[photoptimist]

The point that answered my own personal objections was:




I can accept that there are ways in which the final viewing size is scalable back to the original sensor size in digital photography, even though there is no optical enlargement taking place in the same way that it does with film, and even allowing for resampling and the fact that digital data itself has no physical size. It's quite a subtle distinction, but it does the job for me within the context of this thread.

I'm glad I can help.

Maybe one way to look at digital things is in terms of how far the various parts move as they read, process, and output the pixels.

In the K-1, the read-out circuit essentially moves over about 0.005 mm to read each successive pixel.

In the computer, a bit of software might downsample the image 2:1 by moving through the input camera file and skipping every other pixel. Now the downsampled pixels in the output file come from 0.010 mm apart on the sensor.

Finally a 300 PPI printer, would deposit the downsampled pixels on 8" x 12" paper with the printhead moving 0.084 mm (1/300") for each 0.010 mm downsampled pixel in the output file.

The end result of this electro-mechanical scaling is an 8.4X magnification from the original optical image features to the printed image features on paper.

[BrianR]

A FF sensor. Lets say we go and put a Nikon lens on and we all know don't we that, because we have payed three times as much for it, it has twice the resolution of the glunky ol pentax one
So we now have a FF image with twice the resolution of a normal FF. Should we double the coc on the calculator accordingly.?

Nope. DoF doesn't change just because you slapped on a sharper, more expensive lens. DoF is really just measuring how points get mapped to larger and larger discs (in your end output) as your point moves away from the focal plane. It's never claimed to take into account lens abberations, diffraction, or anything else. It was never meant to be the end word on the sharpness of your image, it's just measuring this one effect. A good example is when you stop down a crazy amount, like into f/32 on an aps-c camera. The blur discs from being away from the plane of focus get small, but the garbling from diffraction starts to be a dominating factor and tears your sharpness apart.

If you're so inclined, I'd suggest having a look at the derivation of the DoF equations in a bit more detail. One of my favourites is no longer on the web, but here's a link in the internet archive. From a pretty basic optics assumption, all you need is some highschool geometry and algebra. Having a look at the equations the calculators are using can be a big help in understanding what they are and are not saying.

The DOF equations

[GUB]

I am quite familiar if not fluent with the equations
So are you going with ;

A ; The dof calulator application of coc is wrong

B ; A digital image has a physical size.

or C ; another explanation for why they change it

[swanlefitte]

Let me present an example;
A FF sensor. Lets say we go and put a Nikon lens on and we all know don't we that, because we have payed three times as much for it, it has twice the resolution of the glunky ol pentax one
So we now have a FF image with twice the resolution of a normal FF. Should we double the coc on the calculator accordingly.?
.

If a lens can focus on the plane exceptionally sharp, it will still air the disc off the plane. Not sure how this could change anything. I air a very sharp dot or I air an airy dot the result is an airy dot. Remembering that as I air the airy part it is less noticeable like fog dispersion but the part that looks solid becoming airy is just as noticable. I am not saying there is no difference but for a subject dof its of little consequence.

[BrianR]

I am quite familiar if not fluent with the equations

Perhaps I misunderstood - you were saying a sharper lens means you should change the CoC weren't you? If so, the sharpness of the lens should come into the equations somewhere, no?

A ; The dof calulator application of coc is wrong

B ; A digital image has a physical size.

Under the assumption that your viewing size, distance, eyesight etc. stay the same and all you change is the sensor/film size, then yes, the CoC changes.

A digital file has no physical size, but it was certainly made by a chunk of silicon that has a physical size. Again, DoF is looking at how large the blur discs are in your final image. These come from points in our image space being projected as little blur discs onto your sensor or film. The magnification from the sensor size to the viewing size is a very real thing we can measure to get from these projected discs to our final print - it doesn't matter that we passed through a digital representation or a piece of film, the math is the same.

[GUB]

Perhaps I misunderstood - you were saying a sharper lens means you should change the CoC weren't you? If so, the sharpness of the lens should come into the equations somewhere, no?



Under the assumption that your viewing size, distance, eyesight etc. stay the same and all you change is the sensor/film size, then yes, the CoC changes.

A digital file has no physical size, but it was certainly made by a chunk of silicon that has a physical size. Again, DoF is looking at how large the blur discs are in your final image. These come from points in our image space being projected as little blur discs onto your sensor or film. The magnification from the sensor size to the viewing size is a very real thing we can measure to get from these projected discs to our final print - it doesn't matter that we passed through a digital representation or a piece of film, the math is the same.

I have been festering on this all afternoon - I am done- I got to go and buy a tray of eggs.
I had been stuck on the blonde concept that if it was degree of enlargement that the coc applied to then the computer would need to know the size of the image. That was dumb, the sensor faithfully records the image cocs and all and dutifully lays it out in whatever medium needing proportion only. We ourselves put the sensor format into the calculator and forecast how the computer readout is inevitably going to look like.
There is still bits like the resolution difference of the two formats I am uncomfortable with - I am going to experiment a bit.

[Dartmoor Dave]

Naaaa wipe that egg off your face Dave -- I haven't heard a fat lady singing yet. Firstly I agree with Photoptimist it is not to do with pixel size but it still has nothing to do with format either. (except in the practical analogue area)
Let me present an example;
A FF sensor. Lets say we go and put a Nikon lens on and we all know don't we that, because we have payed three times as much for it, it has twice the resolution of the glunky ol pentax one
So we now have a FF image with twice the resolution of a normal FF. Should we double the coc on the calculator accordingly.?
I say yes and so it is not format based but resolution based.
Your pixels were too small in relationship to the coc to have any real input but the practical lpm is much closer to the size of the coc so lpm has a large effect.

For me the only question in all this has been whether or not the viewing size of a digital photo can be physically scaled back to the physical size of the sensor in any way at all. For my money, online depth of field calculators are so simplistic in the assumptions they make they are never going to be much use. My own approach has just been to have the final viewing size in mind when I shoot, and go by what I know works from experience with the lens I happen to be using. That's dependent on the CoC projected onto the sensor by the lens, the size of the sensor pixels, and also on the pixel size on my 24" 1920x1080 monitor -- which is about 0.25mm and of course displays a downsampled image.

And downsampling an image is not a simple arithmetical process. I've only got a very basic layman's understanding of it, but I have checked that understanding with my father, who designed computer graphics hardware for a living and was a member of mpeg before he retired. So I like to think he's a fairly reliable sounding board when I want to double-check if I'm understanding something correctly. My understanding is that in most cases, when you're looking at a digital photo, it's done by scaling the waveform and most definitely not by simple processes like skipping pixels. Which is why I still think that @Photoptimist is oversimplifying downsampling in his examples, even though I'm happy to go along with him in general that the CoC at the final viewing size does have a physical size, and so it is scalable back to the physical size of the CoC on the sensor.

I do accept your point that the pixels on a current generation sensor are small enough that we can think in terms of the physical size of the CoC on the surface of the sensor. So once we know the final viewing size, resolution, and distance we can then scale back in a meaningful way. But if people start asserting again that the viewing size of a digital photo is an optical enlargement in the same way that a print from a negative is then I'll be right back in the fray.

[Dartmoor Dave]

In the computer, a bit of software might downsample the image 2:1 by moving through the input camera file and skipping every other pixel. Now the downsampled pixels in the output file come from 0.010 mm apart on the sensor.

That's not really how it's done, but I'm happy to accept your overall argument that we can scale back from viewing size to sensor size in useful ways.

[Dartmoor Dave]

My main conclusion as I bow out of the thread is that nothing that's been discussed will have any effect at all on the practical way that I think about depth of field when I'm doing my own photography. So I'm not sure that I've actually achieved anything other than to irritate myself and probably a few other people. Although I did end up making a really nice Spanish omelette for dinner last night.

[MrKodak]

Okay, I understand the focal length of a lens doesn't change if you mount the same lens on a difference size sensor body but I'm trying to understand how the field of view (I believe I'm using the correct term) changes from medium format, to full frame, to APS-C. I asked this question in another thread and don't want to get into a major discussion there, so as not to thread on that thread.

So, let me be specific. It is my understand if you mount a full frame 150mm lens on an APS-C body you get a crop factor of 1.5, so the focal length equivalent is 225mm. Okay, now let's use the example of a 150mm 67 format lens. If you mount this lens on a APS-C camera, what is the focal length equivalent? From the following chart, it would appear it is in the area of 450mm. Where an I wrong?

Field of View Tables, APS-C, 24x36, 645D, 645, 6x7 - PentaxForums.com

Let me explain this: the focal length is ALWAYS the same focal length, no matter what, period.
The different dedicated lenses, excluding contacts and mechanical differences, differ optically how wide the lens covers or project an image.
The medium format lens needs to be wider to allow it to project an image wider than full frame, as is the full frame wider than APS-C.
That is why the APS-C lens crops the corners or in optical terms vignette as crazy on a larger sensor camera. This is also the reason why a lens for APS-C can be made smaller than the same focal length zoom for full format lens given the same aperture (you don't need the extra wide glass to project an image in the corners).

So no; a medium format 150 mm lens IS always 150 mm, which would be equivalent to the crop factor of 1.5 when mounted on your APS-C camera. The only difference is the flange focal distance is much longer so it would be further away (bigger adapter) from the sensor to give you the possibility to focus from close to infinity.
It would also be one big ass lens on your body as it is wider and therefore heavier.

[photoptimist]

That's not really how it's done, but I'm happy to accept your overall argument that we can scale back from viewing size to sensor size in useful ways.

The downsampling algorithm I mentioned is known as "nearest neighbor interpolation" and is the simplest, fastest way to resample a raster image. It's still used where speed of resampling matters.

But for better IQ, especially with non-integer resampling ratios, there are better algorithms such as those mentioned at Image scaling - Wikipedia

[MossyRocks]

Or you can just use your 400mm lens on any of the different cameras, and then crop the picture to APS-c size in post processing. Pixel density aside, a bigger sensor always contains the smaller area of a smaller sensor. Just crop if you want a smaller field of view.

Kjell

Well that kind of defeats the purpose of having that larger sensor, especially when dealing with photon sparse images for astrophotography.