Depth of Field Due to Sensor Size

[NaOH]

So I'm taking a digital photography class. My professor was saying how you should look at the megapixel rating when picking the camera. As well as the aperature size. I ask her if the sensor size would limit your creativity (bad choice of words) with your depth of field. Basically I was asking if the sensor size of the camera would limit how much blur you would have in a picture (say you wanted focus on the subject and blur the background without getting super close to the subject). She looked at me confused and asked me if I knew what depth of field was. Then she said that it only relates to the mega pixels. Then she continued. I was left confused. Someone enlighten me.

 

[destrekor]

do you know what depth of field is?
are you speaking in terms of physical sensor size? most censors are the same size, just the amount of light they can capture and process changes. normally they are equivelent to a 35mm film negative (at least in DSLRs). the number of megapixels also do not control depth of field. depth of field is controlled single-handidly by the lens and the aperture in the lens.

edit:
to add.. number of megapixels only increases the size of the raw picture, when unedited. this gives the ability to increase DPI when printing to create a print that is of higher quality than a lower megapixel print of the same size.

 

[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.

 

[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?

 

[NaOH]

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.

 

[vi edit]

Originally posted by: destrekor
do you know what depth of field is?
are you speaking in terms of physical sensor size? most censors are the same size, just the amount of light they can capture and process changes. normally they are equivelent to a 35mm film negative (at least in DSLRs). the number of megapixels also do not control depth of field. depth of field is controlled single-handidly by the lens and the aperture in the lens.

edit:
to add.. number of megapixels only increases the size of the raw picture, when unedited. this gives the ability to increase DPI when printing to create a print that is of higher quality than a lower megapixel print of the same size.

I don't think that's correct. I'm pretty certain a Canon "Elph" SD400 has a much different sensor size than a DSLR like a XT or a 30D.

 

[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

 

[GTaudiophile]

Most DSLRs (Rebel, EOS-20D, D50, D70, etc.) have cropped sensors. Meaning, they are like 60% smaller than their 35mm-sized counterparts. Imagine looking at an object through a hole in cardboard the size of a silver-dollar. Now imagine doing the same looking through a hole the size of a quarter. There is like a "zoom" affect to the quater-sized hole. This is the same thing that happens with 1.6x crop sensors. So, putting on a 50mm prime lens on a 1.6x crop camera gives you the same DoF as an 80mm lens on camera with a 35mm-sized-sensor.

 

[NaOH]

Originally posted by: GTaudiophile
Most DSLRs (Rebel, EOS-20D, D50, D70, etc.) have cropped sensors. Meaning, they are like 60% smaller than their 35mm-sized counterparts. Imagine looking at an object through a hole in cardboard the size of a silver-dollar. Now imagine doing the same looking through a hole the size of a quarter. There is like a "zoom" affect to the quater-sized hole. This is the same thing that happens with 1.6x crop sensors. So, putting on a 50mm prime lens on a 1.6x crop camera gives you the same DoF as an 80mm lens on camera with a 35mm-sized-sensor.

Yeah i got that part. I'm just too confused to put it all together. So those smaller cameras can't focus on an object in the foreground and blur the background because of it's sensor size? Doesn't it's lens focal length differ too?

 

[Madwand1]

If you want to read a really good text on DoF, and a point of view that goes a bit beyond the standard math and techniques to good effect, I can recommend "The Ins and Outs of Focus" by Merklinger. You can even download it these days.

http://www.trenholm.org/hmmerk/download.html

 

[destrekor]

Originally posted by: GTaudiophile
Most DSLRs (Rebel, EOS-20D, D50, D70, etc.) have cropped sensors. Meaning, they are like 60% smaller than their 35mm-sized counterparts. Imagine looking at an object through a hole in cardboard the size of a silver-dollar. Now imagine doing the same looking through a hole the size of a quarter. There is like a "zoom" affect to the quater-sized hole. This is the same thing that happens with 1.6x crop sensors. So, putting on a 50mm prime lens on a 1.6x crop camera gives you the same DoF as an 80mm lens on camera with a 35mm-sized-sensor.

ahh... i was going under the notion that all DSLRs had the same sensor size to match that of a film negative... hmmm. good to know.

 

[GTaudiophile]

Originally posted by: destrekor

Originally posted by: GTaudiophile
Most DSLRs (Rebel, EOS-20D, D50, D70, etc.) have cropped sensors. Meaning, they are like 60% smaller than their 35mm-sized counterparts. Imagine looking at an object through a hole in cardboard the size of a silver-dollar. Now imagine doing the same looking through a hole the size of a quarter. There is like a "zoom" affect to the quater-sized hole. This is the same thing that happens with 1.6x crop sensors. So, putting on a 50mm prime lens on a 1.6x crop camera gives you the same DoF as an 80mm lens on camera with a 35mm-sized-sensor.

ahh... i was going under the notion that all DSLRs had the same sensor size to match that of a film negative... hmmm. good to know.

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.

 

[NaOH]

Can someone summarize and explain this to my ignorant ass? I read that whole page many times but still am not sure.......

 

[destrekor]

Originally posted by: GTaudiophile

Originally posted by: destrekor

Originally posted by: GTaudiophile
Most DSLRs (Rebel, EOS-20D, D50, D70, etc.) have cropped sensors. Meaning, they are like 60% smaller than their 35mm-sized counterparts. Imagine looking at an object through a hole in cardboard the size of a silver-dollar. Now imagine doing the same looking through a hole the size of a quarter. There is like a "zoom" affect to the quater-sized hole. This is the same thing that happens with 1.6x crop sensors. So, putting on a 50mm prime lens on a 1.6x crop camera gives you the same DoF as an 80mm lens on camera with a 35mm-sized-sensor.

ahh... i was going under the notion that all DSLRs had the same sensor size to match that of a film negative... hmmm. good to know.

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.

so do the specialized lenses for the body's with the 1.6x cropped sensor, like... counterbalance the effect of the crop so it ends up performing the same to a body that has a FF sensor and a regular lens?

 

[mchammer]

Originally posted by: destrekor

Originally posted by: GTaudiophile

Originally posted by: destrekor

Originally posted by: GTaudiophile
Most DSLRs (Rebel, EOS-20D, D50, D70, etc.) have cropped sensors. Meaning, they are like 60% smaller than their 35mm-sized counterparts. Imagine looking at an object through a hole in cardboard the size of a silver-dollar. Now imagine doing the same looking through a hole the size of a quarter. There is like a "zoom" affect to the quater-sized hole. This is the same thing that happens with 1.6x crop sensors. So, putting on a 50mm prime lens on a 1.6x crop camera gives you the same DoF as an 80mm lens on camera with a 35mm-sized-sensor.

ahh... i was going under the notion that all DSLRs had the same sensor size to match that of a film negative... hmmm. good to know.

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.

so do the specialized lenses for the body's with the 1.6x cropped sensor, like... counterbalance the effect of the crop so it ends up performing the same to a body that has a FF sensor and a regular lens?

I would think that would be true. The F number (f2.8, etc) is a fraction showing the relationship between the sizes of various parts of the camera. So on a point and shoot digicam, the F number should always perform as you would expect (at a given focal length). It would only be when a lens is used on a sensor size that it was not originally desogned for that all of the numbers would need conversion.

(That is what I think anyway).

As for whether or not the APS-c sensor size (22mm wide) will go away, I am not so sure. It allows the use of smaller lenses and uses mainly only the center perfect part of the lens, meaning a camera with a full sensor (36mm wide) needs to use higher quality lenses. It is still a big sensor, most consumer cameras have a sensor that is only 5-6mm wide.

 

[NaOH]

hello?

 

[foghorn67]

Originally posted by: GTaudiophile

Originally posted by: destrekor

Originally posted by: GTaudiophile
Most DSLRs (Rebel, EOS-20D, D50, D70, etc.) have cropped sensors. Meaning, they are like 60% smaller than their 35mm-sized counterparts. Imagine looking at an object through a hole in cardboard the size of a silver-dollar. Now imagine doing the same looking through a hole the size of a quarter. There is like a "zoom" affect to the quater-sized hole. This is the same thing that happens with 1.6x crop sensors. So, putting on a 50mm prime lens on a 1.6x crop camera gives you the same DoF as an 80mm lens on camera with a 35mm-sized-sensor.

ahh... i was going under the notion that all DSLRs had the same sensor size to match that of a film negative... hmmm. good to know.

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.

A Canon Exec was interviewed, and they are commited to two formats. We can assume that is FF and APS-C (1.6). APS-C would be used for entry level and prosumer cameras. High end and pro bodies will be full frame.

 

[fuzzybabybunny]

tfinch's link should explain all of it

 

[Madwand1]

This is a somewhat advanced topic, with a multiplicity of points of view and formulae to back that up.

I think the following photo.net article comes close to explaining it well:

http://www.photo.net/learn/optics/dofdigital/

The article summarizes, within certain condidtions (see article), as follows:

Depth of field is proporitional to (circle of confusion) / (focal length) ^ 2

The two factors work against each other in smaller frame cameras -- circle of confusion gets smaller (thus implying lower DoF), but focal length gets smaller, and the division and square factor magnify its effect. This "explains" how digicams get their greater DoF -- because, to get the same field of view in a small sensor, you need to have a smaller actual focal length.

Other explanations emphasize the effective diameter of the lens. But this is focal length / f-stop, so the focal length is an implicit player here.

Yet other explanations imply that focal length makes no difference, but they do this by changing the subject distance greatly. Therefore focal length does matter, because if it didn't, why would they have to change subject distance? Subject distance matters greatly too.

Essentially, subject distance, focal length, aperture, and circle of confusion all matter, and make the topic difficult to grasp. When we factor out aperture and subject distance to compare different format sizes, the smaller circle of confusion penalizes smaller format size, but the smaller focal length for the same field of view turns it around, giving smaller formats greater depth of field.

 

[flot]

I think it's safe to assume your prof didn't have any idea what she was talking about.

You might re-ask the question as it relates to film (there are many film sizes other than 35mm) and see what answer you get.

 

[NaOH]

So......I had lecture again today and she handed out packets which included information on how digital camera works. I quote from there...."Some cameras have smaller sensors which let them use smaller lenses, but this also means that the camera will only have a deep depth of field...."

 

[fuzzybabybunny]

Originally posted by: AMDUALY
So......I had lecture again today and she handed out packets which included information on how digital camera works. I quote from there...."Some cameras have smaller sensors which let them use smaller lenses, but this also means that the camera will only have a deep depth of field...."

I don't think she's very well versed in certain aspects of digital photography...

EDIT: Ask her how cameras like the 20D, XT, and 30D with their 1.6x crop sensors can take pictures with very shallow DOF, as well as, erm, ALL Nikon cameras with their 1.5x crop sensor...

 

[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.

 

[Zenmervolt]

Originally posted by: destrekor
so do the specialized lenses for the body's with the 1.6x cropped sensor, like... counterbalance the effect of the crop so it ends up performing the same to a body that has a FF sensor and a regular lens?

Nope. A 10mm lens on an average (APS-C size sensor) gives an equivalent field of view to a 16mm lens on a full frame sensor. Full frame is (nominally) 36x24mm, APS-C is (again, nominally) 22.5x15mm, and the smaller sensor has a narrower field of view.

For a conventional full frame lens, it must pass a circle of light large enough to cover all of a 36x24mm area without shadow appearing in the corners. Using a lens like that with the smaller 22.5x15mm sensor results in a lot of "excess" area in the circle of projected light. The "digital only" lenses are designed only to cover a 22.5x15mm sensor and will show a "circle effect" if used with a full sensor (think of how looking through a telescope is shown on a TV screen). However, the crop factor of the smaller sensor is still in effect.

The advantage to throwing a smaller circle of light is that the lens then requires less glass and can be made smaller and lighter.

 

[Zenmervolt]

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.

Actually, focal length, per se, has nothing to do with DOF. DOF is controlled by the overall size of the aperture. Since F-number is a ratio of focal length over aperture diameter, DOF appears to be greater on shorter lenses due to the fact that a shorter lens achieves the same F-number as a long lens with a smaller absolute aperture size.

A 50mm lens at f2.8 has the same DOF as a 100mm lens at f4. The reason is that the absolute diameter of the aperture of a 50mm lens at f2.8 is the same as the absolute diameter of the aperture of the 100mm lens at f4.

ZV