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Thread started 16 Aug 2015 (Sunday) 15:24
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Sensor size affect DOF? Methinks not.

 
GeoKras1989
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Aug 16, 2015 15:24 |  #1
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One of these is a full as-shot frame. The other is a heavy crop. Both shot from the same spot. Neither the tree nor the cornfield moved while I switched cameras. One from a G15, the other from a 6D. Both at f/2.8. There is a slight difference because I guessed poorly and had the G15 set to 23mm. FF lens is 28mm. Lots of folks are under the impression that sensor size somehow affects DOF. Not true.

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LincsRP
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Aug 16, 2015 15:45 |  #2

http://www.amateurphot​ographer.co.uk ...h-of-field-explained-4196 (external link)


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LincsRP
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Aug 16, 2015 15:47 |  #3

http://photo.stackexch​ange.com ...ion-for-macro-photography (external link)


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DreDaze
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Aug 16, 2015 15:51 |  #4

there seems to be a lot of B.S. in that link, i'm not sure if anything there can really be taken seriously...

the pictures showing the difference between f1.4, and f1.6 have to be a joke...


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AJSJones
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Aug 16, 2015 16:07 |  #5

DreDaze wrote in post #17671185 (external link)
there seems to be a lot of B.S. in that link, i'm not sure if anything there can really be taken seriously...

the pictures showing the difference between f1.4, and f1.6 have to be a joke...

Or you can develop a skill to spot typos - that's pretty obviously a typo for f/16 :D But if they have typos like that, one does need to check the text as well, I would agree.


Some people object to the "the sensor size alters the DoF" statement. What is usually meant by that is "If I want to take the same picture (and this means the same perspective, aka camera position) with cameras that have different sensor sizes, then the relationship between DoF and f stop is different because I must use a different FL lens on the cameras to make FoV the same. The use of a different FL is the cause of the change in DoF if I use the same aperture."


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P4ulG
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Aug 16, 2015 16:10 |  #6

I agree with that comment also I've not seen a lens that opens to f1.6


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GeoKras1989
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Aug 16, 2015 16:16 |  #7
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Perpetuating the myth. From the article:

"The depth of field achievable can also be affected by the size of the camera’s sensor. If your camera has a small sensor, such as those found in compacts, then the depth of field will be greater and less shallow. However, the larger sensor found in a DSLR is capable of increasingly amplified shallow depth of field, assuming you have the appropriate lenses to match."

The comparison above is between a 6D and G15. There is effectively no difference in DOF. This article makes unfounded claims, with no supporting evidence. The author appears to be echoing what he heard somewhere else, and believes to be true. Guys like him are why I posted the photos above. Sensor size has no effect on DOF.

This guy knows what he is talking about. From the 2nd article:

"In general shooting, provided you keep angle of view, camera position and size of entrance pupil the same, depth of field and diffraction will be the same regardless of sensor size!"


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GeoKras1989
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Aug 16, 2015 16:19 |  #8
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P4ulG wrote in post #17671211 (external link)
I agree with that comment also I've not seen a lens that opens to f1.6

How about just about every f/1.4 or faster lens in existence? Why wouldn't they do f/1.6?


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GeoKras1989
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Aug 16, 2015 16:45 |  #9
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A little better shot. The focal lengths are much closer. I stood in the same spot for both shots. The 6D is heavily cropped. The G15 is as shot. Again, neither the barn or the tree moved during this test. Both shot at the same focal length (~30.5mm) and both at f/4. DOF is nearly identical. There is extremely minimal difference in DOF is these shots.

You'll never see a DOF difference when comparing full frame to APSC shot at the same aperture, focal length and distance to subject/background. How does this myth keep getting perpetuated?

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sploo
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Aug 16, 2015 16:47 |  #10

GeoKras1989 wrote in post #17671142 (external link)
One of these is a full as-shot frame. The other is a heavy crop. Both shot from the same spot. Neither the tree nor the cornfield moved while I switched cameras. One from a G15, the other from a 6D. Both at f/2.8. There is a slight difference because I guessed poorly and had the G15 set to 23mm. FF lens is 28mm. Lots of folks are under the impression that sensor size somehow affects DOF. Not true.

Depth of field is affected by magnification* and aperture; for the same aperture, the larger the reproduction ratio on the sensor the shallower the DOF. Whilst things go a bit strange at macro levels, one easy way to picture it is that a 36mm wide object would need 1:1 reproduction to fill a 36mm wide full frame sensor, but only 1:2 to fill a four thirds sensor (as it's roughly half as wide). With a larger sensor you can stop the aperture down to regain DOF (countering the shallow DOF due to the larger required magnification); and you can also stop down further than a smaller sensor before diffraction starts to bite. In essence, a larger sensor allows the possibility of shallower DOF.

This is the reason why large format cameras can create razor thin DOF, and why it's common to stop down a long way (hence the naming of https://en.wikipedia.o​rg/wiki/Group_f/64 (external link))

See also http://www.cambridgein​colour.com/tutorials/d​epth-of-field.htm (external link), and the depth of field and diffraction sections here http://www.cambridgein​colour.com ...al-camera-sensor-size.htm (external link)

With regard to your specific example, your 6D has a "full frame" 36x24mm sensor. The G15 is a 1/1.7 sensor (approx 7.6mmx5.7mm), with a 6.1 to 30.5mm lens focal range. As the sensor is ~4.6x smaller than full frame its focal length in 35mm terms is 28-140mm.

The shot you took at 23mm on the G15 was, I presume, at 23mm in real terms (as the 35mm equivalent range starts at 28mm). Therefore, the 35mm equivalent focal length was 23mm x 4.6 = 105.6mm. I.e. much more telephoto and not comparing like-for-like.

I suspect it's a case that to achieve the same field of view/framing, you're magnifying 4.6x less than the 6D (so more DOF) but then using a 4.6x longer effective focal length than the 6D (so less DOF) - the end result was a wash. Though - to my eyes, the tree in the 6D shot doesn't look that sharp - and judging by the other settings that won't be due to a soft lens or the shutter speed, so I'm not sure where the focus was. The relative sizes of the trees and relationship of the backgrounds in the two shots don't quite appear to match either so I'm afraid it's difficult to make any real conclusions.

Out of interest - roughly how far were you from the tree, and roughly how far back was the vegetation behind?

* This is why a telephoto lens, or moving closer to your subject, reduces DOF: both are increasing magnification


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john ­ crossley
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Aug 16, 2015 16:48 |  #11

GeoKras1989 wrote in post #17671142 (external link)
One of these is a full as-shot frame. The other is a heavy crop. Both shot from the same spot. Neither the tree nor the cornfield moved while I switched cameras. One from a G15, the other from a 6D. Both at f/2.8. There is a slight difference because I guessed poorly and had the G15 set to 23mm. FF lens is 28mm. Lots of folks are under the impression that sensor size somehow affects DOF. Not true.
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Well if both images are taken at the same aperture, focal length and subject to camera distance, then the DOF will be the same wont it.

The reason sensor size affects DOF is because to frame the images the same you have to change either the focal length of the lens or the subject difference.


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DreDaze
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Aug 16, 2015 16:55 |  #12

john crossley wrote in post #17671250 (external link)
Well if both images are taken at the same aperture, focal length and subject to camera distance, then the DOF will be the same wont it.

The reason sensor size affects DOF is because to frame the images the same you have to change either the focal length of the lens or the subject difference.

but if you go to any online DOF calculator, and enter in the same aperture, same focal length, same distance to subject, and then change the camera from a 1.6X to a FF camera, the DOF changes as well...so in that case, it seems like sensor size does affect DOF...unless all the calculators are wrong for some reason...


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AJSJones
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Aug 16, 2015 17:06 |  #13

GeoKras1989 wrote in post #17671220 (external link)
Perpetuating the myth. From the article:

"The depth of field achievable can also be affected by the size of the camera’s sensor. If your camera has a small sensor, such as those found in compacts, then the depth of field will be greater and less shallow. However, the larger sensor found in a DSLR is capable of increasingly amplified shallow depth of field, assuming you have the appropriate lenses to match."

The comparison above is between a 6D and G15. There is effectively no difference in DOF. This article makes unfounded claims, with no supporting evidence. The author appears to be echoing what he heard somewhere else, and believes to be true. Guys like him are why I posted the photos above. Sensor size has no effect on DOF.

This guy knows what he is talking about. From the 2nd article:

"In general shooting, provided you keep angle of view, camera position and size of entrance pupil the same, depth of field and diffraction will be the same regardless of sensor size!"

Umm, not quite. "The size of the entrance pupil" is not the same as "the f/stop"

For a crop camera with a "crop factor of, say, 5 (relative to 35mm full frame)" shooting with a 10 mm lens (actual FL) will give you a FoV about the same as using a 50 mm lens (actual FL) on a full frame camera. So far so good.

Same tripod location, so perspective isn't changed.
Let's use f/4 on each camera.

The actual aperture opening on the crop camera is 10/4 = 2.5 mm
The actual opening on the FF camera is 50/4 = 12.5 mm

So the physical aperture is 5x different.

The physical aperture calculated this way is not always the same as the "entrance pupil" but they are closely related (Theentrance pupil of a system (external link) is the image of the aperture stop as seen from a point on the optic axis in the object plane.) and for the purposes of most discussions we can say that "physical aperture in mm = entrance pupil in mm" (macro gets weird).

So the entrance pupil is 5x different.

So, for the two situations (same camera position, same f stop, different sensor size, same FoV) we have different FL and different physical apertures.

If we want to keep the physical aperture (entrance pupil) the same, as your "guy who knows what he's talking about" says, we have to change the f stop. To match the DoF we have to stop down to f/20 on the FF (to get 50/20 = 2.5 mm entrance pupil).

So, in this case, to get the same picture (and that means the same DoF, too) we have to stop down the lens on the larger sensor camera to match the physical opening. The other way to look at it is that the lens on the smaller sensor gives more DoF at the same f stop...

(Not sure exactly how you created the pictures above but don't forget that if you enlarge one more than the other from its original size on the sensor to get to the one on the screen or print, then DoF is also altered)

Edit: OK - it's a little clearer now what you are ,presenting. If you take a small piece of the FF sensor to match the size of the crop sensor, then it doesn't matter whether the crop you present came from a FF, a MF or an APS-C. The only thing relevant in your comparison are the two, little pieces of sensor being used. They captured the same optical image and they've been enlarged to the same degree when you put them on the screen in your post. SO, of course, they look the same - D'oh :D But you didn't use the "whole of the larger sensor" to capture the image - that's what my scenario (and most realistic ones) contemplate - "which camera should I use and how do I set it?


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sploo
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Aug 16, 2015 17:11 |  #14

GeoKras1989 wrote in post #17671246 (external link)
The 6D is heavily cropped

Woah. Hang on. That's an instant disqualification. I'm not trying to be difficult/aggressive, but that's indicative that you might have missed how this works.

Imagine I have a lens on a tripod ring, and mount either a full frame or APS-C camera on the back. The lens doesn't move at all (and we keep the same settings). The lens therefore is projecting exactly the same image from its rear. The difference is that the image circle is large enough to fill the full frame sensor, but the APS-C sensor will only see the central portion of the image projected by the lens. Ignoring the pixel resolution differences; I were to take an APS-C sized crop out of the full frame shot it would be identical to the image from the APS-C body.

If we printed both the original (uncropped) full frame and APS-C images at the same size, it would appear that the APS-C image was "zoomed in" (because it's effectively a crop). To get the same framing as the full frame body with the APS-C we'd either need to use a shorter focal length (i.e. zoom out) or move further away from the subject - both of which reduce the magnification and therefore increase DOF.

In practical terms, for a portrait I could use:

Full frame: 80mm, f/4.5, subject distance of 5 feet
APS-C: 50mm, f/2.8, subject distance of 5 feet

Same distance, but a different focal length (to get the same framing), and a larger f-stop is required to match the full frame DOF.

To go to my earlier point about the possibility of shallow DOF with larger sensors, from the same position/framing, to match an 85mm f/1.2 shot on full frame with an APS-C body would need a 53mm lens at f/0.75; i.e. much harder to obtain.

EDIT: Reinforcing one of AJSJones' points in his post above: consider the actual aperture sizes of 80/4.5 vs 50/2.8 (there's a reason why they're essentially the same, therefore giving the same DOF for the same subject distance on the two relative sensor sizes)


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SkipD
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Aug 16, 2015 17:56 |  #15

Folks, it seems that most of the posts here are missing the boat about depth of field calculations.

ALL depth of field (DOF) calculations are basing their variables on creating the same standard sized image from all camera formats ("format" refers to the size of the film frame or digital sensor in a camera) in order to analyze depth of field. All of these standard sized images are to be viewed at some standardized distance. The visual acuity of the "person" analyzing the images is assumed to be some specific value (which of course isn't real-world truth).

The amount of magnification from the in-camera image (the negative from film cameras and the image formed ON THE SENSOR in digital cameras) to the standard analysis image size varies for all different format cameras. This is why there's a different "circle of confusion" factor assigned to the DOF calculations.

If you used the very same focal length to make images with two different format cameras, you would have different fields of view in the resulting images. If you make standard DOF analysis sized prints from both cameras, there will definitely be different depths of field in the two prints. That's because the blur from the smaller format camera is magnified more when making the standard sized large print as compared to making the same sized print from the larger format camera's image.

The "circle of confusion" is what folks need to understand to begin to understand the DOF calculations.

The fact is that if you make different sized prints of the VERY SAME IMAGE FROM THE SAME CAMERA/LENS COMBINATION, each print will have a different depth of field. Many folks don't understand this and assume that the DOF calculations' results for a given focal length, aperture, and camera-subject distance will be the same for all sized prints made from the image. That's totally wrong.


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