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08-31-2009, 12:40 PM   #1
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hi iso, dof, ff, sr, and other alphabet soup

OK, so a recent shooting situation got me lusting after some serious low-light capability. The D700 is usually the recommendation for that, but also the 5Dmk2 is supposed to "beat" APS-C as well. (I'm talking sub-$3K bodies)

BUT, there is the trade-off of the DOF issues. What good is being able to shoot if you've only got a few inches (?) of area in-focus with say a 50/1.4. If I've got to stop down, then I'm losing light.

I assume someone has done the math on the break-even points for all this. SR gets you a stop or two (though doesn't freeze motion). Going from K20d to D700 gets you from iso 1600 to 6400 (with relatively similar iq - I'm talking in broad strokes here). But what is the DOF difference from APS-C to FF? Is that 1 or 2 stops as well?

I'm beginning to wonder if it really is worth the hassle...

08-31-2009, 01:11 PM   #2
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It's exactly 1.3 stop.


In any case, where DoF is concerned, don't sweat the small changes too muchŚwhether this or that sensor is slightly larger or slightly smaller, whether this or that lens is â/1.7 or â/2.4, whether you're focused at 10 feet or 12 feet. When learning to manage sharpness and blur, it's the big swings that make more noticeable differences.

The Online Photographer: Depth-of-Field Hell
08-31-2009, 01:18 PM   #3
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DISCLAIMER: Pure speculations!

Dont have any real-life experience with your hypothetical issue, but i played around with DOF calculator a bit.

Say you have a 50mm and your aperture set to 1.4. Suppose you are shooting something 3m away from you, this will give you about 20 cm worth of DOF.

In case you wanted to shoot the same thing with similar composition ( your subject covering the same area of shot ) and same 50mm and apperture set to 1.4 you would need to get 1m closer to your subject, and DOF would be about 13cm.

1.5x difference in DOF doesnt sound like huge problem to me, hope there are some to report actual experience, like maybe Samsungian?
08-31-2009, 09:43 PM   #4
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I've never used a digital FF, but shot with 35mm film for years.

Too lazy to do the maths but the impression I got was, for the same focal length, same aperture and subject distance, the absolute DOF (minimum / maximum in-focus distance) was actually slightly deeper with FF than with APS-C. Not enough to make much practical difference, though.

Ah, to get the same Viewpoint on an FF body as a shot done with an APS-C body, you have to get in a lot closer to the subject, and that's where it results in quite a bit shallower DOF.

09-01-2009, 03:28 AM   #5
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QuoteOriginally posted by nostatic Quote
I assume someone has done the math on the break-even points for all this.
You are lucky. Because it is all very simple.

If you do the math correctly, you'll find a surprise: all the parameters for DoF cancel out except for the following three:
  1. Physical aperture of lens [in mm, "bigness" of glass, not the f-stop number!]
  2. Field of View (FoV) [in ░]
  3. Distance to subject [in m]
Now, all the parameters for noise cancel out except for the following four:
  1. Physical aperture of lens [in mm, "bigness" of glass, not the f-stop number!]
  2. Field of View (FoV) [in ░]
  3. Shutter speed [in 1/s]
  4. Quantum efficiency (a constant for a given CMOS process)

This means that noise is a direct function of DoF and can only be changed by distance to subject or shutter speed. The size of the sensor doesn't come into play at all!

Or in different words:
A larger sensor allows bigger glass which allows shallower depth of field and less noise, at the same time. But not one w/o the other! Basically, it is all controlled by how big your glass is.

Somewhere else on this forum, you'll find my more detailed view on this including diffraction and optical lens abberations.

Last edited by falconeye; 09-01-2009 at 03:40 AM.
09-01-2009, 08:51 AM   #6
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Falk, you're a legend...
Beautifully summarised.
09-01-2009, 10:01 AM   #7
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Equivalence essay

QuoteOriginally posted by falconeye Quote
This means that noise is a direct function of DoF and can only be changed by distance to subject or shutter speed. The size of the sensor doesn't come into play at all!

Or in different words:
A larger sensor allows bigger glass which allows shallower depth of field and less noise, at the same time. But not one w/o the other! Basically, it is all controlled by how big your glass is.
That's amazingly simply put, although it's working out what this might mean in practice that makes the head swim.

FWIW, here is one person's attempt to make all this more explicit. Can't remember who pointed me to this link in the first place 9 might have been you for all I know - but

Equivalence

One of these days I'm going to really sit down and work through it...
09-01-2009, 10:28 AM   #8
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I think you should forget the math, and go out and shoot.

I have shot for years at ISO 3200 with my *istD and DOF with a 200mmF2.8 wide open (not the biggest DOF in the world) has never been an issue. Subject motion has!

If you are at a point where DOF and absolute sharpness is an issue on a 50mmF1.4 wide open, and you are already at between 1600 ISO and 6400 ISO, then the resolution of the camera is probably hides any errors in focusing and DOF. This is from experience, I have never had issues with sharpness over an entire stage scene shot wide open.

If you are close enought where you are at or near minimum focus, then clearly, limited DOF could be a problem, but you could also do something about lighting or use a flash. High ISO is really for use only where flash is inappropriate and a distraction. if you are approaching minimum focus, these points are NOT (IMO) an issue.

09-01-2009, 10:53 AM   #9
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QuoteOriginally posted by Haakan Quote
But then I started to think what parameters I play with when constructing an image. The parameters I control are FOV, DOF (aperture) and shutter time. The ISO setting comes as a consequence of these parameters (and sometimes as a limitation if noise becomes too high).

So then I thought that in order to have more of an "apple to apple" comparison, I should compare the noise performance under the same image parameters, i.e. under the same FOV, DOF and shutter time.
That's more or less the type of approach taken in the essay I mention.

If I were to take the time to actually work this out, though, I'd do things a bit differently. Instead of worrying about keeping DOF constant so much, I'd be more interested in measuring the effect of noise as sensor size changes while keeping constant FOV, shutter speed, and *total size and weight of camera and lens* (using some sort of metric for relating sensor size to camera size/weight, and focal length, image circle, & maximum aperture to lens size/weight). DOF would act more as a limiter than something I need to keep constant (that is, I'll take any DOF I get as long as it's more than N inches). I'd be even more curious to see the results of an equation where I swapped noise for shutter speed - that is, for a given level of noise and camera size/weight at a given FOV, how does sensor size affect shutter speed?

But ultimately, I'm more with Lowell on the bottom line - I'd rather shoot than work out the math. I know from experience that in the environment I'm most concerned with (concert photography), there's far more to worry about than any of this. Getting a faster shutter speed than I currently am for a given level of noise and camera/lens size/weight would be great, but that in itself wouldn't make me a better photographer.
09-01-2009, 11:02 AM   #10
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Sorry Marc, I thought that my post might be misplaced in this thread so I moved it to a new thread before I realized you had answered it. So in order for people to understand the logic, I put a link to my post here.

https://www.pentaxforums.com/forums/pentax-dslr-discussion/71896-low-noise-be...uals-zero.html

Best regards,
Haakan
09-01-2009, 11:07 AM   #11
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QuoteOriginally posted by Marc Sabatella Quote
That's more or less the type of approach taken in the essay I mention.

If I were to take the time to actually work this out, though, I'd do things a bit differently. Instead of worrying about keeping DOF constant so much, I'd be more interested in measuring the effect of noise as sensor size changes while keeping constant FOV, shutter speed, and *total size and weight of camera and lens* (using some sort of metric for relating sensor size to camera size/weight, and focal length, image circle, & maximum aperture to lens size/weight). DOF would act more as a limiter than something I need to keep constant (that is, I'll take any DOF I get as long as it's more than N inches). I'd be even more curious to see the results of an equation where I swapped noise for shutter speed - that is, for a given level of noise and camera size/weight at a given FOV, how does sensor size affect shutter speed?

But ultimately, I'm more with Lowell on the bottom line - I'd rather shoot than work out the math. I know from experience that in the environment I'm most concerned with (concert photography), there's far more to worry about than any of this. Getting a faster shutter speed than I currently am for a given level of noise and camera/lens size/weight would be great, but that in itself wouldn't make me a better photographer.
And also to answer your comments, I fully agree with you that sometimes there is too much focus on technical details that might not matter so much in the end. But in this case I thought that the result I got was, at least to me, quite counter intuitive, and thus I wanted to see if this was already known, and only me that was "in the dark" (no pun to low light performance intended)

Best regards,
Haakan
09-01-2009, 11:25 AM   #12
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QuoteOriginally posted by Marc Sabatella Quote
Wow! This makes my head swim!

Seriously though, the matter is way too simple to use this many words to explain it.

Gives me the idea to try a blog article about it myself
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