Question about depth of Field with a 400 f2.8

Cryve

EOS 90D
Jul 4, 2018
115
74
Germany
Im interested in the abiility of the 400 f2.8 to blur the background (for wildlife).
But while looking into this with a depth of field calculator i noticed some contradictions.

when i type 400mm f2.8 into the dof calculator it says that i get about 62cm of in focus area with a subject at 25m distance.
however when i type in 560mm f4 (400 f2.8 with 1.4 teleconverter) it says i get a dof of about 45cm.

how is it possible that the dof is smaller with the teleconverter? All the sources i could find online said that the teleconverter doesnt
affect dof at all and that the teleconverter basicaly only performs an "optical crop".

Does anyone know more about this?
 
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Czardoom

EOS 90D
Jan 27, 2020
134
315
Just tried it with a 1.4 teleconverter and yes, the DOF changes and gets shallower. There are indeed some online links that say no, it doesn't change the DOF, but many others that say yes it does.

Attached is a file with two pics - one with and one without 1.4 teleconverter showing the change in DOF.
focal DOF.jpg
 

HeavyPiper

6D Mark II
CR Pro
Aug 1, 2018
48
27
70
Don't for get that the f2.8 becomes f3.92 with the 1.4 converter.
Doesn't the field become more compressed with the longer lens?
 

privatebydesign

Garfield is back...
CR Pro
Jan 29, 2011
9,191
3,412
120
DOF is determined by two things, subject magnification and absolute aperture (that is the actual size of the hole of the aperture not the f stop number).

If you keep the distance the same the magnification is bigger with the TC and the aperture is the same physical size, ergo less DOF. If you moved back so the magnification was the same with the TC then the DOF would be the same because the aperture hole is.

Did I explain that in a way it is understandable?
 

Cryve

EOS 90D
Jul 4, 2018
115
74
Germany
DOF is determined by two things, subject magnification and absolute aperture (that is the actual size of the hole of the aperture not the f stop number).

If you keep the distance the same the magnification is bigger with the TC and the aperture is the same physical size, ergo less DOF. If you moved back so the magnification was the same with the TC then the DOF would be the same because the aperture hole is.

Did I explain that in a way it is understandable?
Thanks for the Explanation.

I still have problems with the logic though. I saw a video of a german optics expert and he said that when attaching a teleconverter the apperature stops down inside the lens. He said you can even see that when looking into the lens with a tc attached.

This would mean that the apperature hole wouldnt be the same size as you described.

can you clear things up for me here?
 

Cryve

EOS 90D
Jul 4, 2018
115
74
Germany
To give further information about why im asking these questions;

Im currently photographing with a 600mm f6.3 lens and i want to upgrade to either a 400 f2.8 or 600 f4 in the future.

for my style of pictures i need very good background blur because the subject is very small and far away in my pictures.

At first i thought the 400 f2.8 would be best for me because i could show a lot of environment of the animal and still have somewhat blurry background,
but when i went to a dof calculator to check i saw that, according to the dof calculator, the 400 f2.8 and a 600 f6.3 have basicaly the same dof.

So the only advantage a 400 f2.8 would give me would be that, at the same subject distance, i could show a little bit more environment with the same blur.

idealy though i want MORE blur so i can for example photograph a stag far away in a field, have him small in the picture, and the background still beeing somewhat blurred.

if the dof of the 400 f2.8 with a subject at 25m is 62cm (the same as with my 600 f6.3) then that wouldnt help me out very much for my use case.
However if the dof calculator is wrong and the dof is 45cm in reality (without tc) then it would be substantial upgrade over my 600mm f6.3. I really dont want to get closer and disturb the animals, i just want more blur at the same subject distance.

otherwise i have to go for a 600 f4 i guess.
 

Attachments

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BeenThere

EOS R
CR Pro
Sep 4, 2012
1,216
642
Eastern Shore
To give further information about why im asking these questions;

Im currently photographing with a 600mm f6.3 lens and i want to upgrade to either a 400 f2.8 or 600 f4 in the future.

for my style of pictures i need very good background blur because the subject is very small and far away in my pictures.

At first i thought the 400 f2.8 would be best for me because i could show a lot of environment of the animal and still have somewhat blurry background,
but when i went to a dof calculator to check i saw that, according to the dof calculator, the 400 f2.8 and a 600 f6.3 have basicaly the same dof.

So the only advantage a 400 f2.8 would give me would be that, at the same subject distance, i could show a little bit more environment with the same blur.

idealy though i want MORE blur so i can for example photograph a stag far away in a field, have him small in the picture, and the background still beeing somewhat blurred.

if the dof of the 400 f2.8 with a subject at 25m is 62cm (the same as with my 600 f6.3) then that wouldnt help me out very much for my use case.
However if the dof calculator is wrong and the dof is 45cm in reality (without tc) then it would be substantial upgrade over my 600mm f6.3. I really dont want to get closer and disturb the animals, i just want more blur at the same subject distance.

otherwise i have to go for a 600 f4 i guess.
For you to have a more blurry Background, you would have to either get closer to the in-focus subject or use a wider aperture lens, or put a TC on your lens. You are already at the point of diminishing returns for a wider aperture given what lenses are available.
 

Cryve

EOS 90D
Jul 4, 2018
115
74
Germany
For you to have a more blurry Background, you would have to either get closer to the in-focus subject or use a wider aperture lens, or put a TC on your lens. You are already at the point of diminishing returns for a wider aperture given what lenses are available.
i dont want to get closer, ergo i have to use a wider apperature lens.

my interest is in if the 400 f2.8 can deliver a more shallow depth of field vs a 600 f6.3 at the same subject distance.

the answers in this thread point towards the 400 f2.8 not having this ability without a tc and only having this ability with a tc. but i still dont quite understand, as i have stated in my response to the user "privatebydesign".
 

BeenThere

EOS R
CR Pro
Sep 4, 2012
1,216
642
Eastern Shore
i dont want to get closer, ergo i have to use a wider apperature lens.

my interest is in if the 400 f2.8 can deliver a more shallow depth of field vs a 600 f6.3 at the same subject distance.

the answers in this thread point towards the 400 f2.8 not having this ability without a tc and only having this ability with a tc. but i still dont quite understand, as i have stated in my response to the user "privatebydesign".
Using an app (PhotoPills) to calculate DoF:
your 600/6.3. .41M
400/2.8. .42M
600/6.3 +1.4 TC. .29M
600/4. .26M

all at target distance of 20M
 

RBS

EOS M50
Feb 6, 2020
26
30
The f2.8 with 1.4x tele will provide a 560 f4 lens with a depth of field 243 mm shallower than the 600 f6.3 at 30 meters on a full frame body.

The EF 400 f2.8 is one of the few lenses that provides fairly good results with the 2X resulting in a 800 f5.6 with a 445 mm depth of field at 30 meters. My EF 800 f5.6 is better than my EF 400 f2.8 with the 2X extender but the only glass I found acceptable with the 2X are the 300 and 400 f2.8 primes.
 

privatebydesign

Garfield is back...
CR Pro
Jan 29, 2011
9,191
3,412
120
Thanks for the Explanation.

I still have problems with the logic though. I saw a video of a german optics expert and he said that when attaching a teleconverter the apperature stops down inside the lens. He said you can even see that when looking into the lens with a tc attached.

This would mean that the apperature hole wouldnt be the same size as you described.

can you clear things up for me here?
Your German optics expert is 100% incorrect, or at least what you are deriving from it is. Technically some manufacturer TC's might stop a lens down to the last used aperture or the currently selected one, but that is not true of Canon EF TC's and any manufacturers TC is capable of shooting with the lens wide open. The derived 1 stop loss of DOF when using a 1.4TC is easy to prove mathematically so the lens must be fully open.

400 f2.8 = 400/2.8 = 143mm aperture
560 f4 = 560/4 = 140mm aperture

Allowing for actual focal lengths and true apertures and the fact that we are referring to an apparent aperture/hole size when viewed from the subject the numbers are essentially the same so the maths holds true. To be clear, the aperture inside the lens is not actually 143mm wide, it just looks like it is to light coming from the subject direction.

All you need to do to get your calculation of the optimal background separation is compare the focal length (which is magnification given that you are shooting from a set distance) and the (apparent) aperture. The larger the result of the sum the more separation you will get.

600mm f6.3 = 600/6.3 = 95mm apparent aperture
560 f4 = 560/4 = 140mm apparent aperture

So in your situation, shooting from the same distance but with a larger apparent aperture, you will get more separation with a 400mm f2.8 and a TC than with a 600mm f6.3.

Of course the quality of the glass and the way it renders background blur can impact the results to a very large degree but the truth is you are going 'the right way' in that nobody makes a bad 400mm f2.8.

Addendum: The 600 f4 would give you a 150mm apparent aperture so for your shooting situation that would give you even more separation. Only you could decide on the cost/value/image equation though!
 
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Cryve

EOS 90D
Jul 4, 2018
115
74
Germany
Your German optics expert is 100% incorrect, or at least what you are deriving from it is. Technically some manufacturer TC's might stop a lens down to the last used aperture or the currently selected one, but that is not true of Canon EF TC's and any manufacturers TC is capable of shooting with the lens wide open. The derived 1 stop loss of DOF when using a 1.4TC is easy to prove mathematically so the lens must be fully open.

400 f2.8 = 400/2.8 = 143mm aperture
560 f4 = 560/4 = 140mm aperture

Allowing for actual focal lengths and true apertures and the fact that we are referring to an apparent aperture/hole size when viewed from the subject the numbers are essentially the same so the maths holds true. To be clear, the aperture inside the lens is not actually 143mm wide, it just looks like it is to light coming from the subject direction.

All you need to do to get your calculation of the optimal background separation is compare the focal length (which is magnification given that you are shooting from a set distance) and the (apparent) aperture. The larger the result of the sum the more separation you will get.

600mm f6.3 = 600/6.3 = 95mm apparent aperture
560 f4 = 560/4 = 140mm apparent aperture

So in your situation, shooting from the same distance but with a larger apparent aperture, you will get more separation with a 400mm f2.8 and a TC than with a 600mm f6.3.

Of course the quality of the glass and the way it renders background blur can impact the results to a very large degree but the truth is you are going 'the right way' in that nobody makes a bad 400mm f2.8.

Addendum: The 600 f4 would give you a 150mm apparent aperture so for your shooting situation that would give you even more separation. Only you could decide on the cost/value/image equation though!
Thank you so much. That was a very clear explanation and exactly what i wanted to know.

With this information i think the 600 f4 is the right choice for me.

I will have to save a lot for it but in a few years i will hopefully own it.
 
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AlanF

Hands. Face. Space.
CR Pro
Aug 16, 2012
7,330
7,103
DOF is determined by two things, subject magnification and absolute aperture (that is the actual size of the hole of the aperture not the f stop number).

If you keep the distance the same the magnification is bigger with the TC and the aperture is the same physical size, ergo less DOF. If you moved back so the magnification was the same with the TC then the DOF would be the same because the aperture hole is.

Did I explain that in a way it is understandable?
No. You didn't state qualitatively or quantitatively how it is determined by subject magnification or absolute aperture. Is it linearly proportional, inversely proportional, logarithmic or what?
 
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RBS

EOS M50
Feb 6, 2020
26
30
With this information i think the 600 f4 is the right choice for me.

I will have to save a lot for it but in a few years i will hopefully own it.
The EF 600 f4 is a great lens like its other prime siblings. Keep in mind that with this series, when used with a 1.4X the optical performance is still quite good and even a 2X works pretty well.

The first attached photo is with the EF 400 f2.8 plus 2X, the second is with the bare EF 800 f5.6 and both were taken with a 1DX III and cropped to 12% of the sensor area. You always lose IQ with a converter but with some glass the bare IQ is good enough that the results may still be acceptable with a converter and sometimes this makes sense if you need a combination of focal lengths. If 560 f4 works for you, the EF 400 f2.8 plus 1.4X makes a very high quality and fairly versatile setup. Of course DoF is very shallow with this aperture and focal length.


AS0I4075.JPG
AS0I5985.JPG
 
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