Crop-in Factor for Flash Auto Zoom Heads?


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May 6, 2003
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#1
As I understand it, DSLRs have a focal length crop-in factor, for those which don't have a full-frame sensor. As such, a 50mm effectively becomes a 75mm on a DSLR, for example. However, I noticed the focal length that is automatically set on the flash doesn't reflect the crop-in factor (e.g. a nikon sb80dx). Will this be a problem when it comes to flash exposures? Somehow I 'feel' that I don't get as 'proper' a flash exposure on a DSLR as on a full-frame SLR. Also, will the 'bokeh' of the 50mm on a DSLR be the same as the 'bokeh' of a true 75mm used on a full-frame SLR? Thanks for any replies in advance.
 

Ah Pao

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#2
For a proper flash exposure, the focal length of the zoom must be shorter or equal to the focal length of the lens. So, in practice, it doesn't matter that the focal length of the zoom is not equal to the focal length of the lens, whether with crop factor or not.

E.g. the lens is set at 50mm (effective focal length 75mm with the crop factor), the zoom on the flash should be 50mm or less (it can be 35mm, 28mm or 18mm...doesn't matter).

As for bokeh, it doesn't matter because it is just a crop on your DSLR...the lens' optical qualities doesn't change. Imagine a mask with a black border around your view of vision on a 35mm-film SLR. The DSLR sensor is smaller in size than your 35mm film, therefore the so-called "focal length multiplier". More accurately you should keep in mind it is a crop factor rather than a focal length multipler.
 

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Ah Pao said:
E.g. the lens is set at 50mm (effective focal length 75mm with the crop factor), the zoom on the flash should be 50mm or less (it can be 35mm, 28mm or 18mm...doesn't matter).
i'm confused by this statement....in this case, why do they build auto zoom heads in the first place? why not just an all-purpose 14mm zoom head to cover almost all focal lengths? :think:
 

Ah Pao

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#4
So that you can cover long distances.
At 105mm the flash can cover a longer distance compared when at 28mm.

Wide vs far, get it?
 

May 6, 2003
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i see....sort of get the picture now (pun pun :bsmilie: ) in theory, if i use a focal length that is much longer than is set on the flash zoom head (e.g., 105mm vs 24mm set on zoom head)....i should get a halo/fall-off effect on the subject? or should it be the other way round?
 

Ah Pao

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If you set a focal length on your flash head longer than your lens (say 35mm on lens but your flash head is set at 105mm), you will have a fall-off effect because the sides are not illuminated properly by the flash.

Lens focal length >= Flash focal length --> technically no problem.
But if the flash focal length is set too wide but your lens is set to telephoto your flash may not be bright enough to illuminate your subject if it's too far away.

Lens focal length < Flash focal length --> fall-off effect.

In summary, keep your flash focal length equal or wider than your lens' focal length to get proper flash exposure.
 

Stereobox

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Ah Pao said:
In summary, keep your flash focal length equal or wider than your lens' focal length to get proper flash exposure.
provided the subject is within range of the flash focal length? Excellent, I get it now. thanks Ah Pao for your concise and patient explainations! :thumbsup:
 

Ah Pao

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#8
Yeap, in all cases you need to take in account of the distance the flash can cover.

And thanks, it's my pleasure. :) We're all here to share and learn!
 

May 6, 2003
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Ah Pao said:
If you set a focal length on your flash head longer than your lens (say 35mm on lens but your flash head is set at 105mm), you will have a fall-off effect because the sides are not illuminated properly by the flash.

Lens focal length >= Flash focal length --> technically no problem.
But if the flash focal length is set too wide but your lens is set to telephoto your flash may not be bright enough to illuminate your subject if it's too far away.

Lens focal length < Flash focal length --> fall-off effect.

In summary, keep your flash focal length equal or wider than your lens' focal length to get proper flash exposure.
I get it now, thanks Ah Pao!! :thumbsup: guess I am too spoilt by automatic settings and neglected to understand how things work before there was auto this-n-that :bsmilie:
 

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