Looking at DoF.

[HoppyUK]

"Combination of FL and A only for DOF. Oh how true that is. Naff all to do with the sensor. Altogether now "I`m walking backwards for Christmas""

I'm not too sure if you're being funny or serious in all of that but actually after reading up I think that it is also a factor of sensor size and possibly of pixel size / density but I don't know how that would relate to different sensors of the same size. Maybe the densities aren't different enough yet. At least I've read some pretty convincing stuff that that's the case.

"It's not actually the sensor size that gives you the DoF, but it is the combination of aperture and focal length used to get the shot."

That's what I've not been able to convince myself of. I don't think focal length matters and some web sites seem to be saying that it doesn't. Some seem to be saying that the sensor size and even the size of the pixels and density matters.

The technical aspects of all this do interest me but there are so many web sites saying contradictory things that as I simply don't myself know it's impossible for me to know from reading stuff on the net. Lots of sites says the they're right and the other view is wrong... so I don't think that there's a definite answer yet or if there is it's hidden amongst all the rest and not acknowledged as the truth, the whole truth and nothing but the truth.

Looking at just one format, my 20D, focal length doesn't affect DoF if the framing is the same. I seem to believe that and I seem to be able to prove that to myself in my own little tests. If I change formats and use my LX2 and get the framing right so it looks like the 20D shot the DoF is different. That seems to be pretty conclusive to me so I'm now perfectly willing to believe that two things affect DoF besides the size and relationship of the things in the frame. The sensor (size, pixel size and density maybe if certain web sites are right) and aperture.

I said earlier that none of this makes much practical difference to me as I'm just an amateur but for some specialists I'm sure that this stuff matters and as compacts get better and better I suppose that the implications and possibilities will be there if you need them.

The problem with the internet is that the answer is usually out there somewhere, but hidden amongst a mass of misinformation - needle in a haystack. But one of the beauties of photography is that if you want to see what is happening, just take a picture. It's all there for the looking, which is exactly what you have done, and what you have found is correct. To see what factors are making the difference, all you need to do is change the right things, which you have done, and compare the results.

The truth of the matter is that it is only the magnification of the image, and the f/number, that defines depth of field. (There are different aspects to magnification apart from the size of the sensor, related to output size and viewing distance, but for our purposes here we have to assume they are fixed - which is the concept behind all DoF calculations.)

A lens doesn't have any inherant fixed depth of field, and neither does a sensor. To create measureable DoF you have to form an image, of a certain size with a certain f/number, and for that you obviously need both. However, the way we do it is to take a camera with a fixed size of sensor and vary the focal length of the lens to suit. We don't take a fixed focal length lens and vary the size of the sensor. Well, you could do that, it's called cropping but we know that if you want a short cut to poor image quality, that is the sure way to go.

So the reality is that the size of the sensor drives everything else. That is why we say that you get more DoF on a crop format DSLR than full frame, and yet more still on a compact.

 

[woof woof]

Yup. And these days you don't even have to pay to get pictures developed so experimentation costs nowt. Well, it costs wear an tear on the shutter and pc I suppose.

I've been thinking that as compacts improve they could be the first choice for applications which suit their particular properties. Maybe some compacts are the first choice for some applications now? I dunno. Probably. In which case this should be true even more so if they improve but if the drive to get bigger sensors into smaller bodies holds sway maybe small sensor technology wont improve too much. Time may tell.

I'm not going to go too far into the technicalities as coming from an electronics / computing background if I do it could just take over my life and what I want is a hobby and not yet another obsession :bonk:

 

[arad85]

"It's not actually the sensor size that gives you the DoF, but it is the combination of aperture and focal length used to get the shot."

That's what I've not been able to convince myself of. I don't think focal length matters and some web sites seem to be saying that it doesn't. Some seem to be saying that the sensor size and even the size of the pixels and density matters.

Whoah.. Hang on a sec. There are only 3 things that affect DoF (ignoring this CoC business) - aperture, distance to subject and focal length.

What you are doing with your experiments is mixing focal length and sensor size and coming out with sensor size being the "factor". It isn't - it's just the effect sensor size has on the photos you are taking. What you are missing is that when you have the subject framed on a compact, the focal length used is much shorter than that used for an APS-C sized sensor. Much much shorter...

Forget sensor size - the thing that matters is focal length, subject distance and aperture. Given that you have fixed aperture in your experiments, you have subject distance and focal length. For your 20D experiments, you doubled distance and doubled focal length and you got the same DoF. They cancelled each other out.

When you were taking photos with the compact, what was your distance and what was your focal length? To get the same field of view, your focal length would have been much shorter - thus making the DoF bigger (shorter focal length = bigger depth of field if you don't move the camera). If you set your compact to the same focal length as your 20D, it will provide the same depth of field as the 20D, just you will see much less of the image as the sensor is smaller.

 

[HoppyUK]

Whoah.. Hang on a sec. There are only 3 things that affect DoF (ignoring this CoC business) - aperture, distance to subject and focal length.

What you are doing with your experiments is mixing focal length and sensor size and coming out with sensor size being the "factor". It isn't - it's just the effect sensor size has on the photos you are taking. What you are missing is that when you have the subject framed on a compact, the focal length used is much shorter than that used for an APS-C sized sensor. Much much shorter...

Forget sensor size - the thing that matters is focal length, subject distance and aperture. Given that you have fixed aperture in your experiments, you have subject distance and focal length. For your 20D experiments, you doubled distance and doubled focal length and you got the same DoF. They cancelled each other out.

When you were taking photos with the compact, what was your distance and what was your focal length? To get the same field of view, your focal length would have been much shorter - thus making the DoF bigger (shorter focal length = bigger depth of field if you don't move the camera). If you set your compact to the same focal length as your 20D, it will provide the same depth of field as the 20D, just you will see much less of the image as the sensor is smaller.

LOL just when I thought we'd nailed it! Nothing you have said here contradicts WW's findings.

"If you set your compact to the same focal length as your 20D, it will provide the same depth of field as the 20D, just you will see much less of the image as the sensor is smaller." People often say this, but I fail to see the relevance of the point being made and it's just confusing. You cannot compare the two images because they have completely different framing. The whole point of this thread is the comparison of depth of field in images of similar framing, but when taken on different formats.

 

[woof woof]

I think that another problem is that there are so many ways of saying the same thing that this in itself leads to confusion.

"The whole point of this thread is the comparison of depth of field in images of similar framing, but when taken on different formats."

Yup. The point was to try and take (more or less, not measured to the nearest mm) the same picture with three dfferent sized sensors and see what the results were. There seemed to be so little difference between the GF1 and the 20D that I didn't see any point in going further as a fast lens gets you there or there abouts with the GF1. The LX2 was the challenge and the surprise in that I couldn't get anywhere near the same DoF with the (near as damn it) framing and that interested me and was worth looking at.

 

[arad85]

People often say this, but I fail to see the relevance of the point being made and it's just confusing. You cannot compare the two images because they have completely different framing. The whole point of this thread is the comparison of depth of field in images of similar framing, but when taken on different formats.

The relevance is that you only need to look at subject distance, focal length and aperture when figuring DoF. The DoF calculations show that here: http://www.dofmaster.com/equations.html they only depend on subject distance, focal length and aperture.

The problem is that people (ww included ) think that taking the "same" photo with a compact and an APS-C sensor are interchangeable. They aren't. One is taken at an entirely different focal length than the other, yet the still show the same field of view because the sensor is a different size and hence need different fields of view to fill the sensor.

At the end of the day, framing doesn't matter a jot to the depth of field. Yes, it matters to the look of the photo being taken, but not to the DoF. A 16mm lens at 1m and F2.0 will always be a 16mm lens at 1m at F2.0, whether on a full frame or compact (I'm talking the ideal lens here..). Sensor size will tell you how much or little of the view you will see, but it won't give you any idea of how big or small the DoF will be.

At the end of the day, I can see where you are coming from. You're of the "I'm taking this picture with this framing with my camera" and relating the depth of field to "magnification". The problem with this as I see it is that it's one level removed from what's actually happening. The DoF is dictated by distance to subject, aperture and focal length - none of which are related to the type of sensor you are using directly. They are indirectly as it is a function of sensor size that makes you choose a particular focal length to take the photo you want... but that's a secondary measure - not a primary one. Choose a particular focal length/aperture. I'd get the same DoF whether that lens was on an APS-C, 35mm or even compact sensor....

At the end of the day, you appear to look at your photo taking with the framing as the defining measure. I look at mine with the technical parameters available - trying to understand focal length vs aperture vs shutter speed. Neither approach is right or wrong - just it's easier to do the maths with the ones I work with............ allegedly

 

[arad85]

Yup. The point was to try and take (more or less, not measured to the nearest mm) the same picture with three dfferent sized sensors and see what the results were. There seemed to be so little difference between the GF1 and the 20D that I didn't see any point in going further as a fast lens gets you there or there abouts with the GF1. The LX2 was the challenge and the surprise in that I couldn't get anywhere near the same DoF with the (near as damn it) framing and that interested me and was worth looking at.

Hmm.. I think the 20D is FAR better than either of the other two in terms of DoF (or lack of it).

 

[woof woof]

Yes, the 20D gives you the most control over DoF if you have a fast lens. The GF1 might not give the same amount of control but I think that the difference is largely irrelevant as with a fast lens the GF1 allows enough control and that's why I lost interest in the GF1 in this little test of mine, because it's close enough to the 20D for DoF not to be an issue (unless your going for extremes) as long as you've got a fast lens.

I can see where your coming from and I think that it's best to just see things as they make sense to each of us.

When doing this test I used the following lenses on my 20D, 20mm, 50mm and 150mm, all at f2.8 and the DoF was the same in each similarly framed shot so for me thinking "The focal length doesn't affect DoF" works because if I want "that" shot the focal length doesn't matter and the only control over DoF I have is the aperture. If I have a different camera available with a bigger or smaller sensor I'll have another different control over the DoF. Thinking of it like that works for me.

 

[HoppyUK]

The relevance is that you only need to look at subject distance, focal length and aperture when figuring DoF. The DoF calculations show that here: http://www.dofmaster.com/equations.html they only depend on subject distance, focal length and aperture.

The problem is that people (ww included ) think that taking the "same" photo with a compact and an APS-C sensor are interchangeable. They aren't. One is taken at an entirely different focal length than the other, yet the still show the same field of view because the sensor is a different size and hence need different fields of view to fill the sensor.

At the end of the day, framing doesn't matter a jot to the depth of field. Yes, it matters to the look of the photo being taken, but not to the DoF. A 16mm lens at 1m and F2.0 will always be a 16mm lens at 1m at F2.0, whether on a full frame or compact (I'm talking the ideal lens here..). Sensor size will tell you how much or little of the view you will see, but it won't give you any idea of how big or small the DoF will be.

At the end of the day, I can see where you are coming from. You're of the "I'm taking this picture with this framing with my camera" and relating the depth of field to "magnification". The problem with this as I see it is that it's one level removed from what's actually happening. The DoF is dictated by distance to subject, aperture and focal length - none of which are related to the type of sensor you are using directly. They are indirectly as it is a function of sensor size that makes you choose a particular focal length to take the photo you want... but that's a secondary measure - not a primary one. Choose a particular focal length/aperture. I'd get the same DoF whether that lens was on an APS-C, 35mm or even compact sensor....

At the end of the day, you appear to look at your photo taking with the framing as the defining measure. I look at mine with the technical parameters available - trying to understand focal length vs aperture vs shutter speed. Neither approach is right or wrong - just it's easier to do the maths with the ones I work with............ allegedly

Andy, thanks for that.

I would just say this: when you break it down, the components of DoF are focal length (which you can change), focusing distance (which you can change), f/number (which you can change) and format - which you cannot change. The format is fixed, which is why it is the reference for everything else, and while you can look at DoF from any direction you like, it's just not relevant to the way we work.

 

[classcams]

This all started many moons ago when "X" poster said that a small sensor gives more DOF than a large sensor. On the proviso you walk backwards to equal the frame size. Distance always increases the DOF no matter what lens or sensor. To continue my song "And a brand new tie for dad" He can always use the end to polish his lens.