Dongrappo
Hello everyone,
I noticed something rather annoying during my last few DuKa sessions.
The Nikon 2.8/50mm EL is supposedly a really good lens, but… the last time I checked and adjusted the sharpness using the focus scoping device, I realised afterwards that I’d still had the aperture set to f/8.
So I opened the aperture all the way up and the sharpness was now slightly off. So far, no big deal, although I was already irritated by how ‘far’ the sharpness had to be corrected. But because of the depth of field and also the depth of focus ;-) everything was still fine.
So I focused at f/2.8 and then stopped down. Then I realised that the framing, or the cropping, wasn’t right, so I tweaked the enlarger a bit and checked the sharpness again – this time back at my working aperture of f/8 – and the sharpness was off again!
So I adjusted the sharpness at f/8 using the focus screen, then opened up to f/2.8, and once again the plane of sharpness was somewhere else. OK, given the depth of field/focus screen level, whatever, that might be the case, but somehow it’s still a bit odd?!?!?
So, back to square one: I focused on the grains again at f/2.8, stopped down, checked and... and... and... ACTUALLY, the sharpness was off after stopping down and had to be adjusted slightly at f/8!!!
So have I learnt it wrong and always done it the wrong way??? <--(rhetorical question)
Set the sharpness with the aperture wide open and then stop down?! Surely that’s not right!?! There’s no such thing as flange distance, or zoom lenses etc. like in film/video and all that sort of thing that plays a role, is there??
I tested the whole thing several times with the same result every time.
:idea: Now I was curious, so I swapped the Nikon for my 2.8/50mm Schneider-Kreuznach Componon-S and ran the same test. And and and (drum roll)
NO, no shifting sharpness! :at: The Schneider does exactly what it’s supposed to. The sharpness at f/2.8 is spot on even at f/8 and vice versa; although, of course, the depth of field at f/8 means you can’t focus quite as precisely, it still worked out well enough – set at f/8 and then stopped down, it was always pretty good, which isn’t the case with my Nikon.
:arrow: Now I’d be really keen to know whether this is just the case with my lens, or whether it’s a problem inherent in the Nikon’s design?
In terms of construction, both lenses are clearly very different. The Nikon is very reminiscent of a Nikkor prime lens in terms of size and weight, and also appears to have a more asymmetrical design. In contrast, the Schneider-Kreuznach looks almost delicate and simpler, as well as more symmetrical in design. Although the number of elements and optical performance are supposed to be on the same level.
:arrow: Perhaps someone who owns a Nikon like this, or has one to hand, could check it out this time?
Or test it with other lenses as well.
Perhaps this is another one of those situations where that certain ‘crispness’ in the image can be lost somewhat due to the shift in focus when stopping down? And then, just so nastily and meanly, it goes completely unnoticed.
Oh yes, the negative stage used has glass at the top, so the negative isn’t moving in the focal plane at all!
That really blew me away. As if there weren’t already enough variables in darkroom work that need to be mastered and taken into account.
But perhaps it’s just my Nikon. Perhaps the phenomenon is already known, though I haven’t heard or read anything about it anywhere yet.
At the very least, I really wanted to share this experience.
And; unfortunately, I’ve got carried away again and have once more taken up a great deal of the dear reader’s—that is, your—attention.
Warm regards from Bremen
The chatterbox Werner (DonGrappo)
KlausWehner
Hello Werner,
I’m not familiar with the Nikon lens you mention.
But I think it’s possible that your observation is correct.
At the same time, however, I also believe that the phenomenon you’ve observed is of no practical relevance.
There are a number of physical parameters that define the quality of a lens.
You have only considered one of them. That does not give the full picture.
So the answer lies more in the realm of psychology than physics.
Make enlargements with both lenses.
After a while, you may find that you subjectively prefer one of them.
You can then continue working with that one.
It really is a completely subjective decision. Only if you notice a serious flaw in all your enlargements should the decision be made on objective grounds.
But that won’t be the case here.
Best regards,
Klaus
Dongrappo
Klaus Wehner wrote
: Only when you notice a serious error in all your enlargements should the decision be made objectively.
But that won’t be the case here.
Best regards,
Klaus
Hello Klaus. Thank you very much for your assessment.
However, the error is actually very specific!
So, at least in my view, it is not at all subjective. Because the potential sharpness, if one works meticulously – which, incidentally, requires a great deal of patience and finesse even without a fine-adjustment mechanism on the focus wheel, as found on some other enlargers – is then easily lost. And the sharpness then lies, in the stopped-down state, just short of the optimum achievable sharpness, despite the increased depth of field.
Admittedly, I didn’t notice this at first whilst working. After all, the Nikon’s sharpness is still high and the contrast performance is also impeccable. I actually really like—or rather, liked—the Nikon very much.
It’s just that I’ve had to realise this flaw—or let’s call it a quirk—while working. Sure, I could always sharpen the image in the stopped-down state before enlarging it. But you can forget to do that sometimes, and then the feeling—or rather the thought—creeps in later that I wonder whether it turned out perfectly. That gnaws at me. And that’s something you can avoid with the other lens.
Yes, I’m very fussy about some things, downright pedantic, in fact. Probably too much so at times. On the other hand, I’m really quite chaotic in other respects.
I also find my discovery rather annoying, as I’ve been working almost exclusively with Nikon in the 35mm format since the 1980s and have been very happy with it.
Well, I’ll use the Schneider for now and perhaps later make a large-scale print for a direct comparison.
Margro
Dongrappo wrote
: the last time I checked and adjusted the sharpness using the grain focuser, I realised afterwards that I’d still had the aperture set to f/8. So I opened the aperture all the way up, and the sharpness was now slightly off.
...
So this time I focused at f/2.8 and then stopped down. Then I realised that the framing, or the filtering, wasn’t right, so I made some adjustments on the enlarger and checked the sharpness again – this time back at my working aperture of f/8 – and the sharpness was off again!
Hello Werner,
The effect you’ve described is called focus shift. Some lens types are well corrected for this effect, whilst others are not. It’s also not an indication of a lens’s quality.
The Canon EF 50mm f/1.2 L lens springs to mind as an example (sorry, not a Nikon). I’ve had mine adjusted at the factory several times. But if I focus on a point in Live View and then change the aperture from 1.2 to, say, 5.6, you’d expect: now I have a greater degree of sharpness, everything’s great. Yes, the degree of sharpness is greater, but the focal plane has
shifted. And indeed
, outside the depth of field! It’s hard to believe when you’re looking at an expensive, modern ‘professional lens’, but unfortunately that’s the case.
It’s not a fault with the lens, but an optical characteristic that is usually accepted during lens development in favour of other characteristics, such as high imaging performance and/or light intensity. The focus shift effect is more pronounced at close range than at greater distances.
There is only one thing you can do about it: if you are aware of it, check the focus plane at the desired aperture.
Greetings from Berlin,
Olaf
Dongrappo
Margro wrote
Dongrappo wrote
: The last time I checked and adjusted the sharpness using the grain focuser, I realised afterwards that I’d still had the aperture set to f/8. So I opened it up fully, and the sharpness was now slightly off.
...
So this time I focused at f/2.8 and then stopped down. Then I realised that the framing, or the filtering, wasn’t right, so I made some adjustments on the enlarger and checked the sharpness again – this time back at my working aperture of f/8 – and the sharpness was off again!
Hello Werner,
The effect you’ve described is called focus shift. Some lens types are well corrected for this effect, whilst others are not. It’s also not an indication of a lens’s quality.
Hi Olaf,
Thanks for the helpful reply. Ahh, now it’s coming back from the depths of my brain — ‘focus shift’ — yes, yes, there was something about that. I think I read about it ages ago, or did I even learn about it at school???
I’d already suspected that the effect might be due to the lens design. At least my observation doesn’t seem to have deceived me.
That helps me a great deal.
Then surely this should be a much more widely noted and discussed topic, shouldn’t it?!
Simply the reminder that with this or that lens, one must take care to focus not at the maximum aperture, but at the working aperture. That is anything but trivial information.
It makes me wonder: with projection lenses, which are designed for a very narrow plane, does this ultimately have even more dramatic consequences than for ‘normal’ photographic lenses, which are, after all, intended for imaging three-dimensional objects and subjects?!
And what about autofocus cameras? Don’t they always meter with the aperture wide open and only stop down for the actual exposure?
In practice, the error is probably negligible, as Klaus already noted.
On the other hand, it is often the sum of several small errors that, taken together, can lead to unsatisfactory results.
Or perhaps you’re already satisfied with your own results, and don’t realise – or even know – that, with the equipment you have and use, you could actually achieve even better results by taking such peculiarities into account.
So, thank you very much for your answers, opinions, experiences and expertise!
Margro
Hi Werner,
I don’t think focus shift is something you come across very often in everyday use. I was surprised myself when I first experienced the effect, using the lens mentioned above with a digital SLR camera. I subsequently checked a number of fast lenses and only observed the effect on one other Zeiss Planar 50mm f/1.4. Other lenses, some of which are very sharp and high-resolution, with a depth of field of just 1–2 millimetres at wide open, were free of this issue.
And I believe that when it comes to wide-open apertures and sharpness down to the millimetre, we are in a realm where opinions vary widely. For example, regarding front and back focus, or the reliability of AF systems, one can read very different accounts.
If you are lucky or unlucky enough to be working with a lens that exhibits pronounced focus shift and place great value on optimum sharpness (which is often the case for me), you should be aware of the effect. But in reality, it is more of a marginal issue that is rarely discussed. That is my impression. With a macro lens, however, I do find that a bit much!
Dongrappo
That said, it remains to be seen whether this is always the case with Nikon close-up lenses, or whether – for whatever reason (I can’t think of one) – this might be a one-off issue with my particular lens, perhaps due to a fault or something similar.
That’s why I’d be interested in hearing about further experiences and tests. Provided it sparks the interest of others.
And also whether other lenses, from other manufacturers, are affected by this.
A test like this is actually quite simple and can be done yourself in a short time using a focusing screen, without using up any materials.
What’s more, you don’t even need a second lens for comparison.
I, too, think that this phenomenon occurs very, very rarely. It’s just that the Nikon in question is a sought-after and often highly praised lens that I myself also like very much.
And since, particularly when enlarging, good lenses usually achieve their peak performance at relatively wide apertures (I’ve often read +1–2 stops), in my view it’s precisely about achieving the highest possible precision. Why would you want to miss out on that?!
Well then, I wish you a lovely weekend and say a big thank you
Werner
GeorgK
Hello.
This effect is known as ‘spherical aberration’. Every spherical (spherical-shaped) lens exhibits this property to some extent, because, for physical reasons, the peripheral rays converge (‘focus’) at a different point than the central rays, which are closer to the optical axis. A parabolic lens does not exhibit this behaviour, but it has other problems of its own and is also much more complex to manufacture (spherical surfaces are very easy to grind and polish precisely, but with more complex shapes the process becomes labour-intensive). In multi-lens systems, this error (‘aberration’) can theoretically be compensated for; however, with a five- or six-element lens, it is not so easy to correct all lens errors (distortion, chromatic aberration, field curvature, etc.) simultaneously, so a compromise is made. It may well be that the EL, by design, exhibits higher spherical aberration, but is correspondingly well-corrected in other respects.
Focus shift occurs because, at wide apertures, the image is formed primarily by the peripheral rays, whereas when stopping down, the central rays contribute increasingly to the image. If the two do not share the same focus, the sharpness ‘shifts’ as the aperture is stopped down. With ‘mild’ aberration, the effect is compensated for by the increasing depth of field; that is, whilst the focus shifts, the originally focused area always remains within the (continuously increasing) depth of field.
Portrait lenses are sometimes deliberately ‘under-corrected’ in terms of spherical aberration, because at wide-open aperture a dominant ‘sharp’ area (from the peripheral rays) is then superimposed on a faint, ‘out of focus’ image (from the shifted sharpness of the central rays); which results in a sharp image with a ‘flattering’, subtly out of focus halo.
It is quite possible that modern AF lenses have the extent of the focus shift stored in the lens’s EPROM and that the camera automatically corrects the effect when stopping down. This also explains why third-party lenses often have focus issues.
Dongrappo
GeorgK wrote
This results in a sharp image with a ‘softening’, subtly out of focus halo effect.
Thanks for all the explanations
When I read GeorgK’s quote, I immediately pictured Doris Day films in my mind’s eye, when there’s a cutaway during close-ups, often with Rock Hudson. But that’s just so indecent! :-)
Now I’ve also read from an old lab veteran that he always uses the focus screen at working aperture. Once you’ve practised that as a rule, nothing can really go wrong anymore.
I’m slowly starting to relax a bit more when it comes to these questions. It’s just the inner, ignorant ‘theorist’ and ‘ponderer’ who simply always wants answers to such questions.
Wishing you all good light and many thanks to all forum members,
Werner
OliverKleinhoff
It’s great that you take such care with your lenses.
I’m sure you also use a focusing screen to focus and check the sharpness using at least white light, or better still green or blue light, or both.
Don’t you?
Best regards,
Oliver
mattes
The El Nikkors are among the best enlargement lenses.
The effect you’ve noticed basically occurs with all lenses, but usually goes unnoticed as the aperture is typically stopped down after focusing. I’ve noticed this too; I simply refocused and didn’t give it a second thought.
Years later, I read in an article about the Konica Hexar AF that the Konica engineers had taken this effect into account when programming the firmware. This is probably also one reason for their exceptionally high level of sharpness.
Matthias
