Aperture Query & DOF

[Richard King]

Yes it has. Consider just one of the changes - the size of the new image. It's smaller, so to re-establish the correct viewing distance (which is fundamental to the whole concept) you must move closer. Therefore you can see more detail, and therefore the DoF changes.

duplicated

 

[Phil V]

Regards sensor size can you not then move the argument to a compact size camera and we all know that the DoF is much larger on compacts.... Why is that? Same focal length, distance to subject (call that magnification to be correct) and the same aperture and the DoF is drastically different.....

What's changed?.... Has to be Sensor size.

Wrong
What's changed - focal length
As per my previous post - on a small compact the 'standard lens' focal length could be around 11mm, now consider an 11mm lens on a FF SLR and focussed at about 6feet, even with a fairly large aperture the depth of field would be close to hyperfocal giving you a front to back sharp image. And that 11mm lens on that smaller sensor - same thing

The confusion arises because we insist on comparing 'effective focal lengths':bonk: Rather than 'focal lengths'.

 

[Richard King]

Yes it has. Consider just one of the changes - the size of the new image. It's smaller, so to re-establish the correct viewing distance (which is fundamental to the whole concept) you must move closer. Therefore you can see more detail, and therefore the DoF changes.

When I say image, I use the classically accepted meaning for the word image - the one used by lens designers, Optical engineers, physicists, Opticians and the like - that is the image formed by a lens in a given plane (in this case on a film or sensor)

If the plane is unmoved, then no amount of changing the size of the film or the sensor will change the size of the image THE LENS produces. That is very basic optics

I look at a camera as 2 separate parts, a lens and the body.

This whole issue arises because for some reason, the photographic industry wanted to "normalise" a sensor size to a 35mm equivalence i.e 35mm = full frame (I guess because they built full sized looking cameras and slapped a DX sensor in them, they then realised they could release poor quality lenses that actually performed really well on the limited size DX sensor but didn't perform at all on a real 35mm format). For any of us that shot/shoot large or medium format, or for any of us that has used old 35mm lenses or MF lenses on a new modern digital camera, this was clearly just a anti-negative marketing ploy. The irony with all of this is that we argue about lens quality, when old (and new) MF/LF lenses need to produce a clean sharp image on a comparatively massive area compared to a DX lens

For me photography is where science meets art, and the science of optics is pretty simple, and did not need the marketing guys putting a rug underneath it. When you formally study optics (like I did), you start off with a simple example like: Here is a 10 dioptre lens. You can then form an image with the lens, and measure how far from the lens the image is, and how big it is la la la..... You can the progress to learn about things like COC's and aberrations, and best form, and asphericity, and you can still go and set the thing up on a optical bench, fire a beam of light through it and make real measurements. When you learn like this you very quickly disassociate the lens system form the measuring tools The measuring tools simply measure simple things like where the lens is, and how big the image is, and where was the image formed, how wide was the fringe etc..

What is totally annoying is when the marketing people and then the industry pundits take something that is actually very simple and black and white (like the image is 2 MM high", and then push it through the mould of equivalence and come up with "well if it was on a fx sensor AND we then imagine they were printed at the same size, the image is actually 1.75mm high (or whatever)" when clearly it isn't

For me photography is more than shooting DX or FX digital, I shoot film, MF, the odd large format, pinholes, zone plates and pretty much everything else available to me. To pull this off, I rely on knowing facts, like 50mm = 50mm. The moment we add equivalence into the mix, I then have to skew everything to fit and almost calculate everything twice

 

[mattchewone]

Wow this thread has really gotten in deep. I have a bit of a basic question again, what is hyperfocal? I read briefly its about the whole image being sharp, but how would you focus etc for that.
Also how would one determine what aperture to create full sharpness for a portrait at various distances? Say I was filling the frame with a 50mm and was say 3-5feet and then at 80mm at 10feet away is there a rule of thumb regarding distance and aperture without using what I consider a complicated dof calculator?

 

[Phil V]

Wow this thread has really gotten in deep. I have a bit of a basic question again, what is hyperfocal? I read briefly its about the whole image being sharp, but how would you focus etc for that.
Also how would one determine what aperture to create full sharpness for a portrait at various distances? Say I was filling the frame with a 50mm and was say 3-5feet and then at 80mm at 10feet away is there a rule of thumb regarding distance and aperture without using what I consider a complicated dof calculator?

Simple answer; No!:shake:
This is physics and fairly straightforward. In the old days we had our depth of field preview buttons and practice.

Now you have;
forums full of knowledgeable and (mostly) friendly people,
DoF calculators that you can study in your own home or carry about on your smartphone.
You have exif data from your own and other peoles images, that tells you focal length aperture and focus distance that you can study to your hearts content in the comfort of yourhome.

What would make it easy for you?
I apologise if that sounds like ranting, but photography mixes technical with art and there are times when you have to put some effort in to get the results.

 

[EOS_JD]

Wrong
What's changed - focal length
As per my previous post - on a small compact the 'standard lens' focal length could be around 11mm, now consider an 11mm lens on a FF SLR and focussed at about 6feet, even with a fairly large aperture the depth of field would be close to hyperfocal giving you a front to back sharp image. And that 11mm lens on that smaller sensor - same thing

The confusion arises because we insist on comparing 'effective focal lengths':bonk: Rather than 'focal lengths'.

Good point Phil lol DOH! :bonk:

 

[Phil V]

Good point Phil lol DOH! :bonk:

It's ok it's an easy and common mistake to make.

 

[EOS_JD]

It's ok it's an easy and common mistake to make.

I understand it when discussing FF v crop but for some reason missed that with the compact lol

 

[HoppyUK]

Wrong
What's changed - focal lengthAs per my previous post - on a small compact the 'standard lens' focal length could be around 11mm, now consider an 11mm lens on a FF SLR and focussed at about 6feet, even with a fairly large aperture the depth of field would be close to hyperfocal giving you a front to back sharp image. And that 11mm lens on that smaller sensor - same thing

The confusion arises because we insist on comparing 'effective focal lengths':bonk: Rather than 'focal lengths'.

The key question is why has focal length changed? Because of the sensor size. That's the fundamental driver here, which is why the short-hand 'sensor size affects depth of field' is used. You can change everything else, but the one thing you can't change is sensor size.

 

[HoppyUK]

When I say image, I use the classically accepted meaning for the word image - the one used by lens designers, Optical engineers, physicists, Opticians and the like - that is the image formed by a lens in a given plane (in this case on a film or sensor)

<snip>

But in the context of depth of field, that is the wrong road to take. Until you state the print size and viewing parameters, depth of field does not exist at all in any measureable way.

I could also take issue with plenty more of your post, but another time...

 

[Phil V]

The key question is why has focal length changed? Because of the sensor size. That's the fundamental driver here, which is why the short-hand 'sensor size affects depth of field' is used. You can change everything else, but the one thing you can't change is sensor size.

I don't understand how you can grasp the concept of changing lens focal length, shooting distance and even seemingly print size (CoC is a measure of image sharpness with a relationship to DoF but isn't direct), but not sensor size? Which is apparently fixed.:THINKING:

The shorthand 'sensor size affects depth of field' is based on the focal length required for the field of view. But the focal length, aperture and subject distance are all physical constants we can calculate from. The concept is lazy and has been perpetuated by the industry using 'effective focal length' in its marketing.
In the old days with medium format cameras, and half frame and 110, we just accepted the actual focal length required for a standard field of view, but we knew that our 75mm or 80mm standard lens on MF had a shallower DoF than our 50mm standard on 35mm (assuming the same aperture and focus distance).
We also understood that 110 cameras had no practical use of focus control due to the massive DoF.
We ought to be buying a 'standard' lens around 30mm and understanding that our crop cameras give a slightly larger DoF than a full frame DSLR.

 

[HoppyUK]

I don't understand how you can grasp the concept of changing lens focal length, shooting distance and even seemingly print size (CoC is a measure of image sharpness with a relationship to DoF but isn't direct), but not sensor size? Which is apparently fixed.:THINKING:

The shorthand 'sensor size affects depth of field' is based on the focal length required for the field of view. But the focal length, aperture and subject distance are all physical constants we can calculate from. The concept is lazy and has been perpetuated by the industry using 'effective focal length' in its marketing.
In the old days with medium format cameras, and half frame and 110, we just accepted the actual focal length required for a standard field of view, but we knew that our 75mm or 80mm standard lens on MF had a shallower DoF than our 50mm standard on 35mm (assuming the same aperture and focus distance).
We also understood that 110 cameras had no practical use of focus control due to the massive DoF.
We ought to be buying a 'standard' lens around 30mm and understanding that our crop cameras give a slightly larger DoF than a full frame DSLR.

I don't really get your point with the first para, but I will say that I personally find the phrase 'smaller sensors give less DoF' as a perfectly aceptable shorthand, even if it is incomplete. Regardless of the full explanation, if you use such a camera and shoot side by side with someone using a larger format and take like for like images, you will end up with greater DoF. Simple as that really.

The confusion surrounding this is an inevitable consquence of the multiple formats we have these days, but however it is explained or presented or marketed, it doesn't change the facts.

 

[Richard King]

But in the context of depth of field, that is the wrong road to take. Until you state the print size and viewing parameters, depth of field does not exist at all in any measureable way.

I could also take issue with plenty more of your post, but another time...

We are going to have to agree to disagree here.

My view is that a lens or a lens system has a physical set of measurable characteristics. And the (chosen) camera back records that image, and then we use editing software / or a darkroom to enlarge or crop the image for example

Your view is to look at the system as a whole, from front to back, in one go

My view takes into account real tangiable measurements at each stage

Your view is seeking to be simple but in reality adds in a great area of confusion

 

[Phil V]

I don't really get your point with the first para, but I will say that I personally find the phrase 'smaller sensors give less DoF' as a perfectly aceptable shorthand, even if it is incomplete.

It was based on your statement here, which is clearly just meh!:

You can change everything else, but the one thing you can't change is sensor size.

What it comes down to is the difference between teaching someone a 'simple' principle, that then has to be un-taught later. I understand that to you it's this straightforward:

Regardless of the full explanation, if you use such a camera and shoot side by side with someone using a larger format and take like for like images, you will end up with greater DoF. Simple as that really.

That's really simple to understand for someone trying to understand the difference between their P&S and my DSLR.

Or teaching them that it's simply the:

• Focal length (actual)
• Aperture
• Subject distance,

that creates Depth of Field, in doing this, we are teaching them something that'll remain true, measurable and predictable forever.
As Richard says:

Your view is seeking to be simple but in reality adds in a great area of confusion

Telling someone that DoF is related to sensor size, is something they're eventually going to have to unlearn as their interest in photography grows.
I'm assuming that someone who's bought a DSLR and is trying to learn the technical aspects of photography isn't coming here for the easy answers but for the correct answers.

I never taught my children to call a horse a BoBo, because I'd have had to re-teach them later.

 

[HoppyUK]

We are going to have to agree to disagree here.

My view is that a lens or a lens system has a physical set of measurable characteristics. And the (chosen) camera back records that image, and then we use editing software / or a darkroom to enlarge or crop the image for example

Your view is to look at the system as a whole, from front to back, in one go

My view takes into account real tangiable measurements at each stage

Your view is seeking to be simple but in reality adds in a great area of confusion

We can disagree on matters of opinion Richard, that's fine, but not on fundamental principles.

The concept of DoF is independent of all these photographic attributes. You can apply the same concept of perceived sharpness to any viewable image, such as a printed magazine page, or a TV screen, or the VDU you're looking at now.

They all use the same optical 'trick' that is behind the DoF whole theory. If the dots in those images are smaller than a certain size, when viewed from a certain distance, then they become indistinguishable and therefore any smaller detail cannot be detected.

It's only once the viewing conditions are defined that you can apply measures, and the one that drives all depth of field calculations is 0.20mm in that 10in print viewed from 12in. It's not possible to disagree with the principle of that.

What DoF calculator then do is work that back. On DoFmaster for example, the first thing you do is input the camera, which sets the sensor size, and that is then fixed for all future calculations, while the other parameters - focal length, aperture and distance - are varied according the the shot.

It was based on your statement here, which is clearly just meh!:


What it comes down to is the difference between teaching someone a 'simple' principle, that then has to be un-taught later. I understand that to you it's this straightforward:

That's really simple to understand for someone trying to understand the difference between their P&S and my DSLR.

Or teaching them that it's simply the:

• Focal length (actual)
• Aperture
• Subject distance,

that creates Depth of Field, in doing this, we are teaching them something that'll remain true, measurable and predictable forever.
As Richard says:

Telling someone that DoF is related to sensor size, is something they're eventually going to have to unlearn as their interest in photography grows.
I'm assuming that someone who's bought a DSLR and is trying to learn the technical aspects of photography isn't coming here for the easy answers but for the correct answers.

I never taught my children to call a horse a BoBo, because I'd have had to re-teach them later.

There is no un-learning or un-teaching, and no meh.

Saying that sensor size affects DoF is stating a fundamental that can be built on with a fuller explanation, but your suggestion of just focal length, aperture and distance - to the exclusion of sensor size as stated above - is not something that will "remain true, measurable and predictable forever" because the moment you pick up a different camera is goes out of the window.

You either accept the short-hand version, eg smaller sensors deliver greater DoF (which is evidently true, if not the whole story) or you have to give the Full Monty explanation as I did in post #25, which takes four paragraphs.

 

[Phil V]

We clearly have a fundamental difference of opinion, this isn't helping anyone who needs to know the facts, I'll let you get on with it.