Portrait picture help

dbrandon · Jan 19, 2008

Please don't let it seem as if i am trying to argue, because that is not how i'm meaning to come across. I am simply trying to figure all this out, as i am fairly new to photography.

I see exactly what you are saying, and no i dont own a 3 foot long lens, but that shouldn't affect the maths behind aperture size.

Wikipedia ...

"In optics, the f-number (sometimes called focal ratio, f-ratio, or relative aperture[1]) of an optical system expresses the diameter of the entrance pupil in terms of the effective focal length of the lens; in simpler terms, the f-number is the focal length divided by the aperture diameter."

Goes on to give the equation

where f is the focal length, and D is the diameter of the entrance pupil

This clearly shows that f2.8 on a 50mm lens is going to be greatly different to f2.8 on a 200mm lens. Unless i am interpreting the equation completely wrongly.

Wether this means the DOF on a 200mm will be shallower due to the larger pupil, i do not know.

Happy Hour · Jan 19, 2008

you know my grandmother was a Pro photographer for 50+ years I was real into it when i was younger and am just getting back in full throttle. but in all my life you have presented me with more mathematical equations in 5 minutes than I have ever seen in my whole life for Photography. Why do you need all of that? I just called my grandmother with your question. after reading your math to her she replied "what is this ****ing brain surgery? It's photography!! I never used that much math in all my years in the field! at least not to figure out DOF!"I think you are making this WAY WAY WAY more complicated than it ever could be! So let me ask you a question? Before every picture you take, do you sit there with a pen and paper and go - - Ok x - y +d=w Or do you just expose and shoot? because I'm not going to bother doing the math when i can look right in front of me and prove I'm right. do you own more than one lens? hold them side by side open the aperture up the same on both. there is your answer! not x*z+w/c=your ass it's all about feel for me If anyone did all that math for taking a picture there would be no photographers in the world Unless your a total Math GEEK. (LOL) but again your putting way too much thought into a yes or no question. I think you need to go back and read the link you posted over again so you can see what you are over complicating.

jrhaze · Jan 19, 2008

fairly new too, but I've done my share of reading and dbrandon has it right . . .

A 50mm lens at f/5.6 fro a shooting distance of say 8 feet should have a depth of field of (depending on camera sensor size) of about 1.8 feet. A 200mm lens at f/5.6 same shooting distance will have a DOF of about 0.1 feet.

From some of the readinf I've done:
The absolute aperture size depends on the focal length - a 25mm aperture on a 100mm lens has the same effect as a 50mm aperture on a 200mm lens. Aperture when expressed in F/ is aperture (mm) / focal length. Above would be F/4 for both.

F5.6 on a 200mm lens would have an actual aperture of 35mm while F/5.6 on a 50mm would be about 9mm.

It's entriely possible that none of that made any sense too :lol:

jrhaze · Jan 19, 2008

Happy Hour said:
you know my grandmother was a Pro photographer for 50+ years I was real into it when i was younger and am just getting back in full throttle. but in all my life you have presented me with more mathematical equations in 5 minutes than I have ever seen in my whole life for Photography. Why do you need all of that? I just called my grandmother with your question. after reading your math to her she replied "what is this ****ing brain surgery? It's photography!! I never used that much math in all my years in the field! at least not to figure out DOF!"I think you are making this WAY WAY WAY more complicated than it ever could be! So let me ask you a question? Before every picture you take, do you sit there with a pen and paper and go - - Ok x - y +d=w Or do you just expose and shoot? because I'm not going to bother doing the math when i can look right in front of me and prove I'm right. do you own more than one lens? hold them side by side open the aperture up the same on both. there is your answer! not x*z+w/c=your ass it's all about feel for me If anyone did all that math for taking a picture there would be no photographers in the world Unless your a total Math GEEK. (LOL) but again your putting way too much thought into a yes or no question. I think you need to go back and read the link you posted over again so you can see what you are over complicating.

No need to do any math - just know that aperture is derived from focal length . . . so for any given F/number you need to take focal length into account to determine the physical aperture.

A fast 200mm lens at F/2.8 will have less DOF than a 50mm at F/1.8 all other thigs equal. That's a fact.

dbrandon · Jan 19, 2008

Ok, i'm going to forget about the whole thing.

I would like to point out that i am not at all a 'Math GEEK'. If i were, i wouldn't question the matter at all, because i would know the answer. It is rather offputting that you label someone as a geek for showing an interest in a subject matter.

And the equation is quite simple anyway :S

I do not go through mathematical equations before taking shots, but i do have an interest in a number of aspects of photography, and i think it helps when you understand what is going on.

'I'm not going to bother doing the math when i can look right in front of me and prove I'm right' ? I personally wouldn't class that as 'proof' when you have a simple explanation on wikipedia that contradicts what you say.

Pro photographers don't need to know the technicalities, so fair enough if your grandma considers it a load of rubbish, but she didn't invent the rules...

Oh well. I appologise for turning this into an argument, when all i was looking for was an explanation to a possible misunderstanding of a principle.

dbrandon · Jan 19, 2008

jrhaze said:
No need to do any math - just know that aperture is derived from focal length . . . so for any given F/number you need to take focal length into account to determine the physical aperture.

A fast 200mm lens at F/2.8 will have less DOF than a 50mm at F/1.8 all other thigs equal. That's a fact.

Thank you jrhaze, i believe that is what i was trying to say

jrhaze · Jan 19, 2008

dbrandon said:
Thank you jrhaze, i believe that is what i was trying to say

np

Certainly doesn't have to be mathmatical if a person doesn't want to to be - but ignoring the math doesn't mean you get to ignore the concept all together! :lol:

Happy Hour · Jan 19, 2008

dbrandon said:
Ok, i'm going to forget about the whole thing.

I would like to point out that i am not at all a 'Math GEEK'. If i were, i wouldn't question the matter at all, because i would know the answer. It is rather offputting that you label someone as a geek for showing an interest in a subject matter.

And the equation is quite simple anyway :S

I do not go through mathematical equations before taking shots, but i do have an interest in a number of aspects of photography, and i think it helps when you understand what is going on.

'I'm not going to bother doing the math when i can look right in front of me and prove I'm right' ? I personally wouldn't class that as 'proof' when you have a simple explanation on wikipedia that contradicts what you say.

Pro photographers don't need to know the technicalities, so fair enough if your grandma considers it a load of rubbish, but she didn't invent the rules...

Oh well. I apologize for turning this into an argument, when all i was looking for was an explanation to a possible misunderstanding of a principle.

NO,NO,NO I was not directing that toward you! sorry if you took it that way! I thought I did explain it to you several times! I wasn't arguing at all though. But I was trying to say was your over thinking it!Jrhaze answered it in better words than me I guess. but i thought I said the same thing at the beginning. Again sorry if I offended you. I was saying if you had to take so much into consideration for every picture you took. know body would want to take a picture! Unless you were a person who LOVED Math, then you would love to do all the math to take the pictures! a.k.a. Math geek. Not directed toward anyone just a example

Mav · Jan 19, 2008

sorry :mrgreen: but...

Happy Hour said:
(Your a MORON!!!)

:hail:

jrhaze · Jan 19, 2008

Happy Hour said:
NO,NO,NO I was not directing that toward you! sorry if you took it that way! I thought I did explain it to you several times! I wasn't arguing at all though. But I was trying to say was your over thinking it!Jrhaze answered it in better words than me I guess. but i thought I said the same thing at the beginning. Again sorry if I offended you. I was saying if you had to take so much into consideration for every picture you took. know body would want to take a picture! Unless you were a person who LOVED Math, then you would love to do all the math to take the pictures! a.k.a. Math geek. Not directed toward anyone just a example

Just to be clear, my post agrees with dbrandon, and in fact is exactly the oposite of what you were claiming earlier in the thread.

In case anyone is keep track

Jamesy · Jan 19, 2008

dbrandon said:
This clearly shows that f2.8 on a 50mm lens is going to be greatly different to f2.8 on a 200mm lens. Unless i am interpreting the equation completely wrongly.

Wether this means the DOF on a 200mm will be shallower due to the larger pupil, i do not know.

this is right, the aperture size is completely dependent on the focal length of the lens. 70-200 2.8s are popular for portraits partly due to their very shallow dof. these do indeed give a much more blurred background than a wide open 50mm f1.8.

dbrandon · Jan 19, 2008

Happy Hour said:
NO,NO,NO I was not directing that toward you! sorry if you took it that way! I thought I did explain it to you several times! I wasn't arguing at all though. But I was trying to say was your over thinking it!Jrhaze answered it in better words than me I guess. but i thought I said the same thing at the beginning. Again sorry if I offended you. I was saying if you had to take so much into consideration for every picture you took. know body would want to take a picture! Unless you were a person who LOVED Math, then you would love to do all the math to take the pictures! a.k.a. Math geek. Not directed toward anyone just a example

I appologise for jumping to the conclusion that your post was aimed at me. And i appologise if my confusion was frustrating. Up until this thread, i have considered myself very comfortable with most aspects of my photography, aperture and exposure etc. But then this thread triggered my own confusion with regards to focal length affecting all sorts of things. So it is me who is confusing myself.

No offence taken. I am compfortable with my mathematics, but would not say i loved math in anyway, just so thats clear :lol:

Again, appologies for jumping to conclusions.

Happy Hour · Jan 19, 2008

dbrandon said:
I appologise for jumping to the conclusion that your post was aimed at me. And i appologise if my confusion was frustrating. Up until this thread, i have considered myself very comfortable with most aspects of my photography, aperture and exposure etc. But then this thread triggered my own confusion with regards to focal length affecting all sorts of things. So it is me who is confusing myself.

No offence taken. I am comfortable with my mathematics, but would not say i loved math in anyway, just so thats clear

Again, apologies for jumping to conclusions.

You confused me for a while 2. and as far as the Math geek I was actually Thinking of my husband when i wrote it! he is the ultimate Math Geek!! the guy can do anything in his head,including Trig!

KrypticChewie · Jan 19, 2008

LaFoto said:
To tell you the truth, I know too little about the camera you used, and don't quite get if you attached a converter or only just the in-built zoom function... so I cannot REALLY help.

I used the built-in zoom function and screwed a 2x telephoto converter on to the front of my cmaera.