Simple explanation of focal length
Oct 5, 2012 13:43:42 #
I understand f-stop, shutter speed, and ISO, But I can't get through my thick head what is focal length and why one needs to know it? Could you explain this on a fourth grade level?
Oct 5, 2012 13:50:41 #
This is a very complicated subject and hard to easily simplify, and there are so many exceptions to the issue. Here is a fairly comprehensive simple explanation:
http://www.dpreview.com/learn/?/Glossary/Optical/Focal_Length_01.htm
http://www.dpreview.com/learn/?/Glossary/Optical/Focal_Length_01.htm
Oct 5, 2012 13:52:56 #
Oct 5, 2012 13:58:58 #
Greggie wrote:
I understand f-stop, shutter speed, and ISO, But I can't get through my thick head what is focal length and why one needs to know it? Could you explain this on a fourth grade level?
The quick answer is that it refers to the length of the lens and is expressed in millimeters (mm). That, in turn, gives us the field of view of that lens. Generally, 50mm is considered a "normal" lens and lenses shorter are wide angle and the longer ones are the telephoto.
So a 12, 18 24, 35 is a wide angle - it encompasses a wider view than the 50 and an 85, 105, 200 and longer give a progressively narrower field of view, thereby magnifying the view.
Then there are the zoom lenses - that just means that the lens can vary the focal length. A 12-24 has any focal length between the 12 and 24 and a 70-200 has everything between those ends.
Maybe that was fifth grade. I am sure there is a ton of information on the net for you.
Oct 5, 2012 14:25:48 #
Rob O'
Loc: Freakin' Hot Arizona
The Captain is correct again. But now I'm going to complicate things a little.
50mm was considered 'normal' on 35mm film cameras and is still 'normal' on a full frame sensor camera. The reason they are called 'normal' is because it was generally believed that 50mm lenses approximated the field of view of the human eye on 35mm film (which was technically not true - a 35mm lens was closer.) On today's DSLRs, with the exception of the full frame pro DSLRs, a 30 to 35mm lens would be equivalent to the 50mm lenses of yesteryear. This is because the size of the sensor is smaller than a 35mm film frame. So the sensor sees a smaller portion of the picture than the film cameras did. And to make matters worse, 80mm lenses were considered normal for medium format cameras.
If that seems confusing then you should simply ignore it and listen to the Captain!
50mm was considered 'normal' on 35mm film cameras and is still 'normal' on a full frame sensor camera. The reason they are called 'normal' is because it was generally believed that 50mm lenses approximated the field of view of the human eye on 35mm film (which was technically not true - a 35mm lens was closer.) On today's DSLRs, with the exception of the full frame pro DSLRs, a 30 to 35mm lens would be equivalent to the 50mm lenses of yesteryear. This is because the size of the sensor is smaller than a 35mm film frame. So the sensor sees a smaller portion of the picture than the film cameras did. And to make matters worse, 80mm lenses were considered normal for medium format cameras.
If that seems confusing then you should simply ignore it and listen to the Captain!
Oct 5, 2012 14:46:51 #
Rob O' wrote:
The Captain is correct again. But now I'm going to... (show quote)
"Complicate" is the right word. A 50mm lens is a 50mm lens no matter what body you mount it on. You are implying that the lens changes when in fact it is nothing more than the portion of the image that is cast by that lens that is being captured by a smaller sensor. The image cast is exactly the same no matter what downsized sensor is recording whatever portion of that image.
Oct 5, 2012 15:52:40 #
Rob O' wrote:
The Captain is correct again. But now I'm going to... (show quote)
Just a minor correction to your post, Rob O:
The 43mm lens was considered "normal" (later rounded off to 50mm) for the 35mm format not because of field-of-view. The human field-of-view is considerably greater than either a 43mm or 50mm lens. It was considered "normal" because the perceived Perspective was similar to that of human vision.
Oct 5, 2012 16:26:53 #
FilmFanatic
Loc: Waikato, New Zealand
You could take your 18-55 lens, look through it at 18 and then 55 and note the difference in field of view. Then do the same for your 70-300 lens at 70 and 300. Then you will know which focal length looks like which field of view and you will know which focal length to use for any situation
Oct 5, 2012 17:41:33 #
Here is a visual thingy thing, to check out.
http://www.usa.canon.com/app/html/EFLenses101/focal_length.html
Pat
http://www.usa.canon.com/app/html/EFLenses101/focal_length.html
Pat
Oct 5, 2012 22:29:12 #
Oct 5, 2012 22:55:30 #
FilmFanatic
Loc: Waikato, New Zealand
MtnMan wrote:
I see Rob O covered most of my point. But I don't agree with his last sentence.
Why not?
Oct 5, 2012 23:09:50 #
FilmFanatic wrote:
Why not?
MtnMan wrote:
I see Rob O covered most of my point. But I don't agree with his last sentence.
Why not?
The statement was, "If that seems confusing then you should simply ignore it and listen to the Captain!"
The reason I disagree with that is for most DSLRs (those with cropped sensors) 35mm is normal and Captain didn't mention that.
Many people are confused about that.
Oct 6, 2012 06:07:46 #
Pablo8
Loc: Nottingham UK.
I think one would have to go back to film cameras to realise what 'normal' focal length is. If one measures a film negative, diaganol corner to corner, that measurement is the .normal' focal length for that negative size, regardles of whether you are measuring 35mm / 2 1/4" sq / or 5 x 4 negatives.Now that you are all digital users, you do not have negatives to take measurements from. Your loss!!
Oct 6, 2012 06:09:53 #
FilmFanatic
Loc: Waikato, New Zealand
Pablo8 wrote:
I think one would have to go back to film cameras to realise what 'normal' focal length is. If one measures a film negative, diaganol corner to corner, that measurement is the .normal' focal length for that negative size, regardles of whether you are measuring 35mm / 2 1/4" sq / or 5 x 4 negatives.Now that you are all digital users, you do not have negatives to take measurements from. Your loss!!
You don't have a neg, you have a thing called a sensor, and you can see it if you take the lens off! Even better, the dimensions are readily available online, you can measure to your heart's content. Even cut a neg up to that size and measure it with a ruler if it makes you feel better. I love film but statements like yours make me roll my eyes
Oct 6, 2012 06:10:35 #
Jay Pat wrote:
Here is a visual thingy thing, to check out.
http://www.usa.canon.com/app/html/EFLenses101/focal_length.html
Pat
http://www.usa.canon.com/app/html/EFLenses101/focal_length.html
Pat
That was really informative Jay. Thank you for posting it. I enjoyed seeing what a difference it made when the changes were made.
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