OK I think I see your point. The large format DSLRs are exactly challenged in the opposite in that you can't get enough DOF to make the entire subject sharp, as with your spider. So too much light hits the sensor.
Correct. The larger the format, the easier it is to get DOF so narrow it's useless. 4x5 and 8x10 film cameras commonly have lenses that stop down to f/45 or f/64, and they need to in order for such large formats to have usably wide DOF in many cases. That's how Group f/64 (which included Ansel Adems) got its name; Ansel and friends commonly used very small apertures (as well as camera movements and other advanced techniques) to create highly detailed and clearly-focused images. The tradeoff in their case was long exposures, sometimes several seconds, even in broad daylight.
Also, the way I understand the lack of shallow DOF with the F828, or any small sensor camera, is the lack of light that hits the sensor. Is this simplistically right?
Indirectly, yes. The root cause is the small physical aperture diameter (the size of the "hole" light passes through to get to the film/sensor) that is more typical with smaller-format cameras. The larger the format, the larger the aperture must be to let enough light strike the film/sensor for proper exposure in the same time.
I wonder if moving to ISO 200 and shooting wide open might give a shallower depth of field?
Half right. Shooting wide open (f/2 or f/2.
will give you a shallower DOF than shooting at f/8, but increasing ISO at the same time would be a bad move. If you're shooting wide open, and were shooting at f/8 before, you'd want to
reduce ISO in conjunction with opening up your aperture to keep the shutter speed within the capabilities of the camera. Opening up your aperture and raising ISO are both factors that require a faster shutter speed; doing both simultaneously can quickly put you beyond the limits of the camera and cause blown-out exposures.
With any camera, you have 3 basic variables you need to manage for proper exposure: shutter speed, aperture, and ISO. Generally, you want to use the lowest ISO setting you possibly can to get maximum dynamic range and image quality with the lowest possible noise levels. If you can use a slower shutter speed or larger aperture without compromising the image with excessive motion blur or too-shallow DOF, you're generally better off doing so and shooting at a lower ISO setting.
Set your shutter speed so that the image is not being unacceptably blurred by camera or subject motion, but you don't necessarily want to go much beyond that. For static subjects like landscapes, you're often best off stopping down to get a wide DOF (unless you're deliberately using selective focus for creative reasons), shooting at the camera's lowest ISO setting, and shooting from a tripod to keep the resulting longer exposures from causing camera motion blur.
Select your aperture to get the desired DOF, but keep in mind there are drawbacks to using extremely large or small apertures. When shooting wide open (like f/2 or f/2.
, lens aberrations will be most prominent (lenses that perform well wide open are much more expensive than those that do not) and when stopped down all the way (f/22 or f/32) you lose contrast due to diffraction. Most 35mm-format lenses are sharpest somewhere in the f/8-f/11 range, but this varies from lens to lens. Smaller format lenses start running into diffraction sooner, and their "sweet spot" may be more like f/4-f/5.6. But this varies greatly depending on the specific lens; even changing the zoom setting can alter the "best quality" aperture range somewhat. Experience is the best guide here.
