Should I change Exposure when using a 2x Tele Converter

[benjjj6]

Hi
I have an old Minolta SRT-100b film camera and a f3.8 70-150mm Vivitar lens, I also have a 2x Tele Converter. The built in through the lens light meter no longer works so I need to use a hand held light meter.

I know that using the 2x Tele Converter will affect light to the camera but I don't know by how much, or how to compensate.

I have been told that it will affect it by 2 stops, so as long as I don't use the first to stops on the lens then the picture will be exposed correctly. So I don't need to compensate at all apart from not using the first two stops, is this true?

Thanks,
Ben

 

[cgipson1]

A 2x TC can kill anywhere from 2 to 3 stops of light, on average. Try adding one, two and three stops of light... same subject same light, and see what you think when they are developed. Looks at the density / exposure of the film, rather than a print...

 

[amolitor]

Essentially what a 2x teleconverter is doing is making your lens twice as long, right? But it generally doesn't do anything to the aperture blades, so the size of the opening is remaining the same. Given that aperture is focal length divided by the diameter of the aperture, your actual aperture expressed will be twice as big as indicated.

Say you have a 200mm lens, set to f/5.6. The actual diameter of the aperture opening is: 35.7mm (200mm / 35.7 = 5.6).

Now add on a 2x teleconverter. The lens is now 400mm long. The aperture, set to the 5.6 reading on the lens, is still 35.7mm across, right?

400mm/35.7 = 11.2

So mathematically it's two stops, exactly as Charlie indicated, and I'm pretty sure that there are some other losses that can occur in a TC so 2-3 stops in the right number, for sure.

Now you should be able to do it for a 1.4x TC or anything!

 

[benjjj6]

Say you have a 200mm lens, set to f/5.6. The actual diameter of the aperture opening is: 35.7mm (200mm / 35.7 = 5.6).

Now add on a 2x teleconverter. The lens is now 400mm long. The aperture, set to the 5.6 reading on the lens, is still 35.7mm across, right?

400mm/35.7 = 11.2

So mathematically it's two stops, exactly as Charlie indicated, and I'm pretty sure that there are some other losses that can occur in a TC so 2-3 stops in the right number, for sure.

Now you should be able to do it for a 1.4x TC or anything!

Thank you, I am still a little bit confused. Could you just clear one thing up for me, so if my hand held light meter says f/11 should I set the lens with the teleconverter to f/11 or should it be 2 stops higher?

 

[cgipson1]

Say you have a 200mm lens, set to f/5.6. The actual diameter of the aperture opening is: 35.7mm (200mm / 35.7 = 5.6).

Now add on a 2x teleconverter. The lens is now 400mm long. The aperture, set to the 5.6 reading on the lens, is still 35.7mm across, right?

400mm/35.7 = 11.2

So mathematically it's two stops, exactly as Charlie indicated, and I'm pretty sure that there are some other losses that can occur in a TC so 2-3 stops in the right number, for sure.

Now you should be able to do it for a 1.4x TC or anything!

Thank you, I am still a little bit confused. Could you just clear one thing up for me, so if my hand held light meter says f/11 should I set the lens with the teleconverter to f/11 or should it be 2 stops higher?

Does your hand held light meter know about the TC???? If not, there is no way it would automatically adjust for it right? You need to overexpose two stops based on what the meter is telling you. That should put you in the ball park! The TC would cause you to be two stops underexposed (approximately)... so you need to correct for it.

 

[cgipson1]

Essentially what a 2x teleconverter is doing is making your lens twice as long, right? But it generally doesn't do anything to the aperture blades, so the size of the opening is remaining the same. Given that aperture is focal length divided by the diameter of the aperture, your actual aperture expressed will be twice as big as indicated.

Say you have a 200mm lens, set to f/5.6. The actual diameter of the aperture opening is: 35.7mm (200mm / 35.7 = 5.6).

Now add on a 2x teleconverter. The lens is now 400mm long. The aperture, set to the 5.6 reading on the lens, is still 35.7mm across, right?

400mm/35.7 = 11.2

So mathematically it's two stops, exactly as Charlie indicated, and I'm pretty sure that there are some other losses that can occur in a TC so 2-3 stops in the right number, for sure.

Now you should be able to do it for a 1.4x TC or anything!

NICE! The teach a man to fish approach!!!

 

[benjjj6]

Can the EV compensation on the meter do this for me? ie. is changing the compensation to +2 EV the same as overexposing two stops? (or should that be -2?)

 

[cgipson1]

Can the EV compensation on the meter do this for me? ie. is changing the compensation to +2 EV the same as overexposing two stops? (or should that be -2?)

+2... that should do it! The Camera with the TC is already -2... so you need to compensate

 

[KmH]

Yes, in so far that 1 EV = 1 stop.

 

[shefjr]

Essentially what a 2x teleconverter is doing is making your lens twice as long, right? But it generally doesn't do anything to the aperture blades, so the size of the opening is remaining the same. Given that aperture is focal length divided by the diameter of the aperture, your actual aperture expressed will be twice as big as indicated.

Say you have a 200mm lens, set to f/5.6. The actual diameter of the aperture opening is: 35.7mm (200mm / 35.7 = 5.6).

Now add on a 2x teleconverter. The lens is now 400mm long. The aperture, set to the 5.6 reading on the lens, is still 35.7mm across, right?

400mm/35.7 = 11.2

So mathematically it's two stops, exactly as Charlie indicated, and I'm pretty sure that there are some other losses that can occur in a TC so 2-3 stops in the right number, for sure.

Now you should be able to do it for a 1.4x TC or anything!

What I would like to know is where to learn this type of information. Is there a reference book or an online website. I can tell you I don't believe it's in the owners manual for the d7k.

 

[amolitor]

benjj6, you should set the entire lens assembly to f/11 in that case.

The reading ON the lens will read f/5.6 at that point. When you've got the lens itself set to one f-number, the whole lens setup is ACTUALLY at twice that number.

Your meter tells you what ACTUAL f-number you should be using with your shutter speed and ISO. So set the lens+TC setup so that it's ACTUAL number is f/11, which means that the reading on the lens itself has to be 5.6.

Does that make any sense?

 

[benjjj6]

I did a quick test to see if this worked, so I took a reading and then set my light meter compensation to +2 and measured the same spot, given that the shutter speed and ISO were the same the aperture changed from f/5.6 to f/11 when EV was set to +2. This has confused me since you said that I need to overexpose by two stops to compensate, but if +2 EV is causing the aperture to change from 5.6 to 11 then that seems to be underexposing the picture by 2 stops, am I right?

 

[benjjj6]

benjj6, you should set the entire lens assembly to f/11 in that case.

The reading ON the lens will read f/5.6 at that point. When you've got the lens itself set to one f-number, the whole lens setup is ACTUALLY at twice that number.

Your meter tells you what ACTUAL f-number you should be using with your shutter speed and ISO. So set the lens+TC setup so that it's ACTUAL number is f/11, which means that the reading on the lens itself has to be 5.6.

Does that make any sense?

Yes I think so, so if the light meter says f/11 because I need to overexpose by 2 stops (because of the 2x tele converter) the lens itself should be set to two stops lower, ie. f/5.6

 

[Helen B]

benjj6, you should set the entire lens assembly to f/11 in that case.

The reading ON the lens will read f/5.6 at that point. When you've got the lens itself set to one f-number, the whole lens setup is ACTUALLY at twice that number.

Your meter tells you what ACTUAL f-number you should be using with your shutter speed and ISO. So set the lens+TC setup so that it's ACTUAL number is f/11, which means that the reading on the lens itself has to be 5.6.

Does that make any sense?

Yes I think so, so if the light meter says f/11 because I need to overexpose by 2 stops (because of the 2x tele converter) the lens itself should be set to two stops lower, ie. f/5.6

You seem to have got it, though it isn't really 'overexposure'. As amolitor explained, the f-number marked on the lens is no longer correct when a TC is placed behind the lens (it can be correct for TCs designed to go in front of the lens). Just as the focal length is changed (in this case doubled), so is the f-number. Double the focal length marked on the lens, double the f-number marked on the lens.

Once you understand that f-number = focal length/diameter of the entrance pupil* it all falls into place logically. A TC placed behind the lens cannot affect the diameter of the entrance pupil. A TC placed in front of the lens can alter the diameter of the entrance pupil.

*The entrance pupil is the hole in the aperture blades that you see when you look into the lens - ie it is the image of the aperture blades as formed by the lens elements in front of the aperture.