Lenses, TC's, Apertures, DoF, and IQ...

[68lbs]

Ok, the crux of the matter is that my 70-200 isn't long enough on full frame. Was out yesterday and tried taking some shots (candids at a steam rally), and just couldn't get close enough, due to barriers etc.

I don't really need extra length a lot of the time, so am guessing my best bet is a TC, of which there are two IIRC that will fit my 70-200 f/2.8. A canon 1.4x and 2x. I also know that if I was to fit a 2x TC my f/2.8 would become 5.6, and I understand about aperture and it's effects on exposure, DoF, etc.

But what exactly does it mean when they say a TC affects aperture? Surely the aperture can't magically get smaller just putting on a TC? Would my camera still say 2.8 on the lcd, but the DoF changes so is the equivalent to if I had set the lens to 5.6? Or does DoF stay the same, but because of the TC for some reason less light can get through, so it's equivalent to f/5.6? Or is it all linked so that 5.6 is 5.6 and I will loose light AND get increased DoF?

What I am thinking is... I was shooting at 200mm wide open yesterday to isolate my subject nicely. If I fitted a 2x TC, would I be able to get less isolation?

And lastly... what TC would people recommend? I would prefer the Canon 2x as this will obviously give me the longest reach so I can at least try some plane shots at Sunderland Airshow this year. I think 1.4x will still leave me short.

Sorry for so many questions. In some ways would almost be easier buying a new lens!

 

[neil_g]

a TC will generally effect AF speed and IQ. a 2x more so than a 1.4x

like you mention you wont be able to get f2.8 with one attached either. shamelessly stolen quote:

becoming a 98-280mm F4 telephoto zoom lens or a 140-400mm F5.6 telephoto zoom lens

 

[StewartR]

But what exactly does it mean when they say a TC affects aperture? Surely the aperture can't magically get smaller just putting on a TC? Would my camera still say 2.8 on the lcd, but the DoF changes so is the equivalent to if I had set the lens to 5.6? Or does DoF stay the same, but because of the TC for some reason less light can get through, so it's equivalent to f/5.6? Or is it all linked so that 5.6 is 5.6 and I will loose light AND get increased DoF?

Say you're at 200mm. At f/2.8 the aperture is 200/2.8=70mm, right? Now you put on the 2x TC. Your focal length is now 400mm, but the aperture wide open is still 70mm, so you're now at f/5.6. The TC reports its presence to the camera, so the camera knows you're now at f/5.6.

I don't want to get into DoF issues because you're not comparing like with like - 200mm @ f/2.8 vs 400mm @ f/5.6 - so it's not straightforward.

And lastly... what TC would people recommend? I would prefer the Canon 2x as this will obviously give me the longest reach so I can at least try some plane shots at Sunderland Airshow this year. I think 1.4x will still leave me short.

Many people say that the image degradation you get with a 2x TC on the 70-200L is much worse than you get with a 1.4x TC.

 

[68lbs]

Say you're at 200mm. At f/2.8 the aperture is 200/2.8=70mm, right? Now you put on the 2x TC. Your focal length is now 400mm, but the aperture wide open is still 70mm, so you're now at f/5.6

Fantastic explanation Stewart!

That clears everything up beautifully.

 

[Canon Bob]

What I am thinking is... I was shooting at 200mm wide open yesterday to isolate my subject nicely. If I fitted a 2x TC, would I be able to get less isolation?

Adding the 2x T/C and framing the same will mean you'll need to be twice the distance from your subject and the DoF will be larger...meaning less isolation.
If you stood in the same spot, the subject would be twice the size on your sensor and the DoF would be less...meaning more isolation. (You'd also have to modify the distance from your subject to the foreground and background to keep the whole shot the same from a perspective point of view but I'll leave that one alone)

Bob



Bob

 

[towershot]

I have seen this question many times,and the answer given most by those that use them say it's best to use a 1.4 and crop the image than use a 2x converter.

 

[HoppyUK]

Ok, the crux of the matter is that my 70-200 isn't long enough on full frame. Was out yesterday and tried taking some shots (candids at a steam rally), and just couldn't get close enough, due to barriers etc.

I don't really need extra length a lot of the time, so am guessing my best bet is a TC, of which there are two IIRC that will fit my 70-200 f/2.8. A canon 1.4x and 2x. I also know that if I was to fit a 2x TC my f/2.8 would become 5.6, and I understand about aperture and it's effects on exposure, DoF, etc.

But what exactly does it mean when they say a TC affects aperture? Surely the aperture can't magically get smaller just putting on a TC? Would my camera still say 2.8 on the lcd, but the DoF changes so is the equivalent to if I had set the lens to 5.6? Or does DoF stay the same, but because of the TC for some reason less light can get through, so it's equivalent to f/5.6? Or is it all linked so that 5.6 is 5.6 and I will loose light AND get increased DoF?

What I am thinking is... I was shooting at 200mm wide open yesterday to isolate my subject nicely. If I fitted a 2x TC, would I be able to get less isolation?

And lastly... what TC would people recommend? I would prefer the Canon 2x as this will obviously give me the longest reach so I can at least try some plane shots at Sunderland Airshow this year. I think 1.4x will still leave me short.

Sorry for so many questions. In some ways would almost be easier buying a new lens!

When you fit an extender, in terms of focal length and f/number, the lens behaves exactly as if it was built that way in the first place, eg 70-200 2.8 with 2x extender becomes 140-400 5.6. The only difference is that Canon slows down the AF for more reliable focusing, but since you have such a fast focsusing lens in the first place, that's perhaps not a problem.

On the IQ front, I have not tried this combination but from the experience of others I would say go for the 2x if you really need the reach, but if you can get away with less, then the 1.4x will be better and and you could maybe pull in a bit more by cropping in post processing if you had to. One factor here is if you go this route, then you will get more depth of field by enlarging the image in post, than you would if you create the same magnification with the extender, at the same f/number. This is because you are effectively using a cropped format camera and if you enlarge an area half the full frame size then you will gain DoF equal to one f/stop.

I guess the thing to do here is hire both extenders from Stewart and see how you get on. The only thing I would say is, with respect if you are not used to long lenses take great care in getting maximum sharpness with steady support, meticulous focus and the highest possible shutter speed. Many people seem to have trouble with longer lenses, saying they are soft at max focal length, then others post a set of stunningly sharp images from identical kit. Forgive me, but I don't believe the lenses are the distinguishing factor in all cases!

On depth of field, if the image size remains the same f/number for f/number, then so does depth of field. For example, shoot with a 200mm lens at f/5.6 from 10m and DoF is 0.83m. Move back to 20m distance with a 400mm lens, and DoF is again 0.83m. See www.dofmaster.com

However, there is a major change in field of view with the longer lens, which is much narrower. The background immediately becomes less cluttered and distracting, and this will give greater subject isolation. When people talk about telephotos giving less depth of field, they are usually wrong; what they are actually referring to is the effect of reduced field of view.

 

[StewartR]

Adding the 2x T/C and framing the same will mean you'll need to be twice the distance from your subject and the DoF will be larger...meaning less isolation.

On depth of field, if the image size remains the same f/number for f/number, then so does depth of field. For example, shoot with a 200mm lens at f/5.6 from 10m and DoF is 0.83m. Move back to 20m distance with a 400mm lens, and DoF is again 0.83m. See www.dofmaster.com

Pistols at dawn, gentlemen?

However, there is a major change in field of view with the longer lens, which is much narrower. The background immediately becomes less cluttered and distracting, and this will give greater subject isolation. When people talk about telephotos giving less depth of field, they are usually wrong; what they are actually referring to is the effect of reduced field of view.

Interesting point, and one which I hadn't considered before. Thanks.

 

[Martyn...]

On depth of field, if the image size remains the same f/number for f/number, then so does depth of field. For example, shoot with a 200mm lens at f/5.6 from 10m and DoF is 0.83m. Move back to 20m distance with a 400mm lens, and DoF is again 0.83m. See www.dofmaster.com

That is interesting Hoppy, and like Stewart something I had not considered before.

To the OP forget 2x converters, 1.4 will provide an image with little quality loss, particularly on a decent lens, 1.7 will be usable in good light with little quality loss, but the light is more critical.

 

[Canon Bob]

Pistols at dawn, gentlemen?

I'm usually up too late for a dawn shoot out but here goes....using Hoppy's Dofmaster link

5DII 200mm at f/2.8 gives 0.42m DoF at 10m range
5DII 400mm at f/5.6 gives 0.83m DoF at 20m range

Unless I'm mis-understanding what's being said.

Edit...Hoppy is keeping the f number at 5.6 for both examples it seems

Bob

 

[david1701]

a TC on a zoom :O I thought it was just primes that they were good for because of the IQ degradation

 

[Martyn...]

a TC on a zoom :O I thought it was just primes that they were good for because of the IQ degradation

Depends on the zoom Nikon / Canon 70-200 f2.8 no problem, Nikon 200-400 f4 no problem.

SIgma/Tamron/Nikon/Canon variable aperture zooms questionable, also you need to check if the rear element of the lens can foul the front element of the convertor which is more likely to prevent its use, rather than a quality issue.

 

[HoppyUK]

I'm usually up too late for a dawn shoot out but here goes....using Hoppy's Dofmaster link

5DII 200mm at f/2.8 gives 0.42m DoF at 10m range
5DII 400mm at f/5.6 gives 0.83m DoF at 20m range

Unless I'm mis-understanding what's being said.

Edit...Hoppy is keeping the f number at 5.6 for both examples it seems

Bob

LOL, well I was still up at dawn

Clearly there is no disagreement given Bob's example above, which I have taken the liberty of extending to further demonstrate the point that when image size is maintained, so is depth of field, at the same f/number.

5DII 100mm at f/2.8 gives 0.42m DoF at 5m range
5DII 200mm at f/2.8 gives 0.42m DoF at 10m range

5DII 200mm at f/5.6 gives 0.83m DoF at 10m range
5DII 400mm at f/5.6 gives 0.83m DoF at 20m range

I don't know why this fact is commonly overlooked, and people cling to the notion that increased focal length intrinsically gives reduced DoF. It only does that because when you fit a long lens you generally don't move back to a further distance and so image size is magnified, and with it DoF correspondingly reduced. It is field of view that is changing substantiallly, and that is what really gives increased subject isolation. Call that an optical illusion if you like, and it does depend on the visual nature of the background, but that's the way it usually looks.

While I'm on, and before anyone gets picky, I am aware that DoF does in fact alter when you change focal length and focusing distance in this way, but it is only noticeable at much closer distances when comparing extremes like wide angles vs telephotos. Even then the shift is not huge and compared to changes in field of view and perspective, hardly significant. And it is always pertinent to note that DoF is an arbitary concept anyway, subject to a very specific set of shooting and viewing parameters that usually don't apply exactly to every shooting situation. In other words, in practise it's not an exact science at all