Depth of field: Full-frame lens vs. APS-C lens, both on APS-C sensor

[WayneF]

Exactly. My point was that a Full Frame lens on an APS-C sensor will produce less "depth of field" than with a full frame sensor.
This is because the Aperture is not the same on different sensors (using Full Frame as a reference point).

No, the aperture certainly does not change on a cropped sensor. Neither does the focal length (of the same lens). A lens does not change in any way by putting it on a cropped sensor. Only its field of view is reduced when the smaller sensor crops it.

The DOF Reason (greater DOF of cropped sensors) is simply because the lens focal length, or the place where we have to stand, has to change for the DOF of a cropped sensor to see the same view as uncropped.

 

[theraven871]

Start watching around the 20 minute mark.

 

[theraven871]

Mathematically, your "equivalent" aperture DOES change when you mount a full frame lens on a cropped sensor. The sensor isn't as wide as the full frame sensor.
Aperture is the relationship of the width or opening of the lens to the length.
So a f2.8 lens is mathematically identical if you mount it on a full frame or Crop sensor. At least when speaking of the lens itself. The image sensor however lowers the diameter which changes the aperture.
So, a full frame 70-200 f2.8 lens will function like it was a f4 lens on a crop sensor. This is because the sensor size is smaller.
The APS-C (cropped) sensor is not utilizing the entire diameter of the lens. Thus, the math changes.
A crop sensor basically narrows the diameter of a "full frame lens" because there is unused space.

 

[WayneF]

Mathematically, your "equivalent" aperture DOES change when you mount a full frame lens on a cropped sensor. The sensor isn't as wide as the full frame sensor.
Aperture is the relationship of the width or opening of the lens to the length.
So a f2.8 lens is mathematically identical if you mount it on a full frame or Crop sensor. At least when speaking of the lens itself. The image sensor however lowers the diameter which changes the aperture.
So, a full frame 70-200 f2.8 lens will function like it was a f4 lens on a crop sensor. This is because the sensor size is smaller.
The APS-C (cropped) sensor is not utilizing the entire diameter of the lens. Thus, the math changes.
A crop sensor basically narrows the diameter of a "full frame lens" because there is unused space.

 

[CameraClicker]

Well, it looks like the discussion has continued while I have been occupied by automobile issues. It also appears as though the understanding within the group of respondents may be about as good as it is among those to whom I have spoken in person. I think a fact is something that can be demonstrated/proven by observation/experimentation. I don't have an axe to grind and I may well be wrong. It wouldn't be the first time, it won't be the last, either.

This is what I wrote previously: "
Many of the on-line calculators have a problem. They suggest the same prime lens on different bodies FF and APS-C for instance will have different DOF if the camera and subject do not move and aperture is the same.

I have full frame and APS-C bodies, and prime lenses, so I got out a tripod, a target and a tape measure. Sensor size has no effect on DOF.

Sensor size lets you move the camera to frame the scene to match the other body... which changes the parameters that affect DOF. That's a totally different experiment. You can put a 10 mm lens on an APS-C Canon body and a 16 mm lens on a Canon full frame body, and the framing will look the same. View both at 100% and the differences become evident. If you have two APS-C bodies with different photo site densities, you can play other games too. Much attention has to be paid to how the question is framed, and how the reply is framed, or the results can be misleading."

So, during some limited free time this week, I stuck a target on a stool. Put a Sigma 150 mm f/2.8 macro lens on the tripod, and took photos with two different APS-C bodies and a full frame body.

Some random web page that had dimensions said: APS-C is 22.2 x 14.8mm (Full Frame is 36 x 24mm). I don't know without looking in a manual if that is the correct measurements, but it suggests APS-C sensors are about half the size of a full frame sensor. If there is anything to the argument that moving from APS-C to full frame , without doing anything else, affects DOF, it should be readily apparent.

Here are 3 photos, screen captures, to ensure all EXIF data was removed. Which photo is from which body?



........................... A ..................................................... B ............................................................... C ................................

These were all taken through the same lens, at the same aperture, at the same distance. If you see an obvious difference, please tell me which of A, B or C is from the full frame body.

 

[theraven871]

I believe the confusion comes from how lenses are rated.
They are rated using the 35mm format. So a 17-50mm f2.8 is only a 17-50mm f2.8 when mounted to a FULL FRAME (35mm) sensor.
When you mount a 17-50 f2.8 to a crop sensor, it functions as if it were the 35mm equivalent of a 24-70 f4 (approx)

Crop sensor lenses are NOT rated using the crop sensor format (What is it? 21mm???).
They are still rated using the 35mm format, which makes them appear more desirable. This is deceiving to consumers.
This is why Full Frame sensors produce different imagines.

Can you tell the difference between f2.8 and f4? That's a different discussion...

 

[theraven871]

Well, it looks like the discussion has continued while I have been occupied by automobile issues. It also appears as though the understanding within the group of respondents may be about as good as it is among those to whom I have spoken in person. I think a fact is something that can be demonstrated/proven by observation/experimentation. I don't have an axe to grind and I may well be wrong. It wouldn't be the first time, it won't be the last, either.

This is what I wrote previously: "
Many of the on-line calculators have a problem. They suggest the same prime lens on different bodies FF and APS-C for instance will have different DOF if the camera and subject do not move and aperture is the same.

I have full frame and APS-C bodies, and prime lenses, so I got out a tripod, a target and a tape measure. Sensor size has no effect on DOF.

Sensor size lets you move the camera to frame the scene to match the other body... which changes the parameters that affect DOF. That's a totally different experiment. You can put a 10 mm lens on an APS-C Canon body and a 16 mm lens on a Canon full frame body, and the framing will look the same. View both at 100% and the differences become evident. If you have two APS-C bodies with different photo site densities, you can play other games too. Much attention has to be paid to how the question is framed, and how the reply is framed, or the results can be misleading."

So, during some limited free time this week, I stuck a target on a stool. Put a Sigma 150 mm f/2.8 macro lens on the tripod, and took photos with two different APS-C bodies and a full frame body.

Some random web page that had dimensions said: APS-C is 22.2 x 14.8mm (Full Frame is 36 x 24mm). I don't know without looking in a manual if that is the correct measurements, but it suggests APS-C sensors are about half the size of a full frame sensor. If there is anything to the argument that moving from APS-C to full frame , without doing anything else, affects DOF, it should be readily apparent.

Here are 3 photos, screen captures, to ensure all EXIF data was removed. Which photo is from which body?


View attachment 87387 View attachment 87388 View attachment 87389
........................... A ..................................................... B ............................................................... C ................................

These were all taken through the same lens, at the same aperture, at the same distance. If you see an obvious difference, please tell me which of A, B or C is from the full frame body.

Going to take a guess here. Is A the full frame?

 

[WayneF]

Mathematically, your "equivalent" aperture DOES change when you mount a full frame lens on a cropped sensor. The sensor isn't as wide as the full frame sensor.
Aperture is the relationship of the width or opening of the lens to the length.
So a f2.8 lens is mathematically identical if you mount it on a full frame or Crop sensor. At least when speaking of the lens itself. The image sensor however lowers the diameter which changes the aperture (WRONG).
So, a full frame 70-200 f2.8 lens will function like it was a f4 lens on a crop sensor. This is because the sensor size is smaller.
The APS-C (cropped) sensor is not utilizing the entire diameter of the lens. Thus, the math changes.
A crop sensor basically narrows the diameter of a "full frame lens" because there is unused space.

No, this is getting worse. f/stop is focal length / aperture diameter. It is NOT even about the sensor. the lens does not change in any way when placed on any camera. That would be a silly notion.

Nor does the focal length change if on a cropped sensor. You need to realize what Equivalent focal length actually means.

Assuming same lens is used, at same distance, all that a cropped senor changes is the field of view. The reasoning is: The cropped sensor sees a smaller cropped view, a more narrow view than if uncropped. That is what cropped refers to. But to compare it, we can only compare the same size image of the same view on both sensors. To do this (to see the same view), the uncropped sensor would need to use a lens longer than the uncropped sensor. Compared to the uncropped view, that same lens (cropped smaller by smaller sensor) superficially appears as if a longer lens, so which the uncropped needs now to see the came cropped view. That longer lens on uncropped sensor is called the Equivalent (view) of the cropped sensor (which is as is). The term Equivalent applies to the uncropped sensor.

The ideas is, if you crop ANY IMAGE, ANYTIME LATER in your editor, and then enlarge it back to compare as the same size again, that enlargement appears as a psuedo telephoto view (as if a longer lens). Same applies the the smaller cropped sensor too. But instead, all it is a smaller cropped image, enlarged more later.

 

[theraven871]

Mathematically, your "equivalent" aperture DOES change when you mount a full frame lens on a cropped sensor. The sensor isn't as wide as the full frame sensor.
Aperture is the relationship of the width or opening of the lens to the length.
So a f2.8 lens is mathematically identical if you mount it on a full frame or Crop sensor. At least when speaking of the lens itself. The image sensor however lowers the diameter which changes the aperture (WRONG).
So, a full frame 70-200 f2.8 lens will function like it was a f4 lens on a crop sensor. This is because the sensor size is smaller.
The APS-C (cropped) sensor is not utilizing the entire diameter of the lens. Thus, the math changes.
A crop sensor basically narrows the diameter of a "full frame lens" because there is unused space.

No, this is getting worse. f/stop is focal length / aperture diameter. It is NOT even about the sensor. the lens does not change in any way when placed on any camera. That would be a silly notion.

Nor does the focal length change if on a cropped sensor. You need to realize what Equivalent focal length actually means.

Assuming same lens is used, at same distance, all that a cropped senor changes is the field of view. The reasoning is: The cropped sensor sees a smaller cropped view, a more narrow view than if uncropped. That is what cropped refers to. But to compare it, we can only compare the same size image of the same view on both sensors. To do this (to see the same view), the uncropped sensor would need to use a lens longer than the uncropped sensor. Compared to the uncropped view, that same lens (cropped smaller by smaller sensor) superficially appears as if a longer lens, so which the uncropped needs now to see the came cropped view. That longer lens on uncropped sensor is called the Equivalent (view) of the cropped sensor (which is as is). The term Equivalent applies to the uncropped sensor.

The ideas is, if you crop ANY IMAGE, ANYTIME LATER in your editor, and then enlarge it back to compare as the same size again, that enlargement appears as a psuedo telephoto view (as if a longer lens). Same applies the the smaller cropped sensor too. But instead, all it is a smaller cropped image, enlarged more later.

A cropped sensor can NOT utilize the entire diameter of the 35mm lens.
Thus you have to apply (multiply) the crop factor to BOTH the aperture and the focal range.

Following your logic, my iPhone (with a 2.0 aperture) will have the same bokeh as a full frame Nikon at the same focal range.
We ALL know that isn't true.

You are correct that a cropped sensor changes the field of view. But a cropped sensor DOES affect the aperture rating.
If you have a 100mm lens, with an iris (diameter) of 25mm, you have a 100mm f4 lens - On 35mm ONLY.
Since a crop sensor camera cannot use the full opening (in this example it is 25mm) you apply the crop factor to the aperture as well as the focal range.
Aperture ratings are based on 35mm. NOT cropped sensors.

Otherwise, if what you're saying is true, all these "point and shoot" f2.8 cameras out there with crop factors of 3x, or 4x would produce the same bokeh as a 2.8 lens mounted on a 35mm (full frame) camera. Everyone knows that isn't the case.
C'mon buddy. We are talking basic math here. The entire formula for how aperture is determined is based on a 35mm format. The moment you change that format, you have to apply the same ratio to both the focal range and the aperture to find the 35mm equivalent. The point you seem to be missing is that a 17-50mm f2.8 lens for an APS-C camera is NOT rated for an APS-C camera/sensor. That rating is still based on 35mm or full frame. The moment you mount that 17-50mm lens on a cropped sensor, it will function as the 35mm or full frame equivalent of a 24-70 f4.
I'm not saying its not a 2.8 lens. I'm saying the formula you're basing it on has to change.
So, if you're comparing only cropped sensors to cropped sensor lenses, its not confusing. But if you're attempting to say compare a lens for a cropped sensor to the 35mm (full frame) equivalent, you will find that my math is correct. You not only multiply the focal length to the crop factor, you also multiply the aperture by the crop factor. If you change the focal length (becuase you're mounting to a crop sensor) you also change the aperture. Aperture is determined by focal length. You cannot change it without affecting the aperture.
I understand you're looking at this from someone just thinking you're digitally cropping into an image. But the aperture value is also changed.
This isn't as noticeable on the APS-C format. But if you think I'm lying, look at cameras with a f2.8 lens with a 3x or 4x crop.

 

[WayneF]

Mathematically, your "equivalent" aperture DOES change when you mount a full frame lens on a cropped sensor. The sensor isn't as wide as the full frame sensor.
Aperture is the relationship of the width or opening of the lens to the length.
So a f2.8 lens is mathematically identical if you mount it on a full frame or Crop sensor. At least when speaking of the lens itself. The image sensor however lowers the diameter which changes the aperture (WRONG).
So, a full frame 70-200 f2.8 lens will function like it was a f4 lens on a crop sensor. This is because the sensor size is smaller.
The APS-C (cropped) sensor is not utilizing the entire diameter of the lens. Thus, the math changes.
A crop sensor basically narrows the diameter of a "full frame lens" because there is unused space.

No, this is getting worse. f/stop is focal length / aperture diameter. It is NOT even about the sensor. the lens does not change in any way when placed on any camera. That would be a silly notion.

Nor does the focal length change if on a cropped sensor. You need to realize what Equivalent focal length actually means.

Assuming same lens is used, at same distance, all that a cropped senor changes is the field of view. The reasoning is: The cropped sensor sees a smaller cropped view, a more narrow view than if uncropped. That is what cropped refers to. But to compare it, we can only compare the same size image of the same view on both sensors. To do this (to see the same view), the uncropped sensor would need to use a lens longer than the uncropped sensor. Compared to the uncropped view, that same lens (cropped smaller by smaller sensor) superficially appears as if a longer lens, so which the uncropped needs now to see the came cropped view. That longer lens on uncropped sensor is called the Equivalent (view) of the cropped sensor (which is as is). The term Equivalent applies to the uncropped sensor.

The ideas is, if you crop ANY IMAGE, ANYTIME LATER in your editor, and then enlarge it back to compare as the same size again, that enlargement appears as a psuedo telephoto view (as if a longer lens). Same applies the the smaller cropped sensor too. But instead, all it is a smaller cropped image, enlarged more later.

A cropped sensor can NOT utilize the entire diameter of the 35mm lens.
Thus you have to apply (multiply) the crop factor to BOTH the aperture and the focal range.

Following your logic, my iPhone (with a 2.0 aperture) will have the same bokeh as a full frame Nikon at the same focal range.
We ALL know that isn't true.

You are correct that a cropped sensor changes the field of view. But a cropped sensor DOES affect the aperture rating.
If you have a 100mm lens, with an iris (diameter) of 25mm, you have a 100mm f4 lens - On 35mm ONLY.
Since a crop sensor camera cannot use the full opening (in this example it is 25mm) you apply the crop factor to the aperture as well as the focal range.
Aperture ratings are based on 35mm. NOT cropped sensors.

Otherwise, if what you're saying is true, all these "point and shoot" f2.8 cameras out there with crop factors of 3x, or 4x would produce the same bokeh as a 2.8 lens mounted on a 35mm (full frame) camera. Everyone knows that isn't the case.
C'mon buddy. We are talking basic math here. The entire formula for how aperture is determined is based on a 35mm format. The moment you change that format, you have to apply the same ratio to both the focal range and the aperture to find the 35mm equivalent. The point you seem to be missing is that a 17-50mm f2.8 lens for an APS-C camera is NOT rated for an APS-C camera/sensor. That rating is still based on 35mm or full frame. The moment you mount that 17-50mm lens on a cropped sensor, it will function as the 35mm or full frame equivalent of a 24-70 f4.
I'm not saying its not a 2.8 lens. I'm saying the formula you're basing it on has to change.
So, if you're comparing only cropped sensors to cropped sensor lenses, its not confusing. But if you're attempting to say compare a lens for a cropped sensor to the 35mm (full frame) equivalent, you will find that my math is correct. You not only multiply the focal length to the crop factor, you also multiply the aperture by the crop factor. If you change the focal length (becuase you're mounting to a crop sensor) you also change the aperture. Aperture is determined by focal length. You cannot change it without affecting the aperture.
I understand you're looking at this from someone just thinking you're digitally cropping into an image. But the aperture value is also changed.
This isn't as noticeable on the APS-C format. But if you think I'm lying, look at cameras with a f2.8 lens with a 3x or 4x crop.

I don't know how much more clear I can make it.

Not very clear obviously. OK, I don't care what wrong stuff you want to think. I was just reacting to someone on the internet being so wrong. It just seems harmful to others to spread it around publicly though.

The iPhone (5S) has a focal length of 4.2mm. I think crop factor is about 7x, a senor size almost too tiny for numbers. That is the effect you discuss.

 

[CameraClicker]

I don't know if this will help, or just muddy things further.
Here are a couple of size reduced frames


Full Frame sensor .......................................... APS-C sensor



Full Frame 100% crop ............................................APS-C 100% crop


On the full frame body, 16 mm lens. On the APS-C body, 10 mm lens. For the whole frame, the field of view looks the same, except I didn't aim perfectly.

So, you might conclude 10 mm on APS-C equals 16 mm on full frame. But if we grab 700 px by 500 px at 100% out of each frame, and I tried to use the mail truck's bumper and the white vertical line as my reference, we see 16 mm is longer than 10 mm, which is what we would expect since the crop is done by having less sensor in the image circle.

Likewise, if you have a 100 mm lens with an aperture of f/2.0, that's a huge aperture of 50 mm! If you have a 4 mm lens with an aperture of f/2.0, that's not so big, at 2 mm.

Small sensor bodies, like almost any P&S under 2 or 300 dollars, have very short lenses. Cell phones have about 4 mm. P&S typically between 4 mm and 6.5 mm at the wide end. The sensor is so small, you get a regular field of view, but the focal length is very short, so getting bokeh is fairly difficult because the hyperfocal distance for 4 or 6.5 mm is somewhere less than 3 feet. Everything is always in focus. True, some of the P&S cameras like a Canon G11 have fairly large apertures available, Rockwell's page states "6.1~30.5mm f/2.8-4.5.", it is possible, but you have to work at it, to get something out of focus.

Last thought for now: If you have a full frame sensor and 100 mm lens, and you set f/5.6, and you have enough light for a given exposure at say, 1/200th. Changing the sensor will not affect any of that if you don't move. If it did, you would have a problem getting an even exposure across a full frame sensor.

 

[Braineack]

Start watching around the 20 minute mark.

This is the same guy the reviewed the D810, and loved it in every respect but couldn't ditch Canon because he thinks inferior products are better.

His math is flawed: The focal length never changed.

therefore there's no need to change the equation. He changed the definition of aperture being N = f/D to N = (f x crop factor) / D

This is seriously one of the more retarded thing i've seen all day; and I work for the government.


also aperture does not equal DOF.

I put the mathematical formula for DOF in post #15. where the subject distance, focal length, aperture, and CoC all come into play.

Aperture determines the angle of light rays that can pass through to the sensor. It's a factor of a lens, and it's fixed--whatever camera body you bolt it to doesn't not change this value. It's written in stone.

 

[Ysarex]

Derrel and Wayne are correct and Braineack's math is correct. In an appropriate comparison of like photos smaller sensor cameras produce more DOF.

To make any meaningful comparison in this discussion you must make a comparison of similar photographs. The content and perspective in the two compared photographs must be identical as must also be the photographic conditions -- same exposure, which means same f/stop etc. Therefore both photographs must be taken from the exact same location and the angle of view of the camera/lens systems must be adjusted to produce identical content. Both photographs must be taken using the same photographic conditions, i.e. exposure. All other comparisons are worthless.

Comparing like photographs using different format cameras a photo taken with a smaller format sensor will have more DOF than the same photo taken with a larger format sensor.

..........f^2
H = ----------
..........Nc

The above equation is standard. If you dispute it then you're denying accepted math and are certifiable.

"H" is hyperfocal distance, "f" is focal length, "N" is f/stop and "c" is circle of confusion. It is likewise accepted math that c is determined by the size of the format. If you think sensor size does not effect DOF please show examples of this equation in which changing "c" has no effect on "H."

If you do the math for the above equation so that two different format cameras are taking the exact same photograph you get different values for "H" proving mathematically that you get more DOF from a smaller sensor camera. The math in the illustration below is correct. Hyperfocal distance for the 7D is closer to the camera and that means there's more DOF.

Joe

 

[Braineack]

 

[theraven871]

Start watching around the 20 minute mark.

This is the same guy the reviewed the D810, and loved it in every respect but couldn't ditch Canon because he thinks inferior products are better.

His math is flawed: The focal length never changed.

therefore there's no need to change the equation. He changed the definition of aperture being N = f/D to N = (f x crop factor) / D

This is seriously one of the more retarded thing i've seen all day; and I work for the government.


also aperture does not equal DOF.

I put the mathematical formula for DOF in post #15. where the subject distance, focal length, aperture, and CoC all come into play.

Aperture determines the angle of light rays that can pass through to the sensor. It's a factor of a lens, and it's fixed--whatever camera body you bolt it to doesn't not change this value. It's written in stone.

He didn't say he thought inferior products were better. He never said Canon was better.
He wanted to switch to Nikon but stated that the Nikon 70-200 has a focal breathing problem. Whereas the Canon 70-200 lens does not.
He stated that the 70-200 lens he uses too much and felt the focal breathing issue presented a greater issue than gaining the benefits of switching to Nikon.