Depth of Field Calculators.. Are they all wrong??

bald_eagle

Hi All,
Firstly, this is my first post on the forum, so please be gentle with me.. I hope this question hasn't been addressed before (I couldn't find it in a search of the forum).. So here goes..

I sometimes use Depth of Field Calculators (I have a couple of apps on my windows phone but I've also used an online one that I've often seen recommended on the forum.. http://www.dofmaster.com/dofjs.html
I've noticed with the phone apps there's a huge difference in the values output for any given focal length/f-stop/focus distance and C of C, so I was trying to find out which is the most accurate.. Then I discovered something a little confusing..

When I switch between the FX CofC and DX CofC (without changing any other values), ALL the DoF calculators (including the online one linked above) give a LARGER DoF for the FX sensor..

How can that be? As I understand it, DoF is SMALLER on an FX sensor (0.03 CofC) than it is on a DX sensor (0.02 CofC) for the same focal length/F-stop/Focal Distance?
Regards,
Bald-Eagle

sevencrossing

welcome to NRF bald_eagle

yes it has been discussed here

http://forum.nikonrumors.com/discussion/510/dof-fx-vs-dx/p1


and on the old forum

http://nikonrumors.com/forum/topic.php?id=2165

warning after reading the above threads, you might be even more confused

Msmoto

While we have looked at the issue of DOF comparing formats, maybe it is a good idea to revisit this in general. My personal experience is one can only decide what DOF is required for the results one wants to have. For a very large print, i.e., larger than 75 cm square, then DOF which is acceptable on a print half as large may be unacceptable. But, the use of a DOF calculator is IMO used as a reference point for your experience to be relied upon only when early in the learning curve.

Interestingly enough, on a shot of the moon, I found the DOF at f/6.3 with 800mm, inadequate to have the edges and the close point in shape focus.

bald_eagle

Hi,
Thanks sevencrossing for the reply, maybe I wasn't quite clear in my original post..
I've read through the above threads (thanks for pointing them out) but they don't really address my question..

I understand Depth of Field and the factors that change it (or at least I think I do ) My question isn't really related to whether DoF changes between FX and DX, but rather whether ANY of the DoF calculator apps and online DoF calculators are correct and/or accurate..

Every one I have tried gives me a LONGER DoF for a FX than for an DX sensor circle of confusion for the same Focal Length, F-stop, Focus Distance combination..

Isn't that supposed to be the other way round? Isn't the DoF with an FX sensor supposed to be shallower? What am I missing?

Regards,
Bald-Eagle

sevencrossing

I can only agree with Msmoto with dof, things not suddenly go from being sharp to unsharp dof tails off gradually. I suggest you try different f stops on several lenses and see what, if any, difference you can see

sevencrossing

Interestingly enough, on a shot of the moon, I found the DOF at f/6.3 with 800mm, inadequate to have the edges and the close point in shape focus.

by the "close point" do you mean something on the earth e.g. trees

if so I suggest you "cheat" and take two shots and merge in Photoshop

https://www.facebook.com/photo.php?fbid=10152041840735766&l=54fd728b9a

bald_eagle

Hi Guys,
Thanks for the replies...
As I've said, my question isn't about DoF really, it's about the accuracy of DoF calculators (apps for mobile phones and online like the one on dofmaster.com)...
I understand DoF is all a little subjective, however I sometimes use a DoF calculator to ESTIMATE the DoF for a given situation (as I say, I use it only as a guide).. for example when doing Macro Photography...

So.. Are the calculators correct when they give a SHALLOWER DoF for DX as they give for FX (with everything else remaining the same?
Shouldn't the DoF be shallower for FX???

Regards,
Bald-Eagle

Nuke

I think the problem is your not keeping the same angle of view between the two formats. If you do the same framing between the two formats the DX camera will give you a greater DOF.

clskelton

@bald_eagle said:
Isn't that supposed to be the other way round? Isn't the DoF with an FX sensor supposed to be shallower? What am I missing?

Please see my comment and Ade's comment (and link) afterwards here. Also read the Wikipedia article as that was very helpful for me.
http://forum.nikonrumors.com/discussion/comment/87104#Comment_87104

Those DoF values are calculated based on enlargements from the size of the sensor to the 8"x10". Pixel density doesn't matter here. The DX size is smaller so it requires greater magnification than the FX size, and therefore out of focus areas will be more apparent, which means smaller DoF. This is why I don't like general statements such as "FX has smaller DoF".

For example, if you framed your subject 10 feet away in FX mode, and then changed your camera to DX mode, you must move backwards 5 feet in order to frame the subject the same way. Yes, the image has to be further magnified so DoF will get smaller, but moving 5 feet back increases the DoF even more than the effect of magnification, so the net effect is the DoF increases.

Another way to look at this, If you are 10 feet away from your subject in FX mode with a 50mm lens, you will need to use a 35mm lens (33.33mm actually) in DX mode order to get the same field of view so the subject is still in the frame. The DoF will get smaller from the DX mode, and will get larger from the wider angle lens. The wider angle lens will over compensate for the greater magnification.

As @Msmoto was saying, making enlargements larger than 8"x10" will produce an even smaller DoF, because what looked in focus on a 8"x10" print may look out of focus on a 16"x20". The DoF calculators use 8"x10" for their calculations.

MikeGunter

Hi all,

@bald_eagle - Interesting - and I'm not the guy that would know; I don' t know if the DOF of _all_ 50mm lenses for 35mm cameras at f5.6 is exactly the same or they vary by elements - lenght of the lens and distance of the surface to the focal plane or some other factor or brand or something else.

A fellow who might know off the top of his head is @Ade - who I bet would just love for me to cheerfully volunteer his busy time (just kidding Ade - I sure we would understand if you hit the red {ignore} button).

While precision does count, in bright light, the depth of field preview button gives a great preview of the shot, and most preview guides give an account of accurate focus, with a shoulder of the DOF.

My best to all,

Mike

Ade

@Nuke already provided the right answer

@bald_eagle you're not taking field-of-view (FoV) into account, i.e., the 1.5x crop effect. E.g., a 35mm lens on a DX sensor has roughly the same FoV as a 50mm lens on an FX sensor.

With that in mind:

DX sensor, 35mm f/2 at 10ft has a DoF of 1.99 ft
FX sensor, 50mm f/2 at 10ft has a DoF of 1.45 ft

Hence FX has a shallower DoF than DX for the same FoV or "equivalent focal length".

Ironheart

Required reading before posting on this thread:
http://en.wikipedia.org/wiki/Nikon_FX_format
http://en.wikipedia.org/wiki/Nikon_DX_format

One more equiv to Ade's list:
DX sensor, 50mm f/2 at 15ft has a DoF of 2.18 ft (and the same FoV as the other two)

TaoTeJared

Hi Guys,
Thanks for the replies...
As I've said, my question isn't about DoF really, it's about the accuracy of DoF calculators (apps for mobile phones and online like the one on dofmaster.com)...
I understand DoF is all a little subjective, however I sometimes use a DoF calculator to ESTIMATE the DoF for a given situation (as I say, I use it only as a guide).. for example when doing Macro Photography...

So.. Are the calculators correct when they give a SHALLOWER DoF for DX as they give for FX (with everything else remaining the same?
Shouldn't the DoF be shallower for FX???

Regards,
Bald-Eagle

OK so I started to write a statement about maybe you are reading the wrong values - BUT then the online Calc is now doing what you described (which is not the correct values). I have been refreshing - no change, next I'll clean my cache out and see if that works.

You are not crazy - something is hanging on the webpage.


In the general question of DOF calculators they are relatively accurate. There is some rounding differences that move the values slightly, but nothing that could be seen. COC can get a bit out of line between different calculators but again it is not something that is really noticeable.

Gareth

The calculators are correct.

But taking a shot at a set distance, focal length and f stop on a DX camera will not give you the same image as a shot taken using the same settings on an FX camera. One will be "cropped" the other will not be.

If you want your DX shot to be framed the same as your FX shot you will need to walk back quite a bit. This increases your distance to subject, and therefore your DOF.

It's actually quite simple to understand when you have bother type of camera.

bald_eagle

Hi Guys,
Thanks for all the answers, especially @Ade, @clskelton and @Nuke..
So, it appears that the "shallower depth of field" hangs more from the field of view than anything else.. as I understand it now:
For equivalent focal lengths (e.g. DX - 35mm and FX - 50mm) with the same F-stop and focus distance OR,
The same focal length and f-stop but changing the focus distance to get the same field of view (crop) (e.g 50mm 10ft FX -15ft DX), FX has a shallower depth of field than DX..

However, it appears if everything apart from sensor size remains the same, then DX actually gives the shallower DoF..

Thanks again for all the replies guys, it's been most helpful.

Best Regards,
Bald-Eagle

Msmoto

@sevencrossing

By close point I mean the closest point of the moon… it would appear that the fattest edge is not in focus when one focuses on the center.

TaoTeJared

I cleared my cache and everything started working again. Chrome updated as well - not sure if that had something to do with it. I had a bunch of things crashing all day though.

sevencrossing

@sevencrossing

By close point I mean the closest point of the moon… it would appear that the fattest edge is not in focus when one focuses on the center.

my dof calculator recons the dof is infinite at 384,400 km
so it may well be wrong

haroldp

The concept of DOF is dependent on ones individual toleration for unsharpness since at any focal length / aperture/ image size, there is exactly one 'plane of focus ( Idisregarding lens curvature of field etc.), so any guide or calculator is only an approximation.

The apparent decline of DOF since going digital is mostly due to the greater magnifications at which we typically view digital images as compared to film.

... H

Msmoto

@sevencrossing
I was using the extreme magnification of live view to manually focus and my experience was that the edges of the moon would not be in focus when the closest point was. However, as always, my technique may be the issue as old folks sometimes cannot do it with the preciseness of those youngsters….. )

Oh, this "moon shot" will be played with…the D4, 400mm/2.8 VRII and TC-20EIII. My goal is to see just how sharp I can get this with all the confounding factors being present, the atmosphere and its temperature variations, dirt, clouds, being the issue.

haroldp

Msmoto:

I am currently using the 400/2.8 VRII with tc-17eII with very good results. I am intensely interested in your experience with this lens and tc-20eIII.

Regards ... H

Ade

@Msmoto

Not sure of your exact situation, but sometimes if the edge of the moon (or other object) is brightly lit compared to the center, then you might get some softening towards the edges as some channels get progressively more saturated / clipped, resulting in loss of detail.

So the edge softness might not have anything to do with DoF, but an artifact of the very high dynamic range present in the frame. E.g., for a half-moon the tonal range might go from virtually black near the center of the moon to extremely bright at the lit edge. Using HDR techniques might be required to get center-to-edge sharpness.

sevencrossing

I was using the extreme magnification of live view to manually focus and my experience was that the edges of the moon would not be in focus when the closest point was.
.

Out of interes,t was there any noticeable difference in the final image?

(before you guys got up there for a closer look. Lunar mapping was done by photographing (or more often drawing ) long the terminator)

Just been on to Google please to see the telescope I use to work on the 60s is still there
http://www.strellis.com/SAS/graphics/news/thorrowgood.jpg

spraynpray

While we have looked at the issue of DOF comparing formats, maybe it is a good idea to revisit this in general. My personal experience is one can only decide what DOF is required for the results one wants to have. For a very large print, i.e., larger than 75 cm square, then DOF which is acceptable on a print half as large may be unacceptable. But, the use of a DOF calculator is IMO used as a reference point for your experience to be relied upon only when early in the learning curve.

Interestingly enough, on a shot of the moon, I found the DOF at f/6.3 with 800mm, inadequate to have the edges and the close point in shape focus.

Are you sure it isn't edge drop-off? I find it more than a little hard to believe it could be DoF too shallow at that distance.

Msmoto

Well, the technical issues with the shot, simply getting the image in the frame, vibrations, but most of all the multiple refractions created by the fact we are shooting through a wide variety of temperature changes, all create a difficult situation for capturing a moon shot from an Earth elevation of about 900 ft above sea level. However, this is such an interesting challenge, i will continue to pursue this. And, in a couple months I will have an opportunity to attempt this from Colorado at 6,000 ft of elevation, pretty much clear of ground light.

Of course, operator error is always an issue, especially with some of us who are a bit older.

As to some of the interesting things that occur with extremely high contrast, here are some examples of flare or whatever, when shooting the sun with a dark red filter:
https://www.flickr.com/photos/fantinesfotos/8955727576/sizes/o/

Look at the sun behind the building and compare the edge flare to the building more up and to the right. Lots of things happen. Oh, the building is about 1.25 miles from the camera. The POET plant near Lennox, South Dakota, where they convert corn to ethanol.

But, back to DOF, maybe 8 - 12 inches on the front rider…?
https://www.flickr.com/photos/fantinesfotos/8918348982/sizes/o/
400mm f/2.8 + TC-20EIII, 1/800, f/11, D4, ISO 900. And, IMO, the image is fairly crisp, a good example of my "economy" 800mm…. LOL The bikes are over 200 feet from the camera.

spraynpray

"But, back to DOF, maybe 8 - 12 inches on the front rider…?"

That is a helluva good catch Tommie