DoF with different lenses?

[Kev]

Is the depth of field the same, for the same Fstop with 2 different lenses of the same focal length?
For example would the DoF be the same at F4 for the Nikon 300mmF2.8 lens and the Nikon 300mm F4 lens?

cheers
Kev

[puddleduck]

Yes

[pearce_jj]

great answer :thumbs:

[Canon Bob]

Yes

great answer :thumbs:

But wrong.

[puddleduck]

OK, how so?

[pearce_jj]

Now then Bob, no need to confuse matters!

[Canon Bob]

Perceived DOF is related to the circle of confusion. A sharp lens will have greater DOF than a soft lens as the soft lens will disolve the detail sooner.

Bob

[pearce_jj]

regardless of whether that is true or otherwise, it doesn't seem to related to the OP's question though - unless all copies of one type of lens are meteorically rubbish (like Canon's 75-300)??

[puddleduck]

Perceived DOF is related to the circle of confusion. A sharp lens will have greater DOF than a soft lens as the soft lens will disolve the detail sooner.

Bob

Well I dis-agree.

Neither of the lens mentioned are soft. In fact, the lenses mentioned are pretty much one and two in Nikons MTF racks.

Sorry, there is going to be no percievable DoF difference with both of these 300mm lenses at f/4.

[tappilappi]

I would be interested in Canon Bobs reasoning as well. The online DOF calculator does not mention the actual lens, just the camera type, f stop, focal length, and subject distance.

[kiasdad]

If both lenses are the same focal length, and the same aperture setting is used on both lenses, then the DOF will be the same. However, if you use both lenses wide open, then the 2.8 will have less DOF than the f4.

The factors that influence depth of field are:
focal lenght
aperture
format - the larger the film / sensor area, the less DOF for a given focal length / aperture combination. Something to do with hard maths and circles of confusion.

[Canon Bob]

I was answering the theoretical question rather than the "for example"....relative sharpness of two Nikon lenses is outside my sphere of interest (unless I change my forum name ;)).

Bob

[puddleduck]

I was answering the theoretical question rather than the "for example"....relative sharpness of two Nikon lenses is outside my sphere of interest (unless I change my forum name ;)).

Bob

So, I think my rather concise answer was in fact accurate ;)

[Canon Bob]

If both lenses are the same focal length, and the same aperture setting is used on both lenses, then the DOF will be the same. However, if you use both lenses wide open, then the 2.8 will have less DOF than the f4.

The factors that influence depth of field are:
focal lenght
aperture
format - the larger the film / sensor area, the less DOF for a given focal length / aperture combination. Something to do with hard maths and circles of confusion.

The circle of confusion is a measure of whether that region of the image is sharp enough to be considered in or out of focus. A soft lens (like the 75-300 quoted by JJ) will enter the OOF region early than a sharp prime when moving from the plane of correct focus.

Bob

[Canon Bob]

So, I think my rather concise answer was in fact accurate ;)

If you were answering the example and not the question in the top line then you may be right....again, I can't speak about those two lenses.

Bob

[Kev]

after reading Tappilapi's response I googled "depth of field calculator" and got the website with even more interesting info as well :thumbs:

Thanks for all the responses chaps

cheers
Kev

[pearce_jj]

superb thread gents :lol:

[tappilappi]

Ah, I think I understand Bobs thinking. In theory, yes they would be the same. BUT, the actual optical quality of the two lens could lead to a difference in the the perceived DOF.

[kiasdad]

I was answering the theoretical question rather than the "for example"....relative sharpness of two Nikon lenses is outside my sphere of interest (unless I change my forum name ;)).

Bob

What's "relative sharpness" got to do with anything?
Any lens has one point of focus. Depth of field defines the area either side of that point of focus which is acceptably sharp.

[Canon Bob]

Ah, I think I understand Bobs thinking. In theory, yes they would be the same. BUT, the actual optical quality of the two lens could lead to a difference in the the perceived DOF.
If the lenses had the same MTF then the DOF would be the same. The lens with a lower MTF will see the circle of confusion maximum exceeded earlier than the lens with a higher MTF.....I think that's what your saying.

Bob

[tappilappi]

That I agree with :thumbs:

[kiasdad]

The circle of confusion is a measure of whether that region of the image is sharp enough to be considered in or out of focus. A soft lens (like the 75-300 quoted by JJ) will enter the OOF region early than a sharp prime when moving from the plane of correct focus.

Bob

No, that's wrong,:cuckoo: as are all of the other attempts to rewrite optical theory on this thread...
To quote from the Focal Encyclopaedia:
"DEPTH OF FIELD - Strict geometrical theory of image formation by a lens..."

"Circle of confusion can be defined as the patch of light a lens produces when it images a point source. This patch, which is circular on the lens axis, is at minimum size when the image of the object point is critically focused. The largest circle that is seen as a point rather than a circle at a specified viewing distance is identified as the permissible circle of confusion."

Now, the important bit...

"Depth of field is the distance between the nearest object where points on the objects are imaged at the film plane as permissible circles of confusion"

The circle of confusion is dependent on the viewing distance for a standard (6x8 inch) print, for which the "standard" viewing distance is about 10 inches and, according to the Focal encyclopaedia is about 0.145mm.

It goes on to say that the circle of confusion on the negative will be proportiately larger/smaller depending on the negative size, as a smaller negative will need to be magnified more to get to the standard print size.

The scales you see on prime (and some zoom) lenses are a guide to what will be rendered as "acceptably sharp" at a given aperture, with "acceptably sharp" being defined using whatever circle of confusion the lens designer deems appropriate.

Finally...

Let's just think about this business of MTF being a factor in depth of field...
MTF, or Modulation Transfer Function is a methodolgy for measuring the resolving power of a lens using a standard target. The lens is critically focused on the target, typically at a distance of (IIRC) 300x focal length.

Typically, the sharpness of a lens is lowest at wide apertures, improves to it's best level two or three stops down from the widest aperture, then deteriorates as diffraction takes effect. So, to take the premise that MTF is a factor in DOF would mean that it would increase as the aperture is stopped down but then start to decrease as the resolution of the lens drops due to the factors already mentioned.

So...to conclude:

MTF or ANY other measure of lens resolution (which is measured when the lens is critically in focus), has ABSOLUTELY NO BEARING WHATEVER on depth of field. What does change however, is the lens's ability to resolve at different apertures.

[pearce_jj]

So, to take the premise that MTF is a factor in DOF would mean that it would increase as the aperture is stopped down but then start to decrease as the resolution of the lens drops due to the factors already mentioned.

... which admittedly entirely supports Bob's argument, since the 2.8 is obviously stopped down one stop at F4 ...

[kiasdad]

... which admittedly entirely supports Bob's argument, since the 2.8 is obviously stopped down one stop at F4 ...

No it doesn't. If both lenses are at f4, then DOF will be the same, irrespective of the maximum aperture of the lens.

[pearce_jj]

Unfortunately my 5D is 'on holiday' at the moment, otherwise I'd test this!

[33L]

i think Canon bobs argument has merit however in real life we may be talking a few milimeters depending on the how far the object focused on is.

[kiasdad]

i think Canon bobs argument has merit however in real life we may be talking a few milimeters depending on the how far the object focused on is.

How? I'd be delighted to know :thinking:

[Canon Bob]

So, to take the premise that MTF is a factor in DOF would mean that it would increase as the aperture is stopped down but then start to decrease as the resolution of the lens drops due to the factors already mentioned.

This statement would only be true if the diffraction limitation was so extreme as to negate (and then a bit more) the expected increase due to aperture reduction.

More soon ;)

Bob