Opinions on lens sharpness

[rodeo_joe1]

"Any idea why I have so much trouble focusing the f/1.2?"

 

-There are (at least) three things at work to prevent easy focus. Firstly the lens has a high degree of residual spherical aberration, and SA spreads the plane of best focus. There's a plane of highest resolution - called paraxial focus, and a plane of highest contrast. They don't co-incide when there's SA, and the worse the aberration, the further apart the planes of focus are. The eye tends to pick the plane of best contrast, which isn't the plane of best resolution.

 

Secondly: Any lens that suffers from SA necessarily shows a focus shift on stopping down.

 

Thirdly: The viewfinder geometry used by Nikon in every one of its (D)SLR cameras doesn't allow viewing a lens at an aperture greater than f/1.8. In other words it doesn't matter if the lens is at f/1.4 or f/1.2; the screen/mirror/prism arrangement cuts off the effective aperture to around f/1.8. The viewfinder is exactly the same brightness, and shows exactly the same depth-of-field as if the lens was stopped down to f/1.8.

 

Small wonder you can't focus the 50mm f/1.2 lens easily by eye.

 

P.S. The above viewfinder restriction is met with skepticism and denial by some, but it's easy to prove. Just set the aperture of an f/1.2 or f/1.4 lens to f/1.8, and hit the aperture preview button. Do you see the slightest decrease in viewfinder brightness, or any change in the depth-of-field? I thought not!

[conrad_hoffman]

That makes sense, and I had never heard about the viewfinder limitation. Is that true for the full frame bodies too? Way back when i bought the D200, I chose it so I could take advantage of my old lenses. It turns out I rarely use any of them, as the micro and the kit zoom do most of what I need to do.It might be different if I had some very good long lenses and a reason to use them, but that's not where my interests lie. BTW, I rarely do lens tests, as the winners quickly become obvious when I look at the results.

 

One note on the 50 mm lenses. Long ago I had a brand new 50 mm f/1.4, about 1972. I think it was the S, not sure if it was Ai. It was OK, but never delivered what I considered the crisp images that a borrowed f/1.8 did. At some point I sold it and got the newer one with the black front that I tested above. Much better.

[ben_hutcherson]

That makes sense, and I had never heard about the viewfinder limitation. Is that true for the full frame bodies too? Way back when i bought the D200, I chose it so I could take advantage of my old lenses. It turns out I rarely use any of them, as the micro and the kit zoom do most of what I need to do.It might be different if I had some very good long lenses and a reason to use them, but that's not where my interests lie. BTW, I rarely do lens tests, as the winners quickly become obvious when I look at the results.

 

Most AF bodies are even worse than MF bodies when it comes to showing DOF and focusing properly.

 

In the '80s, even a low end body from any make came with a 50mm ~1.8 as its standard lens, so a screen with a focus aid and a good grind to show DOF was a must. Looking at cameras like the F3, Canon New F-1, and even lower end bodies like the A-1 and FM/FM2 I can see a fair bit of "tooth" in the focusing screens which tends to give them fairly good contrast even without using a focus aid. I've talked before about the subtle "pop" I can see even when a matte screen comes into focus. The standard screens(K on Nikon, C on Canons) is optimized for fast lenses, but for the pro-level cameras from both makes it's easy to swap screens that work well with different lenses. I have a New F-1 screen meant for use with long slow lenses, especially with TCs-it gives an aerial image(does not show DOF or any focus) over most of the area but has a ground spot in the center for focusing.

 

AF also marked a big shift toward zoom lenses, both for amateurs and pros. I don't think one is necessarily related to the other-it's just that good zoom lenses started showing up around the same time AF became viable. Even big heavy pro zooms usually top out at 2.8, while a typical consumer zoom might be 4.5 or 5.6 on the long end. Since manual focusing isn't really a priority, modern screens tend to have a VERY smooth grind(I often can't see any texture in a modern camera) which gives an acceptably bright image with a slow zoom and a pentamirror, and quite a brilliant image with a prism and 2.8 lens. Even playing with the D2H with a 50mm 1.4 I have sitting here, though, I can't see any difference in brightness past 2.5 or so.

 

I will add that as an exception to the AF "rule" on screens, I find the stock F4 screen to be as good as an F3 matte screen, and the L screen I have for an F5 is also excellent on the matte areas(I don't like the 45º split). I'd love a K screen for an F4, but they bring more than I want to pay-I lucked out getting the L screen in an F5 from KEH, esp. since the standard screen(which I use most of the time) runs $20 on Ebay.

 

Also, on the other extreme, my Speed Graphic has an honest to goodness ground glass in the back of it. There's no fresnel or anything on it, which means that it's somewhat dim and also vignettes terribly. Still, though, I can focus with that screen more accurately than I can anything else. Outside, I need a full blown dark cloth for critical focus(the built-in hood doesn't help a lot) and of course also use a 4x loupe for critical focus, but there's no question about what's in focus and what's not. Granted f/4 would be a fast 4x5 lens and most folks aren't going to use them wider than f/16 or so, but even there DOF is often shallow enough that you need to check EVERYTHING.

[conrad_hoffman]

Back in my Nikon F days I used a plain matte screen. Young eyes had no trouble focusing on it and I think the lack of focusing aids made for better composition. I've ground a number of replacement focusing screens for various 4x5 cameras but have to admit not shooting a piece of 4x5 film for many years. Maybe someday- my darkroom is still set up.

[ben_hutcherson]

One of the interesting ones for me is the D screen for the F/F2. I have one that came in my first F2, and when I first tried to use the camera I thought there was something wrong with it.

 

The D screen is dim, course(I have some relatives that are like that) and it vignettes badly. After I looked it up and realized that was normal, I went to my screen collection and fortunately had a spare A screen.

 

Still, though, the D screen is easy to focus provided that it's bright enough to see. A magnifier does help it.

 

One of the great things about 60s and 70s SLRs from the major makes is that most of the screens for them are easy to find and are cheap. I have quite a few for the F/F2 as well as for the Canon F-1. I have a bunch that are definitely specialized, but are none the less great when it's appropriate.

 

As an example, not too long ago I photographed one of my nephew's baseball games using my F2AS/MD-2 and 80-200 2.8D. The 80-200 of course works fine on the F2A/F2AS, and unlike a lot of AF lenses it's pretty nicely damped for manual focusing. I used an H1 screen, which is a bit of an oddball in that the entire surface is covered with microprisms(there are 4 different H screens optimized for different focal lengths and maximum apertures). It's great for tracking action, though, as it effectively blocks everything not in focus.

[rodeo_joe1]

"Is that true for the full frame bodies too?"

 

- True for all the Nikon SLRs and DSLRs I have at my disposal. (D700, D800, FM, FE, F2, F3HP, F801s, F4) The DX DSLRs are even worse, with an effective viewfinder 'aperture' somewhere in the region of f/2.8.

 

BTW. I've confirmed the lack of brightness change with lenses > f/1.8 using an eyepiece photometer. That too shows only a 2% variation in screen brightness between full aperture and f/2 with the 50mm f/1.2 lens. This makes me wonder if Nikon's viewfinder-based metering has some sort of 'fudge-factor' built in, because I really can't see how the metering can work properly otherwise.