The Falacy of FF Cameras having Flatter DOF

[Stagecoach]

John,

In undertaking a practical comparison as this there are a number of variables and values involved and their impact on the final result may or may not have any significance.

For these two shots the following are/were apparent;

a) We do not know the accuracy of indicated FLs.
b) We do not know the affect of any lens distortion changes between the indicated FLs.
c) There has been minor movement of the camera position (indicated most by movement of the centre nut position)
d) Focus on the nut may have varied slightly whatever method used.
e) We do not know the accuracy of the '1.5x crop' rectangle frame shown in the viewfinder.
f) Aligning the frame within the '1.5x crop' rectangle is a bit iffy as varies somewhat due to your eye position.
g) There is a very very minor difference in frame size ratio. (But not as much if I compare D800/D300)

Now, if someone wanted to consider the impact of a) to g) on the images as far as their apparent difference in DoF goes, good luck

[ajohnw]

Unusual for me I sort of agree.

What struck me when I first looked that them was that there was a difference. So I downloaded them and put them in layers so I could flick from one to the other. Then thought gif and was rather surprised by how much nearer the building and back ground tree appeared in one of them. It shows that the crop factor used must be out because the lens focal lengths are correct. If you look at the tree it's almost like zooming in with more or less the same view angle. Same with the car and building.

Maybe there is a big hole in so called reach as well. I doubt it though.

Maybe the problem is zoom creep or what ever it's called. A la one newer nikon lens that reduces the view as the focal lenth is increase so to get the expected magnification change it has to be moved further away.

John
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[Stagecoach]

Unusual for me I sort of agree.

What struck me when I first looked that them was that there was a difference. So I downloaded them and put them in layers so I could flick from one to the other. Then thought gif and was rather surprised by how much nearer the building and back ground tree appeared in one of them.

Whilst there is a discrepancy to me it does not appear that the background building and tree look 'much nearer'. It may appear nearer in one image because it is not as blurred as much as the other?

It shows that the crop factor used must be out because the lens focal lengths are correct.

Yes, there are some anomalies with the crop factor exact figures reading various information.

How do we know the FLs are correct in these two images?

One way to more accurately assess/set the FLs would be to set the 75mm (which is at the stop) take a shot of a grid, (I have a useful concrete block wall for this) then replicate this same framing in 1.5x mode at around 50mm adjusting until equal on the image and noting the 'indicated' FL to use in the test proper.

................and the result comes out again at 48 mm indicated FL is the closest

Further thoughts on the discrepancy is that knowing the focal point dropped very slightly this small change would be magnified at farther distances. In addition, the tree behind the satellite dish is near 900 mtr distance it's pretty well near enough the same framing

[ajohnw]

I may be able to try it with a 25* and 50mm prime lenses on different cameras in a week or so. Thinking about the best way to try that I can't see any easy way to put the sensor plain in exactly the same position on both cameras however I can ensure that the are both pointing in the same direction and are focused at the same point. The positional differences will only be mms. The aspect ratio's will differ though so it seems to me that there is a need to make some object in the shot the same size when they are reduced. Still leave the problem of what the actual crop ratio really is so before doing that it needs checking to see just what that actually is.

There is always a possibility that the how much blur calculator isn't correct but that leaves me wondering about my experiences using M 4/3 for macro. There isn't really any reason why macro shouldn't follow the same rules. The problem is that the usual approximations don't work with sufficient accuracy, There are others but in the extreme they don't work because we have insufficient information about the characteristics of the lenses.

* Good excuse to buy the 25mm. I have the 45mm but nothing to compare it with other than zooms.

John
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[Stagecoach]

For the shots I did I used the centre focus point (nut) and a point at the bottom right as datums for alignment. I chose these because it enabled me to sit at my coffee table in comfort whilst playing

[ajohnw]

For the shots I did I used the centre focus point (nut) and a point at the bottom right as datums for alignment. I chose these because it enabled me to sit at my coffee table in comfort whilst playing

Think I will have to go outside with a tripod and swing a slatted bench through some angle. Framing should approximate a waist up shot and some to avoid lens drop off near the corners and edges. The M 4/3 lens has way more resolution than the FF one but probably not enough to make it truly 1:1 in that respect.

John
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[ajohnw]

I don't know who wrote this but it seems that for macro work the dof is less for smaller sensors providing the final image size is the same.

https://www.cambridgeincolour.com/tu...cro-lenses.htm

There is a DOF calculator down the page based purely on magnification. It must assume that the circle of confusion is constant, However if I compare the Canon 60mm macro at F11 with the Olympus 60mm also at F11 the Canon resolves 38 lp/mm and the Olympus 50. The Nikon 60 is the same as the Canon.

However I am not convinced that this explains the points I have noticed. It may do in part as I do tend to use slower apertures than say Geoff but that seems to be more to do with not always achieving precise focus.

The Olympus lens appears to be diffraction limited at F11 according to tests, it and others give 82 lp/mm wide open but both lenses with that figure give 50 at F11.

John
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[ajohnw]

Well which one is which ? It turns out that the 5D111 was in 16-9 mode so I had to crop the m 4/3 lens to the same. I've carefully adjusted the reduction so that the bottle of the T Cut is the same size on both. That was used as the focus point in live view and they are both sharp at full resolution. The film plains are are close to the same position on both shots. It sits a touch higher on the 5D as it's a bigger camera. Both at F1.8 and 50mm on the 5D and 25mm on the E-M1. No PP at all. They need to expanded in the light box to see the difference as they turn out to be different sizes.





The reduction ratio on the shots turned out to be 1/4 on the 4D and 1/3 on the E-M1.

I'm not sure if the 5D has any other mode than 16:9, would have thought it had. Not my camera. The aspect ratio of M 4/3 is different anyway.

This one is the 5D at F4



Just noticed that the camera angles are different. Should have locked the tripod head up more firmly. Maybe tomorrow.

John
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[george013]

I don't know who wrote this but it seems that for macro work the dof is less for smaller sensors providing the final image size is the same.

https://www.cambridgeincolour.com/tu...cro-lenses.htm

There is a DOF calculator down the page based purely on magnification. It must assume that the circle of confusion is constant, However if I compare the Canon 60mm macro at F11 with the Olympus 60mm also at F11 the Canon resolves 38 lp/mm and the Olympus 50. The Nikon 60 is the same as the Canon.

However I am not convinced that this explains the points I have noticed. It may do in part as I do tend to use slower apertures than say Geoff but that seems to be more to do with not always achieving precise focus.

The Olympus lens appears to be diffraction limited at F11 according to tests, it and others give 82 lp/mm wide open but both lenses with that figure give 50 at F11.

John
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Every dof-calculator is is based on magnification, and viewing distance.
Read the note under it

Note: Depth of field defined based on what would appear sharp in an 8x10 in print viewed from a distance of one foot; based on standard circle of confusion for 35 mm cameras of 0.032 mm. For magnifications above 1X, output is in units of µm (aka microns or 1/1000 of a mm).
*If you are using a Nikon SLR camera, you will want to check this box; otherwise leave it unchecked.

George

[ajohnw]

Every dof-calculator is is based on magnification, and viewing distance.
Read the note under it


George

You need to think what an 0.032mm circle of confusion on the sensor means George when it's enlarged to fill a 10x8 print. There is a problem because in one direction that a 7x magnification and in the other it;s 8.46. The numbers it gives may have been work on the basis of some dpi across a diagnonal. I don't know what was used.

Another quote for you from the same page. You need to couple this and the above with the resolution figures I mentioned. The circle of confusion of a typical m 4/3 lens is a lot smaller than a typical full frame lens. That's of any type. It's why they can compete with the usual crop factors but sadly fall a bit short of full frame, or do they at long focal lengths. Any way comparing the 50mm and 25mm lenses used in the shots I posted the 25mm even beats the 50 mm with both at F22

Note that depth of field is independent of focal length; a 100 mm lens at 0.5X therefore has the same depth of field as a 65 mm lens at 0.5X, for example, as long as they are at the same f-stop. Also, unlike with low magnification photography, the depth of field remains symmetric about the focusing distance (front and rear depth of field are equal).

Technical Notes:
Contrary to first impressions, depth of field isn't inherently better with smaller camera sensors. While it's true that a smaller sensor will have a greater depth of field at the same f-stop, this isn't a fair comparison, because the larger sensor can get away with a higher f-stop before diffraction limits resolution. When both sensor sizes produce prints with the same diffraction-limited resolution, both sensor sizes have the same depth of field. The only inherent advantage is that the smaller sensor requires a shorter exposure time to achieve that depth of field.

The real problem with all of these areas is that they are based on rough approximations - as was the blur calculator. It's also interesting to note that the full resolution reduction ratio's for the shots that were posted don't relate to the crop factor. It's a combination of pixel count and sensor size. I haven't fully got my head round if some advantage could be taken of that fact. It might be just a case of stepping back and cropping.

John
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[george013]

You need to think what an 0.032mm circle of confusion on the sensor means George when it's enlarged to fill a 10x8 print. There is a problem because in one direction that a 7x magnification and in the other it;s 8.46. The numbers it gives may have been work on the basis of some dpi across a diagnonal. I don't know what was used.

Another quote for you from the same page. You need to couple this and the above with the resolution figures I mentioned. The circle of confusion of a typical m 4/3 lens is a lot smaller than a typical full frame lens. That's of any type. It's why they can compete with the usual crop factors but sadly fall a bit short of full frame, or do they at long focal lengths. Any way comparing the 50mm and 25mm lenses used in the shots I posted the 25mm even beats the 50 mm with both at F22



The real problem with all of these areas is that they are based on rough approximations - as was the blur calculator. It's also interesting to note that the full resolution reduction ratio's for the shots that were posted don't relate to the crop factor. It's a combination of pixel count and sensor size. I haven't fully got my head round if some advantage could be taken of that fact. It might be just a case of stepping back and cropping.

John
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The basic idea is that a person can distinguish a point of 0.25mm from a circle at a certain distance. I think that distance was an arm length at first. And I'm not sure if the size of 0.25mm was there from the very first time. So if we talk about a coc of 0.032, the example, then we may use a print amplification of 0.25/0.032=7.8. Since we are talking about a point and a circle there is only a radius, no length, width or diagonal.
Goniometric it means the minimal aov of a human eye is 0.014 degree. When I didn't make a miscalculation.

If you enlarge a sensor, the ratio between length a width won't change. If that ratio is different from a A4, well that's petty and not important.

The dof is the range your optical system could imaginair moved for- or backwards limited by the coc , changing the image distance. Recalculating that to the subject distances gives you the dof.

Dof-calculators dont't take in account diffraction, basicly. But they can. I don't know about the calculations in that technical note.

George