Q3 Crop Modes and Equivalent Depths of Field Question

[Dr. G]

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I've never been able to figure out how using the crop modes effects the perceived depth of field of the final images.  If I'm shooting at 28mm f/1.7 at a set distance and then I use one of the crop modes (35, 50, 75, 90), how does the approximate perceived depth of field f stops change in each of the resulting images?

[David Wien]

1 hour ago, Dr. G said:

.  If I'm shooting at 28mm f/1.7 at a set distance and then I use one of the crop modes (35, 50, 75, 90), how does the approximate perceived depth of field f stops change in each of the resulting images?

It doesnt change. It stays the same as a 28mm f/1.7 shot.  The only difference is that you are looking at part of the original photo.

David

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[Dr. G]

2 minutes ago, David Wien said:

It doesnt change. It stays the same as a 28mm f/1.7 shot.  The only difference is that you are looking at part of the original photo.

David

So you're saying that if I shot the same framing with a 90mm f/1.7 lens as the 90mm crop from the Q3's 28mm f/1.7 lens the depths of field in the resulting images would appear exactly the same?  

[SrMi]

18 minutes ago, David Wien said:

It doesnt change. It stays the same as a 28mm f/1.7 shot.  The only difference is that you are looking at part of the original photo.

David

I do not think so.

The magnification changes and therefore the DOF (determined by acceptable sharpness). Shooting with crop modes is the same as shooting with a smaller sensor (DOF, noise).

[Daniel C.1975]

vor 48 Minuten schrieb David Wien:

It doesnt change. It stays the same as a 28mm f/1.7 shot.  The only difference is that you are looking at part of the original photo.

David

Technically correct, practically not helpful for the question.

 

To the initial question: 

When you crop, you are only using a portion of the sensor, and with the new diagonal of the used sensor area, there is a change of the angle of view. E.g. an 28mm lens on a 35mm sensor does have an angel of view of ~75 degree, with a larger sensor the angle of view would be wider, with a smaller it would be more narrow. So it is the diagonal of the sensor and the focal-length which together defines the angle of view - simply spoken. 

As the Q does use a 35mm sensor, we stick to that equivalent. And yes, even if some people always try to say something different: If you use a crop out of an 28mm photo to simulate 50mm, you have the perspective and image impression of an 50mm lens, used at the same distance with an equivalent aperture. Its physics and mathematics - period. 

To understand what the equivalent aperture at a given crop to a comparable focal length would be, you can use simple math (yes, you can also use more complicated math, but the following is more than sufficient  )

A = aperture (no dimension)

F = focal length mm

d = aperture diameter in mm 

d = F/A = 28mm/1.7 = 16,471mm

 

Now we just solve the equation towards A, while we use F for the via crop simulated focal length (I know technically not the correct terminus, but again, for simplification sufficient):

35mm: A=F/d = 35mm/16,471mm = 2,125 = f2.1

50mm: A= 3,036 = f3

75mm: A=4,554 = f4.6

90mm: A= 5,464 = f5.5

 

Of course you also lose resolution and therefore need to magnify the image more for the same output size, which also leads to a higher magnification of image errors of the lens or the sensor (e.g. chromatic aberrations, noise, ... )

 

 

 

 

 

Edited November 3, 2023 by Daniel C.1975

[01af]

vor 2 Stunden schrieb Dr. G:

If I'm shooting at 28 mm [and] f/1.7 at a set distance and then I use one of the crop modes (35, 50, 75, 90), how does the approximate perceived depth of field f stops change in each of the resulting images?

Compared to what?

.

vor 1 Stunde schrieb David Wien:

It doesn't change.

It does.

.

vor 1 Stunde schrieb Dr. G:

So you're saying that if I shot the same framing with a 90 mm f/1.7 lens [on 35-mm full-frame format] as the 90 mm crop from the Q3's 28 mm 1:1.7 lens the depths of field in the resulting images would appear exactly the same?  

Yes, that's what he's saying. But no, he's wrong.

.

vor einer Stunde schrieb Daniel C.1975:

Technically correct, practically not helpful for the question. [...]

Daniel, this statement applies to your post exactly.

Okay—when taking the same picture from the same point-of-view with lenses of equivalent focal lengths on different-format cameras then, at the same apertures, the smaller formats will yield more depth-of-field. To get the same depth-of-field, the aperture numbers must be proportional to the linear format size (i. e. format diagonal)—that is, smaller apertures on the larger-format camera.

So, to figure out which aperture you'd have to use on a 35-mm full-frame-format camera to get the same depth-of-field as with your Leica Q, multiply the Q's actual aperture number by the crop mode's virtual focal length, then divide by 28 (i. e. the actual focal length).

Example:
Shooting at f/4 in the Q's "75 mm" crop mode will yield the same depth-of-field as shooting at 4 * 75 / 28 = f/10.7 ... or ~f/11 on a 35-mm-format camera with a real 75 mm lens.

 

Edited November 3, 2023 by 01af

[Qwertynm]

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Can I add another spin? 35/28 = 1.25 x 1.7 = 2.125. same result as @Daniel C.1975 but without the diameter of the aperture leads to the same answer. 
 

applicable real world use can be to needing to stop down beyond f/6.3 in crop mode for landscape images for same DoF

[jaapv]

 

2 hours ago, 01af said:

Compared to what?

.

It does.

.

Yes, that's what he's saying. But no, he's wrong.

.

Daniel, this statement applies to your post exactly.

Okay—when taking the same picture from the same point-of-view with lenses of equivalent focal lengths on different-format cameras then, at the same apertures, the smaller formats will yield more depth-of-field. To get the same depth-of-field, the aperture numbers must be proportional to the linear format size (i. e. format diagonal)—that is, smaller apertures on the larger-format camera.

So, to figure out which aperture you'd have to use on a 35-mm full-frame-format camera to get the same depth-of-field as with your Leica Q, multiply the Q's actual aperture number by the crop mode's virtual focal length, then divide by 28 (i. e. the actual focal length).

Example:
Shooting at f/4 in the Q's "75 mm" crop mode will yield the same depth-of-field as shooting at 4 * 75 / 28 = f/10.7 ... or ~f/11 on a 35-mm-format camera with a real 75 mm lens.

 

Yes, perfectly correct assuming that you are enlarging the cropped image to the same size as the original one and viewing it from the same distance.

We have been over this so often. There must be at least a dozen threads explaining this.

[nariza7]

So I always thought that crop mode for the Q3 merely does a post-crop of the 28mm f1.7 shot, but saves that crop in the jpeg file and leaves the full 28mm f1.7 shot in DNG. In that sense, no matter what crop value (50/75/90) is selected, it will always have the rendering of the 28mm f1.7. I dont believe there is a sensor crop itself, but someone please correct me if I'm wrong.

To clarify it further, crop mode to me is the same as if you took a shot in 28mm f1.7, opened it in lightroom or something, and zoomed in.

I don't believe the crop mode is implemented the same as how sony does with APS-C mode where it has a physical limitation crop on the sensor itself

Edited November 4, 2023 by nariza7

[SrMi]

1 hour ago, nariza7 said:

So I always thought that crop mode for the Q3 merely does a post-crop of the 28mm f1.7 shot, but saves that crop in the jpeg file and leaves the full 28mm f1.7 shot in DNG. In that sense, no matter what crop value (50/75/90) is selected, it will always have the rendering of the 28mm f1.7. I dont believe there is a sensor crop itself, but someone please correct me if I'm wrong.

To clarify it further, crop mode to me is the same as if you took a shot in 28mm f1.7, opened it in lightroom or something, and zoomed in.

I don't believe the crop mode is implemented the same as how sony does with APS-C mode where it has a physical limitation crop on the sensor itself

It does not matter whether you crop in post (Q3), the crop is applied in-camera (Sony) or you are shooting with a smaller sensor (CL). The effect on DOF is the same.

[LocalHero1953]

To add a comment which, like all comments on DoF threads, will probably add to the confusion and the argument......

The confusion arises because there are two different ways of considering DoF: what I would call 'optical' and 'perceptual'. The optical DoF in a Q does not change at all with crop mode: the lens and sensor show exactly the same bits of the scene out of focus to exactly the same amount, no matter what the crop. In fact the only part which is in focus is exactly at one distance: the focal point. What does change is the size of the scene you view. By cropping and displaying the results at the same screen or print size, the eye sees the OOF more clearly in the cropped image because it is enlarged.

Depth of Field formulae and scales on lenses are based on what I have called the perceptual notion of DoF, and so incorporate a fudge factor called the circle of confusion (sic), which makes assumptions about the distance at which you view the image, and about what is an acceptable degree of out of focus.

Edited November 4, 2023 by LocalHero1953

[01af]

vor 57 Minuten schrieb LocalHero1953:

The confusion arises because there are two different ways of considering depth-of-field: what I would call 'optical' and 'perceptual'.

No. There is no 'two different ways'. What you call 'optical depth-of-field' doesn't exist. The concept of depth-of-field is, by nature, purely perceptual.

[LocalHero1953]

1 hour ago, 01af said:

No. There is no 'two different ways'. What you call 'optical depth-of-field' doesn't exist. The concept of depth-of-field is, by nature, purely perceptual.

Yours is a dogmatic view that you are welcome to hold to. It doesn’t change optics. I do agree, though, that when most people talk about depth of field, they are usually referring to subjective perception, although they sometimes expect it to have a more objective basis. 

[jaapv]

I agree with Gordon that DOF will always be subjective as it can be defined as "the zone of acceptable sharpness" which obviously depends on the subject, contrast, enlargement/viewing distance and perception of the viewer.

However, Olaf is right that traditionally certain values have been assigned to the CoC enabling calculations.

Below I demonstrate that the only difference is the DOF (and possibly minor technical considerations like noise) and that the concept of "wideangle rendering"(whatever that may be ??) is of no impact whatever.

 

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28 mm

70 mm

28mm cropped to 70mm AOV

[SrMi]

6 hours ago, LocalHero1953 said:

To add a comment which, like all comments on DoF threads, will probably add to the confusion and the argument......

The confusion arises because there are two different ways of considering DoF: what I would call 'optical' and 'perceptual'. The optical DoF in a Q does not change at all with crop mode: the lens and sensor show exactly the same bits of the scene out of focus to exactly the same amount, no matter what the crop. In fact the only part which is in focus is exactly at one distance: the focal point. What does change is the size of the scene you view. By cropping and displaying the results at the same screen or print size, the eye sees the OOF more clearly in the cropped image because it is enlarged.

Depth of Field formulae and scales on lenses are based on what I have called the perceptual notion of DoF, and so incorporate a fudge factor called the circle of confusion (sic), which makes assumptions about the distance at which you view the image, and about what is an acceptable degree of out of focus.

The 'optical" DOF has depth zero since only an infinitesimally thin plane is optically in focus. Everything behind and in front of that focal plane is 'optically' out-of-focus. The only DOF that matters is what we perceive as being in focus.

[SrMi]

6 hours ago, LocalHero1953 said:

Depth of Field formulae and scales on lenses are based on what I have called the perceptual notion of DoF, and so incorporate a fudge factor called the circle of confusion (sic), which makes assumptions about the distance at which you view the image, and about what is an acceptable degree of out of focus.

FYI, the Circle of Confusion is a scientific term in optical sciences used since the 19th century.

[LocalHero1953]

1 minute ago, SrMi said:

FYI, the Circle of Confusion is a scientific term in optical sciences used since the 19th century.

As a scientist myself, I recognise this. 'Fudge factor' is another way of describing something that includes quite a lot of indeterminate parameters. 

[LocalHero1953]

28 minutes ago, SrMi said:

The 'optical" DOF has depth zero since only an infinitesimally thin plane is optically in focus. Everything behind and in front of that focal plane is 'optically' out-of-focus. The only DOF that matters is what we perceive as being in focus.

Indeed, but to a degree that depends on the distance from the focal plane. I could have written, more exactly, "The optical focus behaviour of a Q does not change at all with crop mode", but I thought that readers would take the point: the optical behaviour of the lens does not change with cropping, but the way it is viewed does.

[SrMi]

30 minutes ago, LocalHero1953 said:

Indeed, but to a degree that depends on the distance from the focal plane. I could have written, more exactly, "The optical focus behaviour of a Q does not change at all with crop mode", but I thought that readers would take the point: the optical behaviour of the lens does not change with cropping, but the way it is viewed does.

To stress again, 'optical' DOF is neither a scientific nor a practical term.

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[LocalHero1953]

2 hours ago, SrMi said:

To stress again, 'optical' DOF is neither a scientific nor a practical term.

Please read what I wrote. Recognising that the optical focus performance of a lens remains constant when cropping (and is measurable - Newton did it), is a useful and necessary starting point for understanding the photographic DoF concept.

Scientific terminology is valuable for scientists, but it appeared to me that the OP was not looking for an explanation in scientific terms.