D70s Mathematicians (distance to subject vs focal length)

[Oz_Beachside]

Hi,

A question for the mathematicians/physicists in here.... I am trying to work out a method to calculate how I can measure, workable focal lengths, in my studio floor area, for full body shots.

For example, if my studio allows me to be 7 meters from my backdrop, and my subject it 180cm tall, what focal length lens would allow me to get them in full frame?

I know I can do this by trial and error, but I thought it would be easier, via maths. But I dont know the relationship between distance to subject, and image size on my D70s.

The reason I ask, is I am trying to assess whether I get a 17-35, 17-55, or 28-70 (based on focal length), please I am aware of other pros and cons of each, but this topic is simply asking about distance to subject, full frame, and focal length. I'm not after a Newtonian relationship, but if someone has an approximate formula would save me time.

thanks in advance
Oz

[gstark]

I know I can do this by trial and error, but I thought it would be easier, via maths.

What's easier than just getting somebody and taking photos and viewing the results?

You're looking in the wrong place, and for the wrong answer.

Forget the maths; just go with what looks good in the viewfinder.

[sheepie]

The specs on maxwells site show angle of view for each lens - you should be able to use that to work out what you're wanting. There's also a few utilities around the 'net which will probably give you the answers.

[MHD]

have a read of:
http://en.wikipedia.org/wiki/Lens_(optics)


The magnifaction of a lens is M=f/(f-d_subject)

As you will note for most distances the magnifaction is negative and small as your image (on your CCD) is reduced and inverted

the height of your image will be h_image=h_subject*M

[Yi-P]

You said you have a D70s, so Im assuming you already have the famous "kit lens" (18-70)

From the list of lenses you mentioned about, the kit lens covers them all (except the 1mm shorter with 17mm).

Go get your subject to sit down, pose in your studio, use your 18-70 and adjust the zoom until you are happy enough with both the working distance and angle of view.


In studio, I think you will worry about distance of the lights to the subject rather than the lens and FOV. Different focal length can achieve different perspectives and meaning to each image, whereas light is more important in a studio IMO.

[Oz_Beachside]

THanks so much MHD, perfect answers, and helped me solve my question in 2 minutes, rather than an hour or so taking pics, thanks heaps!

I needed to do this since my lounge/studio has about 5 meters of depth.
it shows me that in landscape, a focal length over 35mm, is not usable in my studio for full body shots, and half body shots are achievable up to 85mm.

In portrait orientation, I can go up to 70mm full body, and upto nearly 150mm half body shots.

This helps me be practical that within my studio, I will get more usable range in the 17-35, than the 28-70.

results below, thanks again.

Oz.

orientation image height Subject Height (person) focal length magnification distance to subject distance to subject
(D70s sensorsize) (mm) (mm) (mm) (m)
landscape 15.6 1850 full body shot 17 0.008 1999 2.0
15.6 1850 full body shot 20 0.008 2352 2.4
15.6 1850 full body shot 24 0.008 2822 2.8
15.6 1850 full body shot 28 0.008 3293 3.3
15.6 1850 full body shot 35 0.008 4116 4.1
15.6 1850 full body shot 50 0.008 5879 5.9
15.6 1850 full body shot 70 0.008 8231 8.2
15.6 1850 full body shot 85 0.008 9995 10.0
15.6 1850 full body shot 200 0.008 23518 23.5
15.6 900 half body shot 17 0.017 964 1.0
15.6 900 half body shot 20 0.017 1134 1.1
15.6 900 half body shot 24 0.017 1361 1.4
15.6 900 half body shot 28 0.017 1587 1.6
15.6 900 half body shot 35 0.017 1984 2.0
15.6 900 half body shot 50 0.017 2835 2.8
15.6 900 half body shot 70 0.017 3968 4.0
15.6 900 half body shot 85 0.017 4819 4.8
15.6 900 half body shot 200 0.017 11338 11.3
0.0
portait 23.7 1850 17 0.013 1310 1.3
23.7 1850 20 0.013 1541 1.5
23.7 1850 24 0.013 1849 1.8
23.7 1850 28 0.013 2158 2.2
23.7 1850 35 0.013 2697 2.7
23.7 1850 50 0.013 3853 3.9
23.7 1850 70 0.013 5394 5.4
23.7 1850 85 0.013 6550 6.6
23.7 1850 200 0.013 15412 15.4
23.7 900 17 0.026 629 0.6
23.7 900 20 0.026 739 0.7
23.7 900 24 0.026 887 0.9
23.7 900 28 0.026 1035 1.0
23.7 900 35 0.026 1294 1.3
23.7 900 50 0.026 1849 1.8
23.7 900 70 0.026 2588 2.6
23.7 900 85 0.026 3143 3.1
23.7 900 200 0.026 7395 7.4

[myarhidia]

you should also take into consideration distortion using a wide angle.

No point in taking portraits @ 17mm if it's going to make the person look unproportional.

[Oz_Beachside]

yes, true, am considering that, I dont like the look, very close, at 17mm. THats why I may go a little longer, 28-70. I like all the great reviews/thoughts on both.

thanks for your input!
Oz

[Greg B]

Photographs of people where the objective is to make the subject look good are usually best taken at a focal length longer than "standard"

On a 35mm film camera, the "standard" lens was usually 50mm, although some would argue that slightly shorter was standard. Standard was the focal length which would approximate the field of view of your eyes.

A 34 mm lens on a DX format camera has the same field of view as a 50 mm lens on a film camera (using the 1.5 multiplier). The 50mm lens gives 75mm and so on. I would think that 50mm to 70mm on the DX camera would be ideal portrait focal lengths.

I like you mathematical approach to the problem Oz, good luck.

[MHD]

No worries Oz...

Glad to see that 10 years of Uni helped someone