Fast lenses, and High ISO

Started Jul 18, 2014 | Discussions thread
bobn2 Forum Pro • Posts: 59,141
Re: "fast" is relative

EinsteinsGhost wrote:

bobn2 wrote:

EinsteinsGhost wrote:

bobn2 wrote:

EinsteinsGhost wrote:

bobn2 wrote:

Fine. That's what I said too. What determines how fast a shutter speed?

"Sensitivity" of the media, scene brightness and aperture. Nothing to do with media size.

That's a strangely abstract way of determining your shutter speed. Do you have a table of available shutter speed against media, scene brightness and aperture? As I remember, my camera allows me to set 1/8000 for any scene brightness, 'sensitivity' and aperture, why should I not?

Glad that you have a camera you know how to turn dials of. Now, let us try something more with it. For that matter, let us use one of your images with EXIF:

So, why did your camera, or you, choose 1/250s, f/4.5 at ISO 100? Your response will help you get my point, on why you can't just turn the dial to 1/8000s just because you happen to have it.

Can't remember why I chose that exposure at that time. I was just trying out the camera anyway. But if I reconstruct my thinking. I aim for the largest exposure that I can get, subject to my pictorial constraints. This was hand held, and I was trying to avoid shake, so I set a shutter speed of 1/250 for 85mm and f/4.5 is the largest aperture that lens has at 85mm. Does that answer your question?

That wasn't a question. It was a step to help you understand what you (and/or your camera) do when it comes to exposure.

I already know what my camera does when it comes to exposure. I don't think that I have anything to learn from you.

In this case, if you can't remember, or tell, chances are, it was probably your camera's decision-making in the process.

Chances are it wasn't, since I generally use 'M' mode.

And that is perfectly fine. NOW, I have questions:

1-You speak of "large exposure". Could you explain how your idea of: "since I have 1/8000s, I could use it", fits in? In other words, you could have just as easily used 1/8000s shutter speed, right?

1/8000 would have given me a smaller exposure than 1/250 and I didn't need any faster than 1/250 to avoid motion blur, so I wouldn't have chosen 1/8000s.

2-Would your selection of a different shutter speed guarantee a change in exposure?

If I had used a different EV in the same lighting conditions, it would have course resulted in a cange of exposure.

3-What makes 1/250s, f/4.5 the "largest exposure" that you could?

1/250 was the longest shutter speed that I wanted for the reasons of camera shake, as explained. Twice the FL of the lens is a decent guide at 36MP, that gave 1/170, 1/250 is the nearest whole stop above that. f/4.5 is the largest aperture at that FL on that lens. hence, I couldn't have had a larger exposure and met my pictorial constraints, to do with shutter speed in this case.

4-What would be the issues with 1/8000s shutter in this particular scene?

I would have had five stops less exposure and thus a lot more noise.

That was my original post. With high ISO do we need lenses with larger apertures just to allow more light when high ISO can achieve it for you with lesser light.

The only thing 'ISO' achieves is applying a given exposure to brightness function so that the image taken with your chosen exposure looks right at the end.

Which is the point of exposure, to achieve a specific brightness level.

But, you can achieve that specific brightness level with a different exposure just by setting the ISO differently. Does that make exposure pointless?

Yes, you can. In fact, I recall that being a major issue with a collective that I've often argued against.

You keep on saying this, but I can't remember myself or the 'collective' ever having an issue with setting the ISO differently.

The issue wasn't setting the ISO differently, but how ISO has no relevance in determining exposure was this collective all about. And I am fairly confident, more of it just might show up soon.

If you'd said that, I'd have understood what you are getting about. Choosing ISO first is a back to front way of going about selecting your exposure, and will sometimes lead to getting a smaller exposure than you could have had. If a suitable UI were made for cameras, you wouldn't have to do it.

But, since you seem to believe that you can't take ISO out of the equation when considering exposure, would you prefer lowest possible ISO if you can use it? Or, does it not matter?

As I said, I aim for the maximum exposure that I can, subject to my pictorial constraints.

Define "maximum exposure",

Most lux seconds at the image plane. The maximum you can used is either defined by your pictorial constraints or by what your camera can take.

and please do tell how the aperture and shutter speed combinations provide that,

Both of those controls affect the amount of light and thus the exposure.

and that the same exposure would have been impossible at any different speed.

Don't know what you mean by that. Maximising the exposure means largest aperture and slowest shutter spped, but there are competing reasons why you'd want to put limits on that (mainly motion blur and DOF requirements, sometimes using the lens' optimum aperture).

Which ISO is your particular favourite?


Quite often motion blur is more destructive than noise.

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