Depth of Field Due to Sensor Size

[Madwand1]

Originally posted by: Zenmervolt
Actually, focal length, per se, has nothing to do with DOF.

Nonsense. If you're going to post attempted corrections, get your facts straight. Read the previous posts, look at the math.

 

[ElFenix]

Originally posted by: tfinch2

Originally posted by: destrekor

Originally posted by: tfinch2
Sensor size affects DOF.

A FF image at f/3.5 will have less DOF than my 4/3 2x crop sensor at f/3.5.

That's why you can have a P&S with a tiny sensor at f/2.8 and the DOF is very big.

may I ask how so?

http://www.cambridgeincolour.com/tutorials/digital-camera-sensor-size.htm

according to that, the short answer is yes, and the long answer is no, but, "[a]s sensor size increases, the depth of field will decrease for a given aperture (when filling the frame with a subject of the same size and distance). This is because larger sensors require one to get closer to their subject, or to use a longer focal length in order to fill the frame with that subject."

 

[fuzzybabybunny]

Originally posted by: Madwand1
I think she was talking about P&S digicams. In context, I don't see a significant problem with her statement; it's more right than wrong. Read my previous note if you don't see the point -- focal length is the dominant factor in DoF, other factors constant. Digicams have small sensors and short focal lengths, hence greater DoF.

Where she went wrong is stating "some cameras" and not specifying what "some cameras" were.

 

[Madwand1]

When you change the distance, bets are off, because distance itself is a factor in DoF calculations -- you can't do a fair comparison of focal length X and focal length Y when you change distance to compare the two, as distance itself is a significant factor.

Of course it doesn't hurt to understand the effects of distance and focal lengths combined, which is the point of those notes. However, they just confuse the sensor size issue.

If you don't change distance, and change sensor sizes, then to get the same field of view, you must change the focal length. And this does affect the DoF, significantly. Again, look at the actual formulas and their derivation for the most reliable explanations.

 

[Lucky]

But I predict this will change. In 5 years and all DSLRs will be sold with FF sensors. Specialized lenses like Canon's EFS series will be history.

The smaller sensor sizes may have begun due to low yields and high costs on FF sensors, but there are quantifiable benefits to smaller sensors. They have enabled lenses like Nikon's 17-55 and 12-24 to have compact size and still have excellent or reasonably fast aperture sizes.

For professional sport shooters, it's a boon to have that 70-200 2.8 become (essentially) a 300 2.8. Or for that 300 to become a 450 2.8.

 

[Madwand1]

Originally posted by: Lucky
For professional sport shooters, it's a boon to have that 70-200 2.8 become (essentially) a 300 2.8. Or for that 300 to become a 450 2.8.

It doesn't actually do that. A sensor crop is a sensor crop; it doesn't get you magnification for free. If you have the same lens, the same pixel density, and essentially the same sensor but different sensor sizes, then you could get the same pictures from the bigger sensor by chopping off pixels in the edges, whereas you can't do the reverse.

If you get magnification from a sensor, it's due to greater pixel density, not due to the crop factor. That said, cropped DSLRs often have greatest pixel density among "serious" cameras, which is one reason that they're favored by such photographers.

However, if you compared for example a 300D/10D to a 1Ds II, and thought that the 1.6x crop factor would give you a magnification advantage, you'd be dead wrong -- the 1Ds II has greater pixel density; you could crop its image down to the 300D sensor size and still have greater magnification in the result.

 

[ZOXXO]

Originally posted by: GTaudiophile


Nope. Canon's EOS-5D and EOS-1Ds MkII have Full-Frame (35mm) sensors. Yields are MUCH lower/cost is MUCH highter than the smaller 1.6 Crop counterparts. But I predict this will change. In 5 years and all DSLRs will be sold with FF sensors. Specialized lenses like Canon's EFS series will be history.

Interesting prediction.

I can almost see the full frame misnomer becoming the standard for consumer models and a new sensor and lens combo taking the pro market.

We shall see.

 

[jiwq]

okay so to clear things up, if you had a ff camera and a 1.6x crop camera and took a picture of a subject with a 50mm f/1.8 lens at f/1.8, would you get the same amount of background blur?

 

[fuzzybabybunny]

Originally posted by: Madwand1

Originally posted by: Lucky
For professional sport shooters, it's a boon to have that 70-200 2.8 become (essentially) a 300 2.8. Or for that 300 to become a 450 2.8.

It doesn't actually do that. A sensor crop is a sensor crop; it doesn't get you magnification for free. If you have the same lens, the same pixel density, and essentially the same sensor but different sensor sizes, then you could get the same pictures from the bigger sensor by chopping off pixels in the edges, whereas you can't do the reverse.

If you get magnification from a sensor, it's due to greater pixel density, not due to the crop factor. That said, cropped DSLRs often have greatest pixel density among "serious" cameras, which is one reason that they're favored by such photographers.

However, if you compared for example a 300D/10D to a 1Ds II, and thought that the 1.6x crop factor would give you a magnification advantage, you'd be dead wrong -- the 1Ds II has greater pixel density; you could crop its image down to the 300D sensor size and still have greater magnification in the result.

Agreed with all your points, however price of course needs to be considered.

A 1Ds MKII can certainly be cropped down to around 8MP to give you about the same magnification and field of view as a 1.6x crop camera when using the same lens, but then again the 1DsMKII is $6,000-$7,000, whereas the 1.6x crop bodies range from $1500-$500. Are you getting extra reach with a 1.6x crop body for free? Not really. But for cheap? Yup. Therefore it IS a boon for anyone who doesn't have enough money to spend on a 1DsMKII.

Oh, how I would LOVE to own a 1DsMKII... cropped to 8MP you have about the reach of a 1.6X crop camera, and at full 16MP you've got stunningly detailed shots.

 

[fuzzybabybunny]

Originally posted by: jiwq
okay so to clear things up, if you had a ff camera and a 1.6x crop camera and took a picture of a subject with a 50mm f/1.8 lens at f/1.8, would you get the same amount of background blur?

Specify how you're taking the picture.

A 50mm at 1.6x would be equivalent to an 80mm lens' field of view (narrower than 50mm).

Would you be taking the first shot with FF at 50mm, then taking the second shot with 1.6x and 50mm, but backed up to match the field of view as the first shot? In this case no.

 

[jiwq]

Originally posted by: fuzzybabybunny

Originally posted by: jiwq
okay so to clear things up, if you had a ff camera and a 1.6x crop camera and took a picture of a subject with a 50mm f/1.8 lens at f/1.8, would you get the same amount of background blur?

Specify how you're taking the picture.

A 50mm at 1.6x would be equivalent to an 80mm lens' field of view (narrower than 50mm).

Would you be taking the first shot with FF at 50mm, then taking the second shot with 1.6x and 50mm, but backed up to match the field of view as the first shot? In this case no.

standing in the same spot

 

[fuzzybabybunny]

Originally posted by: jiwq

Originally posted by: fuzzybabybunny

Originally posted by: jiwq
okay so to clear things up, if you had a ff camera and a 1.6x crop camera and took a picture of a subject with a 50mm f/1.8 lens at f/1.8, would you get the same amount of background blur?

Specify how you're taking the picture.

A 50mm at 1.6x would be equivalent to an 80mm lens' field of view (narrower than 50mm).

Would you be taking the first shot with FF at 50mm, then taking the second shot with 1.6x and 50mm, but backed up to match the field of view as the first shot? In this case no.

standing in the same spot

Standing in the same spot... someone correct me if I'm wrong... but yes, the depth of field will be the same, BUT the two pictures will be different.

Taken from here: http://www.photo.net/learn/optics/dofdigital/

"I'm sure some people will say, OK, but what if you don't take angle of view into account. What's the relative DOF if you use the SAME lens on an EOS 10D and a 35mm film body?

Now you run into the problem of what you are comparing to what. The same lens on the two formats will give you different fields of view, so if you enlarge each image to the same size (say 8x12), you won't have the same print so you really can't compare DOFs. If you crop the 35mm negative to give you the same print as the digital image the answer is easy. The DOF in the cropped 35mm print and digital image print will be exactly the same. You're using the same lens and same size image (cropped 35mm or digital), so you get exactly the same DOF."

 

[Madwand1]

Originally posted by: fuzzybabybunny
Agreed with all your points, however price of course needs to be considered.

Of course. If price didn't matter, then there'd be no great reason for cropped sensors. Price is the most significant advantage. I also think that this factor will probably be significantly present for a long time. I.e. entry-level DSLRs going full-frame is probably a long long ways off. As to affordability, a 5D is already "affordable" to many photographers, and people generally expect that price to come down further in the future, and the performance to go up.

But IMO, one reason that full frames cameras aren't more common is the myth of the sensor crop giving free magnification / "reach". Hence the following semi-rant.

It simply doesn't; it's a crop, and IMO that's the best way to understand it. To get it sunk in, do the exercise of looking through the viewfinder of a full frame camera (any film camera will do) and compare the view using the same lens and focal length using a cropped DSLR. The image magnification does not change (beyond slight variations in the viewfinders themselves, which has nothing to do with the photographs). What changes is that much of the outside is lost.

Now, when you take that same image, and fill your monitor / page with it, then of course the cropped DSLR will give you an illusion of greater magnification, because you just did that effectively in post-processing. This of course you can do with any camera by cropping the image and magnifying it in post-processing. It's always been that way, and will always be this way. How much detail is left depends on a number of factors, including your technique, lens quality, and pixel density. How much detail do you need for typical web images? Not much..

The inexpensive DSLR sensors crop for you without you asking or trying, and camera vendors position this as a "magnification factor" because that sounds better. But you're better off understanding what's really going on, including the impact of pixel density.

 

[Zenmervolt]

Originally posted by: Madwand1

Originally posted by: Zenmervolt
Actually, focal length, per se, has nothing to do with DOF.

Nonsense. If you're going to post attempted corrections, get your facts straight. Read the previous posts, look at the math.

I stand by what I posted. Assuming a constant aperture size (note that aperture size is NOT F-number), a constant distance to subject, and a constant negative size, the focal length of the lens will not affect DOF. A 50mm f2 lens has a 25mm aperture opening and it will have the same DOF properties as a 100mm f4 lens (which also has a 25mm aperture).

The issue is that "aperture" is generally referred to by F-number, which is technically inaccurate as the F-number is the ratio of aperture size over focal length.

The reason that "focal length" can be used in the math in your link is because it is a proxy for the changes in absolute aperture size. The math assumes a constant F-number, and therefore the change in focal length is used as a convenient proxy for the absolute aperture size, which would otherwise need to be back-calculated for the equations. This is why the math specifies that the "DOF ~ Circle of Confusion/[focal length]^2" can only be used to compare two different lenses used at the same F-number. If you compare the lenses at different F-numbers, focal length can no longer be used as a proxy for the change in aperture size.

If you're going to post mathematic proofs, at least understand basic math.

ZV

 

[ElFenix]

Originally posted by: jiwq
okay so to clear things up, if you had a ff camera and a 1.6x crop camera and took a picture of a subject with a 50mm f/1.8 lens at f/1.8, would you get the same amount of background blur?

from the same distance away? yes. because you'd have to crop off the FF image to get the same image. i think. i might be wrong.


as zenmervolt has said, DOF is related to absolute aperature size. the larger the absolute aperature, the shorter the DOF. the smaller the absolute aperature, the larger the DOF. make a pinhole camera and you'll see it. i know that one for a fact.

 

[Madwand1]

Originally posted by: Zenmervolt
Assuming a constant aperture size (note that aperture size is NOT F-number), a constant distance to subject, and a constant negative size, the focal length of the lens will not affect DOF. A 50mm f2 lens has a 25mm aperture opening and it will have the same DOF properties as a 100mm f4 lens (which also has a 25mm aperture).

Please prove your assertions and provide formulas for DoF calculations that do not incorporate focal length directly or indirectly.

 

[fuzzybabybunny]

Originally posted by: Zenmervolt
Assuming a constant aperture size (note that aperture size is NOT F-number), a constant distance to subject, and a constant negative size, the focal length of the lens will not affect DOF. A 50mm f2 lens has a 25mm aperture opening and it will have the same DOF properties as a 100mm f4 lens (which also has a 25mm aperture).

Ehhhh... I'm not so sure, but I'm far from saying "you're wrong."

Take the Canon 50mm f/1.0 lens. 50mm aperture, EXTREMELY shallow DOF. Then take a 200mm f/4, like the 200mm end on the 70-200mm f/4L. Still a 50mm aperture, but it will not have the same EXTREMELY shallow DOF as the 50mm f/1.0

 

[brandonbull]

Originally posted by: fuzzybabybunny

Originally posted by: Zenmervolt
Assuming a constant aperture size (note that aperture size is NOT F-number), a constant distance to subject, and a constant negative size, the focal length of the lens will not affect DOF. A 50mm f2 lens has a 25mm aperture opening and it will have the same DOF properties as a 100mm f4 lens (which also has a 25mm aperture).

Ehhhh... I'm not so sure, but I'm far from saying "you're wrong."

Take the Canon 50mm f/1.0 lens. 50mm aperture, EXTREMELY shallow DOF. Then take a 200mm f/4, like the 200mm end on the 70-200mm f/4L. Still a 50mm aperture, but it will not have the same EXTREMELY shallow DOF as the 50mm f/1.0

Cold with solution?

 

[fuzzybabybunny]

Originally posted by: brandonbull

Originally posted by: fuzzybabybunny

Originally posted by: Zenmervolt
Assuming a constant aperture size (note that aperture size is NOT F-number), a constant distance to subject, and a constant negative size, the focal length of the lens will not affect DOF. A 50mm f2 lens has a 25mm aperture opening and it will have the same DOF properties as a 100mm f4 lens (which also has a 25mm aperture).

Ehhhh... I'm not so sure, but I'm far from saying "you're wrong."

Take the Canon 50mm f/1.0 lens. 50mm aperture, EXTREMELY shallow DOF. Then take a 200mm f/4, like the 200mm end on the 70-200mm f/4L. Still a 50mm aperture, but it will not have the same EXTREMELY shallow DOF as the 50mm f/1.0

Cold with solution?

huh? Cold... solution... what?

 

[Madwand1]

DON'T GO THERE. You'll never get the time that you spend reading the "Cold with solution thread" back.

Edit: Explanation: "Cold with solution" is an ATOT short form for "huh?" based on an old thread with exemplified it.

I got FBB's cautious post, and think it's time for ZV to respond.

 

[fuzzybabybunny]

Originally posted by: Madwand1
DON'T GO THERE. You'll never get the time that you spend reading the "Cold with solution thread" back.

Edit: Explanation: "Cold with solution" is an ATOT short form for "huh?" based on an old thread with exemplified it.

I got FBB's cautious post, and think it's time for ZV to respond.

Ah, ok. At first I was like "what does this have ANYTHING to do with CPU cooling?" I actually remember that thread somewhat, but I wasn't aware the phrase turned into an... expression of confusion.

When I get into these kinds of disagreements or whatever I tend to do real life thought experiments. From the equations it certainly looks like focal length affects DOF, but I guess proven through thought or experimentation just makes it that much more concrete.

On a side note, does all this even matter? Is this DOF thing going to be applicable in any way, to the debaters and the listeners? Suppose a 200mm f/4 has greater DOF than a 50mm f/1.0. Uhhhh.... great?

*goes out and shoots pictures*

 

[JulesMaximus]

Originally posted by: AMDUALY
Yeah, this is what i heard last time in another discussion so i believed it to be true. Then I asked my prof if we should also pay attention to the sensor size and she looked at me like i was crazy.

Most consumer level digital SLR cameras have a 1.6x crop factor (or close to that 1.5x for Nikon). The full frame DSLR cameras are uber expensive, like $3.5k and up. That's pretty much it. I don't understand why you are asking the question unless you are using P&S cameras in your class.

 

[Zenmervolt]

Originally posted by: fuzzybabybunny

Originally posted by: Zenmervolt
Assuming a constant aperture size (note that aperture size is NOT F-number), a constant distance to subject, and a constant negative size, the focal length of the lens will not affect DOF. A 50mm f2 lens has a 25mm aperture opening and it will have the same DOF properties as a 100mm f4 lens (which also has a 25mm aperture).

Ehhhh... I'm not so sure, but I'm far from saying "you're wrong."

Take the Canon 50mm f/1.0 lens. 50mm aperture, EXTREMELY shallow DOF. Then take a 200mm f/4, like the 200mm end on the 70-200mm f/4L. Still a 50mm aperture, but it will not have the same EXTREMELY shallow DOF as the 50mm f/1.0

Same focusing distance? If you're shooting a subject at 5 meters with the 50mm f/1 and a subject at 50 meters with the 200mm f/4, you will have a larger DOF for the photo taken with the 200mm f/4 because of differing focusing distances.

I'm interested now. I still think I'm mathematically correct, and several "margin note" comments in what I've read about DOF lead me to think I'm right, but I think this calls for me to take a roll of film and actually test this because you have me doubting. I will set up the tripod tonight, run through a roll of film, get a photo CD with it and post the results tomorrow. I'll run my 35mm and 135mm lenses through their aperture ranges and I'll probably prove myself wrong, but it'll be an interesting thing to test.

As for a calculation without focal length included, I haven't seen one. However, I've also never seen one that didn't also include F-number, and focal length is necessary to back-calculate the absolute aperture diameter from F-number, so I don't think this proves anything either way, call the equation part a draw.

ZV

 

[fuzzybabybunny]

Originally posted by: Zenmervolt

Originally posted by: fuzzybabybunny

Originally posted by: Zenmervolt
Assuming a constant aperture size (note that aperture size is NOT F-number), a constant distance to subject, and a constant negative size, the focal length of the lens will not affect DOF. A 50mm f2 lens has a 25mm aperture opening and it will have the same DOF properties as a 100mm f4 lens (which also has a 25mm aperture).

Ehhhh... I'm not so sure, but I'm far from saying "you're wrong."

Take the Canon 50mm f/1.0 lens. 50mm aperture, EXTREMELY shallow DOF. Then take a 200mm f/4, like the 200mm end on the 70-200mm f/4L. Still a 50mm aperture, but it will not have the same EXTREMELY shallow DOF as the 50mm f/1.0

Same focusing distance? If you're shooting a subject at 5 meters with the 50mm f/1 and a subject at 50 meters with the 200mm f/4, you will have a larger DOF for the photo taken with the 200mm f/4 because of differing focusing distances.

ZV

Yup, same focusing distance. 5m for the 50mm and 5m for the 200mm, or Xm for the 50mm and Xm for the 200mm. It involves cropping the 50mm image to match the FOV of the 200mm image (a pretty substantial crop, and you may run into resolution limitations?), which is making it hard for me to visualize concretely which would have the greater DOF. A test would be great, but from general experience, a large part of me just feels that the 200mm f/4 will have greater DOF.

 

[ElFenix]

Originally posted by: fuzzybabybunny
Yup, same focusing distance. 5m for the 50mm and 5m for the 200mm, or Xm for the 50mm and Xm for the 200mm. It involves cropping the 50mm image to match the FOV of the 200mm image (a pretty substantial crop, and you may run into resolution limitations?), which is making it hard for me to visualize concretely which would have the greater DOF. A test would be great, but from general experience, a large part of me just feels that the 200mm f/4 will have greater DOF.

i think that after the crops, the picture taken with the 50mm lens will look exactly the same as long as the absolute apertures are the same, the focusing distances are the same, and overall exposure is the same.