All 50mm lenses created equal?

[Krosya]

Hi,
This maybe a stupid question, but I'll ask anyway.
If you take several 50mm lenses, which are the same - well not the same fingerprint, I'm sure, but same as far as all the specs go - let's say 50/2. For example - Summicron, Planar, Hexanon. Will they all have the same range of DOF at the same F-stops? Or will they vary? It seems to me that with some lenses at the same settings I have seen different DOF range. Or am I misteken?
Can someone make it clear? What about different lenses - still 50mm but if one is 50/2, other is 50/2.8 - will they have same DOF at let's say f4, f5.6, etc.

 

[sepiareverb]

Where is Roland when you need him?

 

[Meleica]

In theory they will all have the same DOF at the same aperture...In practice, lenses display different types of out of focus and in-focus characteristics ( their signature ), so they may appear to have slightly different DOF.

On photo.net there is a posting from someone who tested the Summicron IV 35mm lens agains the ASPH 35 Summicron and at the same apertures, the ASPH appears to have more DOF.

In addition, manufacturers differ in what they define "in-focus" or DOF to be...meaning not all use the same measurement for the circle of confusion.....so DOF scales on different lenses may appear differently. What one person calls in-focus may not agree with some others person view...

Dan

 

[richard_l]

.....If you take several 50mm lenses, which are the same - well not the same fingerprint, I'm sure, but same as far as all the specs go - let's say 50/2. For example - Summicron, Planar, Hexanon. Will they all have the same range of DOF at the same F-stops?

Yes.

Can someone make it clear? What about different lenses - still 50mm but if one is 50/2, other is 50/2.8 - will they have same DOF at let's say f4, f5.6, etc.

Yes. They will have the same DOF at the same F-stops.

The DOF is determined by a mathematical formula and will be the same for lenses having the same focal length, provided the format is the same (e.g. 35mm). It has nothing to do with the sharpness or definition of the lens. However, the DOF scales on lenses from different manufacturers may indicate different DOFs. This is because of the COC (circle of confusion) variable in the computations. For example, I believe Leica uses COC=.033mm, whereas Zeiss uses COC=.024. The COC has nothing to do with the lenses themselves, but is (in the final analysis) related to the degree of enlargement of a print, the viewing distance, and the acuity of vision of a typical human eye.

The above is not my opinion of what the DOF should be, but is rather the way in which DOF scales are calibrated.

What Dan (Meleica) wrote is correct.

Richard

 

[richard_l]

By the way, just to head off any objections... (I have bad memories of this topic from a couple years ago.) As far as DOF computations go, there is a tacet assumption that the lens has perfect definition in the plane of focus. Likewise the enlarging lens is perfect, and there is no grain in the film or print, etc. DOF is a theoretical distance range which uses ideal optics to approximate what one might call the actual depth of field.

Richard

 

[varjag]

Adding to the above, some designs (Sonnar, Ernostar) with undercorrected spherical abberations might exhibit more actual DOF at wide apertures. They are never as sharp in plane of focus as better corrected designs, but they have longer in and out transitional areas.

 

[markinlondon]

Adding to the above, some designs (Sonnar, Ernostar) with undercorrected spherical abberations might exhibit more actual DOF at wide apertures. They are never as sharp in plane of focus as better corrected designs, but they have longer in and out transitional areas.

Is this not the reason for the famous Sonnar focus shift at wider apertures?

 

[photophorous]

Actual focal length can vary too, right? For example, some 50mm lenses might actually measure to 52mm or 49mm? That could have a real effect on DOF, although I would expect it to be minimal.

That's my contribution.

Paul