FF shallow DOF question

[craig.walton2]

Can someone please explain to me why you can get a shallower DOF using a FF over a crop. I don't understand how sensor size relates to DOF. Many thanks in advance.

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[lensflair]

Can someone please explain to me why you can get a shallower DOF using a FF over a crop. I don't understand how sensor size relates to DOF. Many thanks in advance.

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Oh no not this debate again. Some people will tell you sensor size doesn't have an effect, but in the real world, the way we actually tend to use our gear it does.

 

[craig.walton2]

Oh no not this debate again. Some people will tell you sensor size doesn't have an effect, but in the real world, the way we actually tend to use our gear it does.

Thanks for the cryptic response but what does that actually mean? I did do a search before posting and could find anything of use, hence this post.

 

[Dogfish_magnet]

This may help linky

 

[lensflair]

This may help linky

Thanks ill keep that link for future reference.

Sorry.I'm just tired of reading this debate about whether sensor size efffecta dof. There is some huge discussions on TP about it, refine your search maybe, try blurred background.

 

[HoppyUK]

The simple answer to the question, in a word, is magnification.

The smaller sensor dictates the use of a shorter focal length lens to get the same framing, and then in turn that image has to be enlarged more (smaller circle of confusion) to deliver the same sized print. So the DoF formula is completely changed.

The difference between formats is easily calculated using the crop factor, ie f/4 on a crop format Canon gives the same DoF as f/6.4 on full frame (4 x 1.6 = 6.4). About one and a quarter stops difference.

Edit: the reason why people say sensor size affects DoF is basically shorthand, assuming all the other factors as a given consequence of that, because we always start with the camera and the sensor size is fixed.

 

[WillyB]

Thanks ill keep that link for future reference.

Sorry.I'm just tired of reading this debate about whether sensor size efffecta dof. There is some huge discussions on TP about it, refine your search maybe, try blurred background.

Well, if you see it is the same old subject again, you don't have to read it!

 

[Richard King]

This may help linky

Thats a good explaination

Essentially IF YOU USE EXACTLY THE SAME LENS on a crop and a FF camera, your view of what is happening appears to be different. To make the view you see through the view finder appear to be the same, you either have to move your shooting position, or zoom. Because you have moved (or zoomed) the DOF calculation is different

The optical reality is that the lens you are using is unchanged, it is just the full frame camera uses more of the image the lens is producing for you. The crop sensor is essentially taking only the central part of the same image. Because when you look through "any" camera you frame the image with "what you see" the 2 cameras appear to offer you a different images. In reality if you took the full frame camera, and discarded the peripheral part of the image (leaving the equivalent of the crop) then the image size, DOF, distortion, IQ and any other optical quality will be identical. Think of the crop camera as the equivalent of overlaying a print with a 3/4 apreture mount that obscures the edges of a print

 

[lensflair]

Well, if you see it is the same old subject again, you don't have to read it!

Fair call. I'm just a sucker for punishment I guess. LOL.

 

[craig.walton2]

Thanks for the detailed explanations. I have read (and re-read) the link and replys and it's still as clear as mud, but I thinks that's just me.

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[ash59uk]

Thanks for the detailed explanations. I have read (and re-read) the link and replys and it's still as clear as mud, but I thinks that's just me.

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DITTO!! what on earth is a crop camera? I know what cropping is in photoshop or even cropping inside the camera in view mode. I have seen things like 100% crop in some articles usually in a camera review, how can you have a 100% crop? surely that would be the whole picture.

 

[Jelster]

Reverting back to what I said in a previous thread, if you put the same lens on both a full frame and crop sensor camera (in Canon terms, a 7d, xxD,xxxD or xxxxxD) and keep all the parameters the same ( F stop, subject distance and focal length) the resulting image from the full frame camera will appear to have a shallower DoF.

Some will say that the sensor has no effect, but in practical terms it does, simple as. As Hoppy suggested, to get the same DoF on a crop body you need to reduce the aperture by about 1.6 stops.

Steve

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[EdBray]

DITTO!! what on earth is a crop camera? I know what cropping is in photoshop or even cropping inside the camera in view mode. I have seen things like 100% crop in some articles usually in a camera review, how can you have a 100% crop? surely that would be the whole picture.

A so called 'crop camera' is basically a camera that has a sensor that is smaller than that of a camera that has the same size sensor as a 35mm camera ie 36mm x 24mm which is commonly called full frame.

Different camera manufacturers have different sensor sizes thus the proportion of the original sized sensor has to be taken into account when using lenses that were originally designed for 35mm/FF digital cameras.

For Nikon DSLRs other than the D3 series and D700 the crop factor is 1.5x (thus a 200mm lens would give the same AOV as a 300mm lens on a FF Camera from the same position) and for Canon DSLRs other than the 1D and 5D series the crop factor is 1.6x. Canon also have another crop often called 1.3x (actually 1.25x) on their 1D cameras as opposed to their 1Ds Cameras which have the full frame 36x24mm sized sensors.

The reason the cameras are called crop cameras is that if you used a Full Frame DSLR with a 200mm lens and cropped the image to be the same as that that would be obtained from shooting on the 'crop camera' with the same lens from the same position you would end up with a ratio of the mathmatical difference of the size of the two sensors, this would equate to using a longer lens from the same position to get the same result on the full frame camera. So if a Nikon, 300mm.

Any clearer?

Basically, whatever camera you are using you should just select the correct lens to give you the result you want and not get too bogged down with crop factors.

 

[ash59uk]

A so called 'crop camera' is basically a camera that has a sensor that is smaller than that of a camera that has the same size sensor as a 35mm camera ie 36mm x 24mm which is commonly called full frame.

Different camera manufacturers have different sensor sizes thus the proportion of the original sized sensor has to be taken into account when using lenses that were originally designed for 35mm/FF digital cameras.

For Nikon DSLRs other than the D3 series and D700 the crop factor is 1.5x (thus a 200mm lens would give the same AOV as a 300mm lens on a FF Camera from the same position) and for Canon DSLRs other than the 1D and 5D series the crop factor is 1.6x. Canon also have another crop often called 1.3x (actually 1.25x) on their 1D cameras as opposed to their 1Ds Cameras which have the full frame 36x24mm sized sensors.

The reason the cameras are called crop cameras is that if you used a Full Frame DSLR with a 200mm lens and cropped the image to be the same as that that would be obtained from shooting on the 'crop camera' with the same lens from the same position you would end up with a ratio of the mathmatical difference of the size of the two sensors, this would equate to using a longer lens from the same position to get the same result on the full frame camera. So if a Nikon, 300mm.

Any clearer?

Basically, whatever camera you are using you should just select the correct lens to give you the result you want and not get too bogged down with crop factors.

Any clearer? :shrug: all it has done is make my brain hurt :bonk: I do understand what it means now thanks to your post but I`ll leave the mathematics bit though, Its been a long time since I was at school, I think I`ll cross this bridge if I ever come to it.

Bye the way, how is Plymouth (Gus as we used to call it) I had a look on Google earth and didn`t recognize the place but it is nearly 30 yrs since I was last there?

 

[HoppyUK]

Reverting back to what I said in a previous thread, if you put the same lens on both a full frame and crop sensor camera (in Canon terms, a 7d, xxD,xxxD or xxxxxD) and keep all the parameters the same ( F stop, subject distance and focal length) the resulting image from the full frame camera will appear to have a shallower DoF.

Some will say that the sensor has no effect, but in practical terms it does, simple as. As Hoppy suggested, to get the same DoF on a crop body you need to reduce the aperture by about 1.6 stops.

Steve

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Please pay attention Steve

If you actually do the comparison in your first paragraph, not only do you end up with a completely different picture, the full frame image actually (and perhaps surprisingly) has more DoF.

And Hoppy didn't say the difference between crop format and full frame is 1.6 stops. It is f/number x crop factor, which in the case of Canon is 1.28 stops, and Nikon fractionally less. (A crop factor of 1.4x would yield a difference of exactly 1.0 stops.)

 

[EdBray]

Bye the way, how is Plymouth (Gus as we used to call it) I had a look on Google earth and didn`t recognize the place but it is nearly 30 yrs since I was last there?

Still here it has changed an amazing amount in the last 30 years I must agree, not all for the better either.

 

[AndyB1976]

....... I have seen things like 100% crop in some articles usually in a camera review, how can you have a 100% crop? surely that would be the whole picture.

A 100% crop is a crop taken from a full resolution image from a given camera/sensor/lens combo.

The crop itself can be of any size with regards to the original image. For example a portrait could be cropped on an eye, both eyes, smile, face.

As long as the original image hasn't been resized and then the crop applied, it will be a 100% crop.

It does not mean crop 100% of an image, think more like crop part of an image that is 100% untouched.

I'll give an example of an images then a 100% crop of same image both at 800x533 pixels

Original (resized 800x533)

100% Crop (resized 800x533)

Andy

 

[Jelster]

Please pay attention Steve

If you actually do the comparison in your first paragraph, not only do you end up with a completely different picture, the full frame image actually (and perhaps surprisingly) has more DoF.

And Hoppy didn't say the difference between crop format and full frame is 1.6 stops. It is f/number x crop factor, which in the case of Canon is 1.28 stops, and Nikon fractionally less. (A crop factor of 1.4x would yield a difference of exactly 1.0 stops.)

well in my first paragraph I do relate it to all the 1.6 Canon crop bodies (you were paying attention weren't you

And I must have read everything that was discussed on the previous thread in my sleep then, as I'm sure that it was actually you that agreed that the same camera, focal length, distance from subject and aperture with 2 different sensors would give the image taken with FF body less DoF. I am fully aware that the images will look different due the FoV, but then I never said they would.

Steve

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[AJQS]

I doubt this will help, but...

Take my camera with a 2x crop factor (it's easier!) so that a 50mm lens on FF would be equal to a 25mm on mine.

Right so I stick the camera on a tripod and point it at something - both cameras show the same size image. The lenses are both f1.8. Which has most depth of field? Yes, the 25mm. Why? because dof is to do with the actual focal length, not the equivalent focal length (remmeber longer lenses have lss dof?). If I was to put 50mm f1.8 on both cameras, and simply move the 2x camera further back would I get the same dof? No, because it's a different distance, rmember the closer you are to something the shallower the dof.

What will give the same dof? Putting the same lens on the 2 cameras at the same distance - but on the 2x crop the image wil of course be a lot more magnified than on the full frame.


Helped? No thought not!

 

[HoppyUK]

well in my first paragraph I do relate it to all the 1.6 Canon crop bodies (you were paying attention weren't you

And I must have read everything that was discussed on the previous thread in my sleep then, as I'm sure that it was actually you that agreed that the same camera, focal length, distance from subject and aperture with 2 different sensors would give the image taken with FF body less DoF. I am fully aware that the images will look different due the FoV, but then I never said they would.

Steve

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Not me. I gave real figures to show in that example it is actually the opposite, in this thread http://www.talkphotography.co.uk/forums/showthread.php?t=306636&page=3 post #85.

I doubt this will help, but...

Take my camera with a 2x crop factor (it's easier!) so that a 50mm lens on FF would be equal to a 25mm on mine.

Right so I stick the camera on a tripod and point it at something - both cameras show the same size image. The lenses are both f1.8. Which has most depth of field? Yes, the 25mm. Why? because dof is to do with the actual focal length, not the equivalent focal length (remmeber longer lenses have lss dof?). If I was to put 50mm f1.8 on both cameras, and simply move the 2x camera further back would I get the same dof? No, because it's a different distance, rmember the closer you are to something the shallower the dof.

What will give the same dof? Putting the same lens on the 2 cameras at the same distance - but on the 2x crop the image wil of course be a lot more magnified than on the full frame.


Helped? No thought not!

Longer lenses don't give inherantly less depth if field. It's a common falacy. They are often used to give a bigger image, and that will deliver less DoF, but if you move back so that the subject remains the same size, then DoF also stays the same.

 

[lensflair]

Not me. I gave real figures to show in that example it is actually the opposite, in this thread http://www.talkphotography.co.uk/forums/showthread.php?t=306636&page=3 post #85.

Longer lenses don't give inherantly less depth if field. It's a common falacy. They are often used to give a bigger image, and that will deliver less DOF than the full frame shot, but if you move back so that the subject remains the same size, then DoF also stays the same.

Really? I'm pretty sure if you move back on a crop sensor to get the same fov as that same lens would give on a full frame, you will get a greater DOF than in the full frame shot. Think I might have to do some tests and see what happens real world for sure.

 

[HoppyUK]

Really? I'm pretty sure if you move back on a crop sensor to get the same fov as that same lens would give on a full frame, you will get a greater DOF than in the full frame shot. Think I might have to do some tests and see what happens real world for sure.

Yes, you would. But you have changed the size of the sensor (and the perspective) in your example. Check it all here www.dofmaster.com

 

[lensflair]

Yes, you would. But you have changed the size of the sensor (and the perspective) in your example. Check it all here www.dofmaster.com

Yeah I know, but that's kind of my point. When people ask this question they usually want to know if using a larger sensor will make it easier to blur the background. And in reality it often will because of the way we work. I have to be careful shooting large groups because I can get away with a larger aperture on my crop cameras than I can on my FF for precisely this reason.

 

[EdBray]

I doubt this will help, You are correct it won't but...

Take my camera with a 2x crop factor (it's easier!) so that a 50mm lens on FF would be equal to a 25mm on mine. No, if your camera has a x2 crop factor the lens would equate to 100mm rather than a 25mm.

The below is all rubbish because of the above being wrong:
Right so I stick the camera on a tripod and point it at something - both cameras show the same size image. The lenses are both f1.8. Which has most depth of field? Yes, the 25mm. Why? because dof is to do with the actual focal length, not the equivalent focal length (remmeber longer lenses have lss dof?). If I was to put 50mm f1.8 on both cameras, and simply move the 2x camera further back would I get the same dof? No, because it's a different distance, rmember the closer you are to something the shallower the dof.

What will give the same dof? Putting the same lens on the 2 cameras at the same distance - but on the 2x crop the image will of course be a lot more magnified than on the full frame. This would be correct provided that the image is only enlarged to the same magnification. thus the image from the smaller sensor might only be 5"x4" whereas the image from the larger FF sensor would be 10" x 8" but if you put the image from the smaller sensor on top of the larger sensor it would fit on top perfectly. See image below:

Helped? No thought not!

Probably because it makes no sense!

 

[AJQS]

What will give the same dof? Putting the same lens on the 2 cameras at the same distance - but on the 2x crop the image will of course be a lot more magnified than on the full frame.

That's exactly what I already said. And the first bit meant 25mm on crop equals 50mm on 2x crop, I worded it the wrong way round

 

[Jelster]

Hoppy, below is a quote from the thread from last week (well I think it was last week) in which you gave a very good explanation.

On reading the other thread again it was really based on a discussion about sensor sizes relative to DoF and the fact that the originator had a very small sensor on his Fuji.

So, whilst I know that if you take a shot on 2 cameras with different sensors while focal length, aperture and subject distance remains constant you will get a different FoV for each, the DoF on each will also be different.

I do agree however that, to get the same output from both cameras (effectively both having the same FoV) you will have to change either the focal length or subject distance (or a mix of both) to keep the same DoF for the aperture.

F/number 5.6, focal length 50mm, subject distance 10ft.
DoF on DX = 2.76ft
DoF on FX = 4.25ft

The Dof is evidently different (and I also said earlier how it appeared to be the reverse of what you might expect, because the framing is completely changed).

If you change the subject distance, you have changed the perspective and the image is not the same, is it? The only way to maintain equivalence is to adjust focal length.

So, if you want the picture framed the same, from the same viewpoint with the same perspective and DoF, in other words two 100% identical images it looks like this:

F/number 5.6, focal length 50mm, subject distance 10ft.
DoF on FX = 4.25ft

F/number 3.6 (to nearest quarter stop), focal length 33mm, subject distance 10ft.
DoF on DX = 4.11ft

Changing the size of the sensor dictates changes in both focal length and f/number to re-establish equivalence. You can change the focal length, the distance or the f/number, but you can't change the size of the sensor, which is why it is the primary driver of DoF.

Steve

 

[EdBray]

That's exactly what I already said. And the first bit meant 25mm on crop equals 50mm on 2x crop, I worded it the wrong way round

But Alan, that is only if you do not enlarge the image to a size that would be larger than it would be as seen on the FF image. Thus if the FF image was printed at 10 x 8 then the x2 image would be printed considerably smaller ie no different than cutting the 10 x 8 image to suit.

 

[AJQS]

What I was saying wasnt specifically referring to with printed results per se, but yes you are right, the same lens on both cameras at the same distance, the 2x crop camera result would be the same as taking the full frame image and chopping the edges off, and most relevantly to this discussion ,with the same depth of field

As for how far back you'd have to stand to get the same field of view I must admit I don't know that calculation, but I'd guess it would hing around a factor of root2, so say 1.4x further back? but of course then you increase your depth of field...

And off we go again, you can see why it takes so much work to get your head round eh?

 

[HoppyUK]

Yeah I know, but that's kind of my point. When people ask this question they usually want to know if using a larger sensor will make it easier to blur the background. And in reality it often will because of the way we work. I have to be careful shooting large groups because I can get away with a larger aperture on my crop cameras than I can on my FF for precisely this reason.

Yes, that's exactly right. That's how we work and it is the only meaningful comparison. Two images that are identical in every sense, in terms of framing, perspective and DoF - the full frame camera with require both a longer lens and an f/number that is fractionally over one stop higher than the cropper.

And the calculation for both is the crop factor - focal length x crop factor and f/number x crop factor. (DoF calcs take no account of sensor resolution or pixels or anything, and you must be looking at same sized prints.) As detailed in my quote reposted in #26 above

 

[POAH]

Oh no not this debate again. Some people will tell you sensor size doesn't have an effect, but in the real world, the way we actually tend to use our gear it does.

its not a debate, he is asking a question :nono:

 

[ash59uk]

A 100% crop is a crop taken from a full resolution image from a given camera/sensor/lens combo.

The crop itself can be of any size with regards to the original image. For example a portrait could be cropped on an eye, both eyes, smile, face.

As long as the original image hasn't been resized and then the crop applied, it will be a 100% crop.

It does not mean crop 100% of an image, think more like crop part of an image that is 100% untouched.

I'll give an example of an images then a 100% crop of same image both at 800x533 pixels

Original (resized 800x533)

100% Crop (resized 800x533)

Andy

Thank you for your excellent post,everything makes sense now

I THINK :bonk:

 

[markosparko1994]

so not technically really, still just depends what lenses you use

 

[hoohaaa]

So, whilst I know that if you take a shot on 2 cameras with different sensors while focal length, aperture and subject distance remains constant you will get a different FoV for each, the DoF on each will also be different.

I think this is really the point (and the point of last weeks thread).

In the scenario above you'd get a different FoV but the same DoF (ignoring sensor resolution and all that gubbins). However in the real world people do want to compare the same FoV so it is a convenient shorthand to say that sensor size affects DoF but this is only because you need to use a wider lens to get the same shot and this does affect DoF and actually has nothing directly to do with the sensor size.

However, as this is the talk basics forum I think it's a bit misleading and confusing to say that sensor size affects DoF because its the change in lens necessary to get the same shot that affects it, not the sensor itself.

 

[HoppyUK]

I think this is really the point (and the point of last weeks thread).

In the scenario above you'd get a different FoV but the same DoF (ignoring sensor resolution and all that gubbins). However in the real world people do want to compare the same FoV so it is a convenient shorthand to say that sensor size affects DoF but this is only because you need to use a wider lens to get the same shot and this does affect DoF and actually has nothing directly to do with the sensor size.

However, as this is the talk basics forum I think it's a bit misleading and confusing to say that sensor size affects DoF because its the change in lens necessary to get the same shot that affects it, not the sensor itself.

But why is it necessary to change the lens? What is the primary driving factor? It is because you have changed the size of the sensor.

I don't think that is confusing at all, because that's the way we work. We start with a camera, and everything else is changed to fit around it.

And focal length does not determine DoF at all - it is the combination of lens and sensor size, without which you don't have an image. You need both, plus f/number and distance, to make any calculation at all.

All four factors. But as a shorthand, to say that sensor size affects DoF is key and logical, which is why it's commonly used.

 

[hoohaaa]

I don't think that is confusing at all, because that's the way we work. We start with a camera, and everything else is changed to fit around it.

Well I agree that the statement 'if your camera has a smaller sensor you will usually have much more depth of field in your images' is an easy idea to grasp and remember but it doesn't help anyone to understand.

For anyone trying to learn (and this is the talk basics forum) it just seems misleading to state that sensor size affects DoF when in reality it's the change of lens associated with the small sensor that affects it, not the sensor size itself.

I suspect that quite a few people do not quite understand that a crop sensor is just that - a large sensor with the edges cropped off. Jelster said earlier that "I know that if you take a shot on 2 cameras with different sensors while focal length, aperture and subject distance remains constant you will get a different FoV for each, the DoF on each will also be different". This isn't true, the DoF will be the same. The image projected onto the film plane will be identical it's just that the larger sensor will collect more of the image. It is just this sort of misunderstanding I was trying to avoid.

For everyone who properly understand what's going it seems a reasonable abbreviation but those trying to comprehend what's going on it seems, at least to me, misleading.

 

[Jelster]

Well I agree that the statement 'if your camera has a smaller sensor you will usually have much more depth of field in your images' is an easy idea to grasp and remember but it doesn't help anyone to understand.

For anyone trying to learn (and this is the talk basics forum) it just seems misleading to state that sensor size affects DoF when in reality it's the change of lens associated with the small sensor that affects it, not the sensor size itself.

Well, as it is the "Talk Basics" forum, most people hear will have a basic camera, some even without a DSLR, so pointing out that a smaller sensor makes it more difficult to generate a shallow DoF is important. When people new to the hobby user these wonderful shots with really narrow depth of field and fail to recreate them, thru need to understand why.

In practical terms, smaller sensor means an increased depth of field.

Steve

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[hoohaaa]

When people new to the hobby user these wonderful shots with really narrow depth of field and fail to recreate them, thru need to understand why.

I agree they do want to understand why. But explaining that to get the same image on a crop sensor camera will require a different lens and this will affect DoF comes closer to what I would call understanding rather than merely stating that sensor size affects DoF.

In practical terms, smaller sensor means an increased depth of field.

Quite agree with that.

 

[HoppyUK]

<snip>

Quite agree with that.

That's a relief!

If you're interest is not to mislead people, then you should stop making incorrect statements like the one in para three of post #35 above. Jelster is right, you are not. Why don't you actually do the calc and see what comes out?

 

[Jelster]

I suspect that quite a few people do not quite understand that a crop sensor is just that - a large sensor with the edges cropped off. Jelster said earlier that "I know that if you take a shot on 2 cameras with different sensors while focal length, aperture and subject distance remains constant you will get a different FoV for each, the DoF on each will also be different". This isn't true, the DoF will be the same. The image projected onto the film plane will be identical it's just that the larger sensor will collect more of the image. It is just this sort of misunderstanding I was trying to avoid.

Sorry, I'm not wrong. Go do the test yourself. The outputs will be very different as the the FoV will change, and so will the DoF. If the subject is a ruler showing the 10cm mark as point of focus, the full frame camera will show a different DoF to the crop body, FACT !

Steve

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[hoohaaa]

Jelster is right, you are not. Why don't you actually do the calc and see what comes out?

Not again.

There isn't a calculation to be done. Light from the lens falls on the film plane. A larger sensor collects lots of the image, a smaller sensor collects just the middle bit of it.

Tell me how on earth the light passing through the same lens, aperture and focal distance can be affected so that it's DoF changes depending only on how much of the image you collect. (Please don't give me a link to a DoF calculation thingy.)