Sensor size vs max DOF

[james 0]

Hi Andy,

 

That would work if the small sensor cameras could achieve high f-stops, but because of the effects of diffraction, all of them go to a max of F11

 

So, the short focal length helps, but imagine a 10.5mm fisheye photo shot at f22...That's some serious DOF.

 

Cheers

James

 

PS, I agree that it's the photographer that matters, equipment aside. On the other hand, it's important to pick the right tool for the job, and that's what this thread is about. "Talking technical" is important, and doesn't demean the photographer's vision, or the art, but that's just my opinion.

[Rocha 0]

It’s not about the camera but about the photographer’s vision in capturing time and light. 

<{POST_SNAPBACK}>

 

Bingo!

[Rocha 0]

PS, I agree that it's the photographer that matters, equipment aside.  On the other hand, it's important to pick the right tool for the job, and that's what this thread is about.  "Talking technical" is important, and doesn't demean the photographer's vision, or the art, but that's just my opinion.

<{POST_SNAPBACK}>

 

I completely agree that the right tool helps! But I think that a great misconception out there (especially in these days of annual new camera announcements) is that a better camera will make you a better photographer. Bottomline, a fast, high resolution, accurate and expensive camera will certainly help us take technically perfect photos. Now, what we all strive to obtain (I think) are artistic eye-grabbing photographs that reflect the beauty of the underwater world; the closer to technical perfection those photographs are, the better.

[CeeDave 0]

Since Herb wisely suggested that number mavens not stomp on Rand's lovely thread:

 

<nerd>I shoot whenever I can, and dink around with equations when I can't.

 

I don't think it makes sense to compare DOF of a cropped sensor to a FF sensor at the same distance. I think it's more reasonable to use the same FOV. If you buy that, then the greater sensor-object distance increases DOF for the cropped sensor, partially (but usually not fully) offsetting the larger minimum aperture size for the smaller COC on the little sensor. Of course, this also changes the magnification (less for the little sensor), but to me it makes more sense to hold composition constant when making these comparisons. You are free to disagree, of course.

 

It's an interesting fact that all focal length lenses have the same DOF for the same FOV -- WA or tele (for the same format and f-stop). If you step back far enough with a 200mm to get the same view as a 14mm, the DOF at the subject plane will be identical. Really.</nerd>

[UWphotoNewbie 1]

PS, I agree that it's the photographer that matters, equipment aside. On the other hand, it's important to pick the right tool for the job, and that's what this thread is about. "Talking technical" is important, and doesn't demean the photographer's vision, or the art, but that's just my opinion.

 

Sure but I think we're beyond the point of diminishing returns here. All the current DSLRs provide a system of tools to make to quality images. The differences are miniscule and shouldn't hinder a creative photographer from acheiving his/her vision.

 

Having seen this debate go back and forth I'm convinced that whatever little difference there is here, it is insignificant compared to other factors that make a great photo.

 

Don't get me wrong, I love gear for gear's sake but I won't go so far as to confuse my gear lust with what is needed for taking great photos.

[james 0]

I agree in principle about photographic tools.

 

However, what the current discussion gets right down to, and one of the main points of the discussion in the other thread is that you get .25 inches of DOF with one system and .5 with the other - retaining the same level of sharpness. If you're shooting super-macro, that is a HUGE advantage for the latter system.

 

It has nothing to do with the photographer's creative abilities - it's the difference between having the subject in focus or not.

 

Cheers

James

[CeeDave 0]

you get .25 inches of DOF with one system and .5 with the other - retaining the same level of sharpness. 

<{POST_SNAPBACK}>

 

That's true at the same MAGNIFICATION (who cares?) but not with the same COMPOSITION. The constant-composition calculations (which, I admit, are more complicated) give a much smaller (circa 50 vs 100%) advantage to FF. (You have to recompute distance to preserve the fraction of the diagonal filled in various formats -- always causing you to move back / increase DOF in cropped sensor vs FF cameras, for a given f-stop; then, the effect of larger critical <diffraction> aperture is partly offset by larger distance to object).

 

CDW

[james 0]

Keep in mind that backing up to achieve the same composition has other consequences underwater.

 

Cheers

James

[herbko 0]

I put in some numbers awhile ago and forgotten all about it. So I went back and did it again. A good place to start is the photo.net lense tutorial:

 

http://photo.net/photo/optics/lensTutorial

 

I'll first skip to the bottom line for those not so interested in the nerdy equations, and write the equations at the latter part of this post. For a 36mm subject that fills the frame, 1:1 in FF format, 1:1.5 in dx format, the FF sensor has 1.2x DOF of the dx sensor. For this subject at F/22, DOF is 2.64mm for FF. To get the same diffraction limited resolution the dx camera should have the F-stop set to 15 and will have DOF of 2.2mm, not a big difference. These results are independent of the focal length of the lens.

 

For the shots that Rand was showing us from a 1:1 lens with a 2x teleconverter, the magnification is 2. He shot at F/25, assuming this already takes the teleconverter into account, the DOF is 0.75mm. To get this much DOF, Rand gives up about half the resolution that his 12Mpixel camera is capable of imaging. Had he used a FF camera at F/38 he would have gotten a whopping 1mm DOF, a 33% difference. He would need a lens with 1.5x better magnification to do it.

 

Here're the calculations. From the photo.net lens tutorial, for macro subjects

 

DOF = ( c N / M ) * ( 1 + 1/M )

 

where c is the circle of confusion (about 0.03mm for FF), N is the F-stop, and M is the magnification ( Image size / Subject size ). This is an apporixmation valid for M near 1 or greater, and gives the distance in front or behind the subject that's in focus. Multiply by 2 for total DOF.

 

The tricky part is deciding how to compare FF and dx sensor shots and what to plug in to get a good comparison. The assumption I'll make is that both sensor have the same number of pixels and that for the part at the center of focus the image is pixel for pixel identical. Scaling the dx sensor relative to the FF under this assumption we get:

 

c_dx = c / 1.5

N_dx = N / 1.5

and

M_dx = M / 1.5

 

Since c is roughly the length over which the image can blur and still be considered in focus it is 1.5x smaller for the dx sensor. N has to scale the same way to have the diffraction blur over the same number of pixels, and the magnification clearly scales with the length of the sensor to capture the same image. Pluging in these values gives

 

DOF/DOF_dx = (1+M)/(1+M/1.5)

 

from which I calculated some of the numbers above.