Sensor size & focal length question

agp

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Hi guys, I've been watching quite a few different Youtube videos on various things about photography, and one thing that troubles me is sensor size & focal length. I know what both of those things are, but sometimes I hear things like "it's a 35mm focal length but if you factor in the crop/sensor size, it's actually a ##mm focal length". My question is - do different focal lengths produce different zoom/crop/size images when paired with different sensors, and how would you know if Xmm on Y sensor is the same as Zmm on W sensor?
 

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I foresee a 5-10 page thread explaining this simple, yet highly misunderstood/stated topic.


Imagine you printed an 8x10 photo and wanted to frame it.

But you only have a 5x7 frame.

So instead of printing a new photo in the correct size, you crop the photo down in order to fit it in the frame.

Congratulations, you now understand the difference between Full Frame and Crop sensors.
 

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....... My question is - do different focal lengths produce different zoom/crop/size images when paired with different sensors,........

The image projected by a given focal length is always the same size. Lenses do not know what size sensor is behind them.


....... and how would you know if Xmm on Y sensor is the same as Zmm on W sensor?

It's all math, so it can be calculated.

Click here
to see if my explanation helps.


..............Imagine you printed an 8x10 photo and wanted to frame it.

But you only have a 5x7 frame.

So instead of printing a new photo in the correct size, you crop the photo down in order to fit it in the frame.

Congratulations, you now understand the difference between Full Frame and Crop sensors.

What we really need is a Sticky Thread to refer everyone to when the topic comes up.

Keep in mind, taking a scissors or paper cutter to an 8x10 and cutting it down to 5x7 DOES NOT change the focal length of the lens used to take the image. (I'm sure you know this, Braineack, this is for everyone's information).
 

JerryLove

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I foresee a 5-10 page thread explaining this simple, yet highly misunderstood/stated topic.

Indeed.

FOV is usually stated as it relates to "135" or 24x36mm 35mm film pictures. TOday this is commonly referred to as full frame digital (Nikon calls it FX).

For crop-sensor cameras, multiply the ACTUAL focal length on the barrel by 1.5x for Nikon, Pentax, Sony, and 1.6x for Canon.

So, on a Canon 50D or 60D or 70D, a 50mm lens is 50 x 1.6= 80mm equivalent focal length.
 
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agp

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Okay so I get the idea between different sensor size. But how do you calculate those things? If I want to get the same image a 50mm on a full frame would get, what focal length would I use on a APS-C sensor?

I'm currently running a Fuji X-T1 (APS-C) with 23mm. This would be equivalent to using a 34.5mm on a full frame sensor?
 
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JerryLove

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You multiply the image sensor's crop factor by the focal length to determine what the "effective focal length" (in terms of what angle of view is visible) can be seen.

So 50mm on a FF sensor would, on an APC-S sensor (1.6 crop factor) would be 31.25mm.

(*note: though there's a "FF" or "35mm" standard for calibrating focal lengths in most of the interchangeable-lens market, this is not necessarily true with some other camera (say: a lens set designed for iPhones)
 

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I'm currently running a Fuji X-T1 (APC-S) with 23mm. This would be equivalent to using a 34.5mm on a full frame sensor?
Assuming a 1.5 crop factor, yes.
 

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I'm currently running a Fuji X-T1 (APC-S) with 23mm. This would be equivalent to using a 34.5mm on a full frame sensor?

correct. You need a longer lens on a FF, standing in the same physical location, to produce the same image as a crop-sensor since you capture so much more image in the frame.
 
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agp

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Understood, thank you all for the information!
 

Designer

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There are multiple websites that address this issue, such as this one:

Compare camera sensor sizes: full frame 35mm, APS-C, 4/3, 1", 1/1.7", 1/2.5? | PhotoSeek.com | World travel photos, advice, best digital cameras reviewed

Please note: Even though both Canon and Nikon designate their sensors "APS-C", there is a slight difference in actual size between the two.

Now as to how to think of this: The focal length of a lens DOES NOT CHANGE, simply by placing it on a different camera. So the ANGLE of capture (side-to-side) is THE SAME on different cameras.

However, the area of the image that is captured by the sensor will change just because the sensor is a different size.
 

480sparky

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What changes is the apparent field of view between the two. So if you were to take a full-frame and a crop-sensor camera, set them up side-by-side with, say, 50mm lenses on both, and looked through the viewfinders of them, this is what you would see:


FXvDXcomparison.jpg


Neither camera-lens combination 'enlarges' or 'reduces' the apparent size of the subject. The bridge and the flowers are the exact same size in both VFs. What is different is the size of the focus screens, which is in direct proportion to the size of the respective sensors. This, in turn, changes the field of view (measured in degrees).



FFvCropNewSmall.jpg
 

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I know I've posted this before, but can't find it, so here it is again:

Let me try this angle:

The actual, physical size of the FX and DX sensors are different:

1Sensorsizecomparison.jpg




This is readily apparent if you look at an FX body next to a DX body, and can see the actual sensors when both are set to "Mirror Up for Cleaning":

2FXvDXSensorsPost.jpg





So how does this translate into anything meaningful out in the 'real world'? Well, lets' pretend we're out there taking photos. And we come across this peaceful scene:


3Scene.jpg





Now, an FX lens on an FX body has to project a large enough image to cover the larger FX sensor, so it will project an image into the camera that looks like this:

4FXprojection.jpg




(Yeah, I know.... it's right-side-up. In reality, the image would be upside-down, but let's ignore that for the purpose here.)

The lens needs to create a large enough image to cover a sensor measuring 24x36mm (represented by the white rectangle):

5FXSensor.jpg




So an FX lens/body will record the final image as:

6FXFinalImage.jpg







Now let's take the same focal length lens, but only it's a DX-format lens. It will project a smaller circle:

7DXProjection.jpg






Because it only needs to cover a sensor that measures 18x24mm:

8DXSensor.jpg




So the same focal length lens, on a DX body, will record a final image as this:

9DXFinalImage.jpg






Now, if we put the two final images (FX and DX) side by side, we end up with this:

10Finalimagecomparison.jpg





Notice how the subjects in both images are exactly the same size? It's just that the DX sensor recorded a smaller portion of the scene because the sensor is physically smaller! This results in a narrower field of view.

So if you compare the sensor size:


1Sensorsizecomparison.jpg



with the above two images, you should be able to understand the 'crop sensor' effect on field of view.
 

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So you're saying that we should all get an FX camera?

(bookmarking)
 

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