TCs and DOF

This may have been covered before, I have checked the TC thread but can't find anything specific to this.

Now I might get around to actually testing this to see what happens but thought I would ask in case someone knows.

The question is what impact does a TC have on DOF?

My 70-200 f/2.8 looses two stops when I am using my 2x making it a 140-400 f/5.6.

If I take a shot at 200mm at f/2.8 on without TC, and one at 200mm (100mm on the lens) with TC at f/5.6, is there a difference in DOF? I know if I stopped down the lens without the TC to f/5.6 there would be, but with the TC the lens is still effectively wide open.

Does this make sense?

FlyingPete wrote in post #17115009
This may have been covered before, I have checked the TC thread but can't find anything specific to this.

Now I might get around to actually testing this to see what happens but thought I would ask in case someone knows.

The question is what impact does a TC have on DOF?

My 70-200 f/2.8 looses two stops when I am using my 2x making it a 140-400 f/5.6.

If I take a shot at 200mm at f/2.8 on without TC, and one at 200mm (100mm on the lens) with TC at f/5.6, is there a difference in DOF? I know if I stopped down the lens without the TC to f/5.6 there would be, but with the TC the lens is still effectively wide open.

Does this make sense?

Wide open doesn't matter. It doesn't matter how you got to f/5.6. Any way to get to 200/5.6 will give you the same dof as long as you keep the sensor size fixed.

FlyingPete wrote in post #17115009
This may have been covered before, I have checked the TC thread but can't find anything specific to this.

Now I might get around to actually testing this to see what happens but thought I would ask in case someone knows.

The question is what impact does a TC have on DOF?

My 70-200 f/2.8 looses two stops when I am using my 2x making it a 140-400 f/5.6.

If I take a shot at 200mm at f/2.8 on without TC, and one at 200mm (100mm on the lens) with TC at f/5.6, is there a difference in DOF? I know if I stopped down the lens without the TC to f/5.6 there would be, but with the TC the lens is still effectively wide open.

Does this make sense?

Heya,

The problem is that you're assuming that the TC drops aperture by 2 stops, when it doesn't. It halves the light twice. It's not stopping down. It's aperture is still F2.8. It's just half the light, and half that light, but still at F2.8. The TC however will increase your distance to the target, for the same frame up. That physical distance will affect depth of field.


Very best,

MalVeauX wrote in post #17115028
Heya,

The problem is that you're assuming that the TC drops aperture by 2 stops, when it doesn't. It halves the light twice. It's not stopping down. It's aperture is still F2.8. It's just half the light, and half that light, but still at F2.8. The TC however will increase your distance to the target, for the same frame up. That physical distance will affect depth of field.


Very best,

This is what I was wondering was happening.

I always tend to shoot wide open with the 70-200 to I can isolate the subject I am taking photos of, was hoping that I was not increasing DOF by using the TC at effective focal lengths 200 and under.

FlyingPete wrote in post #17115032
This is what I was wondering was happening.

I always tend to shoot wide open with the 70-200 to I can isolate the subject I am taking photos of, was hoping that I was not increasing DOF by using the TC at effective focal lengths 200 and under.

Heya,

Since you're shooting from farther away, you increased distance from you to your target, so the depth of field increased due to that physical distance.

Very best,

MalVeauX wrote in post #17115028
Heya,

The problem is that you're assuming that the TC drops aperture by 2 stops, when it doesn't. It halves the light twice. It's not stopping down. It's aperture is still F2.8. It's just half the light, and half that light, but still at F2.8. The TC however will increase your distance to the target, for the same frame up. That physical distance will affect depth of field.


Very best,

Careful there - the aperture diameter isn't changed. But the focal length is doubled which makes the f-stop go from f/2.8 to f/5.6 because the original aperture was 200/2.8 = 71mm and when used with a 400mm system that gives 71/400 = 1/5.6.

So while the lens is still a 70-200/2.8, the system is a 140-400/5.6.

To stay at f/2.8, you would have needed a TC that could have been mounted to the front of the lens instead - and had a huge front element at least as large as 400/2.8 = 143mm.

pwm2 wrote in post #17115065
Careful there - the aperture diameter isn't changed. But the focal length is doubled which makes the f-stop go from f/2.8 to f/5.6 because the original aperture was 200/2.8 = 71mm and when used with a 400mm system that gives 71/400 = 1/5.6.

So while the lens is still a 70-200/2.8, the system is a 140-400/5.6.

To stay at f/2.8, you would have needed a TC that could have been mounted to the front of the lens instead - and had a huge front element at least as large as 400/2.8 = 143mm.

Thanks for clarifying the wording regarding the relationship to focal length. It's different than if he physically stopped down to F5.6, which is what it sounded like he was stating was happening and isn't.

Very best,

Two scenarios:

EF70-200 f/2.8 at 200mm f/2.8

And

EF70-200 f/2.8 + 2X TC at 200mm equivalent (lens at 100mm) f/5.6

Obviously less light is getting in so there will be a slower shutter to compensate, but will the DOF be the same in both scenarios?

FlyingPete wrote in post #17115090
Two scenarios:

EF70-200 f/2.8 at 200mm f/2.8

And

EF70-200 f/2.8 + 2X TC at 200mm equivalent (lens at 100mm) f/5.6

Obviously less light is getting in so there will be a slower shutter to compensate, but will the DOF be the same in both scenarios?

Your lens plus the extender is a totally different animal from the lens without the extender. This goes for both exposure and depth of field.

FlyingPete wrote in post #17115090
Two scenarios:

EF70-200 f/2.8 at 200mm f/2.8

And

EF70-200 f/2.8 + 2X TC at 200mm equivalent (lens at 100mm) f/5.6

Obviously less light is getting in so there will be a slower shutter to compensate, but will the DOF be the same in both scenarios?

Using the lens at 200/5.6 or using the lens at 100/2.8 + 2x TC (resulting in 200/5.6) gives the same DoF.

Using the lens at 200/2.8 or using the lens at 100/2.8 + 2x TC (resulting in 200/5.6) gives different DoF.

In the end 200/5.6 is 200/5.6 whatever combination of lens elements you use.

pwm2 wrote in post #17115065
Careful there - the aperture diameter isn't changed. But the focal length is doubled which makes the f-stop go from f/2.8 to f/5.6 because the original aperture was 200/2.8 = 71mm and when used with a 400mm system that gives 71/400 = 1/5.6.

So while the lens is still a 70-200/2.8, the system is a 140-400/5.6.

To stay at f/2.8, you would have needed a TC that could have been mounted to the front of the lens instead - and had a huge front element at least as large as 400/2.8 = 143mm.

But you didn't actually change the focal length, you doubled the size of the image that's projected onto the sensor. You're right though, it would stay the same as a 100mm f/2.8 with a 2X crop factor gives the same DoF and background compression as a 200mm f/5.6.

vengence wrote in post #17115224
But you didn't actually change the focal length, you doubled the size of the image that's projected onto the sensor. You're right though, it would stay the same as a 100mm f/2.8 with a 2X crop factor gives the same DoF and background compression as a 200mm f/5.6.

The doubling of the size did happen because the focal length did double. A 2x TC is a focal-length doubler. So 100mm got doubled to 200mm. While f/2.8 became f/5.6 because the aperture diameter stayed the same.

vengence wrote in post #17115224
But you didn't actually change the focal length, you doubled the size of the image that's projected onto the sensor.

Attaching a 2X tele-extender to a lens does, in fact, double the focal length. No, the lens' focal length does not change but the SYSTEM (the lens plus the tele-extender) has a focal length that's twice that of the "naked" lens.

As stated, the maximum aperture diameter does not change when adding a tele-extender so the minimum f-stop number is two stops greater number than for the "naked" lens.

I'm not sure what the conclusion is here, but I would just say that from my experience with using TC's + my 17mm TS-E and then comparing that with a zoom set to the appropriate FL, that I think I disagree with much of what has been said here - but I think there is some confusion and it may be everyone is basically saying the same thing.

Now with the TS-E lens the camera does not recognize the TC, so reports whatever the lens is set at, not lens+TC. Maybe I'm wrong, but is seemed to me that putting a x2 TC on my 17mm TS-E and setting the aperture on the TS-E at f8 gave me the same DOF as using my 17-40 at 35mm and an aperture of f16.

Of course with lenses that report the TC, it may appear different. For instance if I put a TC on my 70-200 set at f4, the camera will report f5.6 (even though the physical aperture has not changed) - so it will be the same as setting the 70-200 at 1/1.4 the FL at f5.6. i.e since the camera reports the TC, 150mm @f5.6 will look the same with or without the TC.

Perhaps this is where the confusion is coming from.

DOF is really determined by aperture and magnification, not by FL directly although that obviously factors into the magnification part.

OK real world testing required, here are three shots to test this scenario: