Does Depth Of Field Change Between FX And DX Lenses?

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Does Depth Of Field Change Between FX And DX Lenses?

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NikoDoby

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This came up in another thread and I could see the debate start to get things off topic so here we are in it's own thread now.

Yes sensor size does effect DOF (Depth of Field) or how much of a picture is in focus. A point & shoot camera will have more of the picture (subject and background) in focus even if you use a low f number like f2.8. A larger sensor will give you less of the picture (shallow DOF) at f2.8. That's why medium format cameras can give you amazingly shallow DOF for isolating the subject from the background.

Now depending on how you measure it, DOF can change when comparing an FX camera and a DX camera. However a 50mm lens is still a 50mm lens regardless of whether it is FX or DX. With a DX lens you just get less of the image.

My original statement regarding DOF was in regards to using the SAME camera body and only switching from FX lenses to DX. All things being the same would mean the same DOF. You wouldn't get medium format like DOF if you could for example mount a medium format lens to a D90. So putting an FX lens on a D90 will not change your DOF.

So let the crazy math calculating begin!

Posted 2 years ago #
spraynpray

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Thanks for this Niko, this forum seems to have a fairly narrow band of acceptance for what is on or off topic and I just keep getting it wrong.

So, if I have a 50mm lens on my D5000 it magnifies the image x 1.5 so is 75mm yes? Does the depth of field I get relate to a 50mm on an FX, a 75mm on an FX, a 75mm on a DX or something else entirely?

I don't suppose you have a handy link to a calculator or table for this do you?

Posted 2 years ago #
PB PM

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I know the feeling spaynpray. Anyway on topic. ;-) A big issue even between DX and FX is focal length. The amount of difference could be hard to figure out, for that very reason. Do you base your calculation on the rated focal length or the approximate focal length?

So, what I mean is, should be try and see if the depth of field is the same between a 35mm DX lens and a 50mm FX lens? Or 50mm on both? The best way to get any idea is to take a series of shots with an FX body, some on crop mode and some not, of the same scene from a tripod.

Posted 2 years ago #
NikoDoby

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I fail to see how putting this in it's own thread is a bad thing? Spray you "complained" about how it's difficult to search for things around here and this is exactly why we try to keep things "on topic".

ANYWAY if you guys have a problem with how we moderate around here then contact me directly via email instead of bringing it up in the forums.

Forum Rule #1 reads in part, "Questioning admin or moderators actions should be done via PM or those post will be deleted." Although you have to email me instead because PM (private messaging) does not work.

So Now back on topic.

Posted 2 years ago #
adamz

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spranay - You are confusing two things mm and f-stop IMHO

Posted 2 years ago #
spraynpray

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I'm prolly confusing lots of things adamz! What I am thinking is that there is probably a fixed or predicatable relationship between some parameters such as focal length, aperture and sensor size that would make it clear for us (even for me).

PB, I think the crop mode thing is a red herring as I understand it is simply a crop of the full size (FX) image).

Posted 2 years ago #
PB PM

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Take away focal length for just a second spraynpray, don't worry about crop factor in terms of field of view or magnification (considering there is no magnification from crop factor). A 300mm lens is 300mm on DX or FX, only the field of view changes.

I don't know how it works from a mathematical point of view, but as I noted in the other thread, I've seen the difference in practice, both on smaller sensors and the difference between shoot with 35mm film and DX sensor cameras. I noted film because the only FX camera I have used was a D700 with a 24-70mm F2.8 demo unit in a store, but the characteristics should be the same regardless. The more you stop down the less noticeable the difference is, and it also depends on the lens as well. The difference is more easily seen in shots taken at wide apertures like F1.4, F1.8 and F2.8 Once you go beyond that the difference starts to gets hard to determine, as the difference is in a matter of half a millimeter or so.

spraynpray said:
PB, I think the crop mode thing is a red herring as I understand it is simply a crop of the full size (FX) image).

Not really, you are creating the same effect. The sensor technology itself has nothing to do with the depth of field, it is the size of the exposed area. So even an FX sensor in crop mode will see the same effect. If you could make a crop of the FX sensor in the size of a P&S sensor you would get the same depth of field as a point and shoot.

Posted 2 years ago #
solid_liq

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Well, according to your diagram:

The diameter of a DX sensor is:
(23.6^2 + 15.8^2)^(1/2) = 28.4mm

The diameter of an FX sensor is:
(36^2 + 23.9^2)^(1/2) = 43.21mm

Keeping that in mind, and considering the crop on an FX sensor for a DX lens is going to end up rectangular (or should), then the image sensor area used on the FX sensor should equal the image sensor area used on a DX sensor unless the camera isn't set to crop mode. In that case, instead of an imager width of 23.6mm, the extremum of the width of image sensor gathering light from the DX lens would instead be 28.4mm. This would shorten the DOF slightly I would imagine relative to using the same lens on a DX camera.

I haven't taken a course in Optical Physics (yet) so this is just speculation.

Thoughts? (btw, there's some of that "crazy math" you seemed to be looking for ;)

Posted 2 years ago #
solid_liq

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Oh, above, that should be "Diameter of a DX/FX sensor LENS PROJECTION UPON THE SENSOR AREA"

Posted 2 years ago #
jonnyapple

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Try this, spraynpray. He's got some other great tutorials, too.
http://www.cambridgeincolour.com/tutorials/depth-of-field.htm

edit: I'll summarize for those who don't feel like going there. Basically, imagine your in-focus plane (sphere? depends on lens) is made of plexiglas. With a marker, go and draw a box that represents the edges of the frame for your shot. No matter which focal length or format you use (DX, FX, medium format, or whatever), for a given f-number you get the same depth of field. Does that clear it up for you? (I have to admit I'm still having trouble wrapping my head around it.)

re: moderation. You should be flattered that Niko moved your comment to its own thread, IMO. I'm always going off-topic and all the mods do is laugh at me because who's going to search for nonsense? ;-)

Posted 2 years ago #
ShadeofBlue

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Depth of field is actually a much more complicated subject than people give it credit for. In a technical sense, your image is only in perfect focus at one distance. Various factors determine how severely the image gets out of focus as you move away from that distance.

Now, the size of your sensor has no effect on the depth of field, since it's just capturing what the lens projects onto it. However, if you have a dx sized sensor, you are likely to move somewhat farther away from your subject when you take your photo compared to using an fx sensor (if you don't you'll probably crop away part of the photo you wanted to keep). Depth of field increases as your subject gets farther away, so to take a similar photo on the two different sized sensors, the dx one is going to have more depth of field. You could also use a lens with a wider focal length instead of moving back with your dx camera, which would also increase the depth of field. This effect is even more drastic on point and shoots where almost everything is in focus because the lenses have such a small focal length (I'm referring to the actual, physical focal length, not the 35mm "equivalent" that is usually stated on the box).

Depth of field is somewhat subjective, though. It depends on what you consider in "acceptable focus." Like I said earlier, only one distance is technically in perfect focus. Since dx cameras tend to have smaller pixels, they will also see the lack of focus at smaller changes in distance than fx cameras. This effectively decreases the depth of field (if you are printing large enough/pixel peeping). Also, if you are only printing small, like say 4x6, then you won't notice things getting out of focus as quickly, and you've increased your depth of field compared to a large print.

This is probably more information than you want. Suffice to say, at a given f-stop, focal length and subject distance, the depth of field is the same no matter the size of the sensor. You will tend to see larger depths of field on dx cameras, though, since you have to change your shooting habits/lens to make sure your photo fits in the smaller sensor.

Posted 2 years ago #
spraynpray

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Great link as usual jonny. Most of it was already well understood by me but what it did clarify well was why 300mm is a 450mm on the DX in short words, and therefore (and what others before on this thread have tried to get me to see [rolleyes]) why it is the magnification that is different not the DoF in reality because the DoF is fairly constant with image size.

So I unnerstan it now, cropping an FX image into a DX size does give the same effect as using a DX camera.

Marvelous, thanks to all.

Posted 2 years ago #
PavelR

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ShadeofBlue, nice explanation. I thought about the same answer - circle of confusion differs between print sizes and viewing distance of an eye looking at the picture. Thus comparing DOF of DX and FX is pretty tough job, you need to specify all other conditions to find out how different will be the DOF on scene captured by DF and FX camera.
The most used difference in DOF between FF and crop camera is using different focal length to achieve the same FOV.
Pretty good comparison I saw in Canon world, 400mm on FF and 300mm on 1.3x crop camera (at the same f-stop). The result pictures surprised me - the difference is so much visible... (I'm sorry I'm not able to find the link to it.)

Posted 2 years ago #
kanuck

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The most important thing to consider about depth of field is that there is significantly less for images shot with the DX format. 24 X 16 compared to 36 X 24. Therefore, I used to just adjust by making sure that I was staying sufficiently deep and used values for the next larger lens aperature. I use FX now so F11 would have been F8 when I had my D300 before.

You have to be careful of diffraction of course as well or else its good bye resolution! Those once straight lines will shoot out at different directions on you and reach the digital sensor and iris diaphram at random times causing light waves to shift and bother others. The general overall softing will just completely ruin an image.

Posted 2 years ago #
spraynpray

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"ShadeofBlue, nice explanation."

Indeed.

"The most used difference in DOF between FF and crop camera is using different focal length to achieve the same FOV."

eggzackerly.

Posted 2 years ago #
NikoDoby

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I'm disappointed jonny. I was hoping at least YOU would throw in some of those crazy equations to scare us away :^)

Here are some visual samples
http://www.have-camera-will-travel.com/field_reports/full_frame_vs_crop_sensor_-.html

http://www.naturfotograf.com/D3/D3_rev06.html

And here is the crazy math side of it all
http://www.normankoren.com/Tutorials/MTF6.html

They don't call it the circle of confusion for nothing :^)
Clear as mud right? Excellent,now lets never speak of DOF between DX and FX lenses again!

Seriously though here's a pretty simple explanation for those who are still lost.
http://www.bobatkins.com/photography/tutorials/crop_sensor_cameras_and_lenses.html

Posted 2 years ago #
jonnyapple

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NikoDoby said:
I'm disappointed jonny. I was hoping at least YOU would throw in some of those crazy equations to scare us away :^)

Listen, Niko. I think that second link is a lot more useful than what I learned in a semester of optics—there wasn't a whole lot in it that was applicable to practical settings like photography. I vaguely remember Jones matrices for polarization and that the focus (haha) was mainly on lasers. It was taught by a spectroscopist, so I guess there's no surprise there.

You know I don't hesitate to post equations when I think I know what I'm talking about. In this case I didn't, but maybe next time. ;-)

Posted 2 years ago #
jbl

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spraynpray said:
So, if I have a 50mm lens on my D5000 it magnifies the image x 1.5 so is 75mm yes? Does the depth of field I get relate to a 50mm on an FX, a 75mm on an FX, a 75mm on a DX or something else entirely?

First, the crop factor does not magnifies the image x 1.5, it just crops. It's like you used a FX body, took the shot, then cropped the center so it looks like 75mm. But it's not.

Depth of field relates to focal length, if every single other setting (aperture, distance between subject and sensor, sensor size) are the same, a longer lens will give you a shorter DOF.

However, that doesn't mean that a 50mm lens on DX that gives you 75mm will give you a shorter DOF because it's still a 50mm lens, you just crop in it.

See it this way: Imagine if you take your 50mm lens on a FX body, take the shot, then crop it at the center, the fact that you crop your image won't change its DOF.

Posted 2 years ago #
spraynpray

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A (rare) moment of clarity has occurred. What I tried to explain (badly) was this:

The reasoning so far has been related to the DoF using the same lens, same subject to camera distance and same aperture so the only difference is the crop factor (magnification).

Try this: If you take a picture of a butterfly at a given distance that fills the image (call it filling the sensor if it helps) completely with a DX camera, to take the same picture where the butterfly fills the image at the same distance using an FX camera you will have to use a longer focal length to fill the image so the DoF will be shallower. Looked at the other way round, the DX has a practically deeper DoF than the FX. If I could underline or use bold on this forum, the word practically would be emphasised here.

Voila. Comments please.

Posted 2 years ago #
PB PM

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Other than the fact that you still don't understand that the crop factor is not magnification you've got it about right. To clarify though, 18mm is 18mm on DX or FX, it is only the field of view that changes. The focal length is measured by the distance from the sensor to the element in the lens. A 300mm lenses rear element is 300mm from the sensor, film, etc, thus it is unchanged by crop factor.

If you are talking about equivalent focal lengths, the depth of field is different, because you are using a different focal length. What people are saying is that depth of field, at the same focal length, on DX and FX results in a different depth of field. Read jbl's post for more info.

Posted 2 years ago #
spraynpray

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"The reasoning so far has been related to the DoF using the same lens, same subject to camera distance and same aperture so the only difference is the crop factor (magnification)."

Perhaps what I should have said was 'apparent magnification' i.e. size of image on the sensor. In order to maintain a full sensor size butterfly at the same distance you would have to use different focal lengths which would result in different DoF's.

Anyway, I am sure I do understand it now, perhaps I am just losing the plot on explaining that.

Thanks PB.

Posted 2 years ago #
jonnyapple

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I think you've got it, spraynpray. For future reference you can bold (embolden?) words and italicize with strong and em tags:

I think you've got it, spraynpray. For future reference you can <strong>bold</strong> (embolden?) words and <em>italicize</em> with strong and em tags:

Posted 2 years ago #
NikoDoby

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PB PM said:
What people are saying is that depth of field, at the same focal length, on DX and FX results in a different depth of field.

That's not what I'm saying. Depth of field is the same it's the field of view that is different. These photos are from the first link I posted above.

"DX" camera with 105mm lens at f2.8

"FX" camera with 105mm lens at f2.8

Also just because you use a telephoto lens doesn't mean that you get less DOF then with a wide angle lens. At the same aperture you get the same DOF but just a different perspective.

http://www.luminous-landscape.com/tutorials/dof2.shtml

Posted 2 years ago #
PB PM

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lol Niko, I love how you pull a quote about something not even related to any of your posts and then make a big deal about it. I should have be clearer. What I mean is that there are people who say that depth of field is different at the same focal length, on DX and FX results in a different depth of field.

Posted 2 years ago #
NikoDoby

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??? how does it not relate? I thought we were talking about DOF and how it relates to lenses? The above photos are from the same focal length, one a "DX" and one "FX". The result is the SAME depth of field.

The 19, 20, 21, and barely 22 are "readable" on both photos. With 20 being the sharpest and focus point for each.

And I'm "making a big deal" because what you are saying isn't correct.

Posted 2 years ago #