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Thread started 01 Nov 2016 (Tuesday) 07:16
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Calculating required MFA values

 
sploo
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Nov 01, 2016 07:16 |  #1

WARNING: Totally unnecessary geek post ahead. Stop now if you're more interested in photography than maths... (and wild guesswork).

I've been experimenting with a 5D4 and some lenses, using a test chart that will show where the focus plane is (in terms of centimetres in front/behind the intended plane). I was wondering how the results might relate to MFA values. The only reference to the "size" of one Canon MFA unit that I've been able to find is that 1 unit roughly equates to 1/8th of the total DOF for the len's focal length & subject distance when shot wide open.

200mm at f/2.8 and 3.5m camera-to-subject distance is about 5.2cm DOF. One MFA unit should then be 5.2/8=0.65cm. The camera was consistently 1.5cm back focusing, so I make that 1.5 / 0.65 = 2.3 MFA units; which would need to be -2.3 to pull a back focusing lens frontwards.

The 1/8th data seems a bit coarse to me, as a standard MFA distance of 50x the focal length (3.5m for a 70mm shot), at f/2.8, has a DOF of 44cm; meaning one MFA unit is a 5.5cm shift. As that lens was back focusing by 3cm, it would indicate it needs an MFA adjustment of 3/5.5=0.54.

Has anyone got any better/different data on the "size" of a Canon MFA unit? I guess I could set up a tripod and run some tests in the absence of a definitive answer.


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Nov 01, 2016 07:48 |  #2

Measure it yourself but I'm guessing that it's going to be quite variable from lens to lens.


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sploo
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Nov 01, 2016 08:16 |  #3

gjl711 wrote in post #18172738 (external link)
Measure it yourself but I'm guessing that it's going to be quite variable from lens to lens.

I'd assume there must be some standard, and fractions of the total DOF would make sense. It's just that a 5.5cm shift for the 70mm lens seems pretty coarse (and I don't think it matches my experience of tweaking MFA values).


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Post edited over 2 years ago by gjl711.
     
Nov 01, 2016 08:24 |  #4

I believe Canon states (external link) the 1/8 DOF number and thats as granular as they get.


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Nov 01, 2016 09:25 |  #5

gjl711 wrote in post #18172769 (external link)
I believe Canon states (external link) the 1/8 DOF number and thats as granular as they get.

Thanks. That's the most "official" source I've seen.

The thing is though, "Each step is a very fine increment, equal to 1/8th of the depth-of-field you'd have with the current lens wide-open" and "Don't expect radical shifts in focus with adjustments like plus 3 or minus 5" are somewhat contradictory; if one MFA unit for a 70mm f/2.8 shot was 5.5cm, "minus 5" would move the plane of focus by 27.5cm (nearly 11")!

There are different standards for DOF calculations; IRC standard circle of confusion figures usually relate to a 10x8" print held at a normal viewing distance, with the viewer having standard eye sight - so perhaps they're using a tighter figure (which will result in smaller calculated DOF values).

EDIT: Later in the article it says "only 1/8th of the forward or backward depth-of-field"; depending on the stress on "or", that might mean it's 1/8th of half the total DOF (1/16th), which would be a bit more subtle.


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Nov 02, 2016 11:49 |  #6

There are different standards for DOF calculations; IRC standard circle of confusion figures usually relate to a 10x8" print held at a normal viewing distance, with the viewer having standard eye sight - so perhaps they're using a tighter figure (which will result in smaller calculated DOF values).

But that makes no sense. The CoC used is a function of the amount of enlargement needed to go from the native (optical) image to the 8x10 print; for an FF image it is around 8.5X and the CoC generally used is 0.03 mm, while an APS-C image, because it needs to be enlarged 13.5X to reach the 8x10, is held to a tighter standard - a CoC of 0.019 mm. Therefore, if the other factors are constant (lens, aperture and distance) but the sensor size is changed, the DoF is different. According to Dofmaster with a 100 mm. lens at f/2.8 and the target at 5 meters (the recommended 50x distance) the FF DoF is 42 cm and the Cropper DoF is 26 cm, a 1:1.6 relationship as you would expect.

So far as I know, the "50 times the focal length" recommendation is not tied to a sensor size; it is not, for instance, "50 times the 35mm equivalent focal length". And it does not seem likely that what you call a "Canon MFA unit" on the 5D4 is 1.6 times larger than a "Canon MFA unit" on the 7D2 or the 80D.

Whatever it is that they mean by DoF and 1/8 thereof, it shouldn't be a definition that is dependent on a standard print. Unless, of course, now that there is MFA on crop-sensored cameras, it really does grind finer on them than on full-frame models.


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sploo
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Nov 02, 2016 12:16 |  #7

tzalman wrote in post #18173839 (external link)
But that makes no sense. The CoC used is a function of the amount of enlargement needed to go from the native (optical) image to the 8x10 print; for an FF image it is around 8.5X and the CoC generally used is 0.03 mm, while an APS-C image, because it needs to be enlarged 13.5X to reach the 8x10, is held to a tighter standard - a CoC of 0.019 mm. Therefore, if the other factors are constant (lens, aperture and distance) but the sensor size is changed, the DoF is different. According to Dofmaster with a 100 mm. lens at f/2.8 and the target at 5 meters (the recommended 50x distance) the FF DoF is 42 cm and the Cropper DoF is 26 cm, a 1:1.6 relationship as you would expect.

So far as I know, the "50 times the focal length" recommendation is not tied to a sensor size; it is not, for instance, "50 times the 35mm equivalent focal length". And it does not seem likely that what you call a "Canon MFA unit" on the 5D4 is 1.6 times larger than a "Canon MFA unit" on the 7D2 or the 80D.

Whatever it is that they mean by DoF and 1/8 thereof, it shouldn't be a definition that is dependent on a standard print. Unless, of course, now that there is MFA on crop-sensored cameras, it really does grind finer on them than on full-frame models.

I've seen sources use different CoC sizes (for the same enlargement). Possibly Zeiss? It seems to be one of those things where some prefer a value that results in a shallower calculated DOF; partly with the argument of guaranteeing settings that maximise sharpness (note that I'm not saying they're right - just that I've seen it done).

I do wonder if MFA actually adjusts something that isn't really measured in any sort of focal length or DOF related units, but has an affect that can be roughly claimed as "1/8th of the DOF per MFA unit" in layman's terms (i.e. what happens when a marketing guy presses an engineer for a one-liner he can use to explain it to customers). I can quite believe it'll be much more complex and variable across different sensor sizes and lenses.


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Nov 04, 2016 22:51 |  #8

This link (which is pretty technical, but seems well documented) probably gives the best overview of the AF system, microfocus adjust, and correction values. Maybe it will answer some of the questions about best focus and CoC: Here (external link)


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Nov 05, 2016 17:39 |  #9

Ok, rank amature here but from what I've read regarding MFA, DOF refers to depth of focus NOT depth of field. So MFA shifts the focal plane at the sensor position. So one MFA unit = 1/8 Depth of Focus. At sensor position this could be some very small number ie: 0.00001 mm. I have never seen a numerical value for what one focus unit represents in linear terms. I suppose you could measure the dispersion of laser light but read the thread about lasers and camera sensors first.

And that's my two cents which covers about all I know about this.

Rod

Edit: right or wrong I view MFA like sighting in a rifle with a scope. You can get it perfect at some distance but plus or minus that distance it is no longer perfect, maybe good enough but not perfect. Use different ammo with different ballistic characteristics and things change again. To me this is a very good approximation of focus accuracy at different distances, different focal lengths and using different lenses. Only you can decide what is good enough for you. For the perfectionist maybe you can set your camera to take multiple shots at different MFA values. Sort of like bracketing for exposure but in this case bracketing for focus.


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Nov 05, 2016 17:59 |  #10

RodS57 wrote in post #18176800 (external link)
Ok, rank amature here but from what I've read regarding MFA, DOF refers to depth of focus NOT depth of field. So MFA shifts the focal plane at the sensor position. So one MFA unit = 1/8 Depth of Focus. At sensor position this could be some very small number ie: 0.00001 mm. I have never seen a numerical value for what one focus unit represents in linear terms. I suppose you could measure the dispersion of laser light but read the thread about lasers and camera sensors first.

And that's my two cents which covers about all I know about this.

Rod

Wheather it's field or focus, the end result will be the same, no?


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Nov 05, 2016 19:01 |  #11

gjl711 wrote in post #18176811 (external link)
Wheather it's field or focus, the end result will be the same, no?

No depth of focus and depth of field are not the same thing at all. I'm not going to go dig up the definitions, but depth of focus is measured at the sensor/film plane and as far as I recall is independent of any viewing conditions. Depth of field on the other hand can only be measured on the final print, and is dependent on both the viewer's visual acuity, and the print size/viewing conditions, all of which have to be specified in order to calculate the DoF.

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Nov 05, 2016 19:02 |  #12

gjl711 wrote in post #18176811 (external link)
Wheather it's field or focus, the end result will be the same, no?

I'm gonna say no. Depth of field is at the target. The math earlier means you can't correct the problem when calculating using depth of field. Depth of focus is where the points of light converge on the sensor surface. Adjusting MFA moves this point of convergence towards/into or away from the sensor. In a sense the camera says "perfectly focused" and via MFA you say ok, now move it by this much.

Sorry for this rather crude example but as I said I don't know a lot about this and this is how I visualize it

Copy variation:
camera = 1, lens = 1; 1+1 = 2 = perfect match, perfect focus.
But camera = 1.1; lens = 1; 1.1+1 = back focus; point of convergence is behind the surface of the sensor
MFA set to -.1 now we have 1.1+.9 = 2 = perfect focus
Of course you can have camera = 1.1, lens = .9; 1.1 + .9 = 2 = no MFA required
Unless you have a perfectly matching lens and body some adjustment may be necessary

I'm gonna stop before I start to look really stupid or something.

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Nov 05, 2016 19:17 |  #13

BigAl007 wrote in post #18176842 (external link)
No depth of focus and depth of field are not the same thing at all. ...

I know the differance between the two and am not claiming that they are the same thing just that MFA will affect both the same and it doesn't really matter which you choose. The number might be different but the end result the same. Each single MFA unit moves the plane of focus about 1/8 of DOF (where F = either field or focus)


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Nov 06, 2016 08:18 |  #14

The value of an MFA (Master of Fine Arts) degree is greatly overstated.


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Nov 06, 2016 08:55 |  #15

Depth of field is not the same as depth of focus... depth of field doesn't involve the focal lens and is calculated at the lens' max aperture. I felt into that trap too, in the beginning, and was quickly corrected. Correcting depth of field is less aggressive than the DOF at longer focal lengths, because focal length isn't used for AFMA.

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