Depth of Field (DOF) and Teleconverters (TCs)???

If you would like to post, you'll need to register. Note that if you have a BCG store account, you'll need a new, separate account here (we keep the two sites separate for security purposes).

Member's gallery
Roy said:
Sensor size does not effect DOF - only aperture and distance do.
This is a mistake usually made by people adjusting the distance or aperture to compensate for a smaller field of view from a smaller sensor...
Click to expand...
Sorry - perhaps you coluld check the physics carefully.

Different sensor sizes need a different circle of confusion, as I mentioned earlier, to produce sufficient detail in the theoretical 10x8 print when determining the zone of acceptable "front to back" sharpness.

The size of the circle of confusion is one of the factors, along with aperture and focus distance, in depth of field formulae.

There are three main components, not just the two you mention, used in depth of field calculations.
 
Member's gallery
Roy said:
Sensor size does not effect DOF - only aperture and distance do.
This is a mistake usually made by people adjusting the distance or aperture to compensate for a smaller field of view from a smaller sensor...
Click to expand...
Agree with the first point.
But you forgot to mention focal length, which also affects DOF, unless you adjust the distance to match the field of view (or magnification factor)...
 
Member's gallery
Len Shepherd said:
Sorry - perhaps you coluld check the physics carefully.

Different sensor sizes need a different circle of confusion, as I mentioned earlier, to produce sufficient detail in the theoretical 10x8 print when determining the zone of acceptable "front to back" sharpness.

The size of the circle of confusion is one of the factors, along with aperture and focus distance, in depth of field formulae.

There are three main components, not just the two you mention, used in depth of field calculations.
Click to expand...
In this case, it would be correct to say: DOF depends on the circle of confusion. But it doesn't depend on sensor size.
 
Member's gallery
This is one representation of DOF calculation (from german literature):
1679399581346.png

where g' is the nearest and the furthest distance within the DOF, f is the focal length, g is the distance, k is the F-number and Z is the circle of confusion.
Hence, there are four independent components. Focal length is one of them.

It is often assumed that shooting at the same magnification (e.g. adjusting the distance to match the field of view) makes DOF independent from focal length.
This is somewhat deceptive, since one could argue in the same manner that F-number has no influence on DOF when the distance is adjusted to compensate for the effect of F-number, which is obviously meaningless.

Sensor size plays no role. However, depending on the sensor's properties and on the purpose of imaging, it can be necessary to adjust Z to match sharpness requirements.
 
Member's gallery
Krümelkraft said:
This is one representation of DOF calculation (from german literature):
View attachment 57262
where g' is the nearest and the furthest distance within the DOF, f is the focal length, g is the distance, k is the F-number and Z is the circle of confusion.
Hence, there are four independent components. Focal length is one of them.

It is often assumed that shooting at the same magnification (e.g. adjusting the distance to match the field of view) makes DOF independent from focal length.
This is somewhat deceptive, since one could argue in the same manner that F-number has no influence on DOF when the distance is adjusted to compensate for the effect of F-number, which is obviously meaningless.

Sensor size plays no role. However, depending on the sensor's properties and on the purpose of imaging, it can be necessary to adjust Z to match sharpness requirements.
Click to expand...
We must keep in mind the basic underlying assumption for calculating DOF which is the output of a certain size image viewed at a certain distance. Circle of confusion is the variable that allows us to understand how detail on the capture media(film/sensor) translates to that standard output image.

So change in sensor size matters only in that it changes FOV. And changing FOV changes circle of confusion. Rather than speaking of sensor size it is better to think in terms of crop factor. Because cropping images in PP has the same net effect on DOF as shooting with a smaller sensor.
 
The bottom line is you can't tell the genie you wish for more wishes. You can't shoot from farther away to get the benefit of better DOF thinking you can crop later to get the framing you want. And you can't switch to crop mode or switch to a crop camera for the same reason either. Of course viewing the original and the crop side by side, one big and one small, the dof would be the same. But once you resize to view both at an equal size the crop would have reduced dof because the magnification impacts the C of C.
 
Of course advanced depth of field calculators online allow you to manipulate the variables to get to any desired circle of confusion. This is because circle of confusion is tied to visual acuity which can be measured. Some of us see 20/40, some 20/20, some 20/10.

Where those acuity numbers come from is not plucked out of thin air. Those numbers scientifically reflect how an individual resolves detail at a standard distance. So someone with better 20/10 vision will need a smaller circle of confusion than someone with not as good 20/20.

Notice how the CoC changes when you switch from standard to 20/20 in the calculator, or when you switch from a full frame to a crop camera (You might have to refresh the browser)

https://www.photopills.com/calculators/dof-advanced
 
Member's gallery
Krümelkraft said:
Agree with the first point.
But you forgot to mention focal length, which also affects DOF, unless you adjust the distance to match the field of view (or magnification factor)...
Click to expand...
Unless you change distance to correct AOV focal length does not directly effect DOF.
At the same distance and aperture Taking 2 images with different focal lengths and cropping the wider image to math the longer lens will show the same DOF...🦘
 
Member's gallery
Roy said:
Unless you change distance to correct AOV focal length does not directly effect DOF.
At the same distance and aperture Taking 2 images with different focal lengths and cropping the wider image to math the longer lens will show the same DOF...🦘
Click to expand...
You've got this backward. For the same aperture if you change distance to keep the same FOV then focal length doesn't matter. DOF will be the same. On the other hand cropping does indeed alter DOF. Playing around with a DOF calculator will reveal this.

For example compare 300mm vs 500mm at f4 and 100ft range. For a 35mm full frame camera the 500mm lens will yield an horizontal FOV of 7.2ft. From the same distance to get the same FOV an image taken with the 300mm must be cropped by the ratio of the focal length(IOW 300/500=0.6). Circle of confusion changes directly with the crop ratio so 0.03 x 0.6 = 0.018. Plug all of that into the DOF calculator and the results will show DOF of the full frame image to be 2.9ft vs 4.8ft for the cropped image.
 
NorthernFocus said:
You've got this backward. For the same aperture if you change distance to keep the same FOV then focal length doesn't matter. DOF will be the same. On the other hand cropping does indeed alter DOF. Playing around with a DOF calculator will reveal this.

For example compare 300mm vs 500mm at f4 and 100ft range. For a 35mm full frame camera the 500mm lens will yield an horizontal FOV of 7.2ft. From the same distance to get the same FOV an image taken with the 300mm must be cropped by the ratio of the focal length(IOW 300/500=0.6). Circle of confusion changes directly with the crop ratio so 0.03 x 0.6 = 0.018. Plug all of that into the DOF calculator and the results will show DOF of the full frame image to be 2.9ft vs 4.8ft for the cropped image.
Click to expand...

Just curious, in your example did you account for resizing the crop to be viewed at an equal size as the non crop? Not saying you did or didn't, just wasn't sure.
 
Member's gallery
Way back, the original question was if adding a teleconverter changes depth of field. Based on simple optics, I had posted a response that it really should not. (I tend to use simple optic systems to let me understand basic concepts and then just try to learn how my lenses work out by using them and seeing what things look like - anything more is just too much math!).

Calculating depth of field, for me, really can’t be as precise as the online calculators suggest due to the subjective assessments of what is acceptable sharp focus, how is the out of focus area rendered, what does the transition from acceptable focus to blur look like (abrupt or gradual), etc. The calculators can be fun to play with to get an idea of what one can expect.

Just for fun I made this set of images. I used a 70-200 zoom at the 200mm setting (which is listed as 190mm in the metadata) and kept the aperture at f/5.6 (i.e. not physically changing the aperture setting on the lens for both images). The lens is on a tripod and is focused on the thumb pick (in all of it’s “clown barf” colors!). The first image is without the teleconverter. I then added a 1.4x teleconverter but kept the lens at the same position, same focus point, same set f-stop and took the second picture. Then for the third image I visited Photoshop to align the two pictures (no TC and with TC) so I could crop the first (no TC) to the field of view of the second (with TC). (poorly lit office so used a high ISO).

To my eye, looking at the images at the same size on screen and at the same distance (and looking at a print of all three together) the actual narrow area of what I think is acceptable sharp focus is pretty much the same but there is a bit of variation. The “19” mark seems similar in all but the “17” mark is not as sharp in the teleconverter image than in the non-teleconverter one (which also is not fully sharp at "17").

What is interesting to me is the more gradual changes seen when the teleconverter is not in place and the more abrupt transition to blur and very different rendering of that blur when the teleconverter is used.

So, if one defines depth of field as the area in front of and behind the plane of focus that is essentially as sharp as the plane of focus then I do not see, in my small test, significant differences. However, if one looks at the overall transition from sharp to sort of sharp to blur then the apparent depth of field when using a teleconverter does appear to be shallower. Interesting!

Dof Tryptic.jpg
You can only see EXIF info for this image if you are logged in.
 
Last edited:
To my eye the cropped one on the right seems to have slightly more DOF compared to the middle one. The one on the left has the most DOF. In theory between the 1st and third the difference should be accounted for by f number times crop factor.
 
Last edited:
Member's gallery
bleirer said:
Just curious, in your example did you account for resizing the crop to be viewed at an equal size as the non crop? Not saying you did or didn't, just wasn't sure.
Click to expand...
All DOF calculations are based on viewing the same sized image. Otherwise they are meaningless. When it comes down to the physics of it DOF is zero. Focus is only spot on at the focal plane perpendicular to the axis of the lens. Every point off that plain is technically OOF. DOF calculations are an attempt to mathematically model human perception of an image created by the optics and capture device. If the viewed image does not remain constant then the entire exercise is moot.
 
Member's gallery
bleirer said:
To my eye the cropped one on the right seems to have slightly more DOF compared to the middle one. The one on the left has the most DOF.
Click to expand...
The left and right pictures are the same image, the right is a cropped version of the left. The one on the left is less magnified and has a more gradual transition from sharp to blurred than the teleconverter image and I agree, it looks like it has more depth of field (even though the acceptable sharp sections of all the images are comparable).

I was mostly impressed by the teleconverter image (middle one) and the more abrupt transition to really blurred. From a practical standpoint it does seem that a teleconverter will produce an image that has less apparent depth of field due to this more abrupt change. (again defining depth of field as the area that seems as sharp as the actual plane of focus).
 
Member's gallery
Robert Ardinger said:
Way back, the original question was if adding a teleconverter changes depth of field. Based on simple optics, I had posted a response that it really should not. (I tend to use simple optic systems to let me understand basic concepts and then just try to learn how my lenses work out by using them and seeing what things look like - anything more is just too much math!).

Calculating depth of field, for me, really can’t be as precise as the online calculators suggest due to the subjective assessments of what is acceptable sharp focus, how is the out of focus area rendered, what does the transition from acceptable focus to blur look like (abrupt or gradual), etc. The calculators can be fun to play with to get an idea of what one can expect.

Just for fun I made this set of images. I used a 70-200 zoom at the 200mm setting (which is listed as 190mm in the metadata) and kept the aperture at f/5.6 (i.e. not physically changing the aperture setting on the lens for both images). The lens is on a tripod and is focused on the thumb pick (in all of it’s “clown barf” colors!). The first image is without the teleconverter. I then added a 1.4x teleconverter but kept the lens at the same position, same focus point, same set f-stop and took the second picture. I then visited Photoshop to align the two images so I could crop the first to the field of view of the second. (poorly lit office so used a high ISO).

To my eye, looking at the images at the same size on screen and at the same distance (and looking at a print of all three together) the actual narrow area of what I think is acceptable sharp focus is pretty much the same but there is a bit of variation. The “19” mark seems similar in all but the “17” mark is not as sharp in the teleconverter image than in the non-teleconverter one (which also is not fully sharp at "17").

What is interesting to me is the more gradual changes seen when the teleconverter is not in place and the more abrupt transition to blur and very different rendering of that blur when the teleconverter is used.

So, if one defines depth of field as the area in front of and behind the plane of focus that is essentially as sharp as the plane of focus then I do not see, in my small test, significant differences. However, if one looks at the overall transition from sharp to sort of sharp to blur then the apparent depth of field when using a teleconverter does appear to be shallower. Interesting!

View attachment 57370
Click to expand...
This is a good example of why some wise soul at some point in the past decided to mathematically model DOF. To my eye there is a very clear difference in DOF. And interestingly exactly as the math shows. The DOF of the cropped image is wider/deeper than it is with the TC. In addition to the drop off being more gradual in the cropped version.
 
Robert Ardinger said:
The left and right pictures are the same image, the right is a cropped version of the left. The one on the left is less magnified and has a more gradual transition from sharp to blurred than the teleconverter image and I agree, it looks like it has more depth of field (even though the acceptable sharp sections of all the images are comparable).

I was mostly impressed by the teleconverter image (middle one) and the more abrupt transition to really blurred. From a practical standpoint it does seem that a teleconverter will produce an image that has less apparent depth of field due to this more abrupt change. (again defining depth of field as the area that seems as sharp as the actual plane of focus).
Click to expand...

I think even though the same image, the cropping and resizing changes the DOF because magnification changes the circle of confusion. If no resizing, then yes they are the same picture and the same DOF. But nor with resizing. Keep all other settings the same in a DOF calculator except swap a full frame camera for a crop camera. The DOF will be less in the crop camera because they adjust for viewing an image of the same size.

Physically hold an 8x10 print at arms length then take scissors and cut out a 4x5 crop. Still the same picture at arms length with the same DOF. But then hold it 6 inches away or blow the cropped 4x5 back up to 8x10 and the DOF decreases. Magnification and f number are the keys to the puzzle , I think. Focal length and distance and cropping all impact magnification, as does our visual acuity and viewing distance and image size. I think circle of confusion is the one number that changes to attempt to reflect many factors.
 
Last edited:
Member's gallery
Robert Ardinger said:
...From a practical standpoint it does seem that a teleconverter will produce an image that has less apparent depth of field due to this more abrupt change. (again defining depth of field as the area that seems as sharp as the actual plane of focus).
Click to expand...
The teleconverter produces the same DOF as a lens of that focal length. So for a given aperture your 200mm lens plus 1.4x TC produces the same DOF as any other 280mm lens set at the same aperture(i.e. lens aperture + 1.0 to account for the TC).
 
Member's gallery
I think that a good deal what has been written in this post is saying similar things but perhaps from different perspectives. In a straight forward, basic optical system it just makes sense to me that all a teleconverter can do is to magnify the image drawn by the lens including magnifying (making more obvious) the circle of confusion, defects in the lens along with magnifying the image itself. The optics in the teleconverter may have a distortion effect (think Spiratone) but overall I do not see how the added convertor in this system can do anything else.

There are ways of obtaining equivalents such as the example of 280mm lens compared with a 200mm/1.4x converter with it’s physical diaphragm opened one stop (lens aperture+1) This changes the image the teleconverter is enlarging so it makes sense that final image from a 280mm at f-stop “x” would look the same as the 200mm with a 1.4 converter set at f-stop “x”+1.

I was interested in seeing what a teleconverter would do to the image if the only variable was just the adding of the converter (what I took as the original question) and all else remaining the same. I had not thought about it until seeing this post as I only recently obtained lenses that have dedicated (more or less) teleconvertors.

It does make sense that an image made with a 200mm lens, cropped and enlarged to equal an image made with a 280mm lens would look different than the image made with a 200mm lens and a 1.4 converter as the converter had it’s effect, good or bad on the image before it was recorded on the sensor or film and the cropped and enlarged image is still an image made without any optical changes (like magnification) being made before being recorded.

One of the problems I think, in trying to show any of these effects with modern camera lenses is that they are not simple optical systems (my 500mm lens is not 500mm long) and the teleconverters I use are specifically designed to work with only a few specific lenses and these lens and converter combinations may yield a different look in a final print than a simple lens and simple (generic) teleconverter.
 
Last edited:
Member's gallery
Krümelkraft said:
In this case, it would be correct to say: DOF depends on the circle of confusion. But it doesn't depend on sensor size.
Click to expand...

Back to my original comment – I attempted to make it clear that the extra death of field I was referring to from a smaller format sensor is obtained when maintaining the same angle of view to achieve the same size image.

As an example - from the same focus distance - 300mm on 24x36 format results in the same size image and image magnification as 200mm on 18x24 format.

In this specific scenario the formulae etc confirm an increase in depth of field moving to a smaller sensor with a shorter focal length lens, or a reduction in depth of field moving to a larger sensor with a longer focal length - while maintaining the same image size.

Digressing - the use of smaller sensor cameras is not limited to their ability to be used to increase subject magnification with a specific focal length lens.

I go back far enough in time to recall when medium format film was needed for high quality images – and that the advice (from Kodak) was to move to a smaller format camera (despite other potential quality limitations) if sufficient depth of field was not possible from medium format.

As to the chicken and egg detail of CoC versus format size - a smaller sensor has to be enlarged more to get the same detail in a print, with a different CoC being needed to achieve this.

It matters little to me whether to take the longer route of saying different size sensors require different size CoC's resulting in different depth of field parameters, or the more direct summary of saying different format sizes result in different depth of field characteristics.
 
Member's gallery
Len Shepherd said:
[...] As an example - from the same focus distance - 300mm on 24x36 format results in the same size image and image magnification as 200mm on 18x24 format.

In this specific scenario the formulae etc confirm an increase in depth of field moving to a smaller sensor with a shorter focal length lens, or a reduction in depth of field moving to a larger sensor with a longer focal length - while maintaining the same image size.
[...]
As to the chicken and egg detail of CoC versus format size - a smaller sensor has to be enlarged more to get the same detail in a print, with a different CoC being needed to achieve this.

It matters little to me whether to take the longer route of saying different size sensors require different size CoC's resulting in different depth of field parameters, or the more direct summary of saying different format sizes result in different depth of field characteristics.
Click to expand...
I cannot contradict here.
But my point in regard of the parts I highlighted is:
The increase in depth of field in the former case is a result of reducing the focal length, and not of reducing sensor size, because reduced sensor size (or increased enlargement) reduces the size of acceptable circles of confusion, which would actually result in a reduction in DOF.
However, since focal length is almost quadratic in the equation, whereas CoC is linear, the effect of reduced focal length outweights the effect of CoC. Therefore you get increased depth of field with a smaller sensor at constant FOV.

By the way, we should create a discussion group and call it "Circle of Confusion" ;)
 
Here is another way to look at it, focusing on magnification. Perhaps a refinement of what was mentioned above, but the DOF varies with the square of the magnification. So if the magnification doubles the dof changes by 4 times. Where dof only changes proportional to f number. Magnification is estimated to be focal length divided by subject distance, not just focal length. Keep the focal length and the subject distance in the same ratio to each other and the dof remains unchanged.

damienfournier.co

DoF and Subject Magnification

The magnification of the subject is the parameter that influences the most the Depth of Field, much more than the aperture. This articles explains what is the magnification of the subject and in w…
damienfournier.co damienfournier.co
 
Last edited:
Also a lot of the above I think is where the simple "double the distance" method comes from. Because magnification has so much more impact than f number, and distance is one of the two components of magnification (The other being focal length). So if you keep focal length the same you impact dof more by changing focusing distance than by changing f number.

Or we could hold in that DOF preview button on the front of the camera.

photographylife.com

Double the Distance Method Explained

photographylife.com photographylife.com
 
Last edited:
You must log in or register to reply here.
Back
Top