24-70 lenses

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Philx1979

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Phil
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can anybody explain to me why 24-70 2.8 lenses are fully extended at 24mm and shortend at 70mm?

I`ve always thought zooms lenses extended when zoomed out but cant understand why these work back to front?

Sorry for the daft question if it is daft lol
 
Generally a design thing to minimise vignetting and be able to optimise the the lens hood for all focal lengths.

The above might not be true.
 
It depends how the lens formula is designed, and indeed where any focal points are within the lens. Remember that even thought the objective lens appears to move forward, when zooming out, internal optics maybe do the opposite. As a result, the relative movement of elements with respect to a focal point within the lens will allow the second set of optics to capture more or less of the light captured by the front most group. This in effect could account for a change in focal length. The remaining optics would probably then be used for collimation towards focusing optics for example. Does that help at all?

Essentially it's simply down to how the optics are designed. The vignetting point made above is probably correct also, as the hood doesnt change size, as we know :)

Very crude drawing to come...

photo-2-1.jpg


So here, lets say the inner optic moves with respect to the objective, the focal point changes position with respect to the second optic. This means the second optic captures a different 'area' if you will of what the objective is letting through. Very crudely speaking, this could account for a change in focal length. Needless to say, real lens formulae are far more complex than this, but you get the general idea :)
 
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Is that right? My 24-105 extends as I zoom to 105, my Tamron 28-75 extends out and the Sigma 24-70 extended out the same...... I own the Canon and Tamropn and sold the Sigma. What lens you talking about Phil?
 
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EOS_JD said:
I think that's the one he's using. lol

Unless there is internal focusing (like the Canon 24-70), the lens has to extend when zooming to 70mm. It can't work the opposite way.......
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The nikon 24-70 has internal focusing, and it extends when zooming out. It most certainly can work the other way...
 
Woodsy said:
The nikon 24-70 has internal focusing, and it extends when zooming out. It most certainly can work the other way...
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If the lens has internal focusing it won't extend - That is the nature of internal focusing lenses like the Canon 24-70, 70-200 range, 100 macro and others. I stand corrected that it can work the other way though. I'm sure my old Sigma extended but it's years since I owned it.
 
Yv said:
Sigma 24-70 2.8 has exactly that design, long at 24mm, pulling in shorter at 70mm. Its a bit weird when yu first use it, but no longer notice tbh ;)
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my Sigma 24-70 2.8 is the other way round i.e long at 70 and short at 24mm , I've just checked as you set me thinking :)
 
EOS_JD said:
If the lens has internal focusing it won't extend - That is the nature of internal focusing lenses like the Canon 24-70, 70-200 range, 100 macro and others. I stand corrected that it can work the other way though. I'm sure my old Sigma extended but it's years since I owned it.
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He's not talking about the lens moving whilst focusing, he's talking about the way the lens moves when zooming with respect to the focal length. :) But yes, you're correct, that is the idea behind internal focusing :D I believe we were barking up different trees! :D
 
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Yv said:
I think it depends which 'vintage' you have, I know my nikon fit does it and Fabs Sony fit does too ;)
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I guessed there might be differences between models etc

It did make me think though and I had to have a look !
 
Woodsy said:
He's not talking about the lens moving whilst focusing, he's talking about the way the lens moves when zooming with respect to the focal length. :) But yes, you're correct, that is the idea behind internal focusing :D I believe we were barking up different trees! :D
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Woodsy said:
The nikon 24-70 has internal focusing, and it extends when zooming out.
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No I got it - You said the lens you have has internal focusing and extends when zooming.... I was only replying to your point saying that's not how internal focusing works.

I obviously got it wrong though about the zoom in when lens is retracting- It can obviously work either way although I'm not sure how that works :)
 
EOS_JD said:
If the lens has internal focusing it won't extend - That is the nature of internal focusing lenses like the Canon 24-70, 70-200 range, 100 macro and others. I stand corrected that it can work the other way though. I'm sure my old Sigma extended but it's years since I owned it.
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as said its internal focusing which means the end does not rotate, but it does not have internal zoom unlike the 70-200mm f2.8, which stays the same length,
 
If I'm wrong I do apologise - You learn something new every day - even when you think youre right lol..... Sorry :)
 
This lens is also my a 'want list' for the Canon version and Ive read that the reverse zoom method used here is due to the the hood being mounted to the barrel as opposed to the extending tube (sorry for lack of technical description lol)

This protects the extending tube by effectively having the hood covering it at all times. Therefore it is 'wide' at full extension, and its drawn back into the barrel at 'zoom'.

It makes sense to have it working this way if the hood isn't moving relative to the tube when at its widest focal length.

Im guessing the main reason for this is the weatherproofing aspect.


Andy
 
Yv summed it up, the sigma 24-70 is what I have coming and the pictures I've seen online always showed the barrel fully extended at 24mm but at 70mm not zoomed out at all, I've never seen this before and thought it was random
 
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