Understanding Depth of Field (DOF) at equivalent focal lenghts of different systems

phou said:

Hi everyone,

i have a question concerning depth of field/bokeh on cameras with different sized sensors. I don't understand how focal length/aperture can be compared between different systems.

I know that if you theoretically could use the same lense (same distance to the subject) on different systems, all that would change is the crop of the image - bokeh will stay the same. But what happens in reality, where you use lenses with equivalent focal lenghts?

Example:

Sony RX100 III (8.8-25.7/f1.8-2.8)

Sony A7 with 28-70/f3.5-5.6

My understanding is that shooting the RX100 at 25.7mm/f2.8 and the Sony A7 at 70mm/f5.6 will result in more bokeh for the RX100. Infact thats not true as i see using a DOF Simulator. But why?

Can someone explain to me why the A7 will produce a shallower DOF at the same equivalent focal length even with a smaller aperure.

Click to expand...

Have a look at the equations for DoF calculation and it's all explained there. It's a combination of sensor size, sensor qualities, focal length and aperture.

http://www.dofmaster.com/articles.html (and the associated calculator)

Simple explanation: Large format cameras with wide apertures have shallow DoF because the constituent rays are at divergent angles. A tiny camera has greater DoF because it is more similar to a pin-hole camera with relatively straight rays.

One tip when considering cameras/lenses: Just forget about "equivalence" as no good will ever come from trying to get your head around it.

BTW, Bokeh is the "quality" of the out-of-focus sections of the image, not just the fact of being out of focus.

phou said:

Hi everyone,

I know that if you theoretically could use the same lense (same distance to the subject) on different systems, all that would change is the crop of the image - bokeh will stay the same.

Click to expand...

phou said:

regards

phou

Click to expand...

Your understanding here is incorrect. In fact, you can use the same exact lens (not lense) on different systems. I have several lenses that are designed for use on cameras with full frame sensors (24mm x 36mm). But they can also be attached to and will work on the same manufacturer's cameras that have APS sensors (approx. 16mm x 24mm). The smaller sensor camera merely captures an image from the center area that is projected by the lens.

If you shoot a subject at the same distance and f/stop with this lens mounted on each camera type, the depths of field will not be the same. (If you think the term for the distance at which blurriness begins is "bokeh", you are wrong about this too, Depth of Field (dof) and bokeh are not interchangeable terms or concepts.)

Many people are confused by why the depth of field is different when you use the same lens (at same distance and f/stop) with different sized sensors. It is too complicated to fully explain here (and entire threads of the 150 post limit have been exhausted debating this point). But the essence is that depth of field is not measured (detected) at the sensor. It is not detected in the viewfinder or the LCD in live view mode (even with the misnamed depth of field preview button activated). Instead, as measured for all the depth of field calculators, DOF is detected ONLY by viewing prints of the entire area of the sensor -- prints that are the same size and viewed from the same distance, regardless of the differing dimensions of the sensors involved.

Equivalence: determine the crop factor. To convert to 35mm equivalent, multiply the FL by the crop factor, multiply the f-number by it, and for noise with the same exposure algorithm: multiply the ISO by the crop factor squared.

This is caused by the fact that on the RX100, the actual lens focal length you use is 25,7mm, and on the A7 it is 70mm.

So, the opening of the lens is 27,5/2.8 = 9.8mm on the RX100, and 70/5.6 = 12,5mm on the A7. This shows that the lens aperture on the A7 is bigger, not smaller than on the RX100, despite of it being f/5.6 vs. f/2.8. This results in less depth of field on the A7.

Both photos will have the same field of view, due to the smaller sensor of the RX100. This is why the lens on the RX100 is called 70mm equivalent at the tele end.

WryCuda said:

phou said:

Hi everyone,

i have a question concerning depth of field/bokeh on cameras with different sized sensors. I don't understand how focal length/aperture can be compared between different systems.

I know that if you theoretically could use the same lense (same distance to the subject) on different systems, all that would change is the crop of the image - bokeh will stay the same. But what happens in reality, where you use lenses with equivalent focal lenghts?

Example:

Sony RX100 III (8.8-25.7/f1.8-2.8)

Sony A7 with 28-70/f3.5-5.6

My understanding is that shooting the RX100 at 25.7mm/f2.8 and the Sony A7 at 70mm/f5.6 will result in more bokeh for the RX100. Infact thats not true as i see using a DOF Simulator. But why?

Can someone explain to me why the A7 will produce a shallower DOF at the same equivalent focal length even with a smaller aperure.

Click to expand...

Have a look at the equations for DoF calculation and it's all explained there. It's a combination of sensor size, sensor qualities, focal length and aperture.

http://www.dofmaster.com/articles.html (and the associated calculator)

Simple explanation: Large format cameras with wide apertures have shallow DoF because the constituent rays are at divergent angles. A tiny camera has greater DoF because it is more similar to a pin-hole camera with relatively straight rays.

One tip when considering cameras/lenses: Just forget about "equivalence" as no good will ever come from trying to get your head around it.

Click to expand...

The OP is exactly why "equivelance" shouldn't be a dirty word. SOOOO many newcomers do not understand the way DOF and angle of view works with different formats. Then they get confused by the "wide angle" and "wide aperture" lenses on smaller formats vs "slow lenses" on larger formats. Obviously no need to be overly technical in this forum. There is another forum where it gets beaten like Tina on Ike's birthday

Mystery member said:

BTW, Bokeh is the "quality" of the out-of-focus sections of the image, not just the fact of being out of focus.

Click to expand...

phou said:

Hi everyone,

i have a question concerning depth of field/bokeh on cameras with different sized sensors. I don't understand how focal length/aperture can be compared between different systems.

I know that if you theoretically could use the same lense (same distance to the subject) on different systems, all that would change is the crop of the image - bokeh will stay the same. But what happens in reality, where you use lenses with equivalent focal lenghts?

Example:

Sony RX100 III (8.8-25.7/f1.8-2.8)

Sony A7 with 28-70/f3.5-5.6

My understanding is that shooting the RX100 at 25.7mm/f2.8 and the Sony A7 at 70mm/f5.6 will result in more bokeh for the RX100. Infact thats not true as i see using a DOF Simulator. But why?

Can someone explain to me why the A7 will produce a shallower DOF at the same equivalent focal length even with a smaller aperure.

regards

phou

Click to expand...

The key to this is understanding that you need to be comparing what's in the final image that you look at, not the image that's actually presented on the sensor. So, imagine that you have a light source which produces a nice big bokeh disc covering 1/10 of the image hight in the image that you're looking at. That means that it has to produce a bokeh disc covering 1/10 the height of the sensor, whatever sized sensor you are using.

To maintain this condition, that the bokeh disc from the same object be the same proportion of the sensor, it turns out that you need to have a particular aperture diameter for any given field of view, since it is the relative sizes of the angle of view and angle subtended by the entrance pupil at the point source making the blur circle which determines the relative size of the blur circle within the image. I hope that this rather scrappy diagram explains that:









--
Bob.
DARK IN HERE, ISN'T IT?

bobn2 said:

phou said:

Hi everyone,

i have a question concerning depth of field/bokeh on cameras with different sized sensors. I don't understand how focal length/aperture can be compared between different systems.

I know that if you theoretically could use the same lense (same distance to the subject) on different systems, all that would change is the crop of the image - bokeh will stay the same. But what happens in reality, where you use lenses with equivalent focal lenghts?

Example:

Sony RX100 III (8.8-25.7/f1.8-2.8)

Sony A7 with 28-70/f3.5-5.6

My understanding is that shooting the RX100 at 25.7mm/f2.8 and the Sony A7 at 70mm/f5.6 will result in more bokeh for the RX100. Infact thats not true as i see using a DOF Simulator. But why?

Can someone explain to me why the A7 will produce a shallower DOF at the same equivalent focal length even with a smaller aperure.

regards

phou

Click to expand...

The key to this is understanding that you need to be comparing what's in the final image that you look at, not the image that's actually presented on the sensor. So, imagine that you have a light source which produces a nice big bokeh disc covering 1/10 of the image hight in the image that you're looking at. That means that it has to produce a bokeh disc covering 1/10 the height of the sensor, whatever sized sensor you are using.

To maintain this condition, that the bokeh disc from the same object be the same proportion of the sensor, it turns out that you need to have a particular aperture diameter for any given field of view, since it is the relative sizes of the angle of view and angle subtended by the entrance pupil at the point source making the blur circle which determines the relative size of the blur circle within the image. I hope that this rather scrappy diagram explains that:

Click to expand...

Another key here is to understand what aperture diameter is.

--

Iliah Borg said:

bobn2 said:

phou said:

Hi everyone,

i have a question concerning depth of field/bokeh on cameras with different sized sensors. I don't understand how focal length/aperture can be compared between different systems.

I know that if you theoretically could use the same lense (same distance to the subject) on different systems, all that would change is the crop of the image - bokeh will stay the same. But what happens in reality, where you use lenses with equivalent focal lenghts?

Example:

Sony RX100 III (8.8-25.7/f1.8-2.8)

Sony A7 with 28-70/f3.5-5.6

My understanding is that shooting the RX100 at 25.7mm/f2.8 and the Sony A7 at 70mm/f5.6 will result in more bokeh for the RX100. Infact thats not true as i see using a DOF Simulator. But why?

Can someone explain to me why the A7 will produce a shallower DOF at the same equivalent focal length even with a smaller aperure.

regards

phou

Click to expand...

The key to this is understanding that you need to be comparing what's in the final image that you look at, not the image that's actually presented on the sensor. So, imagine that you have a light source which produces a nice big bokeh disc covering 1/10 of the image hight in the image that you're looking at. That means that it has to produce a bokeh disc covering 1/10 the height of the sensor, whatever sized sensor you are using.

To maintain this condition, that the bokeh disc from the same object be the same proportion of the sensor, it turns out that you need to have a particular aperture diameter for any given field of view, since it is the relative sizes of the angle of view and angle subtended by the entrance pupil at the point source making the blur circle which determines the relative size of the blur circle within the image. I hope that this rather scrappy diagram explains that:

Click to expand...

Another key here is to understand what aperture diameter is.

Click to expand...

Yes indeed. It is the diameter of the entrance pupil, the aperture stop as seen from the front of the lens.



--
Bob.
DARK IN HERE, ISN'T IT?

But I think you are talking over the head of the OP......

golfhov said:

But I think you are talking over the head of the OP......

Click to expand...

Let the OP work it out for himself. Depends on how much he wants to put into it.

bobn2 said:

bobn2 said:

Another key here is to understand what aperture diameter is.

Click to expand...

Yes indeed. It is the diameter of the entrance pupil, the aperture stop as seen from the front of the lens.

Click to expand...

It turns out that many folks erroneously think that f-number is what solely defines the light collection ability of a lens, DoF, blur, and even bokeh. Sometimes the very obvious and simple geometry of lens workings is proclaimed "over the head" of beginners.

Maybe a good start is explaining what we mean saying f/N. Like if lens focal length f is 50mm, 50/5.6 is close to effective aperture diameter of 9mm. Wiki has https://en.wikipedia.org/wiki/F-number - not ideal, but IMHO useable.

Iliah Borg said:

Iliah Borg said:

Iliah Borg said:

Another key here is to understand what aperture diameter is.

Click to expand...

Yes indeed. It is the diameter of the entrance pupil, the aperture stop as seen from the front of the lens.

Click to expand...

It turns out that many folks erroneously think that f-number is what solely defines the light collection ability of a lens, DoF, blur, and even bokeh. Sometimes the very obvious and simple geometry of lens workings is proclaimed "over the head" of beginners.

Maybe a good start is explaining what we mean saying f/N. Like if lens focal length f is 50mm, 50/5.6 is close to effective aperture diameter of 9mm. Wiki has https://en.wikipedia.org/wiki/F-number - not ideal, but IMHO useable.

Click to expand...

I would go even further. Insist on the notation f/N, not FN or fN. 'f/N' has built in a useful mnemonic. You can say 'the aperture is f/2.8', lengthen the shorthand to 'the aperture is the focal length divided by 2.8', and straight away you understand what 'the aperture' actually is.

bobn2 said:

Iliah Borg said:

bobn2 said:

bobn2 said:

Another key here is to understand what aperture diameter is.

Click to expand...

Yes indeed. It is the diameter of the entrance pupil, the aperture stop as seen from the front of the lens.

Click to expand...

It turns out that many folks erroneously think that f-number is what solely defines the light collection ability of a lens, DoF, blur, and even bokeh. Sometimes the very obvious and simple geometry of lens workings is proclaimed "over the head" of beginners.

Maybe a good start is explaining what we mean saying f/N. Like if lens focal length f is 50mm, 50/5.6 is close to effective aperture diameter of 9mm. Wiki has https://en.wikipedia.org/wiki/F-number - not ideal, but IMHO useable.

Click to expand...

I would go even further. Insist on the notation f/N, not FN or fN. 'f/N' has built in a useful mnemonic. You can say 'the aperture is f/2.8', lengthen the shorthand to 'the aperture is the focal length divided by 2.8', and straight away you understand what 'the aperture' actually is.

Click to expand...

Right.

Incidentally, the German language shorthand inventor was the father of the inventor of the first aperture numbering system.

/2bobn2 said:

Iliah Borg said:

bobn2 said:

bobn2 said:

Another key here is to understand what aperture diameter is.

Click to expand...

Yes indeed. It is the diameter of the entrance pupil, the aperture stop as seen from the front of the lens.

Click to expand...

It turns out that many folks erroneously think that f-number is what solely defines the light collection ability of a lens, DoF, blur, and even bokeh. Sometimes the very obvious and simple geometry of lens workings is proclaimed "over the head" of beginners.

Maybe a good start is explaining what we mean saying f/N. Like if lens focal length f is 50mm, 50/5.6 is close to effective aperture diameter of 9mm. Wiki has https://en.wikipedia.org/wiki/F-number - not ideal, but IMHO useable.

Click to expand...

I would go even further. Insist on the notation f/N, not FN or fN. 'f/N' has built in a useful mnemonic. You can say 'the aperture is f/2.8', lengthen the shorthand to 'the aperture is the focal length divided by 2.8', and straight away you understand what 'the aperture' actually is.

Click to expand...

And what is usually left unsaid is what "f" is, in equivalence comparisons. F/2=F/2 is usually wrong when is used for different formats because we actually have F_1/2 and F_2/2 and F_1 and F_2 are different.

phou said:

Hi everyone,

i have a question concerning depth of field/bokeh on cameras with different sized sensors. I don't understand how focal length/aperture can be compared between different systems.

I know that if you theoretically could use the same lense (same distance to the subject) on different systems, all that would change is the crop of the image - bokeh will stay the same. But what happens in reality, where you use lenses with equivalent focal lenghts?

Example:

Sony RX100 III (8.8-25.7/f1.8-2.8)

Sony A7 with 28-70/f3.5-5.6

My understanding is that shooting the RX100 at 25.7mm/f2.8 and the Sony A7 at 70mm/f5.6 will result in more bokeh for the RX100. Infact thats not true as i see using a DOF Simulator. But why?

Can someone explain to me why the A7 will produce a shallower DOF at the same equivalent focal length even with a smaller aperure.

regards

phou

Click to expand...

The Equivalence Ratio (crop factor) between an RX100 III and an A7 is the ratio of the sensor diagonals: 43.3mm / 15.9mm = 2.7. Thus, the 8.8-25.7 / 1.8-2.8 on the RX100 III is equivalent to a 24-70 / 4.9-7.6 on the A7. So, at 25.7mm f/2.8, the RX100 III is equivalent to the A7 at 70mm f/7.6. Thus the A7 will have a more shallow DOF and greater background blur.

(By the way, "bokeh" is the quality of the blur, not the quantity of the blur).

Great Bustard said:

(By the way, "bokeh" is the quality of the blur, not the quantity of the blur).

Click to expand...

"One of the rare Japanese words to be incorporated in the English language. It refers to the way the lens renders out-of-focus points of light. Depending on the lens aberrations and shape of the lens diaphragm, the out-of-focus quality looks different." -- http://photojpn.org/words/len.html

Iliah Borg said:

Iliah Borg said:

(By the way, "bokeh" is the quality of the blur, not the quantity of the blur).

Click to expand...

Click to expand...

Or maybe it is just the blur itself?

Iliah Borg said:

"One of the rare Japanese words to be incorporated in the English language. It refers to the way the lens renders out-of-focus points of light. Depending on the lens aberrations and shape of the lens diaphragm, the out-of-focus quality looks different." -- http://photojpn.org/words/len.html

--
http://www.libraw.org/

Click to expand...