Sigma lenses for MFT mounts

[dancingcat]

I have Sigma's 30mm f/1.4 DC DN lens and it passes UV well enough for flower portraits.  But in search of more working room I saw a Sigma 56mm f/1.4 DC DN on B&H.

 

B&H specs says that the 30mm has two aspherical elements and one special low dispersion (SLD) element, and the 56mm has two aspherical elements and one high-refractive index element. 

 

The similar number of elements in the 56mm seems like it might pass UV, but I don't know if the asphericals are the same, or if the HRI element in the 56mm would prevent it passing UV.

 

Has anyone tried to use the 56mm?

[dabateman]

I haven't tested either.  I have the Sigma 30mm f2.8 smooth barrel lens for UV.

The Olympus 60mm doesn't pass UV well at all.

 

[Andrea B.]

Let me call this question to the attention of Ulf who has a lot of lens knowledge. He might be able to venture a guess as to how it would perform in UV. 

 

You might also check with Lens Rentals to see if they have these Sigmas for rent. Sometimes renting the lens for a tryout helps one decide whether to purchase it. Can save money in the long run by stopping you from buying an unsuitable lens. Also note that Lens Rentals will sell you any rental lens at a somewhat reduced price. I've bought several items from them.

[Andrea B.]

P.S. I moved this topic to the general Tech section.

[ulf]

I looked at the design of these two lenses at sigma.

The internal lens structure can be seen  in the Construction tab in both pages.

I guess that is what dancingcat already did.

 

https://www.sigma-imaging.dk/30mm-f1-4-dc-dn-c

https://www.sigma-imaging.dk/56mm-f1-4-dc-dn-c

 

Both lenses seam to have 12-14 air to glass transitions that all must have a modern AR-coating that will limit any deeper UV-A transmission.

Without knowing what optical glass materials used, it is not possible to speculate about their blocking properties.

The two lenses are reasonably close in focal length and if we assume that their design is done by the same design team they might favour similar glass types in the designs.

 

The Special Low Dispersion element of the 56mm lens might not be a problem for the UV reach. Some in that group transmit UV reasonably well.

 

The only simple way to know if it is usable is to try it.

Fortunately a very deep UV reach is not needed for flower portraits, even if the exposure times and false colour saturation will be less optimal.

Good enough UV-reach is enough.

If other optical parameters are good instead that might give a better image in the end than a deep reaching less well corrected lens.

 

Sorry for that I cannot give a better answer.

 

Here is an interesting document that can be educational to browse:

https://www.schott.com/shop/medias/schott-tie-35-transmittance-of-optical-glass-eng.pdf?context=bWFzdGVyfHJvb3R8NzYxOTY3fGFwcGxpY2F0aW9uL3BkZnxoM2MvaGQ1Lzg4MTc0MDk5MTY5NTgucGRmfGY0ZWVlNWRmMDczYjliNjA0MWJiMDg3NTlkMDZhODRmZmNmNGNiOTNjOTg0Y2ZiMjY1YmMwNDUwZWU2NjU4NGE

Figure 2-3 show how different the UV reach can be for different materials and glass thickness.

 

 

[dabateman]

I just remembered and posted in an other thread that the Sigma 60mm f2.8 art lens, with smooth barrel should also work well in UV.

 

There are posts about it here.

https://www.ultravioletphotography.com/content/index.php?/topic/2511-q-sigma-60-mm-f28-mft-in-uva/#comment-18591

 

[Andrea B.]

Ulf - thank you for the assessment of the lenses !

 

My experience matches Ulf's statements.

• The only simple way to know if it is usable is to try it.
• Fortunately a very deep UV reach is not needed for flower portraits,
  even if the exposure times and false colour saturation will be less optimal.
• Good enough UV-reach is enough.

 

We often worry too much about "reach" at the expense of enjoying our UV photography in the upper regions 360-400 nm. Reach is nice if you can afford it, but don't let it stop you from trying with what you have. enjoying what you have.

 

 

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Dancing Cat:  Another possibility is to try one of the old Ebay-cheap 105/3.5 or 135/3.5 lenses. LINK

Old Pentax or Galaxy brands come to mind.

[Andy Perrin]

The question is, how sharp is it in UV (assuming it transmits "deep enough.") And does it have any other disqualifying features, like big focal shift? I don't use my Sony 55mm/1.8 even though it passes some UV just because of the focal shift issue.

[Andrea B.]

In case not everyone knows, please describe the Focal Shift Problem. Thx !!!

 

Another potential disqualifier is the chromatic aberrations often seen with lenses uncorrected in UV.

[Andy Perrin]

Well, I found that the focal shift was especially severe for that lens between IR and vis/UV, so you can take a photo and if you have no filter on the lens at night, things reflecting IR will be a giant blur and things reflecting visible will be sharp. In the same photo! Also, I think I remember that in UV the autofocus didn't work properly because when the camera thought the image "should" be focussed, the live view showed it was not.

 

[Andrea B.]

Thanks! One good photo, as they say....

 

I've found that distance plays a role in focus shift. For some of my lenses, it is best to refocus after every filter change through UV, Vis and IR even though some claim to be corrected. 

[Lou Jost]

I have the 56mm Sigma, an amazingly good lens in visible light. I have not tried it yet in UV.

 

But this is a good time to elaborate in something Andy also alluded to. Transmission (percentage of incident UV light that gets through the lens) is not always the most important parameter for daylight UV photography. For example, suppose I had a lens with 80% transmission at 365nm, but it had to be closed down to f/11 to get sharp images. Compare that to a lens with only 50% transmission but that was sharp already at f/5.6. The latter lens will give less noise and shorter exposures than the lens with higher transmission. It's important to know both the transmission and the widest aperture that gives acceptable sharpness. Of course, if your application requires that you stop the lens down anyway (maybe for depth of field) then the lens with the higher transmission will indeed be the best choice.

[Andrea B.]

wise words.....thanks, Lou.

[Andrea B.]

Dancing Cat:  Here is a link to a test I made years ago with the Asahi Pentax 135/3.5. LINK

(I mentioned above that an old 135/3.5 of some brand might be useful to you.)