50mm Lux Asph DOF compared to 50mm Lux Pre-Asph

[frederick_muller]

Just thought I'd post an observation. When shooting wide open and at close to

minimum focus distance, the apparent depth of field of the 50mm Lux Asph is

much, MUCH narrower than the 50mm Lux Pre-Asph (casual experimentation suggests

the 50mm VC Nokton is somewhere in between). The Asph is like a flat-field

macro lens with almost no depth of field. If you are shooting a flat subject

with the lens axis ninety degrees to the plane of the subject, the Asph will

give razor sharp results to die for. If you are shooting a flat subject with

the lens axis at an oblique angle to the plane of the subject, and details in

depth ahead of and behind the plane of focus, the shallowness of the asph's dof

becomes glaringly apparent.

 

I've seen this effect before at medium distances when comparing the 35mm Type 4

'cron to the 35mm Cron Asph, but tonight was the first time I noticed it with

the 50's. Absolutely amazing. That shallow depth of field is an interesting

effect, but it is not always desirable.

[erikhaugsby]

They're both f/1.4, aren't they?

 

Maybe I'm being too simplistic, but I thought that an 50/1.4 is a 50/1.4 is a 50/1.4?

[frederick_muller]

No ... apparently the optical formula has a lot to do with it.

[vivek iyer]

Frederick, Try the Canon 50/0.95 and be amazed at the apparent DOF (I like your usage) at f/0.95 at portrait distances!

 

Albert Smith has posted a nice illustration somewhere (i think, Nikon forum).

 

Field flatness is not that easily discerned from the numbers given by the manufacturers (or measured by others). Their effect on the actual images are much more substantial.

[mark_amos]

Frederick, perhaps a year ago or so, someone made a fascinating post about this exact topic. I believe the topic of discussion was even the two lenses you are talking about. My imperfect understanding of it was that the curvature of field of the pre-asph actually allows objects in the center to be well focused, but the dof is not flat, but is rather shaped with the curvature of field. This has the benefit with the older lens of being able to capture a subject in the context of portions of a foreground that will appear more focused than occurs with a less distorting lens. For my assessment of the prior post to be correct, you should not see improved dof directly in front of a center subject. You would see more dof somewhat to the sides because the dof is essentially a cone instead of a plane. I found it rather astounding, and it might be another one of those reasons that can scientifically explain the reasons that an older design might exhibit a characteristic that can be useful, particularly if it is well understood by the user.

[stuart_richardson]

I have noticed the same thing. It seems to be a characteristic of the ASPH lenses that they

have less depth of field at the same aperture than the older lenses. I think that the extreme

sharpness at the point of focus contributes to the effect as well. The ASPH lenses tend to call

attention to the point of focus more dramatically since the point of focus is sharper, and in

turn, this more clearly separates the true plane of focus from the rest of the image.

[john_r._fulton_jr.]

If I'm not mistaken this then leads to what's called "focus shift". That's when you focus on

something in the middle of the frame, then shift the camera over so the subject is not

centered, and the subject is then not tack sharp (missed focus) in the photograph. The is a

characteristic (feature) of the Noctilux for instance.

[john_r._fulton_jr.]

"This" is a characteristic........ (above) sorry.

[stuart_richardson]

Well, to be fair, that happens with any lens with small depth of field. It is just more apparent

when using rangefinders because you always have to focus in the center and recompose. If

you do the same thing with an SLR, you will have the same problem. But yes, I think it can be

more pronounced with the ASPH lenses since they have smaller apparent depth of field.

[ed_schwartzreich]

I was the person a year ago who wrote about the new 50 Lux, but the comparison was to the 50./1.4 Nikkor on the Millennium S3. The problem with DoF wide open in the close range turned out to be rangefinder focus error. This was after I had personally asked everyone knowlegible I knew of, incl Erwin Puts and even Peter Karbe, the designer of the 50 lux.

 

Lens compatibility with the Leica's rangefinder depends on exact tracking between the lens's cam and the camera's cam follower. It is not that infrequent to find a particular lens sample that does not track exactly perfectly with one's camera, even in this day of CNC machining. It turns out that the 50 Lux I had borrowed for my test tended to back focus just a smidgen -- not enough to be really "off", but enough to show differences with the Nikkor 50/1.4, and later with my 50 Nocti.

 

The DoF business is mostly a mathematical one. You need to check very carefully on your conjoint len / camera's focus accuracy before proceeding further. Try photographing a ruler obliquely, and find a third fast 50mm lens to compare the other two with.

 

Hope this helps.

[alan_wilder1]

Not long ago I discovered the generous curvature of field with a 50 mm pre-ASPH Summilux design when compared to the 50 Summicron, a lens with an even flatter field than the ASPH Summilux. Here's a link to my comparison including pictures: http://www.rangefinderforum.com/forums/showthread.php?t=42310. All but the earliest 2nd version pre-ASPH Summilux share this characteristic. I did however test an early black Summilux from 1965 vintage (ser.# 222xxxx) that had a much flatter field out to the corner due to good correction of tangential astigmatism (like the rim of a wagon wheel) but residual radial astigmatism (like the spokes of a wagon wheel) was still present in high enough amount to soften the outer field sharpness quite a bit until stopped down to f/5.6 or f/8. Later versions of the lens from the early 1970's till the end of production traded off only radial astigmatism for curvature of field (equal amounts of astigmatims in both radial and tangential directions?) thus extending dof in the foreground. Works great for most 3D subjust but not as well for distance landscapes requiring edge to edge sharpness unless stopped down to f/8.

[frederick_muller]

Really interesting phenomenon. Thanks for your observations guys. If one were looking for a reason to own BOTH the asph and the pre-asph, here it is for me! I'm curious about Ed's experience ... wondering if the problem went away for the same lens but with a different body?

[frederick_muller]

One way to distinguish between a back focus problem and shallow depth of field is to shoot a flat subject with the plane of the subject directly perpendicular to the lens axis. In that case there should be no question if the lens and the rangefinder are in alignment.

[35mmdelux]

kool, thanks, Ed.

[torbj_rn_holen]

might be a stupid suggestion, but I se that no one has brought it up.

The older lux'es has closest focussing distance of 1m, while newer have 0.7. the Nokton has 0.9.

 

just thought I'd point it out, since focussing distance greatly affects depth of field