Creating a blurred Background

[HoppyUK]

At the end of the day, that's the answer for the OP.

I just wish I had a FF body as well as my 7D, and then I'd quite happily do the test I suggested to show the results.

Steve

Than heaven for common sense!

And you can do the test with a 7D if you really want. I did it couple of years ago on a similar thread that ran and ran and got unpleaseant. Par for the course on this one, but I got an apology at the end.

Take a picture, longest zoom setting, prefereably a close up so you can see the changes more easily. Then without changing anything else, zoom back and take another snap, and enlarge that second image to match the framing of the first and you will have more depth of field.

If you halve the focal length, you will have exactly one stop more DoF. Or to put that another way, if you lower the f/number by one stop with the second shot, the DoF will be identical.

 

[jimmyriddle]

Than heaven for common sense!

Take a picture, longest zoom setting, prefereably a close up so you can see the changes more easily. Then without changing anything else, zoom back and take another snap, and enlarge that second image to match the framing of the first and you will have more depth of field.

I don't think anyone is denying that what you have written here is correct.

Probably best to leave it there, the OP is probably hospitalised with confusion by now

 

[HoppyUK]

I don't think anyone is denying that what you have written here is correct.

That's kind of you to say JR, but actually the evidence of this thread suggests that there are in fact a few people that think I am not correct. They have said so repeatedly and been offensive in the process. I don't mind argument, but some people's behaviour is just, well, I'll leave it there.

And it's not me that's correct, it's the science. It's the understanding of that which folks have been getting wrong.

Probably best to leave it there, the OP is probably hospitalised with confusion by now

Poor chap hasn't posted since last week, but he did say he was going to change the camera so that's a result I guess.

 

[redddraggon]

Haha, you keep thinking that, and carry on living in your own deluded world. It's your scientific knowledge that is lacking, not mine.

But it's a fact that keeping the F-stop, Focal length and subject distance constant, the DoF will be the same between DX and FX.

I don't disagree that keeping the focal length, aperture and framing constant will result in a smaller DoF with a bigger sensor, but with a bigger sensor to keep the framing the same you need to change the subject distance.

 

[HoppyUK]

And you're wrong on both counts! Since you refuse to do the calculations yourself, here they are.

Haha, you keep thinking that, and carry on living in your own deluded world. It's your scientific knowledge that is lacking, not mine.

But it's a fact that keeping the F-stop, Focal length and subject distance constant, the DoF will be the same between DX and FX.

F/number 5.6, focal length 50mm, subject distance 10ft.
DoF on DX = 2.76ft
DoF on FX = 4.25ft

The Dof is evidently different (and I also said earlier how it appeared to be the reverse of what you might expect, because the framing is completely changed).

I don't disagree that keeping the focal length, aperture and framing constant will result in a smaller DoF with a bigger sensor, but with a bigger sensor to keep the framing the same you need to change the subject distance.

If you change the subject distance, you have changed the perspective and the image is not the same, is it? The only way to maintain equivalence is to adjust focal length.

So, if you want the picture framed the same, from the same viewpoint with the same perspective and DoF, in other words two 100% identical images it looks like this:

F/number 5.6, focal length 50mm, subject distance 10ft.
DoF on FX = 4.25ft

F/number 3.6 (to nearest quarter stop), focal length 33mm, subject distance 10ft.
DoF on DX = 4.11ft

Changing the size of the sensor dictates changes in both focal length and f/number to re-establish equivalence. You can change the focal length, the distance or the f/number, but you can't change the size of the sensor, which is why it is the primary driver of DoF.

 

[jimmyriddle]

That's kind of you to say JR, but actually the evidence of this thread suggests that there are in fact a few people that think I am not correct. They have said so repeatedly and been offensive in the process. I don't mind argument, but some people's behaviour is just, well, I'll leave it there.

And it's not me that's correct, it's the science. It's the understanding of that which folks have been getting wrong.



Poor chap hasn't posted since last week, but he did say he was going to change the camera so that's a result I guess.

Yeah there's no need for people to be offensive. But science actually dictates that sensor size does not affect DoF. What you said which I quoted is true, but I think you are confusing the crop factor of a smaller sensor with magnification of the image being produced. As the sensor is physically smaller, it is cropping out the outer edges of the image projected by the lens, so only the smaller inner region is captured. Image quality is not significantly altered as smaller sensors can have the same number of pixels amongst other things. The actual image projected from the lens is the same regardless of sensor. This includes the out of focus light. Take this diagram.

In this example imagine the çlear image is a FF sensor. If a sensor half the size was used only the pencil, no background would be visible, and the DoF would be the same.

Now I think where we are seeing differently is that you are referring to DoF from a print point of view, in capturing an identically framed image. However to truely take the crop factor of the sensor into account, imagine keeping focal length and aperture the same, if you printed the resulting image from a FF and crop sensor so that the objects in the image were identically sized in print then the FF print would be larger. Crop of the edges physically to acheive the same composition and print size and the DoF would be the same.

BTW the reason I asked earlier if you was a biologist, was I had a very similar converation with a biologist, trying to explain the setup of a confocal microscope, and how DoF was affected or not, depending on what is changed in the setup.

Hope that makes sense

 

[HoppyUK]

I think you post maybe crossed with mine above ut.

Of course what is imaged by the lens and recorded by the sensor is the same, but the (main) bit that is missing from your post is the greater degree of magnification/enlargement required from the smaller sensor to obtain a final image of the same size. Therefore the circle of confusion has to be adjusted according to sensor size.

I have run the fully adjusted calcs above.

 

[jimmyriddle]

I think you post maybe crossed with mine above ut.

Of course what is imaged by the lens and recorded by the sensor is the same, but the (main) bit that is missing from your post is the greater degree of magnification/enlargement required from the smaller sensor to obtain a final image of the same size. Therefore the circle of confusion has to be adjusted according to sensor size.

I have run the fully adjusted calcs above.

Yes they did.

I have used the DoF calulator, and yes it gives that answer, but it doesn't explain how.

Can you please explain how the sensor provides magnification?

I was always one of those people that has to know how something works.

 

[Splog]

Yes they did.

I have used the DoF calulator, and yes it gives that answer, but it doesn't explain how.

Can you please explain how the sensor provides magnification?

I was always one of those people that has to know how something works.

The sensor doesn't provide magnification.....Magnification is a function of the lens.

 

[HoppyUK]

Yes they did.

I have used the DoF calulator, and yes it gives that answer, but it doesn't explain how.

Can you please explain how the sensor provides magnification?

I was always one of those people that has to know how something works.

Because, to deliver the same size output, ie print etc, it has to be enlarged/magnified by a different amount. No other comparison represents true equivalence. That's why the CoC changes.

If you want to explore the detailed forumlae, they're all in the technical sections of DoFmaster. Be prepared to frazzle your brain though. Things sometimes go the opposite way to how you first think logical, and then you can have double adjustments which skew them back in line. Enjoy Oh, and think square roots a lot.

But one of the great things about photography is that whatever is happening, it all nets out in the finished picture. It's easy enough to prove all of this stuff just by taking a few pictures.

 

[jimmyriddle]

The sensor doesn't provide magnification.....Magnification is a function of the lens.

I know, that's my point.

 

[jimmyriddle]

It's easy enough to prove all of this stuff just by taking a few pictures.

Haha, are you sure you're not a biologist!

I will have a look through the tech section of that website