Does converters mean less light?

snakebunk

My idea of how a converter works is that it is an optical crop, for example a 1.4x converter turns your fx camera into a dx camera (roughly). The converter does this by magnifying the image so that a smaller part hits the sensor and more light is lost outside of the sensor. This has lead me to the thought that you get less light on the image when you use a converter compared to using a longer lense without converter.

Observe that I am not talking about the reduction of maximum aperture, which is due to the optical law of focal length / opening lense diameter = maximum aperture. And I am not talking about the small differences in transmission between lenses.

To make it clear I ask the question using an example: If I shoot an image using a 600/4 lense at 5.6 and then shoot the exact same image with equal settings using a 300/2.8 lense with a 2x converter, will the first image be lighter?

This may be basic stuff that I should know, but it has been puzzling me for so long that I must ask the question. If anyone has the equipment to examplify with photographs, it would be fantastic. But any thoughts are appreciated.

Ironheart

A Nikon 1.4x teleconverter gives a 1 stop penalty
A Nikon 1.7x teleconverter gives a 1.5 stop penalty
A Nikon 2.0x teleconverter gives a 2.0 stop penalty

There is no additional penalty, the above is enough and to answer your question, no the example you give are actual equivalents.

snakebunk

@Ironheart: Thank you, that is exactly what I've been thinking but I haven't found the information anywhere (and I've read a few books about bird photography). Important stuff to know if you for example are choosing between a 800/5.6 or a 600/4 + 1.4x tc.

@Ironheart II: You changed your answer. Do you mean that the images in my example will be equal? What do you mean with penalty? (I know of course that the maximum aperture changes)

tcole1983

He says penelty, but it is whatever you want to call it. You lose two stops of light by using the 2x TC.

Besides light reduction it seems to be common thought/results that the larger the TC the worse the images. IE the 1.4 TC produces better images than the 2.0 TC.

Ironheart

I was trying to be more clear. Ha! Yes, the images in your example will be equal, in terms of final EV (amount of light).

snakebunk

Ok! Thank you for your answers! I really must find a good book about optics. I am still puzzled exactly how a converter works.

Paperman

My idea of how a converter works is that it is an optical crop, for example a 1.4x converter turns your fx camera into a dx camera (roughly). The converter does this by magnifying the image so that a smaller part hits the sensor and more light is lost outside of the sensor.

That would have decreased your Mps/resolution which you know is not the case.

Just think of it as an additional extra few elements in your tele lens with which it would have been twice the focal length. Instead, they are added to the end ....

snakebunk

@Paperman: I mean that the converter makes the image circle larger, which makes the sensor relatively smaller, hence effectively turns an fx camera to a crop camera. I agree that it doesn't affect the resolution.

haroldp

It is a magnifying element. It effectively increase the focal length of the lens just like the magnifying groups in a true tele.

Increased focal length and the same actual physical aperture (and Objective), means a smaller f-stop which is the inverse of the effective aperture and focal length.

or ... It spreads the same incoming light over a wider image circle reducing intensity.

.... simple no ?

snakebunk

It spreads the same incoming light over a wider image circle reducing intensity.

Well put, this is what bothers me. Given the same aperture, is the light intensity the same?

Or, doesn't a 400/2.8 with a 2x converter waste more light than a 800/5.6?

Ironheart

It spreads the same incoming light over a wider image circle reducing intensity.

Well put, this is what bothers me. Given the same aperture, is the light intensity the same?

Or, doesn't a 400/2.8 with a 2x converter waste more light than a 800/5.6?

They are 100% equivalent.

snakebunk

I think I may have figured it out. Could it be that because the 400/2.8 has a larger angle (field of view) it collects more light, and therefore makes up for the light it wastes using a teleconverter compared to a 800/5.6?

WestEndFoto

There is another potential loss. If the longer lens achieves a certain resolution that is no better than the sensor, then putting a teleconverter on it will reduce resolution because any smaller area on a lens produces less resolution than a larger area.

Whether this is an issue can only be really determined by considering a specific lens camera (sensor) combination and is not an easy experiment to perform.

For a simple example, you may get 36 megapixels of effective resolution on the 800 5.6 at 5.6, less than 36 megapixels of effective resolution on a 400 2.8 with a 2x teleconverter (equivalent to 800 at 5.6) because the centre of the lens does not achieve 36 megapixels while the entire lens does.

As a guideline, I doubt it matters much on the top line superteles and the specific example that I used, but anything less and it is likely an issue.

Pistnbroke

Try a DX lens on your 1.4 converter and fit to an FX camera ..fills the frame ....Which proves the converter increases the diameter of the image circle .This spreads the image information over a bigger area hence the reduced resolution.

WestEndFoto

Try a DX lens on your 1.4 converter and fit to an FX camera ..fills the frame ....Which proves the converter increases the diameter of the image circle .This spreads the image information over a bigger area hence the reduced resolution.

This point is not often understood well. It is why the benefits of FX over DX will become more apparent as sensor resolution increases.

heartyfisher

Try a DX lens on your 1.4 converter and fit to an FX camera ..fills the frame ....Which proves the converter increases the diameter of the image circle .This spreads the image information over a bigger area hence the reduced resolution.

This point is not often understood well. It is why the benefits of FX over DX will become more apparent as sensor resolution increases.

I am an advocate of DX but this is true. I often use DX and FX lenses on FX and DX cameras. and the DXO PMP carries forward from DX to FX if used with Teleconverter. But I dont use DX lenses with Tele Converters often and even if I do, I understand the equal resolution that I get. Having said that I have compared several equialent DX and FX lenses
eg 18-140 vs 28-300.. ie 18-140+1.4TC vs 28-300 both on D610. My conclusion is resolution wise they are similar, each better at different focal lengths. However, the loss of one stop of light to the already not very bright F5.6 kits lense makes it less desirable, but still fun. I find that it is better to use the DX kit lense without TC on the FX camera because the PMP result is the same ie. you can just crop, and you dont lose the light(aperture) and autofocus weakness of adding a TC. Note that the PMP of the 18-140 is superior to the 28-300. Ie you get sharper images using the 18-140 than using the 28-300, both natively on FX. The 18-200 has similar PMP to the 28-300. Ie using the 18-200 on the the D610 gets you better range!! ie you get close to 24-300 equivalent focal lengths, you just need to crop when not in dx mode :-)

Pistnbroke

It is a shame Nikon don't make a better wedding lens ie 24-210 FX . Tokina made one but without VR it was too risky to use ( at a wedding)

I am sure we have totally confused the OP by now ...

Summary Don't use a 1.4x unless you have to to get the shot and use the same brand as your lens.

snakebunk

Try a DX lens on your 1.4 converter and fit to an FX camera ..fills the frame ....Which proves the converter increases the diameter of the image circle .This spreads the image information over a bigger area hence the reduced resolution.

Thanks, this is very informative for me, and goes in line with how I think it works.

What would happen if we perform the criminal act of shooting with maximum aperture even though we have a converter attached? (it should be possible with a manual aperture ring)

Or, will the aperture be smaller as the converter increases the focal length even though the physical aperture opening stays the same?

Pistnbroke

If you spread the light over twice the area it must be half as bright... and an 18mm DX lens gives the same view on Fx + 1.4 as it did on Dx so 18mm equals 28
In truth you need a 1.5 converter to fully fill the frame but those are a bit rare and old.

I was just looking at the Metabones converter which takes the FX image from an FX lens and concentrates it down to 4/3 thus increasing the brightness..interesting world

snakebunk

Yes, you are right, and that is why I asked the question in the first place. What I didn't understand is that the aperture gets smaller when the focal length increases. I think I understand now that the same physical aperture opening can result in F4 on a 600mm lense and F5.6 on an 840mm lense (600 + 1.4 tc).

Ironheart

Yes, you are right, and that is why I asked the question in the first place. What I didn't understand is that the aperture gets smaller when the focal length increases. I think I understand now that the same physical aperture opening can result in F4 on a 600mm lense and F5.6 on an 840mm lense (600 + 1.4 tc).

Precisely.

haroldp

Many modern lenses work this way without external adapters.

True telephoto's where the physical length is shorter than the focal length use magnifying elements (groups) much like a TC , but whose optical formula is tailored to the lens.

Wide angle lenses (less than 58mm focal length) for FF 35mm format SLR's, are retro focus (distance from optical center to focal plane (film or sensor) is greater than the focal length to clear the mirror, essentially use reducing elements (like the metabones), but whose optical formula is tailored to the lens.

Done right, this does not always result in a loss of resolution, and the magnifying elements as part of the optical formula may actually correct for some abberations.

... H