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Understanding Focal Length

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So a few weeks ago we were gathered at Rusty's place for a photo tech day and there were a few conversations about focal length and what relation it plays with the size of sensor that you have in your camera. Also I have seen these questions pop up from time to time on here as well as other places so I figured that we could take a moment to discuss.

I guess we could start this off by actually define focal length in a very easy to understand way. Focal length is the distance between the point of convergence inside the lens, and the sensor of the camera.

focal-length-cross-section.jpg


So what does that mean in the real world? First it controls how wide or narrow of a field of view that your camera sees. A longer focal length means that you will have a narrower field of view. A shorter focal length will give you a wider field of view.

The second biggest thing that focal length gives you is a different perspective. Along with perspective and certainly not to be forgotten is lens compression. At 24mm you can obviously see that the cans are separated by a certain distance but at 300mm that separation is virtually gone.
focal-length-comparison.jpg


This is a very handy thing to know when shooting portraits, landscapes, or even motorbike travel shots. When shooting a portrait of someone with a large nose, do you want to make it appear smaller? When taking a photo of your bike near the sign of some National Park, to you want it to appear closer to the sign instead of off in the distance? If so, lens compression is your friend.

Finally, since focal length is a measurement between the convergence point on a lens and it's sensor, it also means that no matter what camera body you put a particular lens on, the focal length does not, and will not change. A 50mm lens is a 50mm lens no matter what you size sensor it is sitting in front of.

I know, there are lots of folks that will chime in to say that when they pull a 50mm lens off of their full frame camera and put in onto one with a smaller sensor the effect is that the lens is more zoomed in. While it may appear this way, it is just because the crop body has just that, a smaller sensor. It is not using the entire image that the lens is giving it, but it is most certainly not changing the "zoom" of the lens. It is more accurately changing the field of view, which is an all-together different thing.
Cross-Section-lens-projection.jpg


So putting a 50mm lens on a smaller sensor does not in fact give you a 70mm lens. It merely gives you a 50mm lens with a decreased field of view. The very same thing can be accomplished by taking a full frame camera and simply cropping the image after the fact using any photo editing software that you choose. Remember, there is a reason they call them "crop bodies" and not "zoom bodies". ;)

For more info on any of this stuff, here are some handy links:
https://en.wikipedia.org/wiki/Perspective_distortion_(photography)
http://www.photocrati.com/the-curious-case-of-lens-compression/
http://imaging.nikon.com/lineup/dslr/basics/19/01.htm
 

Tourmeister

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I know, there are lots of folks that will chime in to say that when they pull a 50mm lens off of their full frame camera and put in onto one with a smaller sensor the effect is that the lens is more zoomed in. While it may appear this way, it is just because the crop body has just that, a smaller sensor. It is not using the entire image that the lens is giving it, but it is most certainly not changing the "zoom" of the lens. It is more accurately changing the field of view, which is an all-together different thing.
Cross-Section-lens-projection.jpg


So putting a 50mm lens on a smaller sensor does not in fact give you a 70mm lens. It merely gives you a 50mm lens with a decreased field of view. The very same thing can be accomplished by taking a full frame camera and simply cropping the image after the fact using any photo editing software that you choose. Remember, there is a reason they call them "crop bodies" and not "zoom bodies". ;)

For more info on any of this stuff, here are some handy links:
https://en.wikipedia.org/wiki/Perspective_distortion_(photography)
http://www.photocrati.com/the-curious-case-of-lens-compression/
http://imaging.nikon.com/lineup/dslr/basics/19/01.htm
:tab This is technically correct. There is no optical zoom. However, if you have a 24Mpix crop sensor and a 24Mpx FX sensor you get different results between the shot from the crop and the FX cropped in post. This is because entire sensor is capturing the image on the crop body compared to only part of the sensor of the FX body ends up in the post cropped image. The end result is that for two sensors of the same Mpix but different physical sizes, you will usually get a better image quality from the crop body than you would by cropping the image in post from the FX body. This is why a lot of nature photographers still use crop sensor bodies.
 
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:tab This is technically correct. There is no optical zoom. However, if you have a 24Mpix crop sensor and a 24Mpx FX sensor you get different results between the shot from the crop and the FX cropped in post. This is because entire sensor is capturing the image on the crop body compared to only part of the sensor of the FX body ends up in the post cropped image. The end result is that for two sensors of the same Mpix but different physical sizes, you will usually get a better image quality from the crop body than you would by cropping the image in post from the FX body. This is why a lot of nature photographers still use crop sensor bodies.
Correct. All things being equal you will get more data from using the entire sensor of a crop body over a full frame body. However, in terms of the lens, it's perspective, and lens compression, none of those things change one bit when going between different sized sensors. The only thing that changes is your field of view.

Which is why even with a crop body camera I still wanted an 85mm lens for shooting portraits. I wanted the perspective and compression that comes from that focal length, not necessarily the field of view.
 
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:popcorn:

Very good info, I know in practice what depth of field is. Lower f-stop more bOkEh! But its nice to see the technical reasons behind it. Now leave me alone while I go bokeh whore with my 50mm nifty-fifty on my crop body! :dude:
 
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Now remember, field of view and depth of field are two different things. Depth of field IS affected by sensor size as you are using more of the lens with a full frame camera to create that bokeh. ;)
 
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Its all voodoo and black magic to me...

Take a subject matter that is the same distance from the camera and using the same lens on a DX vs FX body, the subject will look "bigger" on a DX body.

My experiments here: LINK
 
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Its all voodoo and black magic to me...

Take a subject matter that is the same distance from the camera and using the same lens on a DX vs FX body, the subject will look "bigger" on a DX body.

My experiments here: LINK
Duke, you are 100% correct in that it will appear bigger or more zoomed in coming from a crop body. How it is appearing to do that is the question. If you select the function on your full frame camera to switch the sensor to a crop body size you will get the exact same image from either camera. The only different is that with a full frame camera you are using more of the lens so you get more of the surround area in the frame.

Think of it as the crop body is already cropping out the surrounding area of an image, and a full frame is not. So you could achieve the same thing by going into Lightroom and cropping your full frame shots down to a crop body sensor size and they would look just as big at that point.

Clear as mud yet?:lol2:
 
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I do like your simplified explanation [Remember, there is a reason they call them "crop bodies" and not "zoom bodies /] as it is EXACTLY what is taking place.

As for the image, well, its pretty dramatic what the same lens looks like on the different type of bodies, that is the difficult part for me to quantify as they are both "right" but both very "different". And each has a purpose and are neither a detriment or an advantage..... That is the black magic, they work the same but different and you can use to your advantage or disadvantage.... Voodoo I tell ya!
 
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Tourmeister

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:tab My understanding is that a 50mm DX lens on a DX body should give REAL close to the same image as a 50mm FX lens on an FX body. This has to do with the size of the image circle created by the lens on the plane of the sensor. Lenses create a circular image on the plane of the sensor. Ideally, the corners of the sensor lie on the perimeter of the circle at the four points. This has the rectangle of the sensor covering the greatest possible area of the circle without actually going outside the circle. The small areas outside the rectangle are simply lost. There will be slight differences in DOF though.

:tab Where you get the "Zoom" or crop effect is when you take an FX lens and put it on a DX body. This makes a much larger image circle on the plane of the sensor. Now the corners of the sensor are well inside the outer edges of the circle and much more of the image is lost. However, the image will usually still look good because the full sensor is being used to capture that smaller area of the circle.

This shows it really well.
3Ol3CSt.jpg


The left side shows an FX lens on both the FX and DX sensor. This is where you get the "crop" effect. The right side shows a DX lens on a DX and FX sensor. You can see that with the FX sensor and DX lens, you lose the corners of the image because the sensor is larger than the image circle created by the lens. This will create a dark vignetting in the corners unless you switch your FX sensor to DX mode.
 
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Folks that say crop bodies "don't zoom" are begging the question.
For all intents and purposes they do since they show a reduced field of vision compared to human eyesight. There is a lot of technical stuff behind depth of field, circles of confusion etc. that simply don't matter as long as you know how to make it work for you. The simplest way by far is the simple multiplication factor. An APS-C sensor is roughly a 1.6 multiplication factor from the full-frame 35mm used as a standard reference in SLR's. A 100mm acts like a 160mm. Your depth of field is increased by the same multiplier. Bottom line is if it acts like a telephoto then there's no harm in just thinking of it that way. K.I.S.S.
Now if you want to use the mathematical principles then you can expand your photographic horizons - but most folks just want to take nice photos.
 
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Now if you want to use the mathematical principles then you can expand your photographic horizons - but most folks just want to take nice photos.
Hence the reason for me posting this in the "Learning" section. I'm hoping that the people that just want to take nice pictures and don't care about anything else with just skip on by.

You are 100% correct in that a crop body does reduce your effective field of view. But it does not change your perspective in the least. Mathematical or not, that is a very real thing that can make or break a photo. Or simply put, it can make a really nice photo turn into an outstanding photo. I'm just trying my best to help people understand what these lenses and camera bodies are doing, and more importantly what they aren't doing.:thumb:
 
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