Calculating Focal Length / Distance Away From Subject

kennysarmy

kennysarmy

Yeah but can your army do this?
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Jeff
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I took some remotely triggered photographs over the weekend and had the camera set on a tripod.

I filled the frame with my 70-200 lens @ 70mm on my Canon 40d and I was about 3m back from the subject.

How could I have calculated how far I would need to move back if I had changed to a 200mm focal length if I wanted the item that was initially full frame at 3m to remain the full size of the image?

Is there a simple formula?

:thinking:
 
kennysarmy said:
Is there a simple formula?
Click to expand...
Yes, it's linear. If you have a FoV at 70mm and distance of 3 metres, going to 210mm (to make the maths easy) you need to be 3x the distance (i.e. 9 metres) for the same field of view.
 
Ah so...

200/70 = 2.86

So for every (m) I go back that has the item filling the frame @ 70mm I would need to go back 2.86 times for the item to be filling the frame @ 200mm

@ 70mm and 1m back:
Depth of field
Near limit 0.97 m
Far limit 1.03 m
Total 0.06 m

@ 200mm and 2.86m back:
Depth of field
Near limit 2.83 m
Far limit 2.89 m
Total 0.06 m


So for maximizing DOF and filling the frame it really does nt matter whether I put the camera closer or move it away...

Interesting....
 
kennysarmy said:
So for maximizing DOF and filling the frame it really does nt matter whether I put the camera closer or move it away...

Interesting....
Click to expand...
It's great when the maths just works ;)

BTW, what you will have is a different perspective. That is, as you go to longer and longer focal lengths, your field of view narrows. Whilst the subject is still filling the frame, you are now seeing less of the background, but it is still spread over the same frame. This means your subjects will pop more the longer the focal length used.

For a good example, take a look at the 3 shots of the flower half way down the page here: http://www.the-digital-picture.com/Reviews/Canon-EF-100mm-f-2.8-USM-Macro-Lens-Review.aspx
 
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