Continuation: Sensor Size, Present & Possibilities

Started Apr 10, 2013 | Discussions thread
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Re: On DOF
In reply to olliess, Apr 10, 2013

olliess wrote:

EinsteinsGhost wrote:

olliess wrote:

EinsteinsGhost wrote:

2- Exposing part of a full frame sensor (FF in APS-C mode)

Unless there's a special shutter or a black mask for DX crop mode, I'm guessing you "expose" the whole sensor regardless. You just don't record all the data from the FF sensor.

You have only the image circle projected through the lens in DX crop mode.

Supposing you really want to argue about this, do you think that no light shines on the FF sensor outside of the APS-C frame when DX crop mode is activated? Even allowing for a dedicated DX lens?

Image circle needs to be larger than the sensor. But that is completely irrelevant to the issue. If you want to consider the entire circle, you could but what do you expect out of it?

3- Digital cropping

In #1 and #2, you compose accordingly, and likely expose for the composition. And #3 can be done to either, or to any sensor size. The question is, why would you prefer #3 over #1 or #2?

That's like asking, "Why crop at all?" The answer is that you like the picture better after trimming away off some of it.

No, that is asking: Why would you prefer #3 over using a longer FL lens?

Two obvious cases come to mind: 1) you don't have a longer lens with you, and 2) you get to make the final decision later.

You can do this anyway, and as addressing a need. Obviously you realize that it isn't ideal. No?

EinsteinsGhost wrote:

olliess wrote:

EinsteinsGhost wrote:

There is no need for that. This was taken with NEX-3 and 35mm f/1.8 lens (wide open):

Why exactly would you need 50mm f/1.4 on FF?

A) To increase the subject isolation while keeping the same subject-background distance

B) To keep the same subject isolation while decreasing subject-background distance

C) To increase the exposure to reduce noise

D) To increase the shutter speed in case the subject suddenly jumps at the camera.

Do we need to continue?

You didn't use an objective analysis, but went with a theoretical one.

What kind of "objective" analysis is possible? I gave you a list of possible objectives one could accomplish with a wider aperture. The same would apply to a 35 mm f/1.2 on the APS-C sensor.

Objective analysis requires a demonstration of facts. By claiming "you need larger aperture you are able to increase subject isolation" isn't that but simply theoretical (and worse, only a part of it). You can see it for yourself by asking self a very simple question: "Do I get the best isolation at all times by keeping aperture wide open?"

You will realize that the answer is not as cut and dry. You may end up making a part of the subject blend with the surroundings (too shallow DoF). Recognizing that aspect is being objective.

Have you never encountered a situation where your DoF is too shallow for good isolation? With my APS-C bodies, I see that often and feel the need to stop down to have sufficient DoF. After all, I wouldn't want portraits with eyes in sharp focus but blurred out nose.

When I need more DOF, I can always stop down a fast lens. I can't open up that 35/1.8 wider than f/1.8. The same reasoning applies to all fast lenses, even though you don't always (maybe even rarely) shoot wide open.

It is true that you can go one way but not the other. The issue I've pointed at, however, has to do with the practical side of it. I can see an insignificant advantage but larger cost and size of going with larger aperture, but if that is the thread you want to hang your hat on, for FF sensor, well, you have that choice. I don't see a point to it. Or, perhaps you can show me what is it about these yet shallower DoF at wide angles that is dictating the need to spend several times more.

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