Depth of field question- Crop / Full Frame

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Pk8887 Junior Member • Posts: 25
Depth of field question- Crop / Full Frame

I'm looking for a crop set camera for sports to lose some weight and gain and gain some length but am confused about depth of field and would like some help figuring out the following.

I like the background separation look I can get at 100mm f2.8 on my full frame 5dmk iii.

If I were to buy a crop camera and the lens was f2.8 from what I understood the background separation wouldnt be as obvious and the apparent depth of field wouldn't be as shallow (similar to f4 or f4.5 as i have read) due to the smaller sensor.

BUT as the crop would be a longer focal length in reality (1.5 x 100mm) and as focal length impacts the depth of field would it actually be similar?

In practical sense if I'm using a 2.8 lens on crop camera will I be able to achieve a  similar shallow depth of field/backgroudn separation compared to a full frame equivalent at the same 'looking' focal lengths.

I may have just confused myself!

baloo_buc Veteran Member • Posts: 9,872
Re: Depth of field question- Crop / Full Frame
1

If you keep the distance the crop sensor will yield a shallower DOF because the print will be magnified more. If you increase the distance to keep the framing the DOF will be deeper.

E.g. if you have a subject at 4.5 m with 100 mm @ f/2.8 on your full frame camera the DOF will be 336 mm.

If you have the same distance (4.5 m) on a 1.6x crop sensor the DOF at 100 mm @ f/2.8 will be 213 mm. If you increase the distance to keep the framing (1.6x4.5=7.2 m) the DOF will increase to 550 mm.

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Victor
Bucuresti, Romania

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Tom Axford Veteran Member • Posts: 5,241
Re: Depth of field question- Crop / Full Frame
4

Pk8887 wrote:

I'm looking for a crop set camera for sports to lose some weight and gain and gain some length but am confused about depth of field and would like some help figuring out the following.

I like the background separation look I can get at 100mm f2.8 on my full frame 5dmk iii.

The rule you need to remember is that to get the same field of view and the same depth of field and background blur, you need a lens on your crop camera where both the focal length and the f-number are divided by the crop factor.  So, for a camera with a crop factor of 1.5 you need a 67mm f/1.9 lens to get similar pictures.

If I were to buy a crop camera and the lens was f2.8 from what I understood the background separation wouldnt be as obvious and the apparent depth of field wouldn't be as shallow (similar to f4 or f4.5 as i have read) due to the smaller sensor.

BUT as the crop would be a longer focal length in reality (1.5 x 100mm) and as focal length impacts the depth of field would it actually be similar?

In practical sense if I'm using a 2.8 lens on crop camera will I be able to achieve a similar shallow depth of field/backgroudn separation compared to a full frame equivalent at the same 'looking' focal lengths.

I may have just confused myself!

OP Pk8887 Junior Member • Posts: 25
Re: Depth of field question- Crop / Full Frame

Thank you Victor. So the same frame from each camera would have the full frame with a shallower DOF but if the crop was at 100m (150mm effectively) and the full frame as at 100mm the crop would have a shallower depth of field?

nick101 Senior Member • Posts: 1,139
Re: Depth of field question- Crop / Full Frame
2

Pk8887 wrote:

BUT as the crop would be a longer focal length in reality (1.5 x 100mm) and as focal length impacts the depth of field would it actually be similar?

NO!!! The focal length is the same - 100mm whatever the crop. (Sorry for the caps, but this is important)

It's the field of view that changes. So - the focal length stays 100mm but on APS-C cameras, the sensor only records a part of the image and so the field of view looks like a 150-160mm lens.  On M43, the field of view would be equivalent to 200mm.

The physical diameter of the lens aperture is the same, but because the field of view is that of a longer length, the effective aperture (f/number) is smaller and so the depth of field is greater.

If the effective aperture at 100mm is f/2, then making the field of view something like 160mm (APS-C) will make it something like f/3 and on M43 it'll be around f/4.,

Example: say the diameter of the lens aperture wide open is 50mm. The f/number is 100mm/50mm = f/2.

On an APS-C camera, the f/number is still the same (same focal length, same diameter, but you're only seeing the central part of the image circle so it looks like you're using a 160mm lens. So the depth of field is similar to that of a 160mm lens. If the focal length was 160mm, the f/number would be 160/50 - f/3.2 (-ish).

On M43, you get the depth of field of 200/50 = f/4

So - if you want the same DOF at the same "looking" focal lengths, you'd want out a 60mm lens on APS-C and a 50mm lens on M43. Each of this would give you the same look, including DOF, at the same aperture as a 100mm FF.

I may have just confused myself!

We've all done it. This isn't very intuitive and you have to work through it until it sticks.

I hope all that makes sense

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Tom Axford Veteran Member • Posts: 5,241
Re: Depth of field question- Crop / Full Frame

You might find this tutorial helpful.

baloo_buc Veteran Member • Posts: 9,872
Re: Depth of field question- Crop / Full Frame

If you have the same framing the DOF will be shallower on full frame. If you keep the distance (different framing) the DOF of full frame will be deeper.

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Victor
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Bob Contributing Member • Posts: 690
Re: Depth of field question- Crop / Full Frame

Excellent question - a common source of confusion for beginners, and as you will soon see from the flood of responses and the ensuing arguments, it’s a source of confusion for experienced photographers as well.

From what I can tell, answers to this question tend to fall into the following categories (all of which are sort of correct):

  • The Mathematical Answer
    • The “circle of confusion” will be quoted, which is the mathematical definition of depth of field.
    • Here, sensors size does matter, but you also have to factor in other considerations such as the size of the print and how far the observer’s eye is from that print.
  • The Practical Answer
    • Most beginners simply want to know if the background will be just as blurry with a smaller sensor.
    • This point of view suggests the sensor size does not impact DOF directly, but it does effect it indirectly because the small sensor will use a different focal length and different subject-to-camera distance to achieve the same angle of view as a larger sensor camera.
    • But, all things being equal (same lens, same distance, same aperture), cropping the image in Photoshop to match the field of view of the larger sensor image should result in very similar background blurriness.
    • In other words, if you have a small sensor camera, you will find yourself moving closer to the subject with a shorter lens.  Both of those will affect your DOF.
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Alan GT Regular Member • Posts: 228
Re: Depth of field question- Crop / Full Frame

Hi - as others have tried to explain to you - cropping an image doesn't change its depth of field.  (Imagine cropping a photo already taken with your full frame camera.)

The only thing that changes the depth of field when you switch from a full frame to a crop camera with the same lens/aperture is whether or not you move yourself further away from the subject to keep the framing of the shot the same.  It is this assumption (easily missed in the discussion) that you move further away to compensate for the change in framing that is the key thing to understand.

Alan

Tom Axford Veteran Member • Posts: 5,241
Re: Depth of field question- Crop / Full Frame
1

Alan GT wrote:

Hi - as others have tried to explain to you - cropping an image doesn't change its depth of field. (Imagine cropping a photo already taken with your full frame camera.)

Again, this is wrong.  It is a well-known paradox that has been discussed many times before on this forum.

It is very educational to think about the reasons for this.

The only thing that changes the depth of field when you switch from a full frame to a crop camera with the same lens/aperture is whether or not you move yourself further away from the subject to keep the framing of the shot the same. It is this assumption (easily missed in the discussion) that you move further away to compensate for the change in framing that is the key thing to understand.

Alan

Alan GT Regular Member • Posts: 228
Re: Depth of field question- Crop / Full Frame
1

Tom Axford wrote:

Alan GT wrote:

Hi - as others have tried to explain to you - cropping an image doesn't change its depth of field. (Imagine cropping a photo already taken with your full frame camera.)

Again, this is wrong. It is a well-known paradox that has been discussed many times before on this forum.

I guess "circle of confusion" is appropriately named!

Alan

Tom Axford Veteran Member • Posts: 5,241
Re: Depth of field question- Crop / Full Frame

Alan GT wrote:

Tom Axford wrote:

Alan GT wrote:

Hi - as others have tried to explain to you - cropping an image doesn't change its depth of field. (Imagine cropping a photo already taken with your full frame camera.)

Again, this is wrong. It is a well-known paradox that has been discussed many times before on this forum.

I guess "circle of confusion" is appropriately named!

Alan

You're not alone here, by any means.  Four years ago, in a poll, I found that the great majority here believe that cropping an image does not change its depth of field.

However, using the traditional definition of depth of field, cropping an image does change its depth of field.  If you have the perseverance to follow through that poll thread, a fairly typical discussion of the subject ensues.  The key, of course, lies in the way that depth of field is defined.

dsjtecserv Veteran Member • Posts: 3,700
Re: Depth of field question- Crop / Full Frame
1

Alan GT wrote:

Tom Axford wrote:

Alan GT wrote:

Hi - as others have tried to explain to you - cropping an image doesn't change its depth of field. (Imagine cropping a photo already taken with your full frame camera.)

Again, this is wrong. It is a well-known paradox that has been discussed many times before on this forum.

I guess "circle of confusion" is appropriately named!

In more ways than one!

When a part of an image is not in focus, it is blurry, and that blur is represented by the blur circle that would surround a hypothetical point that is out of focus. The size of that circle varies as a direct function of the distance of an object from the focus plane; the circles are smallest at the focus plane and get progressively bigger. There is a threshold of blur circle size below which human can't see them; can't distinguish them from a point. So the point at which a part of an image is considered to be "out of focus" is the point at which the circles are big enough to be perceived. Depth of field is the distance between the nearest object that does not have perceptible blur and the farthest object that does not have perceptible blur.

The term "circle of confusion" (better stated as circle of confusion limit) is used to represent the size of a blur circle on the sensor or film that, when enlarged to the final print size and viewed by an observer, is just at this threshold of perception. Clearly, the degree of enlargement matters for this -- the more the sensor image is enlarged, the larger its blur circles will be, and thus the narrower depth of field will be.

In order to evaluate depth of field with respect to enlargement, the size of the print as well as the viewing distance, are standardized. But there are many different sizes of film and sensors. A different amount of enlargement will needed to be applied to each different size in order to produce a print of the standard size. Consequently, the size of the circle of confusion limit will be different for different sizes of the recording medium.

Thus the sensor size is an unavoidable factor in determining depth of field. For smaller sensors, more enlargement will be needed to produce the final picture, and thus the circle of confusion limit must be smaller. If the shooting conditions are the same for both sensor sizes (that is, focal length [not field of view], f-number, and focus distance distance are the same), then the smaller sensor will have shallower depth of field, due to the need for greater enlargement. And this is also true for a crop from a larger sensor -- in order to get the cropped portion of the image to the same print size, it must be enlarged more.

Since the OP stipulated that he was using the same lens on both cameras, that is the most appropriate answer to his particular question. However, if he had said that he wanted the field of view to be the same from both sensors, then the answer would be different, because one or more of the shooting conditions would need to change in order to equalize the field of view.

Dave

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Bob Contributing Member • Posts: 690
Re: Depth of field question- Crop / Full Frame

Tom Axford wrote:

Alan GT wrote:

Tom Axford wrote:

Alan GT wrote:

Hi - as others have tried to explain to you - cropping an image doesn't change its depth of field. (Imagine cropping a photo already taken with your full frame camera.)

Again, this is wrong. It is a well-known paradox that has been discussed many times before on this forum.

I guess "circle of confusion" is appropriately named!

Alan

You're not alone here, by any means. Four years ago, in a poll, I found that the great majority here believe that cropping an image does not change its depth of field.

However, using the traditional definition of depth of field, cropping an image does change its depth of field. If you have the perseverance to follow through that poll thread, a fairly typical discussion of the subject ensues. The key, of course, lies in the way that depth of field is defined.

No, I'm not getting sucked back into this.

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Iliah Borg Forum Pro • Posts: 24,799
Re: Depth of field question- Crop / Full Frame

Pk8887 wrote:

I'm looking for a crop set camera for sports to lose some weight and gain and gain some length but am confused about depth of field and would like some help figuring out the following.

I like the background separation look I can get at 100mm f2.8 on my full frame 5dmk iii.

If I were to buy a crop camera and the lens was f2.8 from what I understood the background separation wouldnt be as obvious and the apparent depth of field wouldn't be as shallow (similar to f4 or f4.5 as i have read) due to the smaller sensor.

BUT as the crop would be a longer focal length in reality (1.5 x 100mm) and as focal length impacts the depth of field would it actually be similar?

In practical sense if I'm using a 2.8 lens on crop camera will I be able to achieve a similar shallow depth of field/backgroudn separation compared to a full frame equivalent at the same 'looking' focal lengths.

I may have just confused myself!

If you will be using the same lens, and cropping the same part of the scene for the final image (that is, you are not filling the frame currently using your FF camera, and with APS camera you will be framing tighter from the same shooting position), geometrical DoF doesn't change. It happens because the absolute sizes of the projections of the objects onto the sensors are the same, and magnification for final image presentation is also the same.

In other words, if you are not filling the frame and cropping for image presentation now, you are already effectively shooting with a cropped sensor.

The pictorial character of the OOF area may however be a little bit different if pixel density is different between your FF and APS cameras.

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Bob Contributing Member • Posts: 690
Re: Depth of field question- Crop / Full Frame

Pk8887 wrote:

I'm looking for a crop set camera for sports to lose some weight and gain and gain some length but am confused about depth of field and would like some help figuring out the following.

I like the background separation look I can get at 100mm f2.8 on my full frame 5dmk iii.

If I were to buy a crop camera and the lens was f2.8 from what I understood the background separation wouldnt be as obvious and the apparent depth of field wouldn't be as shallow (similar to f4 or f4.5 as i have read) due to the smaller sensor.

BUT as the crop would be a longer focal length in reality (1.5 x 100mm) and as focal length impacts the depth of field would it actually be similar?

No, the focal length does not change. A 100mm lens on a FF would still be a 100mm lens on a crop-frame. So you have a bit of a misunderstanding there.

In practical sense if I'm using a 2.8 lens on crop camera will I be able to achieve a similar shallow depth of field/backgroudn separation compared to a full frame equivalent at the same 'looking' focal lengths.

I may have just confused myself!

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bobn2
bobn2 Forum Pro • Posts: 58,604
Re: Depth of field question- Crop / Full Frame
9

Pk8887 wrote:

I'm looking for a crop set camera for sports to lose some weight and gain and gain some length but am confused about depth of field and would like some help figuring out the following.

I like the background separation look I can get at 100mm f2.8 on my full frame 5dmk iii.

If I were to buy a crop camera and the lens was f2.8 from what I understood the background separation wouldnt be as obvious and the apparent depth of field wouldn't be as shallow (similar to f4 or f4.5 as i have read) due to the smaller sensor.

BUT as the crop would be a longer focal length in reality (1.5 x 100mm) and as focal length impacts the depth of field would it actually be similar?

In practical sense if I'm using a 2.8 lens on crop camera will I be able to achieve a similar shallow depth of field/backgroudn separation compared to a full frame equivalent at the same 'looking' focal lengths.

I may have just confused myself!

As Tom Axford has said, the rule is very simple. The apparent DOF when viewed in the final image (sorry for all the qualifiers, necessary to pre-empt confusing arguments) between two format will be the same for lenses where the focal length and f-number are in proportion of the relative crop factor. So, if you like the DOF at 100mm, f/2.8 on FF, you'll get the same effect on a 1.5x crop camera using a 100/1.5 mm (67mm) lens set to f/(2.8/1.5) (f/1.9). With those settings, the aperture diameter and AoV is the same, so the geometry of all the light rays entering the lens is the same, therefore the apparent DoF in the scene must be the same.

Pretty simple. I'd ignore all the obtuse arguments about the theory of DoF - the proponents and up confusing themselves as much as anyone else. The rule above is simple and works.

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Bob

OP Pk8887 Junior Member • Posts: 25
Re: Depth of field question- Crop / Full Frame

Thank you everyone for the information. Some hard to understand but all appreciated.

Bob from what you're saying then (which seemed pretty straight forward) if I'm trying to get a the background separation look of f2.8 at 100mm on FF but on a dx camera I'm not going to get it with a f2.8 lens?

I would like to swap the 5d mkiii currently using the 24-105 f4 to for something lighter and with a shallower dof (more similar to 70-200mm). My idea of D300s and an old Tokina 50-135 f2.8 probably isn't going to hit the mark background sedation wise. Sounds like it will be more like the 24-105 but obviously with more reach.

scokill
scokill Veteran Member • Posts: 5,245
Re: Depth of field question- Crop / Full Frame

Pk8887 wrote:

Thank you everyone for the information. Some hard to understand but all appreciated.

Bob from what you're saying then (which seemed pretty straight forward) if I'm trying to get a the background separation look of f2.8 at 100mm on FF but on a dx camera I'm not going to get it with a f2.8 lens?

I would like to swap the 5d mkiii currently using the 24-105 f4 to for something lighter and with a shallower dof (more similar to 70-200mm). My idea of D300s and an old Tokina 50-135 f2.8 probably isn't going to hit the mark background sedation wise. Sounds like it will be more like the 24-105 but obviously with more reach.

This isn't related to your DoF question, but I'm assuming you shoot sports indoors and/or at night under lights.  My biggest concern would be light, not Dof.  f4 typically wouldn't cut it, especially on crop sensor.  If I were going to shoot crop one of the lenses high on my list would be the Sigma 50-100 1.8.  Might only be appropriate indoors, but after that certainly a 70-200 2.8 would be the lens a majority of folks would be using.  The D300s wouldn't hit the mark in a lot of ways.  A good camera for its era, but not today IMO.   Don't know Canon that well so can't really comment.

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Bob Contributing Member • Posts: 690
Re: Depth of field question- Crop / Full Frame

When in doubt, consult The Manual of Photography by Jacobson, Ray, Attridge, etc.

There's an entire chapter devoted to understanding the calculations of depth of field.

I've looked through this in detail.

The only things that affect DOF are:

  • Distance to subject
  • Aperture
  • Focal length
  • Distance to film or sensor from back of lens
  • Lens diameter

Sensor size, directly has no impact.  However, indirectly it does because cameras are built around sensors and thus the lens geometry is different.  A Nikon 50mm DX lens is indeed smaller than a 50mm FX lens.

So, if you put a full frame 50mm on a crop sensor, the DOF will be unchanged because all the geometetric properties of the lens and distances are the same. But if you use a smaller 50mm made for that crop sensor, then yes, the DOF will be different.  This subtle point is missed by people using online DOF calculators and then arguing the point.

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