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Sensor size vs max DOF


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#1 acroporas

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Posted 10 September 2005 - 06:03 AM

In the recent thread about the difficulties of extreme macro (i cant find a link) it was noted that the smaller pixels of a small sensor camera cause a small sensor camera to pick up on the reduced resolution at small aperatures due to diffraction sooner than a larger sensor/larger pixels do.

Thus a larger sensor will allow you to use a smaller aperature than a small sensor.

But then it is also well accepted that a larger sensor produces less DOF than a small sensor at the same aperature.

So in order to get the same DOF as a the small sensor camera the large sensor camera MUST use a smaller aperature.

So which effect wins and allows you to have greater DOF before diffraction starts to pose a probelm?

But then to make it even more complicated the different sensor size means that given any subject the magnification is greater on a larger sensor. For example, a subject that it 2:1 on a small sensor, would be 3:1 on a larger sensor. Increased magnification resultes in decreased DOF. Is this on top of the normal decreased DOF of a larger sensor, or is this just the reason that larger sensors allways have less DOF?
William

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#2 MikeVeitch

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Posted 10 September 2005 - 06:11 AM

Huh? Stop thinking William, go take some pictures....:unsure:

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#3 acroporas

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Posted 10 September 2005 - 06:18 AM

Good idea Mike. That's what I was trying to do. I ask the question and make everyone else think about it for me..... :unsure:
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#4 randapex

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Posted 10 September 2005 - 09:11 AM

Hi acroporos,

In my limited experience with the 2x TC last week, maybe 10 dives, here's how I'd answer you:

DOF at those magnifications is so thin, that even if you could, say double it, by any means you choose. the real world difference isn't worth talking about. By that I mean, I'm pretty much convinced that I'd be better off at wider apertures so my lighting issues could be resolved. And apparently at the same time, reduce diffraction. So if the question is the DOF differences between a full and cropped sensor noticeable, I'd say they'd be miniscule with a 105mm and 2x TC. There is no DOF to begin with, so, two times 0 is still...0 :unsure:

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#5 james

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Posted 10 September 2005 - 09:22 AM

Hi William,

I'm with Mike and Rand - go take some photos - none of this really matters much.

But then it is also well accepted that a larger sensor produces less DOF than a small sensor at the same aperature.


I personally don't think this is correct. DOF is only affected by the focal length and aperture size. The myth got started with compact cameras that say you are shooting at f10 but you are really shooting through an f40 aperture. Ever wonder why those cameras don't go above f10 or f11?? That's it. They have an extremely short focal length lens and a tiny aperture, hence the better DOF.

A smaller sensor than 35mm (above water) just meant that people had to step back further, and that added distance at the same f-stop also means more DOF.

So...bigger pixels ARE better, as we learned in previous threads due to diffraction. So a FF sensor with low megapixels is the best right...:-) Well then you should go out and buy a 1Ds.

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#6 Phil Rudin

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Posted 10 September 2005 - 11:21 AM

These issues of DOF are common to all cameras and do not releated only to digital. Film cameras do the same thing. A M/F 6 x 7 film camera has a film area Appox. four times larger than 35 mm.
If you use a 45 MM lens on the 6 X 7 camera you get the same Angle of view as a 22.5 mm lens on 35 mm film. Since we all know that a 22.5 lens would have greater DOF than a 45 mm lens at the same aperture, the larger (6 x 7) format has less DOF. This is one of the reasons you don't see many U/W med/format systems. The same thing applies to digital sensors.

A 100mm lens on a 35 mm full frame sensor at life size (1:1) creates appox. the same size image as a 50 mm lens on a sensor which has a X2 crop factor compaired to 35 mm, because that sensor is one fourth as large. The diffrence is that the 50 mm lens only has to be at 1:2 to give the same size image as the 100 mm lens on 35 mm. Given that the photo is taken while both lenses are a F-22, the 1:1 lens is going to have less DOF than the 1:2 lens. Just as a 1:1 extension tube has less DOF than a 1:2 tube.

Life size or 1:1 remains constant for all format sizes. If you take life-size (1:1) photo on the 6X7 film the object is the same size in life as on the film. If you then cut a 35mm size piece of film from the 6X7 the image is still life size and a 1.6 (Canon) or 1.5 (Nikon) or 2X (Olympus) crop from that is still life size.

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#7 acroporas

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Posted 10 September 2005 - 01:19 PM

Hi William,

I'm with Mike and Rand - go take some photos - none of this really matters much.
I personally don't think this is correct.  DOF is only affected by the focal length and aperture size.  The myth got started with compact cameras that say you are shooting at f10 but you are really shooting through an f40 aperture.

Cheers
James

<{POST_SNAPBACK}>


You cant be serious James. Do you really think the F numbers on digital cameras are some sort of equivolent f number?

The f number on any camera is the focal length in mm divided by the size of the aperature in mm.
William

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#8 james

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Posted 10 September 2005 - 01:27 PM

At f11 on a compact camera, the aperture is about 4 times smaller than the equivalent aperture on a 35mm lens so the same as an f44 sized aperture on a 35mm cam. The DOF and diffraction are closely related to the aperture size.

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#9 acroporas

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Posted 10 September 2005 - 01:52 PM

James, you really have fun with this equivalent measurements.

The F number is defined as the focal length in mm divided by the size of the aperature in mm with no exceptions.

Look at the lens on a p&s camera. It will say something like 6-36mm 1:2.8-3.5. They are not listing equivalent f number (what ever that might mean). They are telling you that at 6mm the aperatue is 6/2.8mm.

Are you saying that you think if you take 2 camera one a p&s and one a dSLR and point them at the same wall. Set both to iso100 f/11 and 1/60th second they the P&S will have a much darker exposure?
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#10 Jolly

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Posted 10 September 2005 - 02:22 PM

aperture is not a measurement of "size", it is a relation number. f11 on a digicam and a 35mm cam result in the same exposure brightness.

magnification is not a function of the sensor size. Just browse thru the "wonderful" 5D / macro thread :unsure:
same lens (for example a 1:1 macro lens) delivers the same magnification on different sensor sizes.

DOF remains bound to aperture / f-number and true lens' focal length (not equivalent which is just a help to to give an idea of the FOV associated with 35mm focal length).

the other point: moving from larger sensors to smaller sensors while maintaining the FOV - you have to use a lens with shorter focal length. On the other hand the circles of confusion become smaller too. But the shorter focal length always outperforms the effect of smaller COCs. This means (same aperture number): more DOF.

It's difficult to say if smaller apertures on 35mm (allowed by less diffraction) can outperform something because I have the impression that those diffraction calculations (and which is optimum aperture) won't exactly predict what comes out in real life. Maybe because lenses are built to different (number of aperture blades, different sensor resolutions, etc.).

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#11 herbko

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Posted 10 September 2005 - 02:39 PM

aperture is not a measurement of "size", it is a relation number. f11 on a digicam and a 35mm cam result in the same exposure brightness.


Julian

<{POST_SNAPBACK}>


A quick google search:
http://www.answers.com/aperture&r=67

Aperture can be stated in units of length. It's perfectly correct to give an answer "5mm" when asked what aperture was set on a shot. Of course, no photographer would ever say it that way. It's also correct to give it in F-numbers which is the ratio of the focal length to lens diaphragm opening. When stated that way an aperture setting at a given F-number will give the same exposure rate independent of lens focal length or sensor size.
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#12 Rocha

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Posted 10 September 2005 - 02:57 PM

But then it is also well accepted that a larger sensor produces less DOF than a small sensor at the same aperature.

<{POST_SNAPBACK}>


Ok, let me see if I got this straight. I found this site, a depth of field calculator:

http://www.dofmaster.com/dofjs.html

I then calculated the DOF of a 35mm camera and the DOF of a 1.5 crop camera. Using the same lens (60mm), same aperture (f2.8), same distance to subject (30cm), the 35mm sensor has more DOF (0.34cm on 35mm vs 0.23cm on 1.5 crop). *But*, if you decrease the lens to subject distance to fill the larger frame with the same subject (in this case to 20cm, or 30cm divided by the crop factor of 1.5), then you have a smaller DOF on the larger frame.

I think this is a disadvantage of larger sensors. Given the same lens, you will have less working space between the camera and the subject and need smaller apertures to increase DOF. My point being, everything is a tradeoff, you gain here and lose there.

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#13 acroporas

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Posted 10 September 2005 - 03:09 PM

aperture is not a measurement of "size", it is a relation number. f11 on a digicam and a 35mm cam result in the same exposure brightness.


Exactly, Where did james come up with this equivalent F number for P&S cameras?

magnification is not a function of the sensor size. Just browse thru the "wonderful" 5D / macro thread  :unsure:
same lens (for example a 1:1 macro lens) delivers the same magnification on different sensor sizes.


If magnification is not a functon of sensor size, what is your defination of magnification. My defination of magnification is sensor size / subject size. Clearly a function of sensor size.

For example the subject is an 11mm shrimp. On a small sensor camera this is 22/11 or 2:1. But on a large sensor camera this is 36/11 or ~3:1

the other point: moving from larger sensors to smaller sensors while maintaining the FOV - you have to use a lens with shorter focal length. On the other hand the circles of confusion become smaller too. But the shorter focal length always outperforms the effect of smaller COCs. This means (same aperture number): more DOF.

It's difficult to say if smaller apertures on 35mm (allowed by less diffraction) can outperform something because I have the impression that those diffraction calculations (and which is optimum aperture) won't exactly predict what comes out in real life. Maybe because lenses are built to different (number of aperture blades, different sensor resolutions, etc.).

Julian

<{POST_SNAPBACK}>


Well then even if it wouldnt work in real life, which effect in theory should be stronger?
William

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#14 herbko

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Posted 10 September 2005 - 03:14 PM

the other point: moving from larger sensors to smaller sensors while maintaining the FOV - you have to use a lens with shorter focal length. On the other hand the circles of confusion become smaller too. But the shorter focal length always outperforms the effect of smaller COCs. This means (same aperture number): more DOF.

It's difficult to say if smaller apertures on 35mm (allowed by less diffraction) can outperform something because I have the impression that those diffraction calculations (and which is optimum aperture) won't exactly predict what comes out in real life. Maybe because lenses are built to different (number of aperture blades, different sensor resolutions, etc.).

Julian

<{POST_SNAPBACK}>


You have to be careful in how you ask the question. Craig Jones and I went back and forth on this awhile back and I think we've managed to converge on the way to look at this question. The best way to look at it is to ask, for the same subject, eg. a 1 cm nudi, a same final print, eg. 24x16", the same diffraction limited resolution, eg 1500 lines across the frame, which sensor has an advantage. For a given resolution, the larger sensor uses a smaller aperture for the same diffraction limited resolution, and this more than offsets the longer focal length lens needed to get the same FOV. The larger sensor has a slight advantage.
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#15 acroporas

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Posted 10 September 2005 - 03:23 PM

Ok, let me see if I got this straight. I found this site, a depth of field calculator:

http://www.dofmaster.com/dofjs.html

I then calculated the DOF of a 35mm camera and the DOF of a 1.5 crop camera. Using the same lens (60mm), same aperture (f2.8), same distance to subject (30cm), the 35mm sensor has more DOF (0.34cm on 35mm vs 0.23cm on 1.5 crop). *But*, if you decrease the lens to subject distance to fill the larger frame with the same subject (in this case to 20cm, or 30cm divided by the crop factor of 1.5), then you have a smaller DOF on the larger frame.

I think this is a disadvantage of larger sensors. Given the same lens, you will have less working space between the camera and the subject and need smaller apertures to increase DOF. My point being, everything is a tradeoff, you gain here and lose there.

<{POST_SNAPBACK}>


Ok I used that websites DOF calculator to make it simpler.

First Digial rebel sensor, lens 60mm, F/16 (The largest aperature I can use on my digital rebel before I notice diffraction limited resolution) and subject distance 1ft.

DOF=.4ft

Then 1Ds sensor, lens 95mm(60x1.6 should be the same FOV) and subject distance 1ft.

In order to get a DOF of .4 ft, you must use F/25

So then my question is, could you use F/25 with a theroretical 6mp FF dSLR?

If not, diffraction was limiting on a 6mp FF dSLR, then a small sensor camera has an max DOF advantage.
William

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#16 james

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Posted 10 September 2005 - 03:25 PM

Thanks for dredging that up Herb. I have talked to Craig about this many times and was convinced that the larger sensor had - all factors considered - better DOF. I just didn't remember how all the factors worked together.

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#17 herbko

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Posted 10 September 2005 - 03:35 PM

Ok I used that websites DOF calculator to make it simpler.

First Digial rebel sensor, lens 60mm, F/16 (The largest aperature I can use on my digital rebel before I notice diffraction limited resolution) and subject distance 1ft.

DOF=.4ft

Then 1Ds sensor, lens 95mm(60x1.6 should be the same FOV) and subject distance 1ft. 

In order to get a DOF of .4 ft, you must use F/25

So then my question is, could you use F/25 with a theroretical 6mp FF dSLR?

If not, diffraction was limiting on a 6mp FF dSLR, then a small sensor camera has an max DOF advantage.

<{POST_SNAPBACK}>


This is a good example to illustrate what I tried to say on the previous post. The diffarction limited resolution is usually given as 1500/N to 1800/N in units of lines/mm where N is the F-number. If you plug what you figured out as the F-number required for the same DOF=.4ft in your example into 1500/N x (width of sensor) , you'll find that the full frame sensor resolves more lines by just a little in this case. The difference is much bigger in going from the small < 1cm point-n-shoot to the 1.6x DSLR's.
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#18 acroporas

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Posted 10 September 2005 - 03:57 PM

Thanks Herb.

I just found this website
http://www.cambridge...photography.htm
it agrees with you. You can get more DOF with a larger sensor, but it takes much more light.

Yet another way a switch to FF would cost more, would need more powerfull strobes....
William

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#19 herbko

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Posted 10 September 2005 - 04:29 PM

Thanks Herb.

I just found this website
http://www.cambridge...photography.htm
it agrees with you.  You can get more DOF with a larger sensor, but it takes much more light. 

Yet another way a switch to FF would cost more, would need more powerfull strobes....

<{POST_SNAPBACK}>


That also depends... by now you must be really tired reading this from me. :unsure:

You'll need lots more light if you want to also get better noise performance. Otherwise, you can just crop the FF or shoot at higher ISO on the FF.

I've got it all worked out. The way I'm going to rationalize myself into buying the 5D is by telling myself that: 1) I'll save money by not having to buy a 10.5mm FE, which does not exist for the Canon. 2) I also won't have to buy a 60mm macro, since the 100mm on the FF will be close to working like a 60mm. See how much money I'll save! :P
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#20 acroporas

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Posted 10 September 2005 - 04:36 PM

Yes but then you need a 150mm macro to replace the functon of your 100.

or even worse, how would I replace the functionality of my 150mm macro? Can you imagine the cost of a 240mm f/2.8 macro lens :unsure:
William

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