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04-12-2012, 04:30 AM   #1
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how many steps do i have to go back to get a FOV of 31mm from my 43mm ltd

hi guys.

just want to know . how many steps do i have to go back to get a FOV of 31mm from my 43mm ltd

thankz

04-12-2012, 04:35 AM   #2
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one big step? or two normal steps?

Actually, it depends on the subject distance, the further away the subject the more steps you have to take. one to two steps for 10-20ft object based on my preliminary experience.
04-12-2012, 05:21 AM   #3
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Note that you can get the same "width" by stepping back, but you will never be able to duplicate the image exactly because you'll be farther away than you would be with the 31mm lens, and thus the perspective will be different.
04-12-2012, 05:50 AM   #4
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The reverse question is how many steps up do you need to take with the 31 to get somewhat the framing of a 43. If you have the kit lens, you can zoom in or out between about 30 to about 40 while standing pat.



04-12-2012, 06:53 AM   #5
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QuoteOriginally posted by RBellavance Quote
Note that you can get the same "width" by stepping back, but you will never be able to duplicate the image exactly because you'll be farther away than you would be with the 31mm lens, and thus the perspective will be different.
This is the relevant comment.

FOV is one thing and is a characteristic of the lens and film/sensor format. You often read the term "zoom with your feet". Unfortunately, doing so will change the relationship of subject and its environment within the frame (perspective) and may even change the prominence of certain aspects of the subject as well. (Think portraits with a wide-angle lens..."My what a BIG nose you have Grandmother!"...)


Steve
04-12-2012, 07:16 AM   #6
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QuoteOriginally posted by nirVaan Quote
hi guys.

just want to know . how many steps do i have to go back to get a FOV of 31mm from my 43mm ltd

thankz
According to the FOV calculator on this page, multiplying the 31mm distance by 1.4 gives the distance required to get the same FOV with the 43mm. For example, at 10 feet, you'd need to step back about 4 feet to get the same FOV.

Jeff
04-12-2012, 08:01 AM   #7
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QuoteOriginally posted by nirVaan Quote
hi guys.

just want to know . how many steps do i have to go back to get a FOV of 31mm from my 43mm ltd

thankz

You could just experiment with a zoom lens and see for yourself.

John
04-12-2012, 08:04 AM   #8
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To take a full body shot of a person (1.80m tall) it's not that much; see Jeff's post above where 43 divided by 31 is approx. 1.4. To take a photo of the Eiffel tower in Paris (320 meters tall), a lot of steps; specially if you don't want to tilt your camera.

Please remind me to finally buy an ultrawide zoom when I go there

PS Another calculator: Lens focal length calculator

04-12-2012, 08:22 AM   #9
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The distance you need to move back is proportional to the distance to your subject. In case it's not obvious from Jeff's example, here's the formula:

[distance to subject with lens 1] * [lens 2 focal length]/[lens 1 focal length] = [distance to subject with lens 2].

10 feet * 43mm/31mm = 13.9 feet. There's the ~4 feet step back Jeff posted.

That only applies to your target. The background is at a different distance and will scale at a different rate. If taking a landscape photo, then the background *is* your subject and use that distance. But you'll find that for distant objects you may need to step back a great distance to get the same framing with a 43mm as with a 31mm. A mountain 5 miles away would require you to shift 2 miles to get the same framing with the 2 lenses!
04-12-2012, 11:53 AM   #10
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QuoteOriginally posted by Jeff Charles Quote
According to the FOV calculator on this page, multiplying the 31mm distance by 1.4 gives the distance required to get the same FOV with the 43mm. For example, at 10 feet, you'd need to step back about 4 feet to get the same FOV.
I believe you have confused FOV with DOF. Yes, you can change position to get the same DOF, but you will never, ever, ever get the same FOV. The FOV of a 43 is *always* narrower than that of a 31. Stepping back will allow you to fit more of any given subject, but will not change the FOV - meaning you won't be changing the amount of background in the image nearly as much as you will be changing the amount of foreground.

So for instance, taking a picture of a person standing in front of a scenic view, stepping back a little with the 43 will allow you to fit as much of the person as with the 31 from the original position, but you'll never get as much of the view. And the amount you need to step back to get the same amount of person depends on your original distance to that person.
04-12-2012, 12:24 PM   #11
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According to wikipedia ( Field of view - Wikipedia, the free encyclopedia )
QuoteQuote:
In photography, the field of view is that part of the world that is visible through the camera at a particular position and orientation in space; objects outside the FOV when the picture is taken are not recorded in the photograph. It is most often expressed as the angular size of the view cone, as an angle of view.
It does not take into account the position of a main subject in the photo to its surroundings.
04-12-2012, 12:33 PM   #12
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As others have noted, firld of view is generally expressed as an angle, and is a function of focal length and sensor format, and htis cannot be equal for different focal lengths.

Also, since

image size = subject size x focal length / distance

as you can see, the answer of how many steps, is going to be dependant upon how big your subject is and how far you are away from it presently.

BUT.......

if you are asking what is the difference in distance required to achieve the same principal subject size, when comparing two different focal lengths.

then it is simple, the new shooting distance is the ratio of focal lengths times the present shooting distance

so, if your lens is 33% longer then you move back 33%.
04-12-2012, 12:51 PM   #13
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QuoteOriginally posted by Lowell Goudge Quote
if you are asking what is the difference in distance required to achieve the same principal subject size, when comparing two different focal lengths.

then it is simple, the new shooting distance is the ratio of focal lengths times the present shooting distance

so, if your lens is 33% longer then you move back 33%.
BINGO! As others have pointed out, such so-called foot-zooming changes the DOF and perspective; but this is how to get the same FOV with various focal lengths. And it's a reminder that every shot is a problem to be solved. Our solutions involve many factors and give varied results. Don't leave home without a slide-rule, eh?
04-12-2012, 01:29 PM   #14
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QuoteOriginally posted by RioRico Quote
BINGO! As others have pointed out, such so-called foot-zooming changes the DOF and perspective; but this is how to get the same FOV with various focal lengths. And it's a reminder that every shot is a problem to be solved. Our solutions involve many factors and give varied results. Don't leave home without a slide-rule, eh?
when I went to school, I had a teacher that spent a long time teaching how to estimate the answer to the correct order of magnitude, mentally, because although a slide rule gives you the most significant digits, the number of zeros to put behind the number is lost.

Unfortunately in today's world of teaching, it seems that there is no emphasis put on being able to do mental calculations.
04-12-2012, 09:05 PM   #15
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QuoteOriginally posted by Marc Sabatella Quote
I believe you have confused FOV with DOF.
No. There are both DOF and FOV calculators on the page I linked to, but I used the Dimensional FOV calculator. The calculator can be misleading in that it does not account for the change in perspective as you move back.

Jeff
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