johnske Loc: Townsville

Normally I shoot with a 5 shot bracket 1 or 2 thirds EV apart and then (because I've noted that these give slightly varying shades and hues from shot to shot) I pick the "best" shot for the subject matter to tweak in PP.

This test was initiated by reading various online reviews saying that the optical stabilization on my Sigma 150-500 works so well that it gives a 3 or 4 stop advantage for hand-held shots i.e. instead of using a shutter speed of 1/500sec at 500mm, you can get equally sharp shots at 1/250, 1/125, 1/60 or even 1/30sec. And yep, I found that to be quite true - there was no notable difference in sharpness at all these shutter speeds.

However, I also noted that there IS a difference, though not in sharpness - in exposure (as already mentioned above).

These two shots were shot 2 minutes apart through a screen door about ten feet in front of the lens, and are of a faded sign across the road - both were treated exactly the same in post.

One is shot at 1/500sec at f6.3 with an ISO of 160, the other is shot at 1/250sec at f6.3 with an ISO of 80.

Now, as they were both shot at f6.3, with the ISO difference (160 vs 80) equivalent to one stop, and the shutter speed difference (1/500 vs 1/250) also equivalent to one stop, the exposure triangle says they should have equal exposure values - but they clearly don't, the 1/500sec 160 ISO shows more shadow detail and better colour than the other.


PENTAX K-5, ISO: 80, 1/250 sec, f6.3, 500mm



PENTAX K-5, ISO: 160, 1/500 sec, f6.3, 500mm

Radioman Loc: Ontario Canada

*********

Hi,

Outdoor light can change, even in 2 mins, esp. if there are clouds in the sky.

Both of these have been through photoshop etc. and are not the originals so it is not possible to really comment on them.

johnske Loc: Townsville

Hi Radioman,

Yes, there were 'clouds in the sky', but not in the manner i believe you're referring to. In fact it was totally overcast all day.

As I said, they were treated exactly the same in post - with autolevels. However you can still see the difference in the originals.

None-the-less, as I mentioned, I've seen such differences many thousands of times before when auto-bracketing - i would suggest you try it yourself under whatever experimental conditions you like to impose before disregarding it altogether :)

N.B. FYI i use autolevels with default options set to "Enhance Monochromatic Contrast" and highlights clipped at 0.01% (i.e. RGB channels are all clipped equally)

rpavich Loc: West Virginia

SOMETHING is changing in your scenario that's for sure...I just ran the same test; one shot and then reset the ISO and shutter speed up/down one stop but left the aperture same.

The resultant shots I got MAY be SLIGHTLY different...but it's not what you're showing.

PS: there is DEFINITELY some light in the second shot that's not there in the first one...try that experiment indoors with ONLY lamp light, don't do auto-anything in post, and see if you get the same result.

Searcher Loc: Kent, England

There was a topic on the Hog a while back about this, unfortunately I can't find it. It said that the lowest ISO available did not always give the best exposure results. If that statement is true, then the differences with exposure will be more apparent if you redo your test but using the two lowest ISO's available on your camera.

rpavich Loc: West Virginia

Same results as expected.

GrahamS Loc: Hertfordshire, U.K

You can tell by the sunlight and shadow on the pole in the second shot that the lighting changed.

Searcher Loc: Kent, England

Well I can't see a difference - good test under controlled lighting.

GHK Loc: The Vale of Eden

There is nothing wrong with your reasoning, but the exposures are not the same. Unless you have changed something without realising, it must be a change in the ambient lighting.
GHK

vendav

If you adjust the exposure triangle to include a variation in the ISO value, then surely you must expect some difference in the quality and appearance of the resulting images.
This is the equivalent to the difference between say a fine grained film and a grainy, fast film. No amount of aperture/shutter speed manipulation would make the two shots look the same.
Try again, altering only the aperture and shutter speed, leaving the ISO alone and see if you still have a problem.
Also, picking up on points mentioned earlier, any such test needs to be carried out under fully controlled lighting conditions - not natural light, which can alter without your noticing - with the camera set to manual everything.

selmslie Loc: Fernandina Beach, FL, USA

Do not assume that your shutter is accurate because it is “electronically controlled”.

If you have it tested you may find that, while it us probably very close as slow shutter speeds, by the time you reach 1/250 or more you might see errors of up to a half stop or more, even in the best professional cameras.

BboH Loc: s of 2/21, Ellicott City, MD

The ISO in the first is listed as 80, in the sceond (the brighter) is listed as 160.

rpavich Loc: West Virginia

But he shutter speed was adjusted to create an equal exposure.

mikemilton

On many cameras (all cameras?) ISOs that are not multiples of the basic sensitivity of the sensor are derived. In other words, they actually use the closest multiple and are then changed by a multiplier. ISO is a combination of analog gain and digital compensation.

So, you cannot necessarily use changes in iso in the way you expect because you are not actually shooting at the specified ISO and the in digital compensation is being un done by the auto levels in both the shadow and highlight areas.

Although this speaks to noise, dynamic range is another effect:
http://shootintheshot.joshsilfen.com/2010/05/13/canon-hd-dslr-native-iso/

selmslie Loc: Fernandina Beach, FL, USA

Unfortunately, that article misstates the relationship between ISO values. Powers of two are actually precise steps based on the cube root of 2, just like 1/3 step increments for aperture or shutter speed. They are only rounded off for the convenience for labeling lenses or shutter speed dials.

For example, lens apertures are rounded off from the series: 1.41, 2.00, 2.83, 4.00, 5.66, 8.00, 11.31, 16.00, 22.63, 32.00 … But they are conventionally labeled: 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32 …

ISO values are actually: … 31.50, 39.69 50.00, 63.00, 79.37, 100.00, 125.99, 158.74, 200.00, 251.98, 317.48, 400.00, 503.97, 634.96, 800.00… But they are conventionally given as: … 32, 40, 50, 64, 80, 100, 125, 160, 200, 250, 320, 400, 500, 640, 800…

And fractional shutter speeds follow the same series and convention as ISO values but expressed in the denominator as 1/30, 1/40, 1/50, 1/60 …

So equivalent offsetting incremental changes in ISO, shutter speed and aperture should result in precisely the same exposure for the same lighting. But errors in actual shutter speed performance or imprecision in lens aperture adjustment (and possibly even sensor response) might produce slightly different exposures.