Sharp at the center ... but what is the center

[TheLaird]

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Possibly a stupid question, but it is always best to ask.

I have Q2 but may get a M in the very near future. I am really looking at a F1.2 lens  but what I am struggling to get right in my mind is at 1.2 the center is nice and sharp and then it tails off. Does that mean the center of the lens or the focus point? So if I want my model in the right hand third for instance, and I focus on her eye at 1.2, does it start to taper off from her eye or from the center point of the lens?

I think this is an optics question so must be from center of the lens. If so, does that mean I need to walk back a bit, put her eye in the center and then crop to make her in the right-hand third?

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[Ecar]

17 minutes ago, TheLaird said:

I think this is an optics question so must be from center of the lens.

Yes.

[Pintpot]

From my own experience, all lenses degrade from the centre outwards, obviously this is dependent on the make of lens. But you will find that from the centre plane to the centre edges of modern lenses that degradation is very slight indeed, it only becomes noticeable at the corners of any frame. There are many posts on the forum, and YouTube, about the quality of various manufacturers, worth checking out before you make any purchases.

[pgk]

It sounds as if you are talking about two aspects of fast optics.

The image plane may not be perfectly flat wide open and this is called image field curvature. The centre may be correctly focused but focus shifts marginally as the sharpest part of the image are on a curve rather than being on a flat plane. On good modern designs this should be minimal though.

Performance variation across the image may also mean that at full aperture the edges and corners are not as perfectly optically corrected as the centre although again, in modern good designs the drop in performance will be minimal.

Really whether either or both will give you real world problems depends on you and your subject matter. I am happy to shoot f/1.4 lenses wide open and whilst I've owned a Noctlux I sold it because I didn't find its speed or rendering useful for me.

[pippy]

4 hours ago, TheLaird said:

...I want my model in the right hand third for instance, and I focus on her eye at 1.2, does it start to taper off from her eye or from the center point of the lens?

I think this is an optics question so must be from center of the lens. If so, does that mean I need to walk back a bit, put her eye in the center and then crop to make her in the right-hand third?

All lenses behave in their own unique way, of course, but it's a pretty common truism that lenses are sharper at the centre than they are at the edges and in the corners. This suggests that even having re-focussed accurately when subject-matter is off-axis the photographer will still be capturing the subject using a less-than-optimal 'part' of the lens' rendering.

Many modern lenses will have fewer abberrations than those present with older optical designs and any inherent softness might well be insignificant.

 

21 minutes ago, pgk said:

It sounds as if you are talking about two aspects of fast optics.

The image plane may not be perfectly flat wide open and this is called image field curvature. The centre may be correctly focused but focus shifts marginally as the sharpest part of the image are on a curve rather than being on a flat plane. On good modern designs this should be minimal though.

Performance variation across the image may also mean that at full aperture the edges and corners are not as perfectly optically corrected as the centre although again, in modern good designs the drop in performance will be minimal.

Really whether either or both will give you real world problems depends on you and your subject matter. I am happy to shoot f/1.4 lenses wide open and whilst I've owned a Noctlux I sold it because I didn't find its speed or rendering useful for me.

😸

I was just about to post pretty much the same thing, Paul, and - as It took me long enough to type - I will still post it out of sheer pig-headedness!

"...Let's consider how a 50mm lens performs when shooting a perfectly flat piece of subject-matter - an exterior brick wall of a building perhaps - situated 1.0m distant from camera. If the centre-line of the subject is at 1.0m the parts at the right and left hand sides within the frame are at roughly 1.04m away from the film (sensor)-plane; a difference of 4%. If there is no in-built correction in the optical design then the central bricks will appear sharp but as we get closer and closer to the edges the bricks go more and more out of focus..."

😺

The situation becomes more complicated as camera-to distance is altered. The laws of geometry don't change, of course, so that '+4%' will always be a constant but as the subject-distance increases so, too, does depth-of-field. Were we to shoot that same brick wall using the same uncorrected (for field curvature) lens at the same aperture then the bricks at the edges wouldn't be as soft, relatively speaking, in comparison to those ones in the centre. This 'flattening-out' continues exponentially until fairly soon the '+4%' difference has no apparent effect on the final image. By infinity the '+4%' has disappeared in all but a mathematical sense.

Philip.

Edited March 18, 2024 by pippy

[TheLaird]

Top replies people. Many thanks indeed.

[patrickcolpron]

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5 hours ago, TheLaird said:

Possibly a stupid question, but it is always best to ask.

I have Q2 but may get a M in the very near future. I am really looking at a F1.2 lens  but what I am struggling to get right in my mind is at 1.2 the center is nice and sharp and then it tails off. Does that mean the center of the lens or the focus point? So if I want my model in the right hand third for instance, and I focus on her eye at 1.2, does it start to taper off from her eye or from the center point of the lens?

I think this is an optics question so must be from center of the lens. If so, does that mean I need to walk back a bit, put her eye in the center and then crop to make her in the right-hand third?

Not a stupid question. 

I can answer for the 50/1.2 Noctilux reissue, it can be sharp toward the corners. 

What people generally mean is a lens can be sharp wide open in the centres and towards the corner it will get softer, that is when everything is on the same plane. 

Unless you plan on photographing brick walls - this shouldn't be too much of an issue. For of cantered subjects, you can get part of them sharp when they are off centred, the problem will be focusing accurately with the Leica 50/1.2 when your subject is off centre even with Live View it is incredibly difficult. There seems to be a slight focus shift towards the corner.

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A photo of my youngest about an hour and a half ago, slightly off centre. Notice how the leaves on the top left corner are in better focus than his left eye, it is not easy when you main subject is off centred to nail sharpish focus. 

 

[patrickcolpron]

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From yesterday, centered but "slightly off centre" due to parallax, for this one I used the range finder patch in the optical viewfinder to focus more accurately with the 50/1.2 but you have to realise that at f/1.2 when a subject is facing you - both eyes might not be in focus with the 50/1.2 Noctilux 

I experienced the same thing in the past with two f/1.2 Nikon lenses, the Noct 58 and the 50/1.2 AI-S it is totally normal 

[patrickcolpron]

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A crop of the centre to show one eye in focus while the other isn't - for the off centre photo above - it was the same, one of two eyes is in focus while the other one is slightly out of focus. 

Hope this helps, the 50/1.2 Noctilux reissue is my favourite Leica 50 lens 

Edited March 18, 2024 by patrickcolpron
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[Jean-Michel]

With a rangefinder, if you focus on something off centre and then recompose you will no longer be in focus on the item you focused on, this is mostly irrelevant unless you are using a lens with a large opening such as f/1.4 or f/1.2 or f/1 or f/.95.  But, that slightly unfocused but lovely image will still look great in a reasonably sized print. Do not be a slave to screen images at 100%. 

[patrickcolpron]

As Jean-Michel mentioned, slightly out of focus subject will look good enough on reasonably sized prints. 

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this said - it would be even more forgiving shooting on film with a Leica MP or an M6 because film is more forgiving. This is my oldest who really loves bing photographed (insert sarcasm here) Made with an M10-P earlier today. It was very difficult to nail off centre focus using live view, might be easier with an M11, who knows. All of this to paint out the crest on his vest is in focus. So, with a 50/1.2 lens with a little practice I am sure anyone can get good with it to get a close subject in focus, 1 meter or two away. This is minimum focusing distance. 

A 5 meters away, or more than 15 to 20 feet away it becomes more difficult to even get sharp focus at f/1.2 even in the centre, but gets easier at f/1.7 and f/2 for more distant subjects. 

You can achieve sharp focus on off centred close subjects with practice and a little bit of luck. This was a fun exercise but since I prefer using the optical viewfinder and the range finder patch to focus - I'll go back to entering my subjects tomorrow  

[patrickcolpron]

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... and a close up crop of the patch to show you can achieve sharpness at f/1.2 close to the corners, but it might not be what you intended to be in focus  

[patrickcolpron]

8 hours ago, Al Brown said:

With M lenses, always take into account the field curvature. It is there because of the small form factor. If you know how to tame field curvature and know if it bulges towards you or away form you, you can usually get stuff pretty sharp wide open off-centre as well, but in no way with central rangefinder patch.

That takes a little practice, the main issue I have is even with the EVF, using Live View, it is difficult to get correct focus off centre with the 50/1.2. EVF tells you it is in focus when it isn't. I know off centre the 50/1.2 back focuses, don't trust the EVF when focusing off centre with it 😉 

[patrickcolpron]

16 minutes ago, Al Brown said:

The EVF’s focus peaking is just a contrast based tool and has nothing to do with critical sharpness measure - it is just a digital approximation aid. Live view with zoom will always give more accurate results.

I am fully aware how the focus peaking works, and that it is more of a hindrance with the M10 than an accurate way to focus on a subject. I usually have it set to off in my menu.

With the 50/1.2 lens focus peaking has a hard time in the centre of the image to grab on much of anything in good light, lest evening was overcast and close to sunset when I made these off centred photos, I did turn on focus peaking and it was useless at full magnification off centre.

It had me thinking maybe it would be easier with an M11 since there is digital image stabilisation with Live View magnification yet, to be proven. I'll test it this week with an M11, with an M10 it is pretty much useless off centre even in full magnification without knowing where he focus actually falls.

What I am pointing out is Live View even in full magnification when used to focus off centre with the 50/1.2 Noctilux at f/1.2 on a subject a meter away, minimum focusing distance, is not an accurate way to proceed. One needs practice to know where the focus actually falls off centre  

9 hours ago, Al Brown said:

With M lenses, always take into account the field curvature. It is there because of the small form factor. If you know how to tame field curvature and know if it bulges towards you or away form you, you can usually get stuff pretty sharp wide open off-centre as well, but in no way with central rangefinder patch.

As you pointed out here, I should have clarified that. One on my way to drop my kids at school this morning I thought I should have added that the focus peaking has a hard time grabbing anything at f/1.2 in the centre, and it is worst when placing a subject at the edge or off centre. It's no help.

It is possible to have off centred subject in focus, but it will require practice even with EVF at full magnification. Focus Peaking highlight will be no help at all off centred. 

[patrickcolpron]

24 minutes ago, Al Brown said:

To point out another fact - some lenses with brutal field curvature, like a fast 35mm Voigtlander I own, I can hardly get to be sharp wide open off center. Here is the approximate field curvature chart of such naughty lens. A true challenge, almost mission impossible.

That's way too technical for me. I have a camera and one lens, I compose my photo, I press the shutter and as if by magic = I get a photo out of that magic box. 

Most of my subjects are more or less in the centre, and when off centre - I close down the lens a bit to make sure it's in acceptable focus.  

I don't look a lens charts, have no clue what a back lit or CMOS sensor is or does. The one time I looked at a lens chart was for the Steel Rim, lit ooked my my electrocardiogram when I am doing a stress test but have no clue how to interpret the results of either my cardiogram and the lens chart.

I like experiencing the lenses and cameras making photos with them, to figure out what can be done with them. This is why this topic peaked my curiosity to no end, hum, how about trying to make a sharp off centre photo with my 50/1.2, that's a cool thing to do. 

About the charts, I strangely prefer remaining clueless about certain stuff or maybe it's just that I'm a little slow  But maybe I'd be better at making photos if I wasn't this clueless... This might lead me down a perilous path. I'm torn... 

[pgk]

FWIW. ALL ultra fast (>f/1.4) lenses can be difficult to focus on anything other than static subjects. I've owned a few (f/1 Noctlux, f/1.2 Canon 50 and 85, 24/1.4 Canon and more). this is (as ever) not down to one simple factor but several interacting which go together to make life difficult trying to ensure absolute focus. In my experience the best solution which I have come across so far is to use live view and 'zoom' in onto the point which you require to be as accurately focused as possible. But this is not an easy way to operate on anything which moves. I ended up just accepting that you had to take a lot of images to ensure one which fulfilled expectations and since I don't like hit & miss photography I did not pursue ultra fast lenses. That said, if you are prepared to practice, practice, practice then I'm sure that the hit rate would climb.