Looking for a correct DOF calculator

[Tout76]

Hi,

It seems very strange, but I have not found a correct DOF calculator. For some reason the ones I found all have the same error. Of course at first I thought that I was wrong. But it seems I am not.

When I try to use the calculators, you have to enter:

Focal Length

Aperture

Distance to the subject

So far so good.

Now bare with me. Next is sensor size. When I take a photo with:

50mm

F1.4

10 meters distance

With a full frame sensor. This gives X cm DOF.

Then I crop that image with a APS-C factor in post.

So the image was ~35x24 and I crop out ~24x18 in the center.

The DOF in cm does not change imho.

Why then, when I use the DOF calculators on the internet, does the DOF change when you have a different sensor size. In all calculators I encountered, the DOF of a medium format sensor is less shallow that a Micro 4/3 sensor at the same distance.

Shouldn't it be the same when the aperture, lens, and distance to the subject are the same? Just more of the environment is in the picture with medium format?

I am confused, I would like to know if I am correct. And maybe someone can point me to a correct calculator?

Thanks

 

[Mako2011]

Why then, when I use the DOF calculators on the internet, does the DOF change when you have a different sensor size. In all calculators I encountered, the DOF of a medium format sensor is less shallow that a Micro 4/3 sensor at the same distance.

Most online DoF calculators assume you want to compare DoF at the same viewing distance and same scene FoV (which requires a change in CoC for different sensor sizes) so you're looking at the same 8X10 (same scene filling the frame) regardless of camera

Shouldn't it be the same when the aperture, lens, and distance to the subject are the same? Just more of the environment is in the picture with medium format?

Not when the DoF calculator states: "The calculator will automatically adjust for any "focal length multiplier" or "field of view crop" for the selected camera."

I am confused, I would like to know if I am correct. And maybe someone can point me to a correct calculator?

To get what you are after...just keep camera used (CoC) constant when comparing DoF using the calculator

Good read on the subject that explains better:

Loading…

alikgriffin.com

 

[Jack Tingle]

For convenience they tie circle of confusion to sensor size. That was fine when 35mm film grain and 8x10 paper grain was pretty close across all brands. You need one that allows manual circle of confusion entry. Good luck figuring that out. That's one reason manufactures just freeze it with sensor size. The experiments to actually find the value are very specific and time consuming.

 

[J A C S]

Hi,

It seems very strange, but I have not found a correct DOF calculator. For some reason the ones I found all have the same error. Of course at first I thought that I was wrong. But it seems I am not.

When I try to use the calculators, you have to enter:

Focal Length

Aperture

Distance to the subject

So far so good.

Now bare with me. Next is sensor size. When I take a photo with:

50mm

F1.4

10 meters distance

With a full frame sensor. This gives X cm DOF.

Then I crop that image with a APS-C factor in post.

So the image was ~35x24 and I crop out ~24x18 in the center.

The DOF in cm does not change imho.

It does. When you view both images the same size, you enlarge the crop, and this reveals softness you may have missed originally.

 

[bmw44]

Depth of field calculators are just an approximation and I do find them not to be very useful. The depth of field depends on the lens design as well as the lens opening etc. You do realize that an image will be sharpest at the focus point or near the focus point. Even if the depth of field covers a certain distance, objects away from the focus point will not be as sharp. Yes at some point that is obscured by the quality of the lens, limits of you eyes, the media on which the image is displayed, and the resolution of your sensor/film. When depth of field is critical to me, I use the stop down technique to see the effects in the view finder or screen. For macro photography where I want a very large depth of field, I will sometimes use focus stacking.

 

[J A C S]

Depth of field calculators are just an approximation and I do find them not to be very useful. The depth of field depends on the lens design as well as the lens opening etc. You do realize that an image will be sharpest at the focus point or near the focus point. Even if the depth of field covers a certain distance, objects away from the focus point will not be as sharp. Yes at some point that is obscured by the quality of the lens, limits of you eyes, the media on which the image is displayed, and the resolution of your sensor/film. When depth of field is critical to me, I use the stop down technique to see the effects in the view finder or screen. For macro photography where I want a very large depth of field, I will sometimes use focus stacking.

DOF is proportional to the CoC. It is useful as such; if you want to halve the CoC, you halve the DOF, etc. So with the reference CoC, it is a good reference point.

If you dig deeper, the DOF does depend on the lens design to some extent, but this is nitpicking.

 

[David Lal]

I am confused, I would like to know if I am correct.

Sorry, I believe you are not correct and the calculators you find on the internet are mainly correct providing they do not approximate the maths.

I have written my own and it agrees with, for example:

• https://www.dofmaster.com/dofjs.html and
• https://bobatkins.com/photography/technical/dofcalc.html

A key determinant is the circle of confusion. Usually standard numbers are quoted as 30 microns for 35mm/FX and 20 microns for DX. COC can be rather subjective though so I have allowed in my calculator for any COC value to be entered.

 

[Bob A L]

Hi,

It seems very strange, but I have not found a correct DOF calculator. For some reason the ones I found all have the same error. Of course at first I thought that I was wrong. But it seems I am not.

When I try to use the calculators, you have to enter:

Focal Length

Aperture

Distance to the subject

So far so good.

Now bare with me. Next is sensor size. When I take a photo with:

50mm

F1.4

10 meters distance

With a full frame sensor. This gives X cm DOF.

Then I crop that image with a APS-C factor in post.

So the image was ~35x24 and I crop out ~24x18 in the center.

The DOF in cm does not change imho.

Why then, when I use the DOF calculators on the internet, does the DOF change when you have a different sensor size. In all calculators I encountered, the DOF of a medium format sensor is less shallow that a Micro 4/3 sensor at the same distance.

Shouldn't it be the same when the aperture, lens, and distance to the subject are the same? Just more of the environment is in the picture with medium format?

I am confused, I would like to know if I am correct. And maybe someone can point me to a correct calculator?

Thanks

Maybe we could have a club of two. I believe you and I are the only people that see it this way. I have given up on trying to explain.

 

[foot]

Hi,

It seems very strange, but I have not found a correct DOF calculator. For some reason the ones I found all have the same error. Of course at first I thought that I was wrong. But it seems I am not.

When I try to use the calculators, you have to enter:

Focal Length

Aperture

Distance to the subject

So far so good.

Now bare with me. Next is sensor size. When I take a photo with:

50mm

F1.4

10 meters distance

With a full frame sensor. This gives X cm DOF.

Then I crop that image with a APS-C factor in post.

So the image was ~35x24 and I crop out ~24x18 in the center.

The DOF in cm does not change imho.

Why then, when I use the DOF calculators on the internet, does the DOF change when you have a different sensor size. In all calculators I encountered, the DOF of a medium format sensor is less shallow that a Micro 4/3 sensor at the same distance.

Shouldn't it be the same when the aperture, lens, and distance to the subject are the same? Just more of the environment is in the picture with medium format?

I am confused, I would like to know if I am correct. And maybe someone can point me to a correct calculator?

Thanks

Maybe we could have a club of two. I believe you and I are the only people that see it this way. I have given up on trying to explain.

the DOF math is about the optics, and the path light takes

cropping, resizing....none of those change the optics and the path light took when the photo was taken

 

[Tout76]

Using your first link:

Canon 7d, 50mm 1.4, 10 meters distance: 2.16m DOF

Canon 5d, 50mm 1.4, 10 meters distance: 3.48m DOF

That is quite a big difference.

 

[Tout76]

Maybe we could have a club of two. I believe you and I are the only people that see it this way. I have given up on trying to explain.

Think the club is a bit larger, but any size is ok, it is nice to found someone with the same insight.

 

[Tout76]

Good read on the subject that explains better:

https://alikgriffin.com/depth-of-field-calculator/

Thanks!

Using that calculator with:

Full frame 50mm 1.4, 10 feet: Dof: 1.01ft

Micro 4/3 50mm 1.4, 10 feet: Dof: 0.5ft

So they just half the DOF. But at the same distance of 10ft the subject is just smaller. The DOF should be the more or less the same.

It is beyond the influence of CoC i guess.

Actually I do not mind about the error that much. It's just that with these results I do not trust the outcome.

And the actual comparison I would like to make is the DOF difference between a 300 2.8 and a 600 5.6.

 

[Mako2011]

Good read on the subject that explains better:

https://alikgriffin.com/depth-of-field-calculator/

Using that calculator with:

Full frame 50mm 1.4, 10 feet: Dof: 1.01ft

Micro 4/3 50mm 1.4, 10 feet: Dof: 0.5ft

So they just half the DOF.

Bit more complicated than that...

Actually I do not mind about the error that much.

Not an error really

It's just that with these results I do not trust the outcome.

Outcome is correct/accurate...given the calculators caveats as stated

And the actual comparison I would like to make is the DOF difference between a 300 2.8 and a 600 5.6.

Then just keep the camera sensor format the same (constant) when doing the calculation. The why of that is explained in the link I provided

 

[David Lal]

Using your first link:

Canon 7d, 50mm 1.4, 10 meters distance: 2.16m DOF

Canon 5d, 50mm 1.4, 10 meters distance: 3.48m DOF

That is quite a big difference.

Indeed it is. The 7D has a sensor size of 22.3 x 14.9mm. The 5D's sensor is 35.8 x 23.9mm. The one is FX the other DX and so the Circle of Confusion dimensions are different; respectively 19 and 29 microns.

Hence the different DOF figures - which are correct.

A lens like that and at the same aperture and subject distance used on a 6x6 film camera would have a COC of 0.045 and DOF of 5.41 metres.

You mentioned 300mm @F2.8 versus 600mm @F5.6. On a DX camera these result in DOFs of respectively 0.12m and 0.06m if the subject is at 10m distance.

 

[Tout76]

Take a picture of person at 30ft. With the same lens on two cameras. Same aperture.

On the 4/3 camera you have a picture of let's say head and shoulders

On the full frame you have lets say the whole person.

Now let's crop the full frame picture in post to the size of the 4/3. You end up with the same picture as taken with the 4/3 camera.

You are saying that cropping the picture changes the DOF?

EDIT: So I understand from another reply that taking a picture with a crop sensor is not the same as cropping a full frame picture in post

This has to sink in. I was not aware that a smaller sensor could make for a shallower depth of field on the same distance. That the picture the lens is projecting is not just cropped by a crop sensor...

 

[Tout76]

Using your first link:

Canon 7d, 50mm 1.4, 10 meters distance: 2.16m DOF

Canon 5d, 50mm 1.4, 10 meters distance: 3.48m DOF

That is quite a big difference.

Indeed it is. The 7D has a sensor size of 22.3 x 14.9mm. The 5D's sensor is 35.8 x 23.9mm. The one is FX the other DX and so the Circle of Confusion dimensions are different; respectively 19 and 29 microns.

Hence the different DOF figures - which are correct.

A lens like that and at the same aperture and subject distance used on a 6x6 film camera would have a COC of 0.045 and DOF of 5.41 metres.

You mentioned 300mm @F2.8 versus 600mm @F5.6. On a DX camera these result in DOFs of respectively 0.12m and 0.06m if the subject is at 10m distance.

So you say the cropping of the picture in post is not the same as taking a picture with the crop camera?

Interesting theory.

The crop sensor records a part of the image a full frame sensor records. It does not change the lens.

In your theory, if you have a 10000 meter x 10000 meter sensor, you have endless DOF. Everything in focus with a 1200mm 1.4 lens?

 

[Tout76]

It does. When you view both images the same size, you enlarge the crop, and this reveals softness you may have missed originally.

The difference in the calculators is not a few %, is like a 100% difference:

DOF with 4/3 is for example 0.5ft

DOF with full frame is 1ft

With the same lens, aperture and distance.

That is not a thing to overlook I guess when looking at the picture.

 

[David Lal]

In your theory, if you have a 10000 meter x 10000 meter sensor, you have endless DOF. Everything in focus with a 1200mm 1.4 lens?

Well, this is not "my" theory - just standard optical physics. A sensor of the dimensions you describe does not exist. Let's take a more modest 10cm x 10cm sensor. The circle of confusion is given by d/1730 where d is the length of the sensor diagonal. From Pythagoras that is square root of (10^2 + 10^2 ) = 14.142. Divide that by 1730. COC is ± 0.0081.

At F1.4 and focal length of 10mm DOF will be infinite for a subject at 9m distance.

Maybe you need to do some Googling - there are lots of good explanations.

 

[Dem Bell]

...

Maybe we could have a club of two. I believe you and I are the only people that see it this way. I have given up on trying to explain.

the DOF math is about the optics, and the path light takes

Not quite. DOF is about blur - when you see it and when you don't. The math just works out the relationship between the amount of blur "burnt" into the image and whether the human eye will perceive it as "blurry" under specific viewing conditions. DOF is as much about the lens optics as it is about the viewing conditions.

cropping, resizing....none of those change the optics and the path light took when the photo was taken

Yet they change what you perceive as "sharp" and therefore DOF changes.

 

[J A C S]

It does. When you view both images the same size, you enlarge the crop, and this reveals softness you may have missed originally.

The difference in the calculators is not a few %, is like a 100% difference:

DOF with 4/3 is for example 0.5ft

DOF with full frame is 1ft

With the same lens, aperture and distance.

This is what it should be. The ratio is the crop factor.

That is not a thing to overlook I guess when looking at the picture.

Indeed.