landru515 said:
Gareth is absolutely right. I study cinematography and have lots of experience shooting in both 16mm and 35mm film (different 'sensor' sizes, very similar to the FX/DX sensor size differences). This is always tough for people to wrap their head around at first but its really very simple once it clicks.

Sensor size has NOTHING to do with depth of field, it has a lot to do with FIELD OF VIEW, however. Lets say you have two cameras, one DX and on FX, and you have one FX lens. In order to frame the subject the same way with both cameras, you will need to be at different distances from the subject with both cameras because with the smaller sensor, you will have to back up in order to frame the subject the same way as you did with the FX sensor. The smaller sensor sees a smaller part of the image the lens is 'projecting' onto it, virtually 'cropping it'. Being at a different distance from the subject, the depth of field changes. Let me put it another way: The sensor size you are using can often dictate which focal length (lens) you will be using. Cameras with smaller sensors (assuming you are using FX full frame lenses, not DX lenses) require shorter focal lengths than FX sensors for the same field of view. Thus, generally you are shooting with wider lenses than you are with full fame cameras, giving you MORE depth of field (more in focus). A 50mm lens will have the same depth of field, the sensor size doesn't matter. However, your subject will appear closer up with the smaller sensor camera, so the photographer backs up, completely changing the depth of field.

I know this has been explained to death in this thread, but I thought I'd give it a go, writing it out helps as well :)

Hmmm, I hope you are saying use an FX lens on both the DX and the FX camera so that you don't get vignetting as that is the only reason to do so? Other than that, your explanation is clear.

Gareth is absolutely right. I study cinematography and have lots of experience shooting in both 16mm and 35mm film (different 'sensor' sizes, very similar to the FX/DX sensor size differences). This is always tough for people to wrap their head around at first but its really very simple once it clicks.

Sensor size has NOTHING to do with depth of field, it has a lot to do with FIELD OF VIEW, however. Lets say you have two cameras, one DX and on FX, and you have one FX lens. In order to frame the subject the same way with both cameras, you will need to be at different distances from the subject with both cameras because with the smaller sensor, you will have to back up in order to frame the subject the same way as you did with the FX sensor. The smaller sensor sees a smaller part of the image the lens is 'projecting' onto it, virtually 'cropping it'. Being at a different distance from the subject, the depth of field changes. Let me put it another way: The sensor size you are using can often dictate which focal length (lens) you will be using. Cameras with smaller sensors (assuming you are using FX full frame lenses, not DX lenses) require shorter focal lengths than FX sensors for the same field of view. Thus, generally you are shooting with wider lenses than you are with full fame cameras, giving you MORE depth of field (more in focus). A 50mm lens will have the same depth of field, the sensor size doesn't matter. However, your subject will appear closer up with the smaller sensor camera, so the photographer backs up, completely changing the depth of field.

I know this has been explained to death in this thread, but I thought I'd give it a go, writing it out helps as well :)

That's a nice set of images to illustrate, Gareth. Thanks for posting them.

^^^yes, as I said sensor size is irrelevant in terms of DOF, but does affect distance to subject.

It seems MANY people get confused by this. that's why I posted the pics in an effort to explain.

From the other Thread:
Oops! Looks like I got this part wrong: “As the sensor/film format gets larger the DOF get shorter. The inverse is true as the sensor/film format gets smaller the DOF gets larger”

mspman is right I got it backwards in my defense I just have to say this has been so ingrained in my head that when I would switch between my 35mm film camera with the normal lens 50mm to my medium format camera with the normal lens 80mm, I lose DOF. Yes more than one variable changing and technically I was wrong but in practice it’s how it usually works out. As others have explained.

^^^as I said in the above thread

DOF is a function of focal length, aperture and distance to subject. nothing else.

why does sensor size affect DOF?
Because you are 1.5 the distance to your subject when framing the same shot on an FX vs a DX camera.

I hope this example makes it clear to everyone.

I used a tape measure to measure distance to subject and live view to match the composition to the alternative shot. You will notice that the framing is actually a bit tighter on the DX shots, this probably had to do with my measuring from the centre of the tripod, not the nodal point of the lens. This should actually give the DX shots slightly less DOF than if they were matched perfectly.

I removed colour in post as I felt it made the difference clearer.

SORRY, I JUST RELISED I EMBEDED THE WRONG INFO. THOSE BOTTOM TWO PICS ARE F/1.4.

This conversation was started up again on the thread linked to below. I don't know a better way to merge the threads than close the other one and link to it here.
http://nikonrumors.com/forum/topic.php?id=5636

spraynpray said:
If the distance and aperture are the same, .....
Agreed?

Absolutely :)
the "same distance" was the missing variable on your post I'd quoted on

Panamon_Creel said:

You got it spot on with the exception of the last statement regarding the 50% longer lens and shallower DOF thereof which is a bit to generic and will likely be misconstrued as longer focal length equals shallower DOF wich is not true but also not false depending on the circumstances :)

Instead of the longer lens lets say we maintain the same 100mm lens on the FX and now attempt to get the same butterfly size on sensor as on a DX and in order to do that we have to get closer --> closer to subject means shallower DOF.
Now we take a 150mm lens and do the same size butterfly on DX and FX sensors again and we again have to be closer to the butterfly with FX thus will have a shallower DOF but the DOF of the DX with 100mm and DX with 150mm will be identical as well as the DOF on the FX with 100mm and the FX with 150mm because in order to maintain on sensor subject size we have to be further away with the 150mm than with the 100mm ;)

The focal length in on itself does not change the overall DOF as long as the on sensor subject size is maintained and you are at a close distance to the subject (below Hyperfocal).

I think you may just be as bad at explaining this as I am! ;-) I'll say the important bit again for us to focus on:

"To then get the butterfly the same size on the FX (i.e. filling the sensor), you would have to use a 50% longer lens so the DoF would be shallower."

If the distance and aperture are the same, the DoF will only be affected by the focal length of the lens (whatever it is) and the focal length would have to be 50% longer on the FX to get the butterfly the same size on the (larger) sensor so the DoF will be shallower.

Agreed?

Depth of field between FX and DX cameras is actually of very great importance whether or not you are willing or able to wrap your head around it. This is a GREAT thread. Being a photo pro using 35mm gear and considerably larger formats (both medium and large formats) details like angle of view and even lens shifting to increase (or decrease depth of filed were practical considerations to be made with almost every shot. I have always been a big fan of larger film or sensor sizes in field applications as well as using the right camera and lens for the specific photo assignment. If you really wanted to do digital photography right you would want access to a variety of gear. Yet due to physical and financial constraints we rarely have the option of selecting the optimum image capture gear for the intended photo. If Hollywood made movies that way many studios would be out of business. Nikon Rumors Forum members are serious amateurs or pros without unlimited budgets. My budgets are smaller than they used to be. My willingness to carry heavy gear in the field is FAR LESS than I used to be willing to accept. Yet Nikon DSLR available today is right up there with as professional gear as a photographer has ever had available to them. I for one DO NOT EVER MISS THE FILM DAYS! Yet depth of field considerations with your Nikon DSLRs is critical. If you are shooting large buildings that are presented pretty much as human eyes would see them you will require shift lens or after photo correction. I use DX DSLR Nikons most of the time.
I do think I need to use FX more. Yet I would WANT to KNOW what impact DX compared to FX has in terms of depth of field BEFORE I took critical photos.

spraynpray said:
Bottom line:

If you took a picture of a butterfly which filled the sensor at 1 metre using a 100mm lens at f16 on a DX then transferred the lens onto an FX camera using the same settings and distance, the butterfly would remain the same size and the DoF the same depth but the FoV would be bigger (so you would see more 'other stuff' round the outside of the butterfly). To then get the butterfly the same size on the FX (i.e. filling the sensor), you would have to use a 50% longer lens so the DoF would be shallower.
....

I'll bring this one up top again.
You got it spot on with the exception of the last statement regarding the 50% longer lens and shallower DOF thereof which is a bit to generic and will likely be misconstrued as longer focal length equals shallower DOF wich is not true but also not false depending on the circumstances :)

Instead of the longer lens lets say we maintain the same 100mm lens on the FX and now attempt to get the same butterfly size on sensor as on a DX and in order to do that we have to get closer --> closer to subject means shallower DOF.
Now we take a 150mm lens and do the same size butterfly on DX and FX sensors again and we again have to be closer to the butterfly with FX thus will have a shallower DOF but the DOF of the DX with 100mm and DX with 150mm will be identical as well as the DOF on the FX with 100mm and the FX with 150mm because in order to maintain on sensor subject size we have to be further away with the 150mm than with the 100mm ;)
The focal length in on itself does not change the overall DOF as long as the on sensor subject size is maintained and you are at a close distance to the subject (below Hyperfocal).

I enjoyed this thread.. made me think.... I think I now fully understand DOF .. maybe not the maths .. LOL!

Don't back down on print size, Shade, because you're right. Print size and viewing distance have to be defined in order to calculate dof—for the reasons you mentioned. It's not a nitpick because circle of confusion depends on those parameters.

BTW, I'm pretty sure this thread would win in a vote for most confusing ever seen on this forum. But I think it's just because it's a complicated subject with a lot of variables to keep track of.

NikoDoby said:
Wait are you saying that printing at various sizes changes DOF???

Yes, absolutely. The bigger you print, the shallower your depth of field becomes, since any point that is even slightly out of focus will no longer look acceptably sharp. On the other hand, on a 4x6 print, you won't notice if something is slightly out of focus. However, this does not change the character of points that are very far out of focus, so for instance the background will look pretty much the same. You are more likely to notice something like slightly missed focus on a large print, though.

This is somewhat of a nitpick, though and is probably just adding confusion to the subject. Depth of Field needs to be defined more rigorously, really. I consider depth of field to be the parts of the photo that are tack-sharp (or at least at the peak of that lens's capabilities). If you focus on someone's eye, but, say you miss by 1cm or something small (maybe the camera focuses on the nose instead). If you view that photo at 100% it is possible that the circle of confusion is bigger than a single pixel, especially if you use a fast telephoto. Thus the depth of field does not include the subject's eye. However, if you print it at 4x6 inches, the subject's eyes will still look sharp, since the print is so small. Thus, I would say that the depth of field does include the eyes in this case.

Wait are you saying that printing at various sizes changes DOF???

zhu zhu pets video said:
So you're saying if I use an FX camera and take an image with a 200mm lens, then crop the image so I have the same *field of view* the lens would have given me with 300mm (which is what a DX body does), I will also have the same background compression I would have had if I shot at 300mm to begin with on the FX body? That doesn't make any sense. It would follow that taking any image and cropping it changes the background compression.

It's counterintuitive, but this is exactly what happens. You will have the same background compression. Background compression is a result of perspective, not the physics of the lens. If you took a shot at 14mm and cropped out a tiny portion of a distant object in that image, it will also look compressed (assuming you have enough pixels to still see anything).

Here's another way to think about it: your lens is projecting an image onto your sensor. The sensor size has no effect on perspective, aperture, or focal length, all it does is crop compared to an FX sized sensor, this is true for literally any camera, ever. The only thing that changes is your shooting habits and what types of lenses you are likely to put in front of your sensor.

There is also one difference which is only relevant if you are printing to very large sizes, and that is that at 100% view, the size of each pixel becomes the maximum acceptable circle of confusion. Since DX cameras generally have a different pixel size (usually smaller) than FX, this can also affect the depth of field for huge prints that are viewed very closely (have you really ever made a print like this?).

@astrophotographer : good stuff .. but if we are going into details, there is another factor that needs to be considered and that is relative distance of the subject against the focal length.
eg if you have a 200 mm lens (on a DX camera) and the subject is 500mm from the front element/hyperfocal it will have a different DOF than for a subject 5000mm away. So for the same subject distance and FOV on FX you will need a 300mm lens for a 500mm subject distance. which will result in a much shallower DOF as the subject distance is now relatively closer. this is also one of the reasons why the tiny P&S sensors and short mm focal lengths produce such great DOF. At say 2m distances on some P&S in effect its like shooting on FX at 50m distances.

PS: I had another look at your maths .. and I think you may have taken that in to consideration .. oh well .. maybe... never was very good at maths anyway ! LOL

Panamon_Creel solved it all for me. Thanks

In the end it all comes down to pixel density.. shooting with a compact is like shooting with a FX that would have the same pixel density as the compact and in which you would only crop the very middle of the frame, in order to make it let's say 12mpx... to do a portrait this way you'd need to shoot with an ultra wide angle lens and stay far from your subject... aka near infinite DOF

I seriously thought that the sensor size had its role... but in fact it affects the DOF only indirectly.

Thanks

OK, I'm going to stick my nose in this. I will claim to understand this, including the math behind it. First a point of terminology that I don't see explained.

Circle of confusion (CoC): this is the amount a point is out of focus. It is an arbitrary value but for 35mm 1/1000 of an inch (40 um) is a common value. Now the point is to enlarge the image! If you enlarge FX, say 8x then you'll need to enlarge DX 12x to get the same size. Therefore your CoC must be 1.5x smaller or 1/1500 inch. So, shoot with a 200mm lens on FX and the same lens on DX keeping distance and f-stop the same, enlarge to the same size, the DX will have 1.5x shallower DOF and a 1.5x tighter crop.

Now what if you change the focal length? This is where it gets interesting. Keeping CoC, distance and f-stop constant DOF decreases with the square of the focal length. So shoot FX with a 200mm and a 400mm, the 400mm shot's DOF is 1/4 that of the 200. Now what if we shoot FX at 300mm and DX at 200mm? They'll both have the same FOV. But FX's CoC is 1.5x larger than DX but the 300mm lens' DOF is 2.25x smaller, (300 / 200)^2 = 2.25. Last 2.25 / 1.5 = a DOF 1.5x greater for DX.

Long story short, for a given FOV, f-stop and enlarging that the same size, the smaller format image gives a greater DOF.

Hope that confuses it for everyone.

i'm unofficially confused. . .but I'm unsure if this will effect my photography :-/

points said:
So you're saying if I use an FX camera and take an image with a 200mm lens, then crop the image so I have the same *field of view* the lens would have given me with 300mm (which is what a DX body does), I will also have the same background compression I would have had if I shot at 300mm to begin with on the FX body? That doesn't make any sense. It would follow that taking any image and cropping it changes the background compression.

No you have got it wrong... Focal length relates to absolute DOF in terms of the number of mm in focus at a given aperture.

If you do a DX crop of an FX image as you stated you get a change in FOV (from 200 to 300 mm lens) but the absolute DOF of the image remains, to all intents and purposes, exactly the same. However, If you had a 300mm lens and took the same picture as the cropped DX they will have the same FOV but the DX(cropped version) will have a greater DOF than the 300mm lens version(as the DOF depends on the Focal lenght)

So you're saying if I use an FX camera and take an image with a 200mm lens, then crop the image so I have the same *field of view* the lens would have given me with 300mm (which is what a DX body does), I will also have the same background compression I would have had if I shot at 300mm to begin with on the FX body? That doesn't make any sense. It would follow that taking any image and cropping it changes the background compression.

jbl said:
is it because you shoot at 6mm to get 35mm? (something like that?)

This is exactly why. It's not the sensor size that gives you big depth of field, it's the fact that you are using much shorter focal lengths with compacts than with FX (or DX for that matter). This is why talking about "effective" focal lengths is so confusing, it makes it very difficult to explain how sensor size changes things.

Bottom line:

If you took a picture of a butterfly which filled the sensor at 1 metre using a 100mm lens at f16 on a DX then transferred the lens onto an FX camera using the same settings and distance, the butterfly would remain the same size and the DoF the same depth but the FoV would be bigger (so you would see more 'other stuff' round the outside of the butterfly). To then get the butterfly the same size on the FX (i.e. filling the sensor), you would have to use a 50% longer lens so the DoF would be shallower.

Upside of DX? If both cameras are 12MP using the same lens, there are more pixels on the butterfly using the DX and the butterfly is larger on the sensor at a given distance with a deeper DoF. DX best for macro.

I think the confusion is that there are 2 different meanings being used for the generic DOF.
* Absolute DOF - which remains the same for any specified focal length and aperture.
* Relative DOF - with includes the FOV + FL + Aperture.