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davide sorasio

Confusion about lenses/sensor size and angle of view. Larger format lens on smaller format sensor.

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Hi everybody,

I was reading a previous issues of "Film and Digital Times" and I came across a portion in wich the editor ask one of the lens engeneers behind the signature primes:

-Q: "What happens with a large format lens like a signature prime on a Super 35mm camera like an Alexa SXT?" 

-A: "The LF lens looks the same as the S35 lens. An 18mm LF lens on the S35 camera looks just like an 18mm S35 lens on the S35 camera"

Maybe I lack of knowledge, but I thought we were going to get a narrower field of view. So following the same logic for example if I put a 50mmn Master prime on an Arri 416 plus my lens would give me the same FOV as, for example, a 50mm Ultra 16 prime.

  I'm really confused about what happens when we put a bigger format lens on a smaller format sensor.

Thanks in advance!

Edited by davide sorasio

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A 50mm lens is a 50mm lens, regardless of the format it is designed for. You could put that Signature prime on any format from Super 8 to Full Frame. Its focal length does not change. What does change is the Field of View. On Super 8, a 50mm is a long telephoto lens, on s35, it's standard, and on FF it's slightly wide angle.

Try drawing a circle. That's the lens image circle.The lens's image circle is everything the lens "sees". Now draw a FF sized box inside the circle. That is everything the FF sensor "sees". If you then draw a s16 sized box inside the circle, you can see that it "sees" less of the circle than the FF box, and therefore has a narrower Field of View. A super 8mm sized box would see even less. The field of view changes with each sensor size, even though the focal length of the lens stays exactly the same.

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The large format lens on small format may have lower contrast and more aberrations and less resolving power than a lens optimized for small format. The field of view is the same like Stuart explained

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7 hours ago, aapo lettinen said:

The large format lens on small format may have lower contrast and more aberrations and less resolving power than a lens optimized for small format. The field of view is the same like Stuart explained

But isn't Stuart saying the FoV will change depending on the sensor size ..?.. the focal length, as a mathematical equation will not change of course.. 

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The focal length is a constant, so an 18mm is an 18mm whether on a Super-8 camera or an IMAX camera, it's just going to have a narrower field of view on a smaller format.  It also may not cover a larger format.  So an 18mm designed for FF35 but put on a Super-35 camera will look the same in terms of view, depth of field, etc. as an 18mm designed for a Super-35 camera, but the 18mm designed for the Super-35 camera might vignette on a FF35 camera.

Yes, you can argue that in terms of sharpness, contrast, etc. that the design compromises depend on the format size. On the one hand, a lens designed for FF35 but put onto a Super-35 camera will be using more of the optical sweet spot in the center of the lens since the sides will be cropped to a narrower view, but on the other hand, sometimes larger format lenses are designed with a lower MTF simply because if your format physically has more "millimeters" overall, then a lens does not have to resolve as many lines per millimeter,  which is the advantage of oversampling.

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11 hours ago, davide sorasio said:

Hi everybody,

I-A: "The LF lens looks the same as the S35 lens. An 18mm LF lens on the S35 camera looks just like an 18mm S35 lens on the S35 camera"

Maybe I lack of knowledge, but I thought we were going to get a narrower field of view. So following the same logic for example if I put a 50mmn Master prime on an Arri 416 plus my lens would give me the same FOV as, for example, a 50mm Ultra 16 prime.

  I'm really confused about what happens when we put a bigger format lens on a smaller format sensor.

Thanks in advance!

He said he was comparing the 18mm LF lens put onto the S35 camera, so yes you get a narrower view compared to being put onto a LF camera, but the same view as an 18mm S35 lens.  18mm is a physical measurement of a lens, it's independent of the format size it is going onto, other than the coverage (image circle) issues.

Yes, a 50mm Master Prime on a S16 camera would have the same view as a 50mm Ultra16 prime -- it's a 50mm!

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Still confusing mostly because people use the term field of view differently.  I'd like to try to explain my understanding without the term field of view.  Please let me know where I'm wrong. I understand that the depth of field and the spacial distortion of a lens is fixed. Regardless of the sensor size those characteristics of the lens are part of the optics and not the camera.  25mm will have a the width, height, and spatial distortion relationships between distances of a 25mm within its imaging circle.  When the sensor is smaller than the imaging circle, there is a crop, or smaller cookie cut out of the 25mm imaging circle.  It is not the same as shooting on a tighter mm.  The distortion and the depth of field is the same as the 25mm, but it is cropped in.  The sensor size the imaging circle that the lens was designed for only impacts the potential cropping.  But the full spacial perception characteristics of depth, width, and height, of a 25mm can be projected onto different sized imaging circles.  You can have a 25mm imaging circle that fits on a small sensor that sees the same exact frame as a 25mm designed for big sensor.   If you take the 25mm with an imaging circle designed for LF, and put it on a s16 sensor.  You are still looking through a 25mm but with a crop in equivalent to the percentage of the surface area of the LF sensor divided by the surface area of the S16 sensor.    Now saying a 25mm is a 25mm is a 25mm is right but its kinda a riddle that perplexes the original question.  You won't use an 25mm with a LF imaging circle on s16 like you would on LF, the experience of shooting on the 25mm with a fixed imaging circle is different on the different sensors.  Due to the crop thats going to effect the distance subjects are from the camera.  While you might have liked subjects 6 feet from the camera for mediums on a 25mm with a LF imaging circle, once you use the s16 sensor with the LF lens, the crop will force you move your subject back to 12 feet lets say for the medium.  You will get the depth of field and distortion characteristics of a 25mm at 12 ft but your frame will be a medium.   The frame will definitely look different even though its the same lens. The entire experience of shooting will be different, BUT it is not the same as a 50mm in terms of depth of field and distortion characteristics setting the focus to 12 ft.  It is important to know what imaging circle size the lens was designed for because of potential cropping.  

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7 minutes ago, Ryan Emanuel said:

Still confusing mostly because people use the term field of view differently. 

It’s only confusing because people imagine that a lens has an innate FoV due to its focal length, whereas it is entirely dependent on the size of the sensor that it is attached to.

11 minutes ago, Ryan Emanuel said:

You won't use an 25mm with a LF imaging circle on s16 like you would on LF, the experience of shooting on the 25mm with a fixed imaging circle is different on the different sensors.  Due to the crop thats going to effect the distance subjects are from the camera.  While you might have liked subjects 6 feet from the camera for mediums on a 25mm with a LF imaging circle, once you use the s16 sensor with the LF lens, the crop will force you move your subject back to 12 feet lets say for the medium.  You will get the depth of field and distortion characteristics of a 25mm at 12 ft but your frame will be a medium.   The frame will definitely look different even though its the same lens. The entire experience of shooting will be different, BUT it is not the same as a 50mm in terms of depth of field and distortion characteristics  area of the S16 sensor.    lens was designed for because of potential cropping.  

You’re right in saying that you wouldn’t use a lens in same way when switching between formats. Using a 25mm lens on FF would give you a very different FoV to using it on s16, but that’s exactly the point. The lens doesn’t change its focal length, but the FoV does change.

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FOV is the term that confuses people because others would argue that the focal length and the FOV do not change, only the crop factor.  Its semantics of terms but thats definitely where people get confused.

 

Separate question.  I own a set of Veydra MFT primes.  Now the imaging circles cover s35mm.  People use these lenses with the Fs7, but when I compare frames between the Veydras on my Blackmagic Pocket 4k and my directors viewfinder set to s35mm, the frames line up between the viewfinder and the P4K.  That leads me to believe that the Veydra imaging circle is designed for MFT, the 25mm Veydra has no crop on the P4K.  But then what is going on for the lenses on super 35mm.  Can you really have an imaging circle where the FoV of a 25mm fits on a MFT sensor, but theres still enough extra 25mm room to cover s35mm. The reverse crop would be like .70 crop, that seems like a lot of room before vignetting.  Or is the lens company lying about the mm and the true FOV is more like a 18 but they marked the lens 25mm

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8 minutes ago, Ryan Emanuel said:

FOV is the term that confuses people because others would argue that the focal length and the FOV do not change, only the crop factor.  Its semantics of terms but thats definitely where people get confused.

If the crop factor, or sensor size changes, the FoV changes. The two are interconnected.

I think you are confusing Field of View, which is dependent on sensor size, with Angle of View, which is not.

 

13 minutes ago, Ryan Emanuel said:

Separate question.  I own a set of Veydra MFT primes.  Now the imaging circles cover s35mm.  People use these lenses with the Fs7, but when I compare frames between the Veydras on my Blackmagic Pocket 4k and my directors viewfinder set to s35mm, the frames line up between the viewfinder and the P4K.  That leads me to believe that the Veydra imaging circle is designed for MFT, the 25mm Veydra has no crop on the P4K.  But then what is going on for the lenses on super 35mm.  Can you really have an imaging circle where the FoV of a 25mm fits on a MFT sensor, but theres still enough extra 25mm room to cover s35mm. The reverse crop would be like .70 crop, that seems like a lot of room before vignetting.  Or is the lens company lying about the mm and the true FOV is more like a 18 but they marked the lens 25mm

Lenses often have an image circle substantially larger than the format they are designed for, particularly ‘long’ lenses (that is to say long for that particular format).

in any case, I think you are getting confused by image circle and FoV. As long as the image circle is larger than the sensor, it’s immaterial how much bigger it is.

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1 hour ago, Ryan Emanuel said:

Still confusing mostly because people use the term field of view differently.  I'd like to try to explain my understanding without the term field of view.  Please let me know where I'm wrong. I understand that the depth of field and the spacial distortion of a lens is fixed. Regardless of the sensor size those characteristics of the lens are part of the optics and not the camera.  25mm will have a the width, height, and spatial distortion relationships between distances of a 25mm within its imaging circle.  When the sensor is smaller than the imaging circle, there is a crop, or smaller cookie cut out of the 25mm imaging circle.  It is not the same as shooting on a tighter mm.  The distortion and the depth of field is the same as the 25mm, but it is cropped in.  

Actually it is the same as using a tighter lens other than the depth of field difference and loss of resolution from cropping.  But if you crop a 25mm lens by half the view, it is identical to a 50mm lens other than the depth of field / resolution issues.  There are mechanical differences in building a 25mm versus a 50mm so it is never exactly the same distortions but they are close.

See this:

http://yedlin.net/lens_blur.html

Field of view is field of view, the how much view the lens sees within the frame -- if you're using the term in some other non-standard manner, then people are going to be confused.

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1 hour ago, Ryan Emanuel said:

Now saying a 25mm is a 25mm is a 25mm is right but its kinda a riddle that perplexes the original question.  You won't use an 25mm with a LF imaging circle on s16 like you would on LF, the experience of shooting on the 25mm with a fixed imaging circle is different on the different sensors.  Due to the crop thats going to effect the distance subjects are from the camera.  While you might have liked subjects 6 feet from the camera for mediums on a 25mm with a LF imaging circle, once you use the s16 sensor with the LF lens, the crop will force you move your subject back to 12 feet lets say for the medium.  You will get the depth of field and distortion characteristics of a 25mm at 12 ft but your frame will be a medium.   The frame will definitely look different even though its the same lens. The entire experience of shooting will be different, BUT it is not the same as a 50mm in terms of depth of field and distortion characteristics setting the focus to 12 ft.  It is important to know what imaging circle size the lens was designed for because of potential cropping.  

A 25mm LF lens on a Super-16 camera will give you the same field of view at the same distance to the subject, the same depth of field, etc. as a 25mm lens made for Super-16.  BECAUSE IT IS THE SAME FOCAL LENGTH.  The frame won't look any different other than any particular lens personality like contrast, flare, edge distortion, etc.  You wouldn't change the camera position just because you switched from a 25mm LF lens to a 25mm Super-16 lens.  

Yes, of course a 25mm LF lens on a larger format camera will be different than a 25mm lens on a Super-16 camera because now the format sizes are different, the 25mm LF lens on a larger format will have a much wider field of view.

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here is a old 22mm oct18 lens on a full frame photo camera (the lens is originally made for academy 35 format for use with Konvas cameras) .

You can see the image circle only covering the full frame sensor partially but there is potential for all types of smaller sensor sizes within the image the lens is able to produce.

32689228257_a554d2ac5e_h.jpg

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21 minutes ago, aapo lettinen said:

here is a old 22mm oct18 lens on a full frame photo camera (the lens is originally made for academy 35 format for use with Konvas cameras) .

You can see the image circle only covering the full frame sensor partially but there is potential for all types of smaller sensor sizes within the image the lens is able to produce.

32689228257_a554d2ac5e_h.jpg

Just imagine changing the size of that yellow framing. that would be equal to changing the sensor size of the camera. the full frame sensor would be yellow frame covering the very edges of the monitor, the current state being  slightly smaller sensor than full frame, and S35 being smaller box, S16 the yellow box only covering the center of the display. 

The image circle of the lens never changed but the different sized sensors just use different portion of it

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On 4/17/2019 at 11:29 AM, David Mullen ASC said:

Yes, you can argue that in terms of sharpness, contrast, etc. that the design compromises depend on the format size. On the one hand, a lens designed for FF35 but put onto a Super-35 camera will be using more of the optical sweet spot in the center of the lens since the sides will be cropped to a narrower view, but on the other hand, sometimes larger format lenses are designed with a lower MTF simply because if your format physically has more "millimeters" overall, then a lens does not have to resolve as many lines per millimeter,  which is the advantage of oversampling.

Is this why my Canon 24-70 L produces a sharp image on my FF 5DmkII (21MP) and yet is blurry with heavy chromatic aberration on my APSC 70D (24MP)? I was shooting product shost on the APSC at f8 and had unimpressive performance with that lens.

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4 hours ago, Stephen Sanchez said:

Is this why my Canon 24-70 L produces a sharp image on my FF 5DmkII (21MP) and yet is blurry with heavy chromatic aberration on my APSC 70D (24MP)? I was shooting product shost on the APSC at f8 and had unimpressive performance with that lens.

No, the difference in resolving power between a lens designed for large format, and one designed for APS-C is really only noticeable when looking at MTF charts on a lens projector. It’s more felt than seen. It’s certainly nowhere near enough to appear as blur.

If your lens works ok with other cameras, I’d say there is something wrong with the lens mount on your 70D

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On 4/23/2019 at 9:34 PM, Stuart Brereton said:

No, the difference in resolving power between a lens designed for large format, and one designed for APS-C is really only noticeable when looking at MTF charts on a lens projector. It’s more felt than seen. It’s certainly nowhere near enough to appear as blur.

If your lens works ok with other cameras, I’d say there is something wrong with the lens mount on your 70D

The problem could also be the diameter of the aperture you're using. The smaller the pixels are the higher the chances of the image to become less sharp due to the higher diffraction smaller apertures produce if compared to bigger apertures. It means that the Airy disc would be bigger than the pixel itself which means that the surrounding pixel would be getting a little bit of the light that was supposed to hit the initial pixel.

Input the camera and the aperture you were using on this calculator from Cambridge in Color to check if this is the case.

https://www.cambridgeincolour.com/tutorials/diffraction-photography.htm

I know there is not much you can do in this case due to the need of a broader depth of field but you would at least know whether this is causing the problem or not.

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An aperture of f8 is not small enough to cause any significant softening from diffraction on these cameras.  Even at minimum aperture the effect would be subtle. Also, as stopping down minimizes lens aberrations, it would not explain the heavy chromatic aberration that Stephen is describing.

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It's really easy to visualize what's going on by taking an image shot on for example a super 35 camera, open it in an editing program, and creating a crop window at different resolutions. Say you start with a 4096x2048 image, i.e. 4K. You want to see what that would look like on a 2K Super 16? Set the crop window to 2048x1080.

The numbers aren't exactly correct, but it's close enough to show the effect of different sensor sizes.

 

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