question about DOF

[Digital Matt]

I didn't put it up for a joke. It supports my point. If you matched your perspective and focus distance between the two lenses, the 50mm at f/2 would have less depth of field than the 200mm at f/4. With the 200m at f/4 and the same distance from the subject however, it would have significantly less.

Oh, and as to me getting back to my other post: I just haven't had time to sit down and really read and consider all of the responses. I will try to do that.

 

[markc]

No, I think he's absolutely right. He just presents it as if it's something new.

To get more DOF do any, or combine several, of the following:
decrease focal length
increase focusing distance
decrease aperture size

To get less DOF do any, or combine several, of the following:
increase focal length
decrease focusing distance
increase aperture size

If I increase focal length, and increase focusing distance, the one DOF factor is equalizing the other.

I'd add to that: viewing distance of final print and size of final print. That has a big impact on acceptable sharpness. He was changing that too.

 

[shingfan]

this site provides a very neat equation for calculating the DOF....it provides a solid understand and calculation of DOF...very nice

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

and regarding the link that matt posted.....the guy was presenting a correct concept i think (need to compare with the equation later).....but presented it in a wrong manner......and make ppl (ppl without experience but only logic) think that the concept is wrong

 

[Alex_B]

....or to be precise....DOF is the focus range

well, DOF is the range of object distance of acceptable (un-noticeable) blurrieness.

At the extreme ends of the field, the image is less sharp than somewhere in the centre...

But then again, I am not allowed to talk about DOF on this forum anymore

 

[markc]

this site provides a very neat equation for calculating the DOF....it provides a solid understand and calculation of DOF...very nice

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

and regarding the link that matt posted.....the guy was presenting a correct concept i think (need to compare with the equation later).....but presented it in a wrong manner......and make ppl (ppl without experience but only logic) think that the concept is wrong

That's not complete. The link I posted to includes image size. I think that's why the article is flawed. They didn't realize that there was another component because many of those calculators use a standard print size (8"x10" @ 25cm, I think).

Mind you, I'm winging this based on recent reading. Makes sense to me though.

--edit--

Fudge. Never posted that link: http://www.cambridgeincolour.com/tutorials/DOF-calculator.htm

 

[markc]

well, DOF is the range of object distance of acceptable (un-noticeable) blurrieness.

At the extreme ends of the field, the image is less sharp than somewhere in the centre...

But then again, I am not allowed to talk about DOF on this forum anymore

Yeah, exactly. The only place that's actually in focus is the focal point. That's why print size and viewing distance is important. If you blow an image up and get close, it's going to change the DOF because you can scrutinize the image in more detail. You'll see that some small detail is actually blurry because now it's larger to your eye. Before it was small enough that you didn't notice. DOF shrinks as the image size gets bigger or viewing distance gets closer.

 

[shingfan]

Yeah, exactly. The only place that's actually in focus is the focal point. That's why print size and viewing distance is important. If you blow an image up and get close, it's going to change the DOF because you can scrutinize the image in more detail. You'll see that some small detail is actually blurry because now it's larger to your eye. Before it was small enough that you didn't notice. DOF shrinks as the image size gets bigger or viewing distance gets closer.

i think you are going a little bit too far relating DOF to image size....image size quality is related to resolution of the sensor.......DOF is just the range of focus....two separate topic here.....

 

[markc]

Nope. It's not just range of focus (at least as I understand it). It's the range of focus that is acceptable to our eye. As I said, the only point that is in focus is that infinitely small focal point. If you have a film with enough resolution, you could keep blowing up the image to show this. If you have two items 1" apart out at 30m and focus on the close one, they will booth look in focus with a 200mm lens at f/4. But if you keep blowing the image up, you will eventually get to the point where you will see that only the close item is in focus. The one further back has some blur. Heck, even how good your eyesight is changes the DOF.

 

[shingfan]

Nope. It's not just range of focus (at least as I understand it). It's the range of focus that is acceptable to our eye. As I said, the only point that is in focus is that infinitely small focal point. If you have a film with enough resolution, you could keep blowing up the image to show this. If you have two items 1" apart out at 30m and focus on the close one, they will booth look in focus with a 200mm lens at f/4. But if you keep blowing the image up, you will eventually get to the point where you will see that only the close item is in focus. The one further back has some blur.

(just for the sake of discussion...not starting an argument...or it is an arguments?...lol )

i kinda dont agree on that......because the object can be in focus...while the sensor is not able to pick up proper signal to show the focus....that is also a problem....say you compare a 6MP DSLR and a 12MP DSLR.....both use the same setting capturing the same image.......mathmatically speaking...they would have the same DOF at that specific setting...but i would think that you would get more out of the 12MP DSLR near the edge of the DOF....but it doesnt make the 12MP DSLR has a wider DOF....because the DOF are the same to the lense.....just get pickup differently by the different sensors

does it make DOF "zero" when i dont wear my glasses?....haha.....i think our topic is DOF of the image as it appears to the camera...not DOF of the image we see in our eyes......we can have terrible eyes and see no DOF (nothing in focus when i have my glasses off)

 

[markc]

(just for the sake of discussion...not starting an argument...or it is an arguments?...lol )

Works for me. My discussion can sound like arguments, unfortunately.

but i would think that you would get more out of the 12MP DSLR near the edge of the DOF...

At smaller sizes, the two sensors would give you prints with similar or the same DOF. If you start making large blow-ups, then the 12MP sensor will give you a larger DOF.

does it make DOF "zero" when i dont wear my glasses?....haha

If you have bad eyesight, then yes! For you. But that's why manufacturers use set standards for measuring this.

Have you read the page I linked to? I'm kind of repeating a bit of that.
http://www.cambridgeincolour.com/tutorials/DOF-calculator.htm

 

[Digital Matt]

But that's why manufacturers use set standards for measuring this.

And so should we when discussing it. We can't subjectively discuss something objective otherwise.

 

[Alex_B]

....say you compare a 6MP DSLR and a 12MP DSLR.....both use the same setting capturing the same

beyond 8 MP it is actually mostly the sharpness of the lens and not theoretical sensor resolution which plays the role here... just to add some more controversy to the discussion here

it is not all that easy to find lenses which satisfy a 12 MP sensor, even if you spend beyond 1000 USD for the lens

 

[shingfan]

If you have bad eyesight, then yes! For you. But that's why manufacturers use set standards for measuring this.

Have you read the page I linked to? I'm kind of repeating a bit of that.
http://www.cambridgeincolour.com/tutorials/DOF-calculator.htm

this site you link doesnt take into account of print size...and yes....i would agree with them about the sensor being a consideration for DOF because the bigger the sensor...the better you get in terms of DOF or sharpness....because you see more........however....you take it a little too far bringing up the print size and eye sight....is not related to print size or eye sight quality

 

[Alex_B]

does it make DOF "zero" when i dont wear my glasses?....

no, for you subjectively it increases the DOF a great deal ... it is all quite blurry .. but the minimum blurry-ness will be spread over a much wider depth

(or would you call it deeper depth to be consistent? )

 

[shingfan]

isnt it lovely that we are all so responsive to the same topic....yet not able to come to an agreement....haha....

hard to say whether it increase or decrease the DOF since NOTHING is in focus here when i take my glasses off.....i would say it is a "zero" DOF.....haha