ANAMORPHIC ARTIFACTS & SQUEEZE RATIO QUESTION

[David Mullen ASC]

The point here, is that focal length is not a factor in DOF. There are only two variables #1 f stop #2 image magnification. Focal length is irrelevant. Bringing it in to the equation only confuses the issue.



That would be the image magnification part. An 18mm and a 300mm have the exact same DOF if the shot is the same.

Nick

I think the concept of "image magnification" is more confusing than focal length. A photographer has to deal with f-stops, focal lengths, and distance to subject relative to the size of the format he is dealing with. Which is why depth of field charts are listed by focal length for the format you are shooting in. What's confusing is to say that an 18mm and 300mm lens has the same depth of field "if the shot is the same". What does that mean??? How do you get the SAME shot with an 18mm and a 300mm? And while in theory you could crop the 18mm image to the view of a 300mm and see that the depth of field is the same on the same format, even that's not completely accurate because you have to factor in degree of enlargement after you blow-up the cropped image to the same viewing size as the 18mm full-frame image and thus are working with a more critical Circle of Confusion.

For all practical purposes, larger formats use longer focal lengths to achieve the same field of view as shorter focal lengths on smaller formats and longer focal lengths have less depth of field for all practical purposes, more or less by the amount of the crop factor, i.e. if you switch to a format that has twice the horizontal width and use a lens that is twice as long to achieve the same field of view as a lens that is half as long on a format that is half as wide, you'd have to stop down by two stops on the larger format to match depth of field.

I know that this is not the scientific explanation, but it's the practical one that works on production when discussing formats, focal lengths, and depth of field characteristics. A 40mm 2X anamorphic lens matches the horizontal view of a 20mm spherical lens and the reason you end up with less depth of field for the same f-stop is not because of the anamorphic elements but because you are having to use a lens that is twice as long to achieve the same field of view. You can say that this is due to image magnification all you want, but the practical effect is that you had to switch to a longer focal length to achieve the same field of view and now you are seeing that your image has less depth of field.

 

[Nick Gardner]

You are saying the same thing in a roundabout way. The absolute simplest way to say it is "tighter shots have less depth of field". The problem with the "longer focal length lenses have less depth of field" mentality is that is suggests that if you do a close up on an 85mm versus doing the same close up on a 32mm, the depth of field will be different, which is not the case. That belief is held by a surprisingly large number of otherwise knowledgeable people and it comes form statements like this. The point has been made.

Cheers,

Nic

 

[David Mullen ASC]

You are saying the same thing in a roundabout way. The absolute simplest way to say it is "tighter shots have less depth of field".

Nic

Except that when using a 40mm 2X anamorphic lens versus a 20mm spherical lens cropped to 2.40, the 40mm anamorphic shot isn't tighter but it does have less depth of field. The distance to subject is the same, the f-stop is the same, the composition is the same, but the 40mm anamorphic shot has less depth of field than the 20mm spherical shot.

 

[Nick Gardner]

Exactly, because you have doubled the negative size. Image magnification. The only problem I have is that you keep saying that longer lenses have less depth of field and they don't ;-)

Nick

Edit-

And to clarify the 18mm 300 thing - If you have four people sitting around a campfire, and you compose a shot that just includes all four on an 18mm, and then compose a similar shot that just includes all four but you back way the hell off on a 300mm lens, then the DOF is the same on both shots. Obviously the angle of view is different, but the DOF is the same, and it's an extreme example to illustrate the point.