21 SEM performance

[rirakuma]

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Hi guys, I recently received the 21SEM and have taken it for a quick spin last week. I've found the lens to be very sharp and resolves detail to the corner however it tends to have nervous bokeh for anything in front of the focus plane. I haven't tried it stopped down but it seems quite consistent wide open. I know this may seem like a trivial problem but I bought the 21SEM because I wanted a strong technical performer to replace my WATE. Is this normal for the 21SEM? You can see an example below where the out of focus area at the bottom of the image (in front of the focus plane) looks quite busy. Any input is greatly appreciated

 

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[jackstraw]

Looks sharp throughout to me. I think you acquired a wonderful lens. Enjoy it.

[rirakuma]

Hi Jack, thanks for your reply. If you view the picture in full size you may notice the slightly nervous falloff in the foreground. Sharpness is great throughout the plane of focus and it performs well to the corners. I know I'm being picky but I just wanted to know if this is an inherent property of the 21SEM or whether mine will need servicing. I'm most certainly still enjoying the lens

[rramesh]

My experience with the 21 SEM is that most things tend to be in focus that there is not much of a bokeh, forced or otherwise.

 

For more of a bokeh, maybe the new CV 21 f/1.8 might be better.

[250swb]

Even wide open a 21mm lens isn't going to give you much bokeh, its DOF is too big under average circumstances like this. So it is often better to make everything equally sharp by stopping the lens down rather than have a transition from sharp to nearly sharp.

 

But in post processing you can help things along. A slight darker vignette around that edge would take the eye away from the 'almost in focus' area and towards the centre. Or you could burn/reduce the just highlights and take some of the busy look away. And you could selectively sharpen just the centre of the image while applying some blur to the bottom or edges. Clearly anything but minimal post processing isn't about demonstrating the lenses inherent characteristics like a technical exercise, but bringing out the point of the photograph and making it easy on the eye. But all good photographs tend to use a similar process of bringing areas of the image forward, or making them visually recede to achieve a balanced picture, so it is worth thinking about.

 

Steve

[pgk]

OK, I have a 21SEM and this is my personal conjecture and goes for some other modern lenses too. The 'nervous' bokeh IMHO may just be as a result of the the aspheric surfaces which lie close to the aperture diaphragm and so are in their 'fullest' use at full aperture - and it is here that the bokeh is 'nervous'. As the lens is stopped down their effect appears to me to be less noticeable and I wonder if here the design might in effect be more 'conventional' with the central section of the aspheric elements possibly being less aspheric and more spherical? So overall technical quality is higher than a conventional design might achieve but the cost is the influence of the aspheric elements on bokeh at full aperture. I don't think that the SEM has 'close range correction' by rear floating elements but I may be wrong. If anyone with more knowledge of lens design can comment and confirm or throw out my conjecture I'd be very interested to hear other explanations.

 

I actually own the 21SEM and a copy of the old 21/3.4SA and the 2 lenses are chalk and cheese. The new design is technically superb but tends towards being clinical whilst the old SA produces 'classical' images - very sharp centrally, soft in the corners at full aperture and improves upon stopping down. Its square aperture gives a very characteristic bokeh like no other. I like them both!

[rirakuma]

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Thanks all for the replies. Paul great to hear your experience with the 21SEM, I haven't really thought about the downfalls of an aspherical design. I have to admit some of the stuff you mentioned flew right over my head because I don't enough technical knowledge but its good to know that my lens isn't a dud. I guess this is just something I will need to be aware of in the future and like Steve said I'll have to stop down at the expense of noise so I can avoid this issue.

[Guest Marc G.]

OK, I have a 21SEM and this is my personal conjecture and goes for some other modern lenses too. The 'nervous' bokeh IMHO may just be as a result of the the aspheric surfaces which lie close to the aperture diaphragm and so are in their 'fullest' use at full aperture - and it is here that the bokeh is 'nervous'. As the lens is stopped down their effect appears to me to be less noticeable and I wonder if here the design might in effect be more 'conventional' with the central section of the aspheric elements possibly being less aspheric and more spherical? So overall technical quality is higher than a conventional design might achieve but the cost is the influence of the aspheric elements on bokeh at full aperture. I don't think that the SEM has 'close range correction' by rear floating elements but I may be wrong. If anyone with more knowledge of lens design can comment and confirm or throw out my conjecture I'd be very interested to hear other explanations.

 

I actually own the 21SEM and a copy of the old 21/3.4SA and the 2 lenses are chalk and cheese. The new design is technically superb but tends towards being clinical whilst the old SA produces 'classical' images - very sharp centrally, soft in the corners at full aperture and improves upon stopping down. Its square aperture gives a very characteristic bokeh like no other. I like them both!

 

Generally speaking the better the correction of spherical aberration the poorer the bokeh (taken from Erwin Puts). Leica somehow found a trick to still offer superb out of focus rendering with some of the modern designs (e.g. 50 1.4 asph) while maintaining a high correction of spherical aberration. I never read about the explanation you're giving here but it doesn't sound logical to me (which doesn't necessarily mean that it could not be true).

 

Usually lenses also have 1 bad bokeh and 1 good bokeh too. There's 2 kinds of bokeh: foreground and background. Usually background blur is far more important regarding rendering than foreground bokeh. If a lens has a pleasing background bokeh it usually has a harsh(er) rendering than something in front of the plane of focus. Read about it somewhere, might have been Puts too. A test with all of my lenses did show that it is true to some degree. The harshness may vary to a level where you can hardly see differences but it definitely is there.