preset the focus via the distance setting?

[investmenttechnology]

I on page 46 of Understanding Exposure, 3rd Ed by B. Peterson, can some tell me how I can preset the focus via the distance settings on my D700 with 50mm? I have no idea what he's talking about.

 

[Overread]

If I have it right look at the barrel of your lens, there should be either a window or markers on the lens noting distances (feet or meters). Preset by the distance scale is presetting your focus using those distance markers on the lens barrel.

 

[KmH]

If the lens has one, the distance scale is not very accurate.

 

[analog.universe]

If the lens has one, the distance scale is not very accurate.

This varies greatly by lens... I would hope a Nikkor 1.4G is up to the test. On cheapo lenses though, yes, it certainly not accurate.

Usually right next to the distance scale you'll notice a symmetrical arrangement of aperture numbers on either side of the focus indicator (i.e. 16 11 8 5.6 4 2.8 2 1.4 | 1.4 2 2.8 4 5.6 8 11 16). This is the depth of field scale. If you're shooting at say f/16, and you focus to 2m using the distance scale, the marks for f/16 on either side point to two distances between which your image is in focus (1.5m to 4m or so). This is also useful for setting the hyperfocal distance. If you're at f/16, line up the far f/16 mark with infinity, and everything from the near one to infinity will be in focus.

 

[investmenttechnology]

Usually right next to the distance scale you'll notice a symmetrical arrangement of aperture numbers on either side of the focus indicator (i.e. 16 11 8 5.6 4 2.8 2 1.4 | 1.4 2 2.8 4 5.6 8 11 16). This is the depth of field scale. If you're shooting at say f/16, and you focus to 2m using the distance scale, the marks for f/16 on either side point to two distances between which your image is in focus (1.5m to 4m or so). This is also useful for setting the hyperfocal distance. If you're at f/16, line up the far f/16 mark with infinity, and everything from the near one to infinity will be in focus.

Excellent explanation, I think I've got it, the aperture is f/16, I get about 2m to infinity. Does that mean anything in the foreground, ie from the camera to 2m will still be blur?

 

[analog.universe]

Usually right next to the distance scale you'll notice a symmetrical arrangement of aperture numbers on either side of the focus indicator (i.e. 16 11 8 5.6 4 2.8 2 1.4 | 1.4 2 2.8 4 5.6 8 11 16). This is the depth of field scale. If you're shooting at say f/16, and you focus to 2m using the distance scale, the marks for f/16 on either side point to two distances between which your image is in focus (1.5m to 4m or so). This is also useful for setting the hyperfocal distance. If you're at f/16, line up the far f/16 mark with infinity, and everything from the near one to infinity will be in focus.

Excellent explanation, I think I've got it, the aperture is f/16, I get about 2m to infinity. Does that mean anything in the foreground, ie from the camera to 2m will still be blur?

Yep, that's correct.

 

[investmenttechnology]

so in order for me to get everything into focus, I must buy a lens with short focal length and a smaller aperture capability (f/22 or higher)?

 

[analog.universe]

A shorter focal length and tighter aperture will always get more in focus, but you'll likely never get "everything". You'll probably also never need everything, per se... I've shot landscapes with close up elements at 16mm f/11, and there is nothing visibly out of focus, from 1m to infinity. There's also a school of thought that says when you photograph a scene like that, it's unnatural for the eye's perception to see everything perfectly sharp, and leaving infinity just slightly out of focus will yield a more realistic and deep feeling scene. If you're pushing your aperture to crazy places like f/22, you end up losing overall sharpness to diffraction, even if everything is in focus. I only ever find that trade off worth it in macro situations where DOF is so short that you don't have a choice. For landscapes and such, I rarely even make it to 16.

 

[Josh66]

If the lens has one, the distance scale is not very accurate.

I guess it would depend on your definition of "very accurate"...

I just tested all of my lenses, they were all perfect - the actual point of focus corresponded exactly to the marked distance on the scale.

For this test, I chose a distance that was clearly marked in the scale, in the range of the lens with the finest control over focus. For example - doing this test with a macro lens on an object 10 feet away would not be a good idea. The distance scale goes from 5 feet to infinity in about 1/4". However, the distance it takes to go from 1.02 feet to 1.60 feet takes up over half of the scale. Choose a distance that is somewhere in that range where small differences take up a lot of the scale (that could be worded better, but I don't feel like sitting here for 10 minutes trying to figure out a better way to say that, lol).

I place an object a set distance from the camera (measured from the film plane), then autofocused on it. In all cases, when I then looked at the distance scale to see what it read - it was exactly the distance from the film plane to the object in question.


Maybe there are lenses that have inaccurate distance scales, but since every single one of mine was dead on I don't think you can really make a blanket statement that the scales aren't accurate...

edit
I suggest that you (anyone reading this) do your own testing before drawing any conclusions.

 

[Dao]

Question: Will the distance scale affected by the recording medium size? (35mm film vs cropped Sensor)

 

[Josh66]

As long as the flange distance is unchanged, I would think not. But like I said, it would be best to do your own testing before just assuming that everything on the internet is true.


I might test it out on a crop body just to see if there's any difference... (The test I did earlier was with a 35mm body.)

edit

Also, the lenses I used were (all Canon):
70-200mm f/4L
100mm f/2.8 Macro
50mm f/1.4
85mm f/1.8
135mm f/2.8 Soft Focus

 

[analog.universe]

The size of the recording medium only determines which part of the image circle is recorded. If you take a shot with a lens on a crop camera and then take another shot with the same lens on full frame, all you need to do is crop the full frame image in post and they will be identical. The focal distances, DOF, etc.. will all remain the same. If you're using an adapter from one lens mount to another, or using extension rings, or in any other way changing the mount to sensor distance, then the distance scale will not be accurate.

 

[Dao]

The reason I ask is when I calculate the Hyperfocal distance or DoF with the DoF calculator, it yield a different result with different recording medium size.

 

[analog.universe]

That's because of a difference in the calculated circle of confusion. As the physics go, technically only the exact distance the lens is focused at will actually be in focus, at any aperture (beside 1/infinity of course). However, the defocused light needs to reach a certain size before it starts to be picked up neighboring pixels. This size is the circle of confusion, and is actually determined not only by sensor size, but also pixel density. When they print the DOF scale on the lens, a certain COC is assumed that may or may not be close enough for your purposes. The difference in calculated DOF from full frame to crop was likely because the pixels are a different size in each format.

That probably was a terrible explanation, you should Wiki Circle of Confusion and Depth of Field if you really wanna get into it.

 

[Josh66]

OK, I was bored, so I just tested a crop body with the 70-200 and the results were the same.

I only tested one lens on the crop body because I never use that body anymore and I honestly don't care if it works, ha. Do your own testing to be sure (your lenses may also be different). From MY testing with MY gear - crop or full frame does not matter.


Oh yeah, I forgot to mention - on the 70-200, I tested it at all marked focal lengths (70, 100, 135, 200) for both tests. In all cases, it was "dead-nuts-on".

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In all honesty, this "they are not accurate" nonsense most likely comes from poor distance estimation, not bad scales on the lens.

I measure stuff all day long every day, so I can usually estimate pretty accurately. Especially measurements of less than an inch ... after 10 or 12 years of working with sheetmetal all day, you can pretty much eye-ball anything under a half-inch and be within .025". Within .005" for anything under 1/8"... Typical metal thicknesses you will find on an aircraft are .015", .020", .025", .032", .040", .050", .063", .071", .080", .090", .100", .125" ... etc. It doesn't take long before you can tell which is which just by looking at it.

By the way, I DID use an actual measuring instrument for these tests, lol! No estimation on the testing.