and bokeh – user5359531 Jan 9 at 18:24
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    My perspective would be: f-1.8 vs f-1.4 is a non issue for bokeh. It's the quality of the optical elements and construction techniques that determines the outcome. Until recently, it was a given that FF lenses were just better but the Sigma is part of a newer generation of hq ASPC lenses = more level playing field but that's all. One flag: historically some Minolta lenses were 'duds'; confirm the 50mm wasn't one of them. A camera as a good as the A6500 with FF optics is a fine combo for your aims. Really make sure you're okay at 75mm though. There's some lovely bokeh at (effective) 50mm :) – Adam Gold Jan 9 at 20:18
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    I strongly agree with Adam. A lot of those budget 50mm designs have pretty harsh bokeh and there's little advantage in having "more" of it. Focus on results, not math. – mattdm Jan 9 at 20:34
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To approximate amount of blur across lenses using the same sensor format, calculate focal length/aperture. This gives the aperture size, which directly correlates with the amount of blur produced in resulting images. Higher numbers ⇒ more background blur.

  • 30/1.4 = 21.43
  • 50/1.8 = 27.78
  • 56/1.4 = 40

From this, we can see that the 50/1.8 lens will have a bit more blur than the 30/1.4 when used wide open. The 56/1.4 lens will have much more blur than either of the others.

Note: Depth of field is different from bokeh, which is different from amount of blur. DOF is specifically about the region that is within acceptable focus given specified viewing conditions. It is possible to have narrow DOF while having only a small amount of blur in out-of-focus areas.

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