[L&D]

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

, oops my bad, by image height, i thought the actual height of captured frame where focus is at its sharpest (the center of DOF), was meant.

[L&D]

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)

Thanks Lowell,

this answers my question fully. I somehow thought the Angle of View would have something to do with this, since the lens and the subject actually do form an isosceles triangle.

In any case, hope you won't get offended but i will test your formula on my next photo-shoot.

thanks again.

[Anvh]

AOV is not needed because you've the sensor and image height.
I'm sure you can also do it with AOV but that won't be any easier....

[JimJohnson]

While not precisely the same question asked by L&D....

Ah, the wonders of my old K-A prime lenses! There are some wonderful scales etched right into the lens barrels that eliminate a lot of this arithmetic but still provide for excellent manual control. Focus to a subject and read the distance off the barrel. Set an aperture and determine the hyperfocal distance by looking down at the lens barrel. Does anybody but me use their green button to do a stop-down preview just like the lever next to the lens on the old ME series cameras? I would prefer to use my green button for other functions, but none of my DA L zoom lens tell me anything about focus other than the current focal length.

Other than macro work, precise answers are generally not needed in photography, and interpreting the analog readings from the lens worked well. Showing my age... with analog watch faces, ask someone for the time, and you get a rounded approximation (10 minutes before the hour). But especially when digital watches came out with their glowing red LEDs, ask a wearer the same question and you got, "7:50pm" or on the east side of the Atlantic "19:50".

[Wheatfield]

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?

You would do far better to forget trying to over analyze this stuff and just go out and take pictures instead. It actually does become possible to look at a scene and just know what focal length and aperture to use with very little experience if you take the time to train your eyes, and you will be a much better photographer by being able to size up a scene intuitively rather than by wrote formulas that you are punching into a laptop (and what will you do when the battery dies on your computer)?

[L&D]

You would do far better to forget trying to over analyze this stuff and just go out and take pictures instead. It actually does become possible to look at a scene and just know what focal length and aperture to use with very little experience if you take the time to train your eyes, and you will be a much better photographer by being able to size up a scene intuitively rather than by wrote formulas that you are punching into a laptop (and what will you do when the battery dies on your computer)?

oh, jeez!!! i did anticipate that there would be someone wise giving me this type of lecture on things )
apparently me being a "new member" in this forum, must have confused some into believing that i am new to photography .

OK, i'll take that. and thank you Wheatfield. i'll try to go by your advice next time

[Asha]

Field of view angle is incorporated as Tangent of the angle, which is approximated by object height/distance.

Tan(angle) = y/x from basic Trig.

[Wheatfield]

oh, jeez!!! i did anticipate that there would be someone wise giving me this type of lecture on things )
apparently me being a "new member" in this forum, must have confused some into believing that i am new to photography .

I tend to presume that when people ask about this sort of thing, they are new to photography.

[L&D]

I tend to presume that when people ask about this sort of thing, they are new to photography.

you're free to presume whatever, but keep in mind that i am also entitled to presume that people giving generic, introductory lecture on photography instead of an answer on technical question - are typical "know-it-all" and "my advice is priceless" type of snobs...

[JimJohnson]

HOLD IT FOLKS. It was a simple misunderstanding. We already have a couple useless flame wars that have destroyed other threads. If either one of you can't drop it right here, PLEASE, for the rest of us, take it private?

[Lowell Goudge]

Thanks Lowell,

this answers my question fully. I somehow thought the Angle of View would have something to do with this, since the lens and the subject actually do form an isosceles triangle.

In any case, hope you won't get offended but i will test your formula on my next photo-shoot.

thanks again.

Feel free to test all you want, I don't get offended all that easily.

you might want to avoid all the testing though, and just go to the 300mm lens club, and look back a few pages. I used this exact approach to determine how far someone was based on the image they posted of a bird. Since I knew the subject size from my bird book, and the image size from the height of the frame, I calculated the distance and also estimated the extension tube used to reduce the minimum focus distance of the lens in question (K1000/8).

In other words, I'm pretty confident in the calculation.

[L&D]

Feel free to test all you want, I don't get offended all that easily.

you might want to avoid all the testing though, and just go to the 300mm lens club, and look back a few pages. I used this exact approach to determine how far someone was based on the image they posted of a bird. Since I knew the subject size from my bird book, and the image size from the height of the frame, I calculated the distance and also estimated the extension tube used to reduce the minimum focus distance of the lens in question (K1000/8).

In other words, I'm pretty confident in the calculation.

hah, i tested your formula yesterday and acknowledge it to be correct, and in fact tested it with different focal length glasses. so thank you

distance = image height * focal length / subject height,

[Lowell Goudge]

hah, i tested your formula yesterday and acknowledge it to be correct, and in fact tested it with different focal length glasses. so thank you

distance = image height * focal length / subject height,

Just remember, there is a disclaimer to the formula. It is an approximation, for when distance is much greater than focal length. Without going back through all my notes etc, I cannot tell you where a term (1- distance/FL) was removed but I think it is actually

distance x (1-FL/distance) = image height x focal length / subject height

When distance is very large compared to focal length, this tends to zero, but when the two are close (I.e. macro) it is Important

Someone correct me if my memory has failed

[Wheatfield]

you're free to presume whatever, but keep in mind that i am also entitled to presume that people giving generic, introductory lecture on photography instead of an answer on technical question - are typical "know-it-all" and "my advice is priceless" type of snobs...

So you don't really want advice, you want to be an arrogant <potty mouth word edited by mod>.
Why are you here.

[L&D]

So you don't really want advice, you want to be an arrogant <potty mouth word edited by mod>.
Why are you here.

why am i here? definitely not to get useless information from someone, who thinks he's giving the advice of a lifetime.
i asked a specific, technical question, and instead of a somewhat related answer you came up with so much nonsense, automatically assuming that i am new to photography. well, if me being new to photography makes you a better photographer or in fact better person, so be it.


and now, please go be "wise" with someone else.