Comments on: Alpha 550: sky noise, exposure and Auto ISO

By: exposur.es

exposur.es — Mon, 23 Nov 2009 16:13:18 +0000

I’m a little confused. First you talk about getting better results by deliberately UNDERexposing, so that more of the gain is generated via the BIONZ circuitry and less from the inferior sensor gain, but the recommendation for less noise with the A550 is to OVERexpose then adjust in post. This seems contradictory, although I am probably being thoroughly dense.

I would add that I really can’t imagine the typical purchaser of a mid range camera like the A550 being the sort likely to mess around with exposure tweaks + post-processing etc to “trick” his/her camera into producing cleaner skies. In fact I can’t see many professionals having the time or inclination to mess with this kind of thing, let alone real ordinary folk.

By: alphaomega

alphaomega — Mon, 23 Nov 2009 10:34:23 +0000

David
Sorry about the confusion. As I understand it now you over exosed the image by two stops and processed in ACR by (amongst other things) clicking on Auto. That latter process simply returned the image to a normally exposed image and eliminated the sky noise. For some reason you then avoided Auto’s “inbuilt” tendency to over expose. As I said I never use Auto because of that tendency and I must confess I do not have the “courage” to over expose by two stops and then come home and discover I cannot recover all highlights. Maybe the wider dynamic range of A550 is more accommodating than my A350/700.

I am actually more interested in hearing your latest views on the usefulness of A550 at ISO200. Do you still maintain that the quality of ISO200 shots are variable and the image quality is inferior to such cameras as A350 and A700 at low ISO? I did contemplate the purchase of this camera and could even live without P shift etc. if ISO200 quality was consistent, resolving power on a par with A350 and the “flat” images could be made to look a bit like A350 through ACR & PS.

By: admin

admin — Sun, 22 Nov 2009 23:25:01 +0000

The Nikon was not a deliberate comparison, I just happened to shoot a similar view some days apart with the two cameras. The Nikon full sized image is a little sharper as it should be from a lens costing £6000; I showed the comparison because the Alpha 550 does surprisingly well, it interested to notice this when editing the images for other purposes.

I still don’t know what you mean about Auto in ACR brightening a light picture. It doesn’t. Auto will brighten a dark picture, or darken a light one. It will correct exposure errors. The result is exactly the same as doing it manually, though it tends to raise the Black Point a bit high and can sometimes add unwanted Recovery which changes the colour of reds and yellows. If I click Auto I nearly always fine tune the result.

Auto will often over-brighten a normally exposed raw (which is why it is not a good idea to use it as a default). This does not mean it will do the same with either a badly overexposed or underexposed raw.

On dPreview I seem to have caused confusion, one person thinking I am putting the Nikon image there to show it’s better than the A550 – not so, it is there to show how very close the A550 with an 18-250mm superzoom comes to the quality of roughly £10k of professional kit. And all of my comments about noise ONLY apply to raw file capture and processing. I am therefore adding an in-camera JPEG clip to the report.

David

By: alphaomega

alphaomega — Sun, 22 Nov 2009 22:34:51 +0000

David
I simply did not understand your statement above “brightness determined by the simple process of using Auto.” You have over-exposed by two stops as you say & you then use Auto to determine normal brightness. I simply opened an old RAW file from my A350 and applied Auto resulting in a very over-exposed image thinking that Auto is no good to return an over-exposed image to normal brightness. I never use Auto in ACR. I find it totally useless. I always do manual adjustments. I do not understand from your description how you achieved normal brightness by using Auto on an over-exposed image and thus removed sky noise. I would have thought that the only way to return the over-exposed image to normal brightness would be through manual manipulation in ACR and then PP. Maybe I am simply missing something.

As stated above the A550 picture looks better (in my opinion) as the D3s castle looks without contrast and a slightly “funny” colour. I am sure that a touch of the shadow slider would have greatly improved on the looks of the A550 image.

By: admin

admin — Sun, 22 Nov 2009 19:12:09 +0000

Auto certainly doesn’t make an overexposed raw two stops brighter! In fact, Auto leaves the exposure entirely untouched in any of the filetypes I’m currently able to look at – which is most of the current DSLRs available. It adjusts brightness and contrast by default, leaving exposure at 0.0 unless there is a clear exposure error. Can’t work out what you are doing which makes Auto appear to brighten all shots. I would never actually use Auto as a default, it’s so quick just to click in the settings window and you can then see exactly what changes it has made (and over-ride them as you want).

David

By: alphaomega

alphaomega — Sun, 22 Nov 2009 18:26:53 +0000

Not sure if I understand this statement: “And here, finally, is what an adjusted ACR process from the overexposed image looks like with sharpening turned off, NR set to 25/50, exposure and brightness determined by the simple process of using Auto (which can be set as a default in ACR if you want to consistently make generous – over – exposures ‘to the right’)”
Using Auto in ACR on a RAW file switches exposure around two stops brighter. So you take an over exposed image and apply Auto. I would suggest we get a wash-out instead of a well exposed image if this is applied to the above over-exposed image.
I prefer the A550 image to the D3X.