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03-29-2014, 06:31 AM   #1
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Understanding "greater DoF control"

Hi,

I read a lot of funny material on the basics of photography and always when I feel I understood I find some things in a forum which challenge this.

Now my question:
Lets say I have two lenses lying around:
  • 20mm F4, which can be stopped down to F16
  • 40mm F4, which can be stopped down to F16
Both are FF-compatible and I have a FF-senor camera and a mFT camera with adapter, so I can use both lenses on both cameras.

Why would anyone say using the FF camera he get's "more DoF control" using any of the lenses?

Doesnt he gain less DoF on the small FNumber side but loose the exact same DoF control on the large FNumber side?
For portraits the mFT user is limited by being unable to open the lens more (to the equal of F2), so here FF gives more DoF control.
For macros or landscapes the FF user is limited by being unable to close the aperture more than F16, while the mFT user reaches an FF-equivalent F32 with ease, gaining more DoF control.
Both times the apertures have physical limits. Both small and large apertures have uses.

No? Did I get something wrong?

03-29-2014, 07:00 AM   #2
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QuoteOriginally posted by HavelockV Quote
Hi,

I read a lot of funny material on the basics of photography and always when I feel I understood I find some things in a forum which challenge this.

Now my question:
Lets say I have two lenses lying around:
  • 20mm F4, which can be stopped down to F16
  • 40mm F4, which can be stopped down to F16
Both are FF-compatible and I have a FF-senor camera and a mFT camera with adapter, so I can use both lenses on both cameras.

Why would anyone say using the FF camera he get's "more DoF control" using any of the lenses?

Doesnt he gain less DoF on the small FNumber side but loose the exact same DoF control on the large FNumber side?
For portraits the mFT user is limited by being unable to open the lens more (to the equal of F2), so here FF gives more DoF control.
For macros or landscapes the FF user is limited by being unable to close the aperture more than F16, while the mFT user reaches an FF-equivalent F32 with ease, gaining more DoF control.
Both times the apertures have physical limits. Both small and large apertures have uses.

No? Did I get something wrong?
Honestly, I don't think these things make a huge difference, but there are two things to remember. First of all, really narrow apertures (narrower than f11) aren't terribly usable on current APS-C cameras due to diffraction. Second, full frame has the option of narrower depth of field, with wider apertures available. This is true because of two things. First of all, if you use the same aperture on full frame and APS-C with equivalent focal length, then there will be a narrower depth of field on the full frame camera. Second, there aren't APS-C equivalents of plenty of full frame lenses. A lens like the Sigma 35 f1.4 is a couple of stops faster than the fastest 23mm lens available for k mount APS-C. And the wider you get, the more you find this to be true.

That said, I don't find APS-C to be particularly restricting and for many types of photography, the lack of super narrow depth of field options is not a big deal.
03-29-2014, 07:19 AM   #3
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The difference comes down to distance at a given focal length. A shot with a 50mm at f1.4 framed exactly the same will have thinner DOF on larger sensors because you are closer.
03-29-2014, 07:19 AM   #4
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The issue you might be missing, is that with the APS_c sensor being half the size, to get the same size image you have to enlarge the image twice as much. IF that makes a difference. The larger your sensor, the less affected by diffraction your image is going to be. When diffraction gets over 1 pixel it starts to become an issue and smaller sensors usually have smaller pixels. SO saying you have a 12 mP K-x and a 12 Mp 5D.. the pixels are twice the size on the 5D, meaning it's going to start suffering from diffraction later than a K-x does. SO the DoF gain is not what you might think it would be on the smaller sensor. With the added magnification and more affect from diffraction CA, etc. because CA is also best when limited to less than a pixel, the benefits of the smaller sensor need to be qualified. I don't own an FF camera, but when I shot film, I used F16 like I now use ƒ11. But shooting film I would also sometimes shoot ƒ32, and I find on APS-c ƒ22 is pretty much the upper limit of what's likely to be useful.

None of that means you can't do just fine with an APS_c camera, it just means, if you're constantly pushing the limits of your camera you can ease the strain by moving up to FF,
although I would recommend a 645D.

But your observation about both stopping to ƒ16 is completely accurate, the images on APS-c and FF will be the same... if you print the APS-c image at half the size of the FF image, and the pixel sizes are the same, to normalize the effect diffraction and the effect of Chromatic Aberrations, but maybe all you wanted to print was an images half the size of a 60 inch print.

All you need to know is that 90% of what FF will do APS-c will do. And that if your interest is in that 10% of what FF does that APS_c doesn't, you need to get an FF camera. There really is no way to make a smaller sensor perform like a larger one in every regard, but 90% is pretty darn good.

I'm not going to attempt to get all scientific here, these are just some observations.

03-29-2014, 07:35 AM   #5
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QuoteOriginally posted by Rondec Quote
First of all, really narrow apertures (narrower than f11) aren't terribly usable on current APS-C cameras due to diffraction.
I understand diffraction. But it's got nothing to do with "DoF control". And I also am talking about the theory. As a sidenote you might also want to consider that wide open (the differences seem only to exist at the extremes and nowhere in the large in between) most lenses underperform and many times badly, so there are weaknesses on both ends.

QuoteOriginally posted by Rondec Quote
Second, full frame has the option of narrower depth of field, with wider apertures available.
Yes, that is what I wrote, or actually one half of what I asked. The other half is just the other way around if nobody can show me the error I guess.

---------- Post added 29-03-14 at 15:40 ----------

QuoteOriginally posted by elliott Quote
The difference comes down to distance at a given focal length. A shot with a 50mm at f1.4 framed exactly the same will have thinner DOF on larger sensors because you are closer.
Yes, that is what I wrote. But doesnt that mean you have less "DoF control" on the other end of the spectrum, as you are completely unable to achieve the large DoF you can with the smaller sensor?
How could you get the same level of DoF control on the stopped down end using a FF if the lens just doesnt stop down more the same as you cant open it up more that the existing limit?
03-29-2014, 07:41 AM - 1 Like   #6
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QuoteOriginally posted by HavelockV Quote
Why would anyone say using the FF camera he get's "more DoF control" using any of the lenses?
That is one of the biggest lies or misconceptions I hear about why people want you to buy a full-frame or why they bought a full-fame camera.

DOF does NOT change! They don't get more control! And a lens being made for FF or crop does not affect DOF nor its control.
OK, there are some differences of refraction due to pixel size, etc causing changes in circles of confusion, etc.... but I don't know if I would consider that changing DOF and I don't see it making that big of a difference in the end.

To make things worse, professionals meaning well, try to simplify explanation of Crop v.s. Full frame by saying "a 50mm lens on a crop sensor is *equivalent* to a 75mm on a full frame". First, the "angle of view" is equivalent*. Unless they get into how DOF is affected as a result, the lenses are NOT EQUIVALENT! I just saw this yesterday from several prominent professionals. It irritates me to no end.


DOF is a result of three physical properties; focal length, aperture and distance to subject. These are basic *physical* properties that cannot be changed without changing how the Universe is built and operates. (doing so would be *bad* )

Basically, the crop sensor does just what it says... it crops in on the circle of light that is created on the sensor plane by the lens.
As a result, the DOF area covers a larger area of the entire resulting image on a crop sense.

Take a full frame camera and aim it at a picket fence running along a road.
Focus on a spot 10 feet away. Lets say at a given focal length and aperture, from where you're standing there ate three out of 20 posts in focus.
You stand in the same location and focus to the same spot with the same focal length and aperture... what will you see?
You'll see the *SAME* three posts in focus! BUT, you will maybe only see 13 posts in total.

Did DOF change? No!
Does the DOF area on the crop sensor take up a larger portion of the screen? Yes.
That is what confuses people. They simply do not understand what is going on.


What is interesting is people also say you can never create the same DOF effect on a crop sensor camera as you can on the Full frame.
May use terms like "you get better control" with Full Frame. BS!
Well that may or may not be true depending on the settings and capabilities of your equipment. Often any limitation is a one sided limitation. Fiip over and it becomes a open window that the other cannot get through. i.e you can make arguments for FF or for crop.

Remember three things affect DOF; distance to subject, aperture and focal length.

- You cannot move, distance to subject dictates perspective, and any change in your location completely changes the resulting image.
- The easiest method to change DOF is to change focal aperture. But as in the example above, you don't get the same image. (left with 12 posts instead of the desired 20)
- The next easiest is focal length. Lets say you are able to zoom out to see the 20 posts but now your DOF changes.

So mix them.
- Decrease your focal length by 1.5x (75mm full frame changes to 50mm crop frame)
- Decrease your aperture by a full stop. (F8 on full frame changes to f5.6 on crop sensor)
You should now have the equivalent image on your crop sensor as was seen on the full frame.
The only problem comes in when you cannot decrease your aperture or change your focal length to compensate.
On the flip side, if your crop sensor is at the long end of the range and you twang to make equivalent photo on FF, they may be limited by not being able to reach another stop higher, or get to a wider focal length to meet the crop'd body's capabilities.

Long story short, the 1.5 change in focal length is only *equivalent* if you also drop your aperture by a stop.
It is really not so difficult, but is is important.
03-29-2014, 07:41 AM   #7
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Just mentally switch "DoF control" to "subject isolation" when you read such things...
03-29-2014, 07:47 AM   #8
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QuoteOriginally posted by normhead Quote
All you need to know is that 90% of what FF will do APS-c will do
I am just asking about the "more DoF control" term being used.
It's overlapping 90% and both have a small section on one extreme end where the other can't go.
And I need to know if it's just plain wrong and I didn't misunderstand the theory or if I missed out something.

03-29-2014, 07:48 AM   #9
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QuoteOriginally posted by HavelockV Quote
Yes, that is what I wrote. But doesnt that mean you have less "DoF control" on the other end of the spectrum, as you are completely unable to achieve the large DoF you can with the smaller sensor?
People are talking about the thin DOF when they mention DOF control. Uselessly thin DOF and extreme subject isolation is a popular look now.
03-29-2014, 07:52 AM   #10
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QuoteOriginally posted by vonBaloney Quote
Just mentally switch "DoF control" to "subject isolation" when you read such things...
Thanks. That is understandable then. "subject isolation" only looks at one end of the scale and there a large image circle does decrease DoF.
It is then not talking about any situation where you want more parts of the picture sharp rather than less.
03-29-2014, 08:18 AM   #11
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Full frame lenses have more DOF control, colloquially speaking, because DOF can be functionally infinite at the low apertures but have much greater ability to go to shallow DOF with inexpensive lenses.

You speak of a hypothetical lens that only has an aperture range of F/4 to F/16. Such a lens would be exceedingly rare, the aperture range would only be four stops! That doesn't happen in practice.

A more realistic range is from F/1.4-2.8 to F/22 for your primes.

Two equivalent lenses for Micro 4/3 and FF would be

20mm F/1.4 - F/16
40mm F/2.8 - F/32

In practice they're more like:

20mm F/1.4 - F/22
40mm F/2.8 - F/22

The practical application where large DOF is desired is typically that 'infinity' needs to be in focus and the near-field also needs to be in focus.

So at 20mm F/22 the micro 4/3rds camera would have everything in focus from 2 ft to infinity.
At 20mm F/16 (equivalent lens) the micro 4/3rds camera would have everything in focus from 2.8 ft to infinity.

At F/32 (equivalent lens) the FF camera would have everything in focus from 2.8 ft to infinity.
At F/22 (practical lens) the FF camera would have everything in focus from 3.9 ft to infinity.

So in equivalent lens terms there is no difference between capability of the lenses. In theoretical-practical, owning-one-lens terms there is a difference. Most of us don't have one lens, and if you only plan on owning one lens then I would suggest a psychiatric forum!

In practice if a smaller front-focus-distance was desired the appropriate thing for a FF user to do would be to use a wider lens and crop, or use a lens with a smaller minimum aperture (they're cheap). Micro-4/3rds has no large-DOF advantage in practical terms for a realistic use case. It has a weight advantage, sure.


On the contrary, it is often the case that users want shallow DOF. People pay a thousands dollars for micro 4/3rds lenses (Voigtlander) that are equivalent to FF lenses that are literally $100, and the actual acuity of the $100 lens exceeds the micro 4/3rds lens.

That's why enthusiastic users who've studied the +/-'s go FF before they go to expensive lenses.
03-29-2014, 08:35 AM   #12
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QuoteOriginally posted by HavelockV Quote
I am just asking about the "more DoF control" term being used.
It's overlapping 90% and both have a small section on one extreme end where the other can't go.
And I need to know if it's just plain wrong and I didn't misunderstand the theory or if I missed out something.
Honestly, I have no idea what "DOF control" means. It makes no sense what-so-ever.
Have whomever is spouting the BS define it.

DOF is DOF, as defined by physical properties; Aperture,Focal length and Distance.
The size of the sensor has no bearing on DOF.
PERIOD

Appearance of DOF, is not DOF!

Changing one of the three properties to match framing will change DOF but that is because you're changing one of the three properties.
It is "push and shove". If you change one property another MUST, and by physical laws, CAN be changed to balance the equation (bring same DOF back into view).

Most important, and why I would say you are correct and not mis-understanding anything... keep the following in mind:
Size of sensor does NOT give anyone super mystical powers to better control the three properties.
The range to which those properties fall to match YOUR DESIRED result, may be better suited by use of a full frame sensor or by a cropped frame sensor.
And as for which one is *better*, is a matter of how it suites your personal preference and requirements.
03-29-2014, 08:55 AM - 1 Like   #13
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QuoteOriginally posted by amoringello Quote
That is one of the biggest lies or misconceptions I hear about why people want you to buy a full-frame or why they bought a full-fame camera.

DOF does NOT change!
I won't comment on your lengthy note, but I think you need to go back to the drawing board. DOF is truly based on three factors,* but not the ones you listed. They are:
  • Absolute (not relative) aperture
  • Magnification required for final image size**
  • Viewing distance for final image
Focal length, taking distance, capture media frame size, and post-exposure crop all factor into magnification.

For a given composition and aperture, available DOF varies according to the media frame size. This behavior is readily demonstrable. Just ask anybody who shoots medium or large format. If you disagree, I suggest that you prepare a series of example images proving otherwise.


Steve


* There is also a fourth (assumed) factor, that being the notion of acceptable blur.
** Any doubts about magnification? Simply compare the DOF of my avatar image to the original: https://flic.kr/p/5SzM3k
03-29-2014, 08:58 AM   #14
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QuoteOriginally posted by amoringello Quote
Honestly, I have no idea what "DOF control" means. It makes no sense what-so-ever.
Have whomever is spouting the BS define it.

DOF is DOF, as defined by physical properties; Aperture,Focal length and Distance.
The size of the sensor has no bearing on DOF.
PERIOD

Appearance of DOF, is not DOF!

Changing one of the three properties to match framing will change DOF but that is because you're changing one of the three properties.
It is "push and shove". If you change one property another MUST, and by physical laws, CAN be changed to balance the equation (bring same DOF back into view).

Most important, and why I would say you are correct and not mis-understanding anything... keep the following in mind:
Size of sensor does NOT give anyone super mystical powers to better control the three properties.
The range to which those properties fall to match YOUR DESIRED result, may be better suited by use of a full frame sensor or by a cropped frame sensor.
And as for which one is *better*, is a matter of how it suites your personal preference and requirements.
Although I'm basically with you as the "equivalence thought police" annoy me to no end, you're starting to be an extremist on the other side. There are some generally accepted givens when talking about DoF that have to do with format size, circle of confusion, assumed print size, viewing distance, etc (and now in the digital age additional factors like pixel size and density). There are values for these which are accepted as "standards" of acceptability but are somewhat arbitrary (which is how they knew where to put the marks for the DoF scale on old lenses) -- after all, only a SINGLE point plane is actually ever in focus and everything else is blurred to some degree. So while it is true that putting the same lens on a different body isn't going to magically change the light going through it, factors such as "how much are we going to enlarge this image for our print" DO matter (and you need to enlarge them more from a smaller sensor to get the same print size) and so in a sense "appearance of DoF" DOES equal "DoF". Because again, only a single point is actually in focus, and the rest is only appearance of "acceptably" sharp, and what is "acceptable" is of necessity somewhat subjective.
03-29-2014, 09:00 AM   #15
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This thread's topic is one of the best reasons to get used to the idea of equivalent lenses.

Equivalent focal length, equivalent aperture, equivalent ISO, you get the same DOF, SNR, and view. It's the same picture with two different size sensors.

It also doesn't matter what your definition of acceptably sharp is, as long as you're using the same definition for both sensor sizes.
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