Depth of Field (DOF) and Teleconverters (TCs)???
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Depth of Field (DoF) calculator | PhotoPills
This Depth of Field (DoF) calculator will help you get the creative control on the design of your photos. Decide the zone of sharpness you need to tell the story you want and get your message across.
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The DOF will be consistent with the focal length and aperture of the lens plus TC. So if you're shooting a 500mm f5.6 lens with 1.4x TC and max aperture the DOF will be for 700mm at f8.
If you stay in the same place then yes it does reduce DOF. If you move so the subject is the same size in both, then no. However in the 2nd case background blur might appear larger due to perspective so it sorta looks like shallower dof.
The way I understand it, when you really get into the weeds, is that DOF runs with the size of the entrance pupil and the magnification and nothing else. In practical terms it's easier for a calculator to estimate magnification using subject distance and focal length and to estimate entrance pupil using focal length and f number. So that is why we say the variables in DOF are subject distance, f number and focal length, it's plenty good as an estimation except for macro.
But to take the long road to answer the question. The tc increases the focal length and so increases magnification and it also affects the effective f number/entrance pupil.
If you took the calculator linked above and instead of clicking a 2.0 TC just doubled the focal length and the f number the answer should be the same as clicking the 2.0 TC. In other words a 100mm f 2.8 lens with a 2x TC should have the same dof as a 200mm f 5.6 lens, at the same distance.
The way I understand it, when you really get into the weeds, is that DOF runs with the size of the entrance pupil and the magnification and nothing else. In practical terms it's easier for a calculator to estimate magnification using subject distance and focal length and to estimate entrance pupil using focal length and f number. So that is why we say the variables in DOF are subject distance, f number and focal length, it's plenty good as an estimation except for macro.
But to take the long road to answer the question. The tc increases the focal length and so increases magnification and it also affects the effective f number/entrance pupil.
If you took the calculator linked above and instead of clicking a 2.0 TC just doubled the focal length and the f number the answer should be the same as clicking the 2.0 TC. In other words a 100mm f 2.8 lens with a 2x TC should have the same dof as a 200mm f 5.6 lens, at the same distance.
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An informative article, except for the unfortunate wording of the DOF question.
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How to Use a Teleconverter to Get Closer to Your Subjects
Wondering about what a teleconverter is and how you can use it? In this article, we go through everything you need to know about using teleconverters.
photographylife.com
Yes
Teleconverters are actual optical elements and actually change the lens focal length and wide open aperture (f/stop). It's an unusual optical formula compared to say designing an 840mm f/5.6 lens from scratch but it's an actual optical change as opposed to say the 'crop factor' that comes from smaller sensors or cropping in post that don't actually change the optics in front of the sensor.
Actually if subject distance is changed to keep a constant FOV then DOF is purely a function of f stop regardless of focal length. So for the same FOV DOF increases when using the TC. Which of course is purely theoretical because who's going to use a TC if they can achieve the same FOV with a bare lens?
Yeah I think I got that part wrong.
Based on your historical content I'm confident you knew what you wanted to say. But by the time it passed through the keyboard and appeared on screen it was lost in translation. Happens to me a lot
Roy
Well-known member
When you use a TC the lens takes on all of the parameters of the longer focal length.
A 1x TC will lose 1 stop a 2x TC 2 stops, a little image quality and maybe some AF speed depending on the camera body and lens.
You will gain focal length at the cost of light and the DOF will de-crease accordingly ...
MadMikeOne
Member
I'm a bit confused by your statement that "the DOF will de-crease accordingly...". Wouldn't it increase as the f stop (aperture) decreases? It's always been my understanding that as the aperture decreases in SIZE , the DoF increases (more of the image front to back in focus). I'm wondering if I'm incorrect in my understanding of the relationship of aperture size to DoF. It would seem to me that, all else being equal except for the addition of the TC, the DoF will bring more of the image into focus.When you use a TC the lens takes on all of the parameters of the longer focal length.
A 1x TC will lose 1 stop a 2x TC 2 stops, a little image quality and maybe some AF speed depending on the camera body and lens.
You will gain focal length at the cost of light and the DOF will de-crease accordingly ...
You've got two things happening when you add a TC to a lens:
- The aperture decreases (f/ stop increases) by the magnification factor of the TC (i.e 1 stop for a 1.4x TC, 2 stops for a 2x TC, etc)
- The focal length of the lens increases by the magnification factor of the TC which means larger subject size (higher image magnification) which decreases DoF
Yeah, if you just decreased aperture without increasing focal length the DoF would increase but that second part has a bigger impact and the longer focal length for the same subject and same distance ends up decreasing the DoF compared to the smaller subject size and wider aperture prior to adding the TC.
Spend some time playing with different lens and TC combos at different subject distances in a tool like this to see the impact of adding a TC: https://www.photopills.com/calculators/dof
MadMikeOne
Member
Thank you so much for that clear explanation. Apparently I was not aware of that 2nd thing happening. So glad I asked the question. I learned something new to me. Playing with that sight was very interesting and informative.
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I think DOF is technically primarily aperture and magnification (with some assumptions about visual acuity of the viewer, image size, and viewing distance). The focal length and subject distance are ways for calculators to estimate magnification for normal distances. It breaks down when trying to use a calculator for macro, for example, and some value for "pupil magnification" has to be added into the calculator.
Roy
Well-known member
Because the teleconverter decreases the effective aperture - DOF is also decreased.
A 400mm f4 with a 1.5 TC becomes a 600mm f5.6
A 600mm f4 with a 2x TC becomes a 1200mm f8
And the DOF is reduced ...
Though if it was only a smaller effective aperture the dof would increase. I think the reason is more that the magnification is greater.Because the teleconverter decreases the effective aperture - DOF is also decreased.
A 400mm f4 with a 1.5 TC becomes a 600mm f5.6
A 600mm f4 with a 2x TC becomes a 1200mm f8
And the DOF is reduced ...
I largely take a different view to what has been written so far.
If instead of using a 1.4x TC on a 200mm lens I shoot on a lens with 280 mm available (perhaps a Nikon 180-400) - one stop of exposure is not lost to the converter - making a faster shutter speed available - though 2 stops depth of field of field is lost.
Using a 1.4x converter has already lost 1 stop of shutter speed - resulting in only 1 stop loss of depth of field compared to loosing 2 stops depth of field without a converter using a longer focal length lens.
When using converters more depth of field is traded against slower shutter speeds.
When using a 200mm with a 1.4x converter to get a 280mm equivalent angle of view, opening the lens up an extra stop (assuming not shooting wide open) there is the same shutter speed as with a lens without converter at 280mm - and also the loss of 2 stops depth of field as when 280mm is available without a converter.
For most telephoto and macro work 2 stops loss depth of field reduces the depth of field to 1 quarter.
Using a longer focal length lens to narrow the angle of view - 400mm compared to 200mm - looses 4 stops depth of field.
When using a 2x on 200mm to get to 400mm angle of view - 2 shutter speeds - and just 2 stops depth of field - are lost.
For relative novices increasing focal length has a proportionately greater effect on "narrow or short" depth of field than opening up the aperture.
This is partly why very diffuse backgrounds are relatively easy to achieve with longer focal length lenses.
If instead of using a 1.4x TC on a 200mm lens I shoot on a lens with 280 mm available (perhaps a Nikon 180-400) - one stop of exposure is not lost to the converter - making a faster shutter speed available - though 2 stops depth of field of field is lost.
Using a 1.4x converter has already lost 1 stop of shutter speed - resulting in only 1 stop loss of depth of field compared to loosing 2 stops depth of field without a converter using a longer focal length lens.
When using converters more depth of field is traded against slower shutter speeds.
When using a 200mm with a 1.4x converter to get a 280mm equivalent angle of view, opening the lens up an extra stop (assuming not shooting wide open) there is the same shutter speed as with a lens without converter at 280mm - and also the loss of 2 stops depth of field as when 280mm is available without a converter.
For most telephoto and macro work 2 stops loss depth of field reduces the depth of field to 1 quarter.
Using a longer focal length lens to narrow the angle of view - 400mm compared to 200mm - looses 4 stops depth of field.
When using a 2x on 200mm to get to 400mm angle of view - 2 shutter speeds - and just 2 stops depth of field - are lost.
For relative novices increasing focal length has a proportionately greater effect on "narrow or short" depth of field than opening up the aperture.
This is partly why very diffuse backgrounds are relatively easy to achieve with longer focal length lenses.
Likely a too simplistic explanation but I basically think of teleconverters as just magnifiers. Teleconverters don't change things like the minimum focusing distance or the actual aperture size to main lens front element size so I never really felt they change overall depth of field. To me, what changes is that the image is magnified and "circle of confusion" is also magnified so some areas that might have looked sharper without the magnification now look fuzzier giving the impression that there is a loss of depth of field (for same subject distance, same focus point, same aperture size). Small changes in depth of field are always harder to pin down as there are a number of factors that result in the perceived zone of sharpness of an image. Practically there is only one plane of truly sharp focus no matter what lens or aperture setting is used but the perceived sharpness of course varies greatly with focal lengths (wide angle verses telephoto) and aperture (stopped down verses wide open) along with size of the viewed image and viewing distance. My use of a teleconverter is limited to extending the reach of a medium aperture, longer lens and the resultant light fall off almost always means I am using the largest aperture so I just take whatever depth of field I get. ( Side note, I just love the term "Circle of Confusion").
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MadMikeOne
Member
Now you have me googling "circle of confusion". It made me giggle, too!
Although the physical size of the aperture is unchanged, the angle of view using a converter is narrower.
A 200mm at f4 has a 50mm aperture (200 divided by 4)
When the lens is changed with a teleconverter to 280mm, 280mm divided by the unchanged 50mm aperture is f5.6.
I am not sure there is a fully precise way of calculating perceived image sharpness (depth of field) using a relationship of shutter speed to aperture and focal length magnification.
The depth of perceived sharpness (area in front of and behind the actual plane of focus) is set before the point of convergence (focal point) of the lens. This really can't change with anything added after that point (teleconverter). The teleconverter would magnify the image on the sensor (or film) and in doing so would leave off some of the image forming light at the outer part of the image circle causing a less light to hit the sensor resulting in the light loss (reduced effective aperture) caused by the teleconverter. Magnifying the image after the focal point also causes the degradation of the image that teleconverters are accused of.
In practice this can be hard to show as exactly how the convertor works with any particular lens is not all that predictable. (thinking more of generic teleconverters than the dedicated convertors for specific lenses).
In the above example I would think that if I took two pictures, the first with a 200mm lens at f/4 and then without changing camera to subject distance and without changing the aperture, took the second after putting on a 1.4x teleconverter (giving a field of view of a 280mm lens with the typical 1 stop light loss yeilding an effective aperture of f/5.6) and then printed the teleconverter image "full frame" (say a 6 x 9 inch print) and printed the non-teleconverter image to the same physical size (6 x 9 inches) but cropped (enlarged) the image to give the field of view of the teleconverter image, the perceived depth of field would be essentially the same in the two pictures.
There has been at least one lens series (APO-Telyt) designed to offer different focal lengths by changing lens components before the lenses focal point. These combinations would act like individual prime lenses and would have depth of field changes at different apertures as the changeable components are not teleconverters.
For the most part it is easiest (though not fully precise) for me to think that a teleconverter just magnifies the image drawn by the lens giving, as a practical result, the final image's subject the same perspective and relationships (like depth of field) that cropping and enlarging would give.
The depth of perceived sharpness (area in front of and behind the actual plane of focus) is set before the point of convergence (focal point) of the lens. This really can't change with anything added after that point (teleconverter). The teleconverter would magnify the image on the sensor (or film) and in doing so would leave off some of the image forming light at the outer part of the image circle causing a less light to hit the sensor resulting in the light loss (reduced effective aperture) caused by the teleconverter. Magnifying the image after the focal point also causes the degradation of the image that teleconverters are accused of.
In practice this can be hard to show as exactly how the convertor works with any particular lens is not all that predictable. (thinking more of generic teleconverters than the dedicated convertors for specific lenses).
In the above example I would think that if I took two pictures, the first with a 200mm lens at f/4 and then without changing camera to subject distance and without changing the aperture, took the second after putting on a 1.4x teleconverter (giving a field of view of a 280mm lens with the typical 1 stop light loss yeilding an effective aperture of f/5.6) and then printed the teleconverter image "full frame" (say a 6 x 9 inch print) and printed the non-teleconverter image to the same physical size (6 x 9 inches) but cropped (enlarged) the image to give the field of view of the teleconverter image, the perceived depth of field would be essentially the same in the two pictures.
There has been at least one lens series (APO-Telyt) designed to offer different focal lengths by changing lens components before the lenses focal point. These combinations would act like individual prime lenses and would have depth of field changes at different apertures as the changeable components are not teleconverters.
For the most part it is easiest (though not fully precise) for me to think that a teleconverter just magnifies the image drawn by the lens giving, as a practical result, the final image's subject the same perspective and relationships (like depth of field) that cropping and enlarging would give.
Shutter speed is not directly relevant except in relation to a possible increased camera shake blur.
Knowing the focal length and modified aperture of an image - after magnification of the original image using a converter - enables depth of field to be calculated.
This is not entirely true.
One of the components of hyperlocal distance is image (format) size.
The "shape" of depth of field in front and behind the point of focus varies from substantially more behind the point of focus at hyperlocal distance to - for practical photographic purposes - close to equal each side of the point of focus by about 1/20th hyperlocal distance or closer.
Photographing at 1/20th of HD or closer is common for birds photographed with long telephoto lenses and for much macro photography.
Although it initially sounds counter-intuitive; while a smaller format sensor requires a smaller circle of confusion to produce enough detail in the image to look sharp in the theoretical 10 by 8 inch print viewed at the comfortable viewing distance assumed in depth of field calculations, the maths of the formula clarify a smaller circle of confusion results in more depth of field for the same angle of view.
This is why smaller formats such as APSC and 4:3 have more inherently depth of field than 24x36 format for the same angle of view with - and medium format has inherently
less depth of field than 24x36 format.
To within plus or minus 3% accuracy a lens with a HD of 400 feet used at 1/20 of its HD (20 feet) yields depth of field equal to 1/19th of the focus distance of 20 feet of 12.63 inches behind the point of focus - and 11.43 inches in front of the point of focus.
A difference of less than 1 inch in depth of field (in this example) between in front and behind the point of focus of 20 feet is for many photographic scenarios close to practically equal.
Depth of field is not always an exact science.
In a photograph of a foggy landscape it can be difficult to determine the boundary between just sharp enough and unsharp from the relatively indistinct detail compared to a high contrast "hard" side lighting landscape.
Roy
Well-known member
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...