Teleconverter f/stop

[scottvd]

Was curious if when using a teleconverter such as the TC-14E II which makes the following statement: "This converter will increase the focal length of the lens by 1.4x, and reduces the lens aperture by one f/stop." - Does that for all practical purposes reduce your aperture by a stop - so increases the depth of field and reduces amount of light let in, or just one of those two characteristics, or something else?

Thanks,
`S

[Pa]

going on my experience with a t/c, when it's cloudy it slows the lens down considerably.....i think the images are a little soft....to counteract this i turn the iso up to allow faster shutter speeds

i would say your assumption is spot on

[Antsl]

Because a tele convetor has the job of magnifying an image it has to do this at the expense of brightness of image..... it is like shining a torch on a wall, if you want to make the spatch of light bigger it will become darker. It is all related to the inverse square law.

Yes.... when you lose a stop of light you are losing a stop of light. The image is half as bright and you will need to increase ISO or drop Shutter Speed one stop to compensate.

As for Depth of Field... assuming the subject to camera distance does not change the Depth of Field will increase simply because you are magnifying the image and seeing more softness about the foreground and background.

[gstark]

As for Depth of Field... assuming the subject to camera distance does not change the Depth of Field will increase simply because you are magnifying the image and seeing more softness about the foreground and background.

Ants,

I think you may have meant decrease here ... more softness ... less sharpness, therefore less DoF.

I'm not sure that this directly answers Scott's original question though, which I think was framed more in the context of the master lens's aperture and DoF characteristics.

Within that context, and as you know, as you change your aperture setting, your available and actual DoF changes in a predictable manner.

With a TC, what happens is the light available to you for exposure changes to the value of (in this instance) a one stop reduction in EV, and thus it has been expressed as "reduces the lens aperture by one f/stop.".

Normally, when shooting at f/8, if you move to f/11 (effectively reducing the lens aperture by one f/stop) then you might expect to see an increase in the apparent "in-focus" area of the image because of an increase in the DoF.

With the TC, no actual change in f-stop has occurred: you have just reduced the available light for shooting by a similar value, and as already discussed, you may need to compensate by increasing ISO or reducing shutter speed. And thus, all things being equal, there will be no direct effect (from using the TC) on your DoF within the context that Scott has described.

There will certainly be effects on DoF as described by Antsl, but these come about through different characteristics - as put by Antsl - rather than those put forward in the original question.

[losfp]

This issue has always intrigued me a little.

As I understand it, DOF is influenced by 3 things:

1) Lens actual focal length
2) Distance to subject
3) Aperture

When you stick a teleconverter (in this example, let's go with a 1.4x TC on a 300/2., Two things "appear" to change: Your widest aperture goes from f/2.8 to f/4 and your focal length goes from 300mm to 420mm.

Now, assuming you stop down to, let's say, f/5.6 both before and after fitting the TC and take two shots of the same subject the same distance away:

1) 300mm, f/5.6
2) "420mm", "f/5.6"

Will there be a difference in depth of field between the two? Or is the second simply a magnification of the first, in which case the DOF would appear to be the same.

The second question: when you fit a teleconverter and the camera stops you from adjusting the aperture higher than f/4 in this case.. When you have it set at f/4, is the lens ACTUALLY set at f/4, or is it set at f/2.8, and the EXPOSURE is that of f/4?

Or, to put it another way... When I stick this TC on, will I have a lens that will act, for all intents and purposes just like a 420/4, or will it simply still be a 300/2.8, but magnified and cropped?

[gstark]

Des,

That's an interesting way of framing this question, and perhaps a more clear way of expressing the problem.

Or, to put it another way... When I stick this TC on, will I have a lens that will act, for all intents and purposes just like a 420/4, or will it simply still be a 300/2.8, but magnified and cropped?

What about a third option? 420mm, f/2.8, BUT 1EV less light transmission through the glass?

Compare this with sticking a 1EV ND filter on the front of the lens? Would you be expecting DoF to change?

Same deal.

Am I making sense, or should I just wipe the drool from my jeans?

[bwhinnen]

To my limited knowledge it is only changing the widest aperture. I.e. from the f2.8 to an f4. Once you dial in f5.6 with the camera that is the aperture you are setting, I don't believe it is changing the lens to f4 and reporting f5.6 I believe it is changing the lens to f5.6 and reporting f5.6.

Does that make sense?

[losfp]

Des,



What about a third option? 420mm, f/2.8, BUT 1EV less light transmission through the glass?

Ahhhh....

So the compression, perspective, DOF etc will be that of a 420/2.8, but the EXPOSURE will be that of a 420/4.

So what'chasayinghere... is that the setting on the camera is really just indicative of the exposure, not the actual aperture opening - I suppose much the same way that for some 3rd party TCs, you can actually set the aperture to f/2.8, and the only real difference is dodgy metering in-camera.

I think that makes sense. You are after all losing light through the TC. Stands to reason that if you know you are losing light through it, you would want f/2.8 at the lens before it becomes f/4 exposure wise at the camera.

Clear as mud!

[ATJ]

I believe you need to go back to the definition of f/stop. The f/stop is the ratio of the focal length of the lens to the diameter of the hole (aperture) through which light passes. The amount of light passing through the lens will be governed by both factors. Increase the focal length, decrease the amount of light. Increase the size of the hole, increase the amount of light.

Now, when you put a TC behind a lens you are increasing the focal length but you are not changing the size of the hole (which is governed by the lens). This means the f/stop will will move to a larger number, less light passing through the lens.

As to depth of field, increasing the f/stop will result in more depth of field, but at the same time increasing the focal length will decrease the depth of field. The net effect will be dependent on distance to subject.

I believe that if you put a 200mm lens on a 1.4x converter and set the lens to f/8, you will have around the same depth of field as you would if you were using a lens at 280mm at f/11. Note that the 1 f/stop is only an approximation.

[DaveB]

So the compression, perspective, DOF etc will be that of a 420/2.8, but the EXPOSURE will be that of a 420/4.

Nope.

A 300/2.8 lens with a 1.4x TC behaves in all ways like a 420/4 lens. Yes, if you look at the formula for calculating DOF, you'll see that you would have more DOF, but only if the magnification was the same. Effectively the matching increase in focal length can cancel out the apparent change in DOF.

[Biggzie]

If you focus on an object, and you set the aperture to give you a DOF that you want around this object. When you put a Teleconverter behind the lens, this lens is still passing on the same DOF to the teleconverter which then magnifies the same DOF to the sensor.
The extra glass takes out some quality and light from the image.
As Gary said it is like putting a ND filter on reducing the light by half.

[losfp]

Errrrr... obviously I need to think about this one a bit more!

ATJ, what you say makes a lot of sense - same size hole, longer focal length, therefore smaller f number.

Furthermore, this means that the DOF etc will follow this line of thinking. (Thanks DaveB)

Sorry for the thread hijack, Scott This has just been bugging me for a while.

[DaveB]

As Gary said it is like putting a ND filter on reducing the light by half.

No, it is distinctly NOT like that. Andrew's explanation is the best technical one so far.
The significant differences between a 300mm/2.8 with a 1.4x TC and a 420mm/4 lens are weight/size and optical quality. That's it.

To demonstrate the effect, if someone has a 200mm (or 300mm) lens with a 2x TC, and a 400mm (or 600mm) lens, it is fairly easy to demonstrate that a 300mm/4 lens with a 2x TC is the same as a 600mm/8 lens (just stop each lens down to the appropriate aperture and use DOF preview).

[gstark]

Dave,

Ok ...

I think that there's more than a small element of confusion creeping into this discussion.

On the one hand, there's the DoF as expressed by Andrew (ATJ) - and which is basically what you're referring to as well, but there's also the loss of light imposed upon the sensor due to the introduction of the TC into the light path.

So the compression, perspective, DOF etc will be that of a 420/2.8, but the EXPOSURE will be that of a 420/4.

Actually, those were Des's words that you quoted, not mine.

I think that Andrew's explanation - going back to the basics and the definition of how an f/stop is calculated - explains the technical aspects of what Scott might see in an image best. Andrew is saying what you're saying, btw, but he's referring to the scientific calculation behind what's occurring.

But none of that explains the (additional?) fact that you will experience the loss of some light, in terms of your exposure, as well.

But your in-camera meter will recognise this automatically, and and should cope regardless.

So what'chasayinghere... is that the setting on the camera is really just indicative of the exposure, not the actual aperture opening - I suppose much the same way that for some 3rd party TCs, you can actually set the aperture to f/2.8, and the only real difference is dodgy metering in-camera.

If you were using an external metering system, I'd say yes. But as noted above, your camera - the internal TTL metering system - will be reading the actual - the reduced - amount of light actually reaching the sensor, and it should be able to cope.

Let me try to give an example ..

Actual EV suggests f/8 and 1/250 for ISO 200 with whatever lens you have mounted, using your in camera meter. An eternal meter would offer the same readings.

Noting my typo above, so too would an external meter.

Introduce a TC with a 1 stop factor, and your in-camera meter should now offer you (using A on Nikon, or Av on Canon) values of f/8 and 1/125 for ISO 200 for that exact same scene, under the exact same lighting conditions.

Note that your external meter will still offer the original settings of f/8 and 1/250.

That, I think, addresses the exposure side of things.

If you then make an image with those settings, the DoF that you would see in your image would be that of your master lens, multiplied by the TC factor, for the selected aperture.

So, for 300mm and 1.4, you would see DoF as if the lens was 420mm shooting at /f8.

And if you were shooting wide open, the maximum aperture available to you - by virtue of the way that aperture is calculated (see Andrew's post) - would be f/4, because your focal length has changed, but the size of the hole through which you're shooting has not.

The original question is actually very complex, and the answers even more so.

[gstark]

As Gary said it is like putting a ND filter on reducing the light by half.

No, it is distinctly NOT like that. Andrew's explanation is the best technical one so far.

it depends upon whether you're trying to explain the effect of a TC upon exposure, or upon DoF, or both.

If it's simply exposure, then my analogy as quoted above is adequate, and I think that was the context within which I introduced that comparison.

But if we go beyond exposure, then yes, Andrew's explanation is spot on.

The problems in understanding this occur because aperture affects exposure, as does light transmission, but aperture also affects Dof, regardless of light transmission.

As I said, it's very complex. Somehow we have to try to bring it all together.

[ATJ]

So, for 300mm and 1.4, you would see DoF as if the lens was 420mm shooting at /f8.

Gary,

What is the f/stop on the lens in this example? If you say the 300mm lens is set to f/5.6, then I agree with you.

[DaveB]

Let me try to give an example ..

Actual EV suggests f/8 and 1/250 for ISO 200 with whatever lens you have mounted, using your in camera meter. An eternal meter would offer the same readings.

Noting my typo above, so too would an external meter.

Introduce a TC with a 1 stop factor, and your in-camera meter should now offer you (using A on Nikon, or Av on Canon) values of f/8 and 1/125 for ISO 200 for that exact same scene, under the exact same lighting conditions.

Note that your external meter will still offer the original settings of f/8 and 1/250.

Actually, when set up the way the manufacturer recommends, a Canon camera would be showing the same as the external meter. It's meant to simplify things for the user, but I suppose in this case it's complicating things by adding another factor.
A "proper" TC for EF/EOS lenses (e.g. the Canon ones and the Kenko Pro300 series) indicate to the lens what type of TC they are, and if the lens is one of those designed for TCs (i.e. the Canon ones with the extra pins on their interface) it will multiply both the focal length and the aperture reported to the camera so that the values the camera sees are the effective values.
For example, with a Canon 300mm/2.8 lens:

• with an EF 1.4x, it will be reported to the camera as a 420mm/4
• with an EF 2x, it will be reported to the camera as a 600mm/5.6
• with a 3rd-party TC (without those extra pins) or if you've obscured the extra pins with tape or an extension tube, it will be reported as a 300mm/2.8

With an EF 1.4x, if your camera tells the lens to use f/8 the aperture will be opened to the same physical size as f/5.6 without the TC. It takes the TC into account for all operations.

Confused yet? I'm not sure how the Nikon system works with TCs in this regard.

[bwhinnen]

Let me try to give an example ..

Actual EV suggests f/8 and 1/250 for ISO 200 with whatever lens you have mounted, using your in camera meter. An eternal meter would offer the same readings.

Noting my typo above, so too would an external meter.

Introduce a TC with a 1 stop factor, and your in-camera meter should now offer you (using A on Nikon, or Av on Canon) values of f/8 and 1/125 for ISO 200 for that exact same scene, under the exact same lighting conditions.

Note that your external meter will still offer the original settings of f/8 and 1/250.

Actually, when set up the way the manufacturer recommends, a Canon camera would be showing the same as the external meter. It's meant to simplify things for the user, but I suppose in this case it's complicating things by adding another factor.
A "proper" TC for EF/EOS lenses (e.g. the Canon ones and the Kenko Pro300 series) indicate to the lens what type of TC they are, and if the lens is one of those designed for TCs (i.e. the Canon ones with the extra pins on their interface) it will multiply both the focal length and the aperture reported to the camera so that the values the camera sees are the effective values.
For example, with a Canon 300mm/2.8 lens:

• with an EF 1.4x, it will be reported to the camera as a 420mm/4
• with an EF 2x, it will be reported to the camera as a 600mm/5.6
• with a 3rd-party TC (without those extra pins) or if you've obscured the extra pins with tape or an extension tube, it will be reported as a 300mm/2.8

With an EF 1.4x, if your camera tells the lens to use f/8 the aperture will be opened to the same physical size as f/5.6 without the TC. It takes the TC into account for all operations.

Confused yet? I'm not sure how the Nikon system works with TCs in this regard.

Makes sense, this is what I was thinking after I hit submit on my post. I thought the Nikon / Canon specific TC's would be smart enough to interpret the aperture on the Lens compared to what it would be through the TC...

[DaveB]

I thought the Nikon / Canon specific TC's would be smart enough to interpret the aperture on the Lens compared to what it would be through the TC...

Just to be clear: in the Canon case it's the lens that does the special treatment. The TC just holds down a pin to indicate if it's a 1.4x or 2x TC (no other options seem to be available in the current interface design) and the lens does the calculations. That's why I mentioned above that it only works this way with Canon lenses designed for TCs (no, Sigma lenses designed for TCs don't do it).