If shooting at f/22, is a fast 1.8 lens still considered fast?

Started Apr 17, 2011 | Discussions thread
SteveL54
Senior MemberPosts: 2,610
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Low Light Electronic Range Finder Test
In reply to edwardaneal, Apr 21, 2011

edwardaneal wrote:

I disagree. The focus system may only need as much light as it can get with an f/5 or 5.6 lens in reasonable light and it may not focus faster or more accuratly with more light, but when you get that f/5 or f/5.6 lens in lower light the focus system can start to miss focus and hunt for focus where as if you had an f/1.4 or f/2.8 lens on the camera the lens would pass as much light to the af system in low light as the f/5 or 5.6 lens did in good light.

sometimes if the light gets too low my 70-300vr will hunt for focus. In these same lighting conditions my primes do not.

If you think I am wrong test it. mount a variable apreture zoom lens and get it in a low light situation at the long end where it is only f/5.6. by sure your AF assist is turned off - I would bet it is fairly easy to get it hunt and not focus well. Then try it with an f/1.4 prime in the same light on the same focus target.

That test has already been done.

See this thread:

http://forums.dpreview.com/forums/read.asp?forum=1030&message=30060787

Despite being skeptical, Kerry did a controlled test and determined low light focusing capability is not improved by larger aperture lenses.

Nikon camera body specifications do not indicate the lens aperture in the minimum EV specified for AF operation. I've never seen Nikon literature say low light AF performance is improved by using larger aperture lenses. Wouldn't you think they would list that as an advantage of their higher priced lenses?

Marianne did another controlled test where she blocked only the f/5.6 region of a 50 mm prime. (See the next post in that thread: Lessons of the Dot) That would block only a very small portion of the entire light coming from that lens. However it rendered the camera incapable of AF operation.

I have since performed the complementary test of closing down the aperture of a f/1.4 50 mm prime while the electronic range finder (ERF) is operating under very poor light. The advantage this test has over comparing an f/5.6 max aperture lens to a f/1.4 aperture lens is that it eliminates all other variables that might come into play when switching lenses.

I had to use an old screw mount (M42) lens on my D300 which allowed me to manually stop down the aperture while the ERF was reading the target. I have an M42 to f-mount adapter which allows me to use the lens from my first SLR (now 40 years old) The camera electronics are completely oblivious to the lens settings.

I used a high contrast target similar to what the Nikon service centers use for AF adjustment tests. Camera was on a good tripod and ball head. At full aperture and under normal room light I focused the lens until the indicator became solid.

Then I turned down the lights until the ERF started to blink. (The ERF in-focus indicator blinks on/off once per second if there is not enough light and/or the target has insufficient contrast.) The room was rather dark at this point. Then I raised the light just enough to get the indicator to come back on steady. That establishes the ambient light limit for the camera's AF operation when the lens is set wide open.

Then I closed the aperture done one full stop at a time until the indicator started to blink at 1 Hz. The aperture at which it started to blink was f/8. Turn the dial back to f/5.6 and the focus indicator came back on steady. So despite the rather dark room the ERF continued to indicate in-focus down to f/5.6

I can repeat the same procedure under normal room light conditions and the same thing happens. The ERF works fine down to f/5.6 but turn it one more stop down to f/8 and it just blinks.

Steve

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