Auto and f stops.

[Derrel]

As far as programmed exposures go there is a pretty much well known characteristic. Let's say we are using 400 ISO. At very dim exposure values, the camera will select pretty much a wide open lens aperture for the lens in use and will select a slow shutter speed. As the light value goes up the camera will close down lens aperture and as it goes up even more the camera will speed up the shutter speed

in a very dim environment with a 50 mm 1.4 lens the camera might select f / 1.4 at 1/8 second. In a very bright environment such as at the beach the camera might select f/16 at 1/1000

You can typically see a graph of this in the instruction manual.

When you are using shiftable programmed automatic exposure, you can just turn the rear command dial to get a series of equivalent exposures with correspondingly equivalent aperture and shutter speed changes.

 

[charlie76]

I've always assumed the auto mode chooses the largest depth of field it can - and uses shutter speed and iso as limiting parameters.

 

[Derrel]

I've always assumed the auto mode chooses the largest depth of field it can - and uses shutter speed and iso as limiting parameters.

I am not sure that is correct. You speak of the auto mode, but there are several different types of automatic exposure mode.

 

[Pixeldawg1]

Most of the auto-aperture settings are now simplified to "Flowers", "Low Light", "Sports" and other basic descriptors. If you want to set things up automatically, try using the shutter-priority in the camera. This is where you set the shutter speed and the camera sets the aperture. This way, you have a much greater control over the depth of field by changing the shutter speed until you are at the desired aperture setting that you want. As far as Bokeh is concerned, there is a distinct difference between it and Depth of Field and in basic photography, there is a lot of confusion about these. Bokeh is the quality of the softness in the background. Some lenses, because of glass, design and other factors, can have a more visually pleasing blur in comparison to other lenses. Each lens is unique in this quality level and again, some lenses are better at this than others.

Now, Depth of Field is a bit more technical. the lower your aperture number, (say, F2.8) the LESS depth of field you'll have. Increase that aperture number, (say, to F16) and you will have a greater apparent sharpness on the Z axis of the image (think here about your high school geometry for a moment. You had X, Y and Z axis. X and Y are vertical and horizontal, and Z goes into the depth of the image, so in a 3D image, you could move toward the subject physically, but since your 2D in a photo, it is only "apparent", but works when we discuss Depth of Field.) There is an old rule too that says that Depth of Field is also broken up into "1/3rd and 2/3rds". Meaning that if you focus at 8 feel away, and you have 9 feet of depth of field at your camera setting, everything from 5 feet to 14 feet will be in focus. 1/3rd BEFORE the point of focus and 2/3rds After the point of focus for that Depth of field number, so 3 feet in front of the focus point at 8 feet (8-3=5) and 6 feet AFTER the point of focus (8+6=14) Now, this is a graduated change. Some photographer disagree with this, which is OK, but generally it is pretty accurate for general situations. Now, this is the basics.

Depth of field gets more complicated when you change lenses and when you move physically closer or farther away from your subject. So, if you are photographing bugs for example and your using F16 with a 105mm macro lens and are 5 inches away from your subject, you may have less than an inch of depth of field. If you take that same lens and photograph something more distant, say 30 feet away at F16, you will have several feet of depth of field with the very same lens. ( BTW, lenses that produce the subject from 1/4 to 1/1 of its' actual size on the film or on the sensor are called "macro". Anything above that- larger than life, is Micro).

Also, lens focal length plays a part in this as well. A wide angle lens has more depth of field at the same aperture (let's say F16 again) than our 105mm at F16 and focused side-by-side at the very same object. Also, there is a change in perspective as well. The wide angle makes things look farther apart in comparison to real life while a telephoto compresses things and makes them look closer together than they really are. When you see photos where the sun looks HUGE in comparison to other objects that it normally appears to be in the frame, it is made with a telephoto lens. When you see a photo of a tree with a lot of detail on the left side of the frame, and mountains far off in the distance and EVERYTHING is sharp, it was made with a wide angle lens.

Most likely more than you wanted to know, but hope it is helpful.

Cordially,

Mark

 

[photoflyer]

Your results surprise me a bit as if I were programming the Auto mode on a DSLR I would use settings that would differentiate the results from the competition: cell phones. Your camera is certainly able to do that but those settings are so "middle of the road.". Maybe the green A stands for " average".

 

[smoke665]

@HowdyMark very well written simplified explanation. One thing not covered is the fact that the focal plane is not flat but concave Petzval field curvature - Wikipedia . Manufactures attempt to correct this fact in their lens construction, but depending on the model or focal length it can still be quite noticeable in the corners.

 

[JBPhotog]

@HowdyMark very well written simplified explanation. One thing not covered is the fact that the focal plane is not flat but concave Petzval field curvature - Wikipedia . Manufactures attempt to correct this fact in their lens construction, but depending on the model or focal length it can still be quite noticeable in the corners.

Unless of course one is using a Micro-Nikkor which has been corrected for field curvature and is considered flat field, not referencing light transmission distribution.

 

[wfooshee]

Not sure why you'd use shutter-priority, then change the shutter until the aperture you want comes up. Why not use aperture-priority and, um... set the aperture you want? If aperture control is what you're after...

 

[Derrel]

If all you are doing is centering the light meter, then you might just as well use programmed automatic and only have the need to move the rear command dial to get the correct shutter speed and aperture you want.

You can of course use exposure compensation and if you like you can even combine this with automatic ISO shifting in some camera systems.

 

[Pixeldawg1]

@HowdyMark very well written simplified explanation. One thing not covered is the fact that the focal plane is not flat but concave Petzval field curvature - Wikipedia . Manufactures attempt to correct this fact in their lens construction, but depending on the model or focal length it can still be quite noticeable in the corners.

Very true, but I consider this more of a lens characteristic than within the discussion at hand. Still, your point is correct and well taken. Thanks!

 

[Pixeldawg1]

This is true in ANY macro lens and not just the Nikons. Canon, Sigma and other macro lenses are all designed as flat field lenses. Additionally, so are enlarger lenses. In fact, in the bad-old-days of photography (Read, 1980) I knew MANY photographers who would take a high quality 75mm enlarging lens, put it on a bellows and that was their macro rig. Worked pretty well in most situations, but not nearly as good as the current crop of macro lenses. And a technical point that is important to remember, Nikon calls their (macro) lenses "Micro", which is technically inaccurate because unless you place it on a bellows or have an extension tube in use with it, it is a true "macro" lens and is capable of 1:1 reproduction (Meaning again, the object size on the film or sensor is exactly the same size as it is in real-life.). The "micro" reference is strictly a marketing thing. And even still, I LOVE my ancient, bakelite 105mm "micro" lens!

 

[petrochemist]

I wonder if the camera "knows" what focal length lens was being used and making decisions based on that? If not perhaps there were assumptions made about what is "typically" used when the algorithms were written?

Personally, I almost always use aperture priority to the the depth (or lack of it) that I want.

It's common for cameras to make use of focal length in their auto calculations. The old 1/focal length rule gives a good idea why.
When I'm adapting lenses the camera usually doesn't have that information.

I often have to settle for a different DOF than would be ideal, to balance noise & shutter speed requirements, but Av is certainly my go-to mode as well.

 

[Pixeldawg1]

I wonder if the camera "knows" what focal length lens was being used and making decisions based on that? If not perhaps there were assumptions made about what is "typically" used when the algorithms were written?

Personally, I almost always use aperture priority to the the depth (or lack of it) that I want.

It's common for cameras to make use of focal length in their auto calculations. The old 1/focal length rule gives a good idea why.
When I'm adapting lenses the camera usually doesn't have that information.

I often have to settle for a different DOF than would be ideal, to balance noise & shutter speed requirements, but Av is certainly my go-to mode as well.

It seems to me that Photography has always had this compromise, and those who are the very best "compromisers", produce the better images. This can be via skill, or intuition.

Cordially,

Mark

 

[AlanKlein]

Aperture priority is good for me as I shoot a lot of landscape so I want deep DOF. SInce I'm shooting on a tripod, I really don't care how slow the shutter is. But if I was a sports or bird shooter I'd use shutter priority. When I travel on vacation shooting shots off the cuff, I leave it in Auto mode and let the camera do all the work while I have fun watching the view and ignoring the camera.

 

[petrochemist]

Aperture priority is good for me as I shoot a lot of landscape so I want deep DOF. SInce I'm shooting on a tripod, I really don't care how slow the shutter is. But if I was a sports or bird shooter I'd use shutter priority. When I travel on vacation shooting shots off the cuff, I leave it in Auto mode and let the camera do all the work while I have fun watching the view and ignoring the camera.

When shooting sports etc. I often want the fastest possible shutter so Av (wide open or one stop down) generally works well.
Propeller aircraft & panning are generally the only common situations I use Shutter priority.