Olympus OM 50mm f1.8, f1.4 and vignetting

[jim_baker6]

<p>I recently took some photos using my OM 50mm f1.8 lens at f1.8 and was struck by the dark edges of my print. I decided to try and understand what was going on and therefore took a test shot (see below). It's apparent in the background how this vignetting comes about. The background out-of-focus highlights (look at the top left dark tree) are ellipses showing that the off-axis 'cylinders' of light passing through the lens are being clipped by the metalwork holding the lens elements in place and this that gives rise to a vignetting which would still occur even if the image were in focus at the edge. Also the ellipses have a ring-like edge which indicates that the lens designers have concentrated on image sharpening at the expense, to my eye, of a pleasing bokeh.</p>

[jim_baker6]

<p>The 50mm f1.8 at f1.8 test shot:</p><div></div>

[jim_baker6]

<p>I also have an Olympus OM 50mm f1.4 lens and noticed that the exit pupil of the f1.4 lens is considerably larger than the f1.8 lens: 30mm vs. 22mm. I guessed therefore that the f1.4 lens at f2 would transmit the off-axis cylinders of light without clipping and that this would therefore improve the vignetting. A second test shot (below) confirms this. The out-of-focus highlights are more circular and the vignetting is correspondingly reduced (look at the plank on the left). The out-of-focus is also more pronounced (f2 vs. f1.8) indicating that the lens designers are using the larger exit pupil of the f1.4 lens. The more pronounced out-of-focus (f1.4 lens vs. f1.8 lens)is apparent at all apertures. There's no doubt that the f1.4 lens at f2 transmits more light than the f1.8 lens at f1.8. This vignetting, f2 vs. f1.8, amounts to about 1/2 a stop i.e. if the lens light gathering power were measured in t-stops and the f1.4 lens at f2 were t2, the f1.8 lens at f1.8 would be about t2.4. Also the f1.4 out-of-focus highlights exhibit a less ring-like structure.<br>

These differences are obvious if present, in even 6x4 prints. They are far more apparent than minute differences in lens sharpness. I had imagined that the f1.4 lens simply had a slightly larger aperture than the f1.8 lens and that-was-that. I now see that the difference in the lens designs produces quite different images, even when the lenses are used at the about the same aperture (f1.8 vs. f2). I think from now on I will be using my f1.4 lens even if I limit the maximum aperture toi f2. At a pinch, f1.4 is always available. </p><div></div>

[craigd]

<p>I have both of these lenses also. In my case, the f/1.4 is also a newer lens: it's the MC Zuiko variety, whereas the f/1.8 is the older single-coated F.Zuiko. Because of this difference, I've never been entirely sure whether my preference for the f/1.4 lens was based mostly in the coating improvements, but I'd never really done an A/B comparison like the one you've posted here. It is interesting how the f/1.4's bokeh, while still noticeably outlined, looks softer and less nervous than that of the f/1.8 lens even when stopped down to f/2.</p>

[machts gut]

<p>Interessting results. Thanks for posting. I never compared them directly, but came to the same conclusion. My samples are a MiJ 1.8 and a MC 1.4.</p>

[jim_baker6]

<p>Craig: as is well know, having anti-reflection coatings made a big difference and in fact made the 'double gauss' lens the most popular 50mm objective. How much difference simply improving the lens coating makes, I don't know. I don't have lenses identical except for the coating so even if I were inclined to, I can't carry out a comparison. Also, how would you know that the coating was the only difference?<br>

In any case, the bokeh/vignetting is a big difference between the f1.4 and the f1.8 lenses. I guessed the f1.4 at f2 would have less vignetting than the f1.8 at f1.8 but what surprised me was that at <em>all</em> apertures the f1.4 lens bokeh is more blurred, even when 'mechanical vignetting' is no longer apparent. The bokeh of the f1.8 lens at f2.8 is about as blurry as the f1.4 lens at f4, and so on. Also, the f1.8 out-of-focus highlights have a distinct (and to my eye slightly distracting) ring like appearance but the f1.4 out-of-focus highlights have a more uniform appearance.</p>

[matthew_newton]

<p>In my opinion I like the Bokeh of my 50/1.4 MC better than my 50/1.8 MIJ, however the 50/1.8 MIJ is signficantly sharper than the f/1.4 at large aperatures. f/1.8 vs f/1.8, f/2 vs f/2 and f/2.8 vs f/2.8 the 50/1.8 MIJ is noticably sharper in larger print sizes are wide open and around f/2. even noticably sharper in smaller prints if you look closely. At around f/4 the differences mostly disappear, but aren't completely gone until around f/5.6.<br>

Earlier 50s faired worse. My old silver ring 50/1.8 performed about on par with my 50mm f/1.4 MC version in terms of sharpness at similar aperatures (the 50/1.4 is a hair sharper at around f/4 on up, but almost esentially a wash, below that they are equivelent). Two early 50mm f/1.4 silver ring (so SC) varients were truely bad performers wide open. Even at f/2 they were noticably worse even in 4x6 prints than my 50mm f/1.4 MC version and looked noticably soft in small prints until you got to at least f/2.8 or better yet f/4 or f/5.6 (and even then f/5.6 was maybe the equivelent of the sharpness of f/2.8 on my 50mm f/1.4 MC version).<br>

I've had two different 50mm f/1.8 SC/silver ring lenses, a pair of 50mm f/1.4 SC/silver ring lenses, a pair of 50mm f/1.8 MIJ lenses (just one now) and a 50mm f/1.4 MC (still have it, for now). I haven't had any of the latest and greatest Zuiko 50mm f/1.4 multicoated lenses, just the earlier multicoated varient, so I can't speak as to whether those are any sharper or better performers.<br>

Overall I prefer my 50mm f/1.4 because of better Bokeh and the extra 2/3rds of a stop if I need it even if I am sacraficing a bit of ultimate sharpness in the f/1.8-f/2.8 range.<br>

That said with converting to m4/3rds I plan on selling my 50/1.4 and just keeping the 50/1.8 MIJ lens. I don't plan on using my OM-1 film kit often anymore (though I won't get rid of it all) and I do like the 50/1.8 MIJ...just not quite as much as the 50/1.4 (and keeping my late version 24/2.8 and 85/2 until you pry them out of my cold dead fingers).</p>

[jim_baker6]

<p>Matthew, I think you are describing the same relationship between sharpness and 'ring-like' out of focus highlights that I referred to. I think these 'rings' are due to over-correcting for spherical aberration or put another way, sharpening the image at the expense of a pleasing bokeh. Spherical aberration is due to rays close to the optical axis being focussed further from the lens than rays far from the optical axis. This tends to unsharpen the image at large apertures. If as they pass through the lens you arrange to 'steer' the rays towards the edge of the lens, they all tend to be focussed in the same place = a sharper image. The flip-side of this particular way of sharpening is, inevitably, 'ring like' out of focus highlights.<br>

My 50mm f1.4 and f1.8 are sharp enough so, at large apertures, the more pleasing bokeh and less significant vignetting of my f1.4 lens is a distinct overall advantage. </p>

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