DOF scale on one lens way off

[Ulophot]

I recently rechecked some of my lenses' DOF scales against an online calculator and found one particularly anomalous. Let me say that I am aware of the various CoC issues respecting DOF determination and magnification in printing, and recognize that manufacturer often create lens scales based on a relatively small enlargement, meaning that a good stop down additionally is likely to be advisable. However, my lenses have aligned pretty well with an online calculator I use. The one that is rather glaringly off is my 55mm f/2.8 Mamiya-Sekor for my 645, and I wonder if anyone else has noticed this. I have three other lenses for the camera that are generally pretty close, as are my Leica M's, so I don't know why this one would not also be.

 

[cgw]

Online calculator aside, how linear is DOF with aperture when stopped down?

 

[Ulophot]

Online calculator aside, how linear is DOF with aperture when stopped down?

I'm not quite sure what you're asking here. Is it a rhetorical question about DOF in general, or something you're asking about the scale on this lens?

 

[cgw]

You said DOF was somehow "off." Have you simply tried to check DOF at a given aperture as indicated on the lens? DOF can be very shallow at close distances on these lenses until at least f5.6-f8. Suspect you already know this?

 

[Ulophot]

Let me clarify. Here are sample comparative values for my 55, approximated as closely as possible for the lens barrel reading. The last column gives the online calculator readout, which is set for a CoC of 0.046mm. Note that the readings in the first row are similar.

LENS	Aperture	Distance in feet	Lens Scale DOF	Calc Scale DOF
55	5.6	6	5’ - 7’	5’2 – 7’
	11	6	4’4 – 10’	4’8 – 8’6
	5.6	10	7’3 – 15’4	8’– 13’5
	11	10	6’ – 40*	6’ 8 – 20’

Regarding the asterisk on the 40-foot in the last row: The f/11 is clearly to the right of the 0 in 40, but the infinity symbol nearly touches the 0, so gauging the distance is impossible. At 10' at f/8, the lens scale conforms to the online calculator.

 

[petrochemist]

Let me clarify. Here are sample comparative values for my 55, approximated as closely as possible for the lens barrel reading. The last column gives the online calculator readout, which is set for a CoC of 0.046mm. Note that the readings in the first row are similar.

LENS	Aperture	Distance in feet	Lens Scale DOF	Calc Scale DOF
55	5.6	6	5’ - 7’	5’2 – 7’
	11	6	4’4 – 10’	4’8 – 8’6
	5.6	10	7’3 – 15’4	8’– 13’5
	11	10	6’ – 40*	6’ 8 – 20’

Regarding the asterisk on the 40-foot in the last row: The f/11 is clearly to the right of the 0 in 40, but the infinity symbol nearly touches the 0, so gauging the distance is impossible. At 10' at f/8, the lens scale conforms to the online calculator.

They don't look that different to me, yes the lens is somewhat more optimistic, but that could easily be from expecting a different output (and hence a different CoC being used). 40 years ago most photos were viewed as 6x4 prints, using their scale should give images looking sharp over the claimed DOF on these prints.

 

[cgw]

Sincerely hoping this isn't costing you any sleep. Agree with petrochemist about the size of the apparent discrepancy and its impact on IQ.

 

[IanG]

DOF scales and tables are very subjective, I always err at least a stop, so using the scale I'd use the markings for say f5.6 when shooting at f8, and remember that you focus 1/3 the way into the range you want sharp.

Ian

 

[cgw]

Think the OP has left the building...