Understanding focal lengths
Apr 27, 2018 03:28:54 #
What does a lens's focal distance tell me to do? In other words, will knowing the focal length of a lens determine my optimal distance in yardage or mileage from a subject? Or is it a complex calculation of factors within the lens itself? I know that there is some relationship between the physical length of a lens and the physical distance it can "reach," so I do know to select a longer lens for air shows and wildlife than for capturing a house pet.
However there are so many different lenses to choose from, and while all are identified by their focal length in millimeters as well as by their aperture ranges, yet some might be 200-400, or 100-300, or 100-400mms for example, so how do those similar ranges advise us to choose among them. Is there a ratio, a formula, for that relationship that guides us in choosing the appropriate lens for a given situation, or are other parameters more relevant? Does a 35mm lens mean we should ideally stand 35mms away from a statue for the best shot? Should we know the distance to a river's shore from a river cruise ship in order to select a lens to shoot the scenery we pass? What about when shooting birds in flight -- does the number relate to our distance to them on the ground or from the ground to where the birds are in the sky?
Maybe there is an obvious explanation that I'm missing, but I would love to know what it is! So thanks in advance for enlightening me :-)
However there are so many different lenses to choose from, and while all are identified by their focal length in millimeters as well as by their aperture ranges, yet some might be 200-400, or 100-300, or 100-400mms for example, so how do those similar ranges advise us to choose among them. Is there a ratio, a formula, for that relationship that guides us in choosing the appropriate lens for a given situation, or are other parameters more relevant? Does a 35mm lens mean we should ideally stand 35mms away from a statue for the best shot? Should we know the distance to a river's shore from a river cruise ship in order to select a lens to shoot the scenery we pass? What about when shooting birds in flight -- does the number relate to our distance to them on the ground or from the ground to where the birds are in the sky?
Maybe there is an obvious explanation that I'm missing, but I would love to know what it is! So thanks in advance for enlightening me :-)
Apr 27, 2018 04:51:25 #
Pablo8
Loc: Nottingham UK.
I would suggest (if it is possible/ convenient) to call in at a local camera shop, with your camera, and try a few lenses to see what coverage each focal length will give you. The suggestion to the shop owner that you are ready to purchase, should make for a more pleasant experience, and show that you are NOT a 'Time-Waster'. Best of luck.
Apr 27, 2018 05:03:47 #
LWW
Loc: Banana Republic of America
If you are using a DSLR with a full frame (FF) sensor a 50mm focal length lens will give you roughly a 1:1 perspective of what you would see with the human eye ... or zero times magnification.
If you are using cropped (DX) sensor DSLR then a 35 mm focal length would provide the same roughly 1:1 equivalence.
Your formula asked is a simple one. If you have a FF sensor then a 200 mm focal length would give you a 4 times normal view, on a DX sensor the same lens would give a 5.7 times point of view.
If you are using cropped (DX) sensor DSLR then a 35 mm focal length would provide the same roughly 1:1 equivalence.
Your formula asked is a simple one. If you have a FF sensor then a 200 mm focal length would give you a 4 times normal view, on a DX sensor the same lens would give a 5.7 times point of view.
Apr 27, 2018 05:06:10 #
LWW
Loc: Banana Republic of America
If you are using a DSLR with a full frame (FF) sensor a 50mm focal length lens will give you roughly a 1:1 perspective of what you would see with the human eye ... or zero times magnification.
If you are using cropped (DX) sensor DSLR then a 35 mm focal length would provide the same roughly 1:1 equivalence.
Your formula asked is a simple one. If you have a FF sensor then a 200 mm focal length would give you a 4 times normal view, on a DX sensor the same lens would give a 5.7 times point of view.
If you are using cropped (DX) sensor DSLR then a 35 mm focal length would provide the same roughly 1:1 equivalence.
Your formula asked is a simple one. If you have a FF sensor then a 200 mm focal length would give you a 4 times normal view, on a DX sensor the same lens would give a 5.7 times point of view.
Apr 27, 2018 06:41:10 #
As follow-on to the earlier comments from Pablo8 and LWW, first, try this Nikon tool, link below, and move the slider between 14mm and 800mm to visualize the field of view and relative closeness of the zoom (telescope) effect of the focal length. Note how the smaller numbers give a wider view than you can see with your eyes. Note how the higher numbers bring distant subjects closer, but within a much narrower field of view. For your river cruise question, you're should be looking for a lens that covers / reaches 300mm to capture details of the shore line and beyond. For BIF and most wildlife, you'll need 400mm or longer. The goal is to fill the frame with the subject, to the extent possible, either by you being physically close to the subject or having the focal length of a lens to 'reach' out to the subject (or both).
http://imaging.nikon.com/lineup/lens/simulator/
Second, these two links provide more of the background discussion in thinking about whether the lens focal length versus your human eyes that are approximately 50mm.
http://www.myfavouritelens.com/focal-length-explained-simply/
http://laughingsquid.com/focal-length-demonstration/
http://imaging.nikon.com/lineup/lens/simulator/
Second, these two links provide more of the background discussion in thinking about whether the lens focal length versus your human eyes that are approximately 50mm.
http://www.myfavouritelens.com/focal-length-explained-simply/
http://laughingsquid.com/focal-length-demonstration/
Apr 27, 2018 07:06:15 #
Very complicated questions, but I will try to answer the 35mm lens question.
"Does a 35mm lens mean we should ideally stand 35mms away from a statue for the best shot?" No.
The physical length of a lens often is different from its optical length.
For simple lenses a lens needs to be physically at its focal length to be focused at infinity if it was a 90mm lens that would be at 90mm from the sensor. At 90mm it would be focusing whats at 'infinity' on the surface of the sensor. kind of like focusing a projector on a screen but in reverse. If the lens is nearer than its focal length then its focusing beyond infinity and the image projected on the sensor is blurry. To focus on something nearer than infinity the lens needs to move further away from the sensor. This isn't a straight line relationship but a logarithmic one.
To focus at 10x the focal length the distance is 1.1 x focal length for a 90mm lens to focus at 900mm that would be 99mm
to focus at 100x the focal length the distance is 1.01 x focal length for the 90mm lens to focus at 9000mm it would be 90.9 mm
to focus at 1000x the focal length the distance is 1.001 x focal length for the 90mm lens to focus at 90,000mm it would be 90.09mm
converting those real world distances
900mm = 90cm or 0.9meters about 3 feet
9000mm = 9 meters about 30 feet
90,000mm = 90 meters about 300 feet
Basically infinity is when the lens is so close to being positioned at its focal length you just can't move it that precisely.
closer than about 10x and the distance the lens has to move away from the sensor increases rapidly. e.g focusing at 180mm would require the 90mm lens to move out by 90mm which is beyond what is practical for most helicoids. You can use extension tubes between the lens and the camera to focus in closer than normal.
Optics can make these physical distances completely wrong, i avoided the 35mm lens in this explanation because the registration distance of an slr (depending on brand) tends to be 44 - 47mm so 35mm is inside the lens mount , so a 35mm lens is not a simple lens :)
ok you should now have a handle on
"beyond infinity" lens closer than its focal length to the sensor
"infinity" when the lens is at its focal length or so close it makes no difference
and what happens when you want to focus nearer than infinity. Unless a lens is a macro lens (designed to focus extremely close up) 10x focal length is a good ball park figure for minimum focus distance.
Shorter lenses focus closer than longer lenses and infinity is nearer for shorter lenses than longer lenses. Depth of field on a short lens will make most of what you see in front of the camera in focus and longer lenses will have a narrower depth of field. Field of view the width and height of what you see also changes between wide angle lenses which give a wide field of view to long telephoto lenses which give a narrow field of view. Because your sensor is a fixed size wide angle lenses give the effect of being further away than telephoto lenses.
Different sensor sizes take a different crop of whats projected on the back wall of the camera for a full frame enlarging that image by about a factor of 7x would give a photo about 8 by 10 for a crop sensor camera enlarge by about 10x for the same size photo and 49x for an iphone sensor.
So a given focal length might be wide angle , normal , or telephoto depending on the sensor size. You need to know the system you are using to be able to be able to say if a focal length is going to be a wide angle , a normal or a telephoto lens on that camera.
When most people were using 35mm cameras around 50mm was considered a normal lens (giving an image with the same view as your eye) there is a more technical definition but we don't care :) a 28mm would be considered wide and anything longer than 50mm telephoto (people will argue that definition too). You could kinda say the 28mm lens was about half the 50mm lens focal lens and would be similar to being twice the distance of your subject and a 100mm lens a bit like being twice as close.
With modern digital sensors the sensor size changes those values an aps-c camera you can multiply the focal length by 1.5 to get an approximation of the 35mm view so a 50mm looks like 75mm (a bit telephoto) the 28mm looks like 42mm (pretty close to normal)
If you only know your own camera system then you will figure it out, but crop factors are useful comparing against different sensor sizes.
A focal length is a focal length on any camera system even though the image you can capture is vastly different across systems.
10x focal length is a fair rule of thumb for the minimum focus distance so expect to be able to focus as close as 50cm with a 50mm lens and 2 meters with a 200mm lens.
Zoom lenses are a compromise generally a smaller zoom lens range will give better IQ than a wider zoom range, better quality lenses should do better than cheaper lenses but you pay for that :) bigger sensors require larger diameter lens elements than smaller sensors in order to make a large enough image circle to cover the sensor (this also makes them more expensive).
A fast lens (say 2.8 or better) lets in more light than a more consumer orientated lens like an f4 lens it helps focus faster and gives faster shutter speeds you pay for that too. Theres also a rule of thumb that says stopping 2 stops down from wide open gives the best image quality another advantage to a fast lens.
It would be nice to be able to afford these fast lenses but you can still get great results from more affordable lenses. It's your eye that makes the composition and the compelling photograph. It's the photographer that makes the biggest difference.
The longer the focal length of a lens the more sensitive it is to movement, which means raising shutter speed. This means less light reaches the sensor so you need to increase aperture and raise the iso sensitivity of the camera. There is a practical limit to this which may make for lousy photos, a shorter lens will capture more of the scene but less of the detail but it will be able to use lower shutter speed. It's a compromise.
Hope this helps some.
"Does a 35mm lens mean we should ideally stand 35mms away from a statue for the best shot?" No.
The physical length of a lens often is different from its optical length.
For simple lenses a lens needs to be physically at its focal length to be focused at infinity if it was a 90mm lens that would be at 90mm from the sensor. At 90mm it would be focusing whats at 'infinity' on the surface of the sensor. kind of like focusing a projector on a screen but in reverse. If the lens is nearer than its focal length then its focusing beyond infinity and the image projected on the sensor is blurry. To focus on something nearer than infinity the lens needs to move further away from the sensor. This isn't a straight line relationship but a logarithmic one.
To focus at 10x the focal length the distance is 1.1 x focal length for a 90mm lens to focus at 900mm that would be 99mm
to focus at 100x the focal length the distance is 1.01 x focal length for the 90mm lens to focus at 9000mm it would be 90.9 mm
to focus at 1000x the focal length the distance is 1.001 x focal length for the 90mm lens to focus at 90,000mm it would be 90.09mm
converting those real world distances
900mm = 90cm or 0.9meters about 3 feet
9000mm = 9 meters about 30 feet
90,000mm = 90 meters about 300 feet
Basically infinity is when the lens is so close to being positioned at its focal length you just can't move it that precisely.
closer than about 10x and the distance the lens has to move away from the sensor increases rapidly. e.g focusing at 180mm would require the 90mm lens to move out by 90mm which is beyond what is practical for most helicoids. You can use extension tubes between the lens and the camera to focus in closer than normal.
Optics can make these physical distances completely wrong, i avoided the 35mm lens in this explanation because the registration distance of an slr (depending on brand) tends to be 44 - 47mm so 35mm is inside the lens mount , so a 35mm lens is not a simple lens :)
ok you should now have a handle on
"beyond infinity" lens closer than its focal length to the sensor
"infinity" when the lens is at its focal length or so close it makes no difference
and what happens when you want to focus nearer than infinity. Unless a lens is a macro lens (designed to focus extremely close up) 10x focal length is a good ball park figure for minimum focus distance.
Shorter lenses focus closer than longer lenses and infinity is nearer for shorter lenses than longer lenses. Depth of field on a short lens will make most of what you see in front of the camera in focus and longer lenses will have a narrower depth of field. Field of view the width and height of what you see also changes between wide angle lenses which give a wide field of view to long telephoto lenses which give a narrow field of view. Because your sensor is a fixed size wide angle lenses give the effect of being further away than telephoto lenses.
Different sensor sizes take a different crop of whats projected on the back wall of the camera for a full frame enlarging that image by about a factor of 7x would give a photo about 8 by 10 for a crop sensor camera enlarge by about 10x for the same size photo and 49x for an iphone sensor.
So a given focal length might be wide angle , normal , or telephoto depending on the sensor size. You need to know the system you are using to be able to be able to say if a focal length is going to be a wide angle , a normal or a telephoto lens on that camera.
When most people were using 35mm cameras around 50mm was considered a normal lens (giving an image with the same view as your eye) there is a more technical definition but we don't care :) a 28mm would be considered wide and anything longer than 50mm telephoto (people will argue that definition too). You could kinda say the 28mm lens was about half the 50mm lens focal lens and would be similar to being twice the distance of your subject and a 100mm lens a bit like being twice as close.
With modern digital sensors the sensor size changes those values an aps-c camera you can multiply the focal length by 1.5 to get an approximation of the 35mm view so a 50mm looks like 75mm (a bit telephoto) the 28mm looks like 42mm (pretty close to normal)
If you only know your own camera system then you will figure it out, but crop factors are useful comparing against different sensor sizes.
A focal length is a focal length on any camera system even though the image you can capture is vastly different across systems.
10x focal length is a fair rule of thumb for the minimum focus distance so expect to be able to focus as close as 50cm with a 50mm lens and 2 meters with a 200mm lens.
Zoom lenses are a compromise generally a smaller zoom lens range will give better IQ than a wider zoom range, better quality lenses should do better than cheaper lenses but you pay for that :) bigger sensors require larger diameter lens elements than smaller sensors in order to make a large enough image circle to cover the sensor (this also makes them more expensive).
A fast lens (say 2.8 or better) lets in more light than a more consumer orientated lens like an f4 lens it helps focus faster and gives faster shutter speeds you pay for that too. Theres also a rule of thumb that says stopping 2 stops down from wide open gives the best image quality another advantage to a fast lens.
It would be nice to be able to afford these fast lenses but you can still get great results from more affordable lenses. It's your eye that makes the composition and the compelling photograph. It's the photographer that makes the biggest difference.
The longer the focal length of a lens the more sensitive it is to movement, which means raising shutter speed. This means less light reaches the sensor so you need to increase aperture and raise the iso sensitivity of the camera. There is a practical limit to this which may make for lousy photos, a shorter lens will capture more of the scene but less of the detail but it will be able to use lower shutter speed. It's a compromise.
Hope this helps some.
Apr 27, 2018 07:29:41 #
fotogal1 wrote:
What does a lens's focal distance tell me to do? I... (show quote)
Fotogal.... '..are other parameters more relevant..'
The only relevant factor for what you are talking about is the 'Field of View' or FoV. The focal length is only a physical dimension of the lens and is virtually meaningless without knowing the sensor size or crop factor of the camera. As you probably know, a 50mm lens on a FF camera will give a FoV of approx 46 degrees or close to the FoV of normal vision. Longer lenses narrow the FoV but the resulting image is stretched out to the full frame size so it appears that you are closer to the subject. A 300mm lens will only have a FoV of about 8 degrees. Calculating the FoV for a particular scene would require knowing the size of the subject, the fraction of the frame that you want the subject to cover and your distance to the subject - and a trigonometry calculator to find the angle of FoV. When you have found the required angle you would need another calculator to find the related lens focal length.
Needless to say - most photographers just go by experience.
The best way to gain experience is to try shooting with different lenses - if you have them. If you don't have a bag of lenses, another way to gain experience is to use a phone App like 'Set my Camera'. This has a FoV viewer. Just hold the camera up like you were taking a picture and zoom in and out until you get it right and then read the Focal Length. You can take the snap too.
Apr 28, 2018 01:50:49 #
Thank you to Pablo8, LWW, CHG Canon, Blackest, and repleo! Your explanations combine to make a valuable guide to the meaning of focal length and its role in creating images along with the other primary settings. I am both grateful and thunderstruck at the detail and depth of your replies. Now i can see that it will take some experimenting for me to work out what I ideally want in terms of FOV, the fastness of the glass, the apertures' influence on image, and as you said, to what extent I'm forced to compromise on these things by price!
Thanks for helping me to understand enough of a more complex subject than I'd realized so I can make a lens selection with some confidence and without sounding clueless at the camera store!
-- Fotogal1
Thanks for helping me to understand enough of a more complex subject than I'd realized so I can make a lens selection with some confidence and without sounding clueless at the camera store!
-- Fotogal1
Apr 28, 2018 08:44:43 #
One thing to add to the definitions already posted is, the longer the focal length of the lens, the farther away you can photograph. If you plan on photographing specific things, such as birds or wildlife, you would need to know the approximate distance they would be at. With that knowledge, you could go into a camera store and ask for a lens that will be good for photographing (example - deer) at 100 yards. If the sales person is intelligent, they would ask you how big you want the deer to be in the frame (fill most of the frame or be a smaller part of the framed image).
Or, you could give that information here and many will give you some choices to think about before going to the camera store. An example is, what camera do you have, what do you plan on using the lens for most often, what is your budget, etc?
Or, you could give that information here and many will give you some choices to think about before going to the camera store. An example is, what camera do you have, what do you plan on using the lens for most often, what is your budget, etc?
Apr 28, 2018 08:51:52 #
fotogal1 wrote:
Now i can see that it will take some experimenting for me to work out what I ideally want in terms of FOV, the fastness of the glass, the apertures' influence on image, and as you said, to what extent I'm forced to compromise on these things by price!
Thanks for helping me to understand enough of a more complex subject than I'd realized
Thanks for helping me to understand enough of a more complex subject than I'd realized
Yes, there is very little you can learn from theory here. Take lenses out into 'the field', see what each does through the viewfinder, take some pictures, and 'study' the results back home. Experience is the best source of understanding.
Apr 28, 2018 08:52:58 #
ole sarg
Loc: south florida
Hey all this is really complex and interesting but I used this rule of thumb. The shorter the lens the wider the view.
Apr 28, 2018 09:02:52 #
fotogal1 wrote:
What does a lens's focal distance tell me to do? I... (show quote)
If no one has mentioned it yet, the focal length is determined by a mathematical formula. The larger the number, the closer it will get you to your subject (optically, that is.
)https://en.wikipedia.org/wiki/Focal_length
Apr 28, 2018 10:03:22 #
gvarner
Loc: Central Oregon Coast
Use a photographic calculator to answer your questions. The one at tabaware.com is great. Plug in the crop factor, focal length, and subject distance to get all kinds of results. I like the one that calculates field of view.
Apr 28, 2018 10:08:07 #
I will try to give you the uncomplicated answer because you don't need a complicated one. Focal length just means the power of the lens to bring in the subject closer, or push it back into the wide angle range. If it is a zoom and not fixed (prime) it can cover various ranges. These ranges can be wide angle only, or telephoto only , or cover both. The numbers involved are not consistent and are affected by the size of the sensor. A 50mm lens is not the same with different sensor sizes. Also, the largest (widest) f stop available varies with the lens and the setting you use on a zoom unless it uses a fixed f stop (more expensive). That's as brief as I can get....
Apr 28, 2018 10:40:54 #
The OP question is a very good question but I don't know how to explain to the OP in a post.
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