[L&D]

I am new to the forum, so firstly, hello everyone

I am wondering if there is a formula to calculate the proper subject distance, using focal length of the lens and actual height of the subject.

[Lowell Goudge]

If you are not considering macro, then there is a very simple approximation that I use

Image height = Subject Height * focal length / Distance

All units in mm

the formula gets more complicated when you focus closer than about 10x the focal length, but at reasonable shooting distances, the approximation is quite good

Edit note, proper shooting distance is a subjective thing, proper does not only include height, but also the perspective of the lens. You need to consider this also, because lens perspective can cause "distortion" of figures especially when shooting with a wide angle lens, so even if you get the "proper" image size, the image itself may be very un-flattering

[L&D]

Hi Lowell,

thanks for the reply. No i am not considering macro. Just full body shots of people, so i'd say image height is slightly greater than subject height by few inches. I know subject distance can be easily obtained through viewfinder, by getting closer or farther from the subject (with prime lens), keeping the difference between subject and image height to minimum. But i was thinking perhaps there is a formula to get the numeric value of subject distance, so that it can be properly used for DOF calculations. I'm sure Angle of View of the lens has also something to do with the subject distance.

[Anvh]

AOV is not needed because you've image and subject height.

here is a calculator
http://www.giangrandi.ch/optics/lenses/focalcalc.html
http://www.tawbaware.com/maxlyons/calc.htm

you need to give object distance so you need to play around with it a bit

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Last edited by Anvh; 05-25-2012 at 09:10 AM.

[L&D]

Yeah, I saw these calculators. They're pretty much useless for my purpose, because they all require numeric value for the subject distance, which i am trying to figure out.

Let me rephrase my question: with 50mm prime lens (with AOV 46 degrees), and a subject height of 175 centimeters (image height 185 centimeters), what should be the distance to the subject?

[MSL]

But there is nothing to stop you from putting in a guess until it pops out the right focal length. I assumed 24mm for the sensor height which gives a distance around 6 meters, but you may want to shoot in portrait mode which would put you closer to 4 meters unless I'm using these calculators completely wrong. I've assumed an APS-C sensor, rather than FF.

[pete-tarmigan]

Let me rephrase my question: with 50mm prime lens (with AOV 46 degrees), and a subject height of 175 centimeters (image height 185 centimeters), what should be the distance to the subject?

"What should the distance to the subject be" to do what? To fill the frame with the subject? To fill half the frame? To fill 1% of the frame?

If you want to fill the frame with the subject, then take the equation that Lowell provided and solve for subject distance, since you know the values for all the other variables.

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Last edited by pete-tarmigan; 05-25-2012 at 09:59 AM. Reason: typo

[L&D]

"What should the distance to the subject be" to do what? To fill the frame with the subject? To fill half the frame? To fill 1% of the frame?

If you want to fill the frame with the subject, then take the equation that Lowell provided and solve for subject distance, since you know the values for all the other variables.

Obviously to fill the frame. Lowell's equation, while useful, is to calculate the image height, which i am not after and which still requires a variable that i am trying to calculate (distance to the subject).

Image height = Subject Height * focal length / Distance

[RioRico]

Rewriting Lowell's equation:

Image height = Subject Height * focal length / Distance
Distance * Image height = Subject Height * focal length
Distance = Subject Height * focal length / Image height

[L&D]

Rewriting Lowell's equation:

Image height = Subject Height * focal length / Distance
Distance * Image height = Subject Height * focal length
Distance = Subject Height * focal length / Image height

distance = 175cm * 5cm (50mm) / 185cm
distance = 4.72972972972973


am I getting this right?

[Lowell Goudge]

Hi Lowell,

thanks for the reply. No i am not considering macro. Just full body shots of people, so i'd say image height is slightly greater than subject height by few inches. I know subject distance can be easily obtained through viewfinder, by getting closer or farther from the subject (with prime lens), keeping the difference between subject and image height to minimum. But i was thinking perhaps there is a formula to get the numeric value of subject distance, so that it can be properly used for DOF calculations. I'm sure Angle of View of the lens has also something to do with the subject distance.

OK if this is what you are looking for, then start with switching this formula around,

Distance = subject height * focal length / Image height

The image height is easy, that is 16mm high and 24mm wide (i.e. the format of your sensor)

then, go on line and take any DOF calculator and consider the DOF vs aperture for the lens you pick. BUT, I did an exercise a while back where I looked at the change in DOF vs shooting distance and found that what it all boiled down to was that in a first order, the Depth of field in actual units of measure did not change as you changed focal length and shooting distance to maintain the subject image the same.

What this means is if you shoot from 5 feet at F2.8 with a 50mm lens, shooting from 10 feet at F2.8 with a 100mm lens will have basically the same depth of field.

[Anvh]

Yeah, I saw these calculators. They're pretty much useless for my purpose, because they all require numeric value for the subject distance, which i am trying to figure out.

Let me rephrase my question: with 50mm prime lens (with AOV 46 degrees), and a subject height of 175 centimeters (image height 185 centimeters), what should be the distance to the subject?

That's why i said play around with the numbers.
i tried several distance and 360 cm is close with 172.8cm

that is with the orientation in portrait.

in landscape you need around 550cm that result in 176cm

[Anvh]

distance = 175cm * 5cm (50mm) / 185cm
distance = 4.72972972972973


am I getting this right?

no image height is your sensor heigth and i doubt your sensor is 185cm heigh, that's as heigh as i'm...

[Lowell Goudge]

Yeah, I saw these calculators. They're pretty much useless for my purpose, because they all require numeric value for the subject distance, which i am trying to figure out.

Let me rephrase my question: with 50mm prime lens (with AOV 46 degrees), and a subject height of 175 centimeters (image height 185 centimeters), what should be the distance to the subject?

Ok, it seems that mathematics are a limiting factor here so I will do some of the work

Having rewritten the formula

distance = image height * focal length / subject height,

take your 50mm lens (5cm to keep units dame), subject height of 175 cm, ( some one standing) and assume vertical format or 2.4 cm for the image height, then you have

distance = 175 * 5 / 2.4 = 364 cm or 3.64 meters (about 10 feet)

[L&D]

That's why i said play around with the numbers.
i tried several distance and 360 cm is close with 172.8cm

that is with the orientation in portrait.

in landscape you need around 550cm that result in 176cm

Hi Anvh,

Thank you for taking the time to answer me , but i am not trying to find out the subject distance, but rather formula through which i can calculate the subject distance using other variables (AOV, Focal length of the lens, subject height etc).